US20250084497A1

US20250084497A1 - Method for Producing Plated Black Heart Malleable Cast Iron Member, and Plated Black Heart Malleable Cast Iron Member

Info

Publication number: US20250084497A1
Application number: US17/439,795
Authority: US
Inventors: Ryo Goto; Takayuki Fukaya; Hiroshi Matsui; Akinori Sawada
Original assignee: Kuwana Metals Ltd
Current assignee: Kuwana Metals Ltd
Priority date: 2019-03-20
Filing date: 2020-03-16
Publication date: 2025-03-13
Also published as: CN113646459A; CN113646459B; JP7375809B2; WO2020189637A1; JPWO2020189637A1

Abstract

Disclosed is a method for producing a plated black heart malleable cast iron member including a black heart malleable cast iron member and a plating layer formed on a surface of the black heart malleable cast iron member, the method including the steps of: performing graphitization in a non-oxidizing and decarburizing atmosphere; performing a particle projection treatment on a surface of the black heart malleable cast iron member obtained after the graphitization such that silicon oxide remains on the surface; immersing the black heart malleable cast iron member obtained after the particle projection treatment in a flux for 3.0 minutes or more; and performing hot-dip plating on the black heart malleable cast iron member obtained after the immersion in the flux.

Description

TECHNICAL FIELD

The present disclosure relates to a method for producing a plated black heart malleable cast iron member, and a plated black heart malleable cast iron member, particularly a pipe joint which is produced by the production method mentioned above.

BACKGROUND ART

Cast irons can be classified into flake graphite cast iron, spheroidal graphite cast iron, malleable cast iron, and the like according to the existence form of carbon. The malleable cast irons can be further classified into white heart malleable cast iron, black heart malleable cast iron, pearlite malleable cast iron, and the like. Black heart malleable cast iron, which is a subject matter of the present invention, is also simply called malleable cast iron and has the form in which graphite is present while being dispersed in a matrix made of ferrite. In a production process of the black heart malleable cast iron, carbon in a cast metal obtained after casting and cooling is present in the form of cementite, which is a compound of carbon with iron. Thereafter, the cast metal is heated to and held at a temperature of 720° C. or higher, so that the cementite is decomposed to precipitate graphite. Herein, the step of precipitating graphite by heat treatment is hereinafter referred to as “graphitization”.
The black heart malleable cast iron is superior in mechanical strength compared to the flake graphite cast iron and also excellent in toughness because its matrix is consisted of ferrite. For this reason, the black heart malleable cast iron is widely used as material for producing automobile parts, pipe joints and the like, which require mechanical strength. The surface of a pipe joint made of the black heart malleable cast iron is often subjected to hot-dip galvanizing to prevent corrosion. The hot-dip galvanized layer has excellent durability and can be formed by plating at a relatively low cost. Thus, the hot-dip galvanized layer is suitable as corrosion prevention means for the pipe joint.
In the prior art, oxides of iron, silicon or the like are more likely to be formed on the surface of a member made of black heart malleable cast iron (hereinafter referred to as “black heart malleable cast iron member”) during graphitization. If a plating layer is formed on the surface with such an oxide thereon, a plating film may not be formed locally and the surface of a base material of the member may be exposed partially (hereinafter sometimes referred to as “bare spots”). Therefore, to form a plating layer with satisfactory adhesiveness to a black heart malleable cast iron member, it is necessary to prepare a black heart malleable cast iron member having a surface on which the formation of oxides is suppressed as much as possible, and to form a plating layer on the surface.
For the purpose of producing a black heart malleable cast iron member having few oxides on its surface, various methods of removing oxides formed on the surface have been studied. For example, Patent Document 1 mentions a method for removing oxides from a black heart malleable cast iron member by immersing it in an acidic solution. This method is sometimes called “pickling”. For example, Patent Document 2 mentions a method for removing oxides formed on the surface of a black heart malleable cast iron member by shot blasting over a long period of time.

PRIOR ART DOCUMENT

Patent Document

- Patent Document 1: JP 2014-19878 A
- Patent Document 2: JP 58-151463 A
- Patent Document 3: WO 2013/146520

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The pickling mentioned in Patent Document 1 has problems, in which the acidic solution itself and gas generated by the reaction with the black heart malleable cast iron are harmful to human bodies and must be carefully handled, and they have a significant environmental impact when the acidic solution is disposed after use or when the generated gas is exhausted outdoors. The method disclosed in Patent Document 2 has a problem of having difficulty in adequately forming a hot-dip plating layer on the surface of a cast iron member.
The present invention has been made in view of the foregoing problems, and an object of the present invention is to produce a black heart malleable cast iron member that allows a hot-dip plating layer to be formed adequately on its surface without performing pickling.

Means for Solving the Problems

A first aspect of the present invention is directed to a method for producing a plated black heart malleable cast iron member including a black heart malleable cast iron member and a plating layer formed on a surface of the black heart malleable cast iron member, the method including the steps of:

- performing graphitization in a non-oxidizing and decarburizing atmosphere;
- performing a particle projection treatment on a surface of the black heart malleable cast iron member obtained after the graphitization such that silicon oxide remains on the surface;
- immersing the black heart malleable cast iron member obtained after the particle projection treatment in a flux for 3.0 minutes or more; and
- performing hot-dip plating on the black heart malleable cast iron member obtained after the immersion in the flux.

A second aspect of the prevent invention is directed to the method for producing a plated black heart malleable cast iron member according to the first aspect, wherein the non-oxidizing and decarburizing atmosphere is an atmosphere in which a partial pressure of oxygen is ten times or less as high as an equilibrium partial pressure of oxygen in chemical formula 1 below and higher than an equilibrium partial pressure of oxygen in chemical formula 2 below.
[Chemical Formula 1]
2Fe(S)+O₂(g)=2FeO(s) (1)
[Chemical Formula 2]
2C(s)+O₂(g)=2CO(g) (2)
A third aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to the first or second aspect, wherein the particle projection treatment is one of shot blasting, shot peening, sand blasting, and air blasting.
A fourth aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to any one of the first to third aspects, wherein a period of time for performing the particle projection treatment is 3.0 minutes or more and 20 minutes or less.
A fifth aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to any one of the first to fourth aspects, further including the step of, before the step of performing the graphitization, preheating the black heart malleable cast iron member at a temperature of 275° C. or higher and 425° C. or lower.
A sixth aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to any one of the first to fifth aspects, wherein the step of performing the graphitization includes first graphitization that includes heating the black heart malleable cast iron member at a temperature exceeding 900° C. and second graphitization that is performed on the black heart malleable cast iron member at a start temperature of 720° C. or higher and 800° C. or lower and at a completion temperature of 680° C. or higher and 780° C. or lower.
A seventh aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to the sixth aspect, wherein at least the first graphitization in the step of performing the graphitization is performed in the non-oxidizing and decarburizing atmosphere.
An eighth aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to any one of the first to seventh aspect, wherein the non-oxidizing and decarburizing atmosphere contains a converted gas generated by combustion of a mixed gas of combustion gas and air.
A ninth aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to any one of the first to eighth aspects, further including the step of, after taking the black heart malleable cast iron member out of the flux, heating the black heart malleable cast iron member to a temperature of 90° C. or higher.
A tenth aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to any one of the first to ninth aspects, wherein the flux is an aqueous solution containing a weakly acidic chloride.
An eleventh aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to any one of the first to tenth aspects, wherein the flux is an aqueous solution containing zinc chloride and ammonium chloride.
A twelfth aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to any one of the first to eleventh aspects, wherein the step of performing the hot-dip plating includes a step of performing hot-dip galvanizing.
A thirteenth aspect of the present invention is directed to the method for producing a plated black heart malleable cast iron member according to any one of the first to twelfth aspects, wherein the black heart malleable cast iron member is a pipe joint.
A fourteenth aspect of the present invention is directed to a plated black heart malleable cast iron member including a black heart malleable cast iron member and a plating layer formed on a surface of the black heart malleable cast iron member, wherein the plating layer is a hot-dip galvanized layer,

- the black heart malleable cast iron member has a work-affected layer on a cast iron surface thereof, and
- the hot-dip galvanized layer contains a silicon oxide.

A fifteenth aspect of the present invention is directed to the plated black heart malleable cast iron member according to the fourteenth aspect, which is a pipe joint.

Effects of the Invention

The method for producing a plated black heart malleable cast iron member according to the embodiment of the present invention enables preparation of the surface of the black heart malleable cast iron member that is suitable for the formation of the plating layer by using the graphitization process essential for production, and can omit the pickling process which is essential in the conventional formation method for the plating layer. Further, by performing the flux treatment defined as the modest particle projection treatment, plating defects can be surely prevented. As a result, the black heart malleable cast iron member having the plating layer can be produced with less burden on the environment and less cost than in the conventional production method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an example of a backscattered electron image of a cross-section near the surface of a black heart malleable cast iron member after the graphitization and before the particle projection treatment in Examples.

FIG. 1B is an element mapping image of silicon in the same area as that in FIG. 1A.

FIG. 1C is an element mapping image of oxygen in the same area as that in FIG. 1A.

FIG. 2 is an example of a backscattered electron image of the surface of the black heart malleable cast iron member after the graphitization and before the particle projection treatment in Examples.

FIG. 3 is an example of a backscattered electron image of a cross-section near the surface of a black heart malleable cast iron member after shot blasting as the particle projection treatment and before immersion in a flux in Examples.

FIG. 4 is an example of a backscattered electron image of the surface of the black heart malleable cast iron member after shot blasting as the particle projection treatment and before immersion in the flux in Examples.

FIG. 5 is an example of a backscattered electron image of a cross-section of a black heart malleable cast iron member that contains a plating layer in its full thickness after hot-dip plating in Examples.

FIG. 6 is an example of a backscattered electron image showing an area near a boundary between the cast iron surface of the black heart malleable cast iron member after the hot-dip plating and the plating layer in Examples.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments for carrying out the present invention will be described in detail below with reference to the accompanying drawings and tables. However, the embodiments mentioned herein are merely examples, and the embodiments for carrying out the present invention are not limited to the embodiments mentioned herein. It should also be noted that the mechanism described herein is merely a hypothesis formulated by the inventors as a reasonable explanation of the facts known at present, and does not limit the technical scope of the present invention.
When plating is applied to a black heart malleable cast iron member such as a pipe joint without pickling in the production process of a plated black heart malleable cast iron member, the quality of the plating depends on the condition of the surface of the black heart malleable cast iron member. The shape of the black heart malleable cast iron member and the plating process conditions also affect the quality of the plating. For example, in the case where a pipe joint is produced as the black heart malleable cast iron member, fine bare spots of several mm or less in diameter tend to be formed on an inner surface of the pipe joint when an immersion time in a plating bath is relatively short. Furthermore, even when the pipe joint with these fine bare spots is immersed again in the plating bath, there is still a problem of difficulty in repairing the bare spots because a plating layer cannot be formed on a part having the bare spots.
Furthermore, the inventors have paid attention to the quality of plating being dependent on the state of immersion of the black heart malleable cast iron member when it is immersed in the plating bath. Specifically, they have paid attention to the fact that plating defects may be caused by the situation in which the black heart malleable cast iron member such as the pipe joint floats upward in the plating bath (hereinafter this phenomenon being sometimes referred to as “plating bath float”) when the pipe joint is immersed in the plating bath. The occurrence of the above-mentioned plating bath float causes problems of plating defects, such as uneven thickness of the plating layer and formation of pinholes caused by bubbles in the plating layer.
The above-mentioned tends to occur especially when the immersion time in a flux is relatively short or when the weight of the black heart malleable cast iron member is relatively light. Further, it also tends to occur when the shape of the pipe joint is complex, as illustrated in Examples below. It is considered that one of the specific reasons for the occurrence of the plating bath float is that immediately after immersing the black heart malleable cast iron member into the plating bath, the flux attached to the surface of the black heart malleable cast iron member is heated quickly in the plating bath, causing any chemical reaction to generate gas at the surface of the black heart malleable cast iron member, and this gas is retained inside the pipe joint as bubbles. However, even when there occurs no bubble or when those bubbles are released to the outside of the pipe joint, the plating bath float may occur in some cases.
The inventors have conducted intensive studies about a method for producing a plated black heart malleable cast iron member in order to solve these problems, particularly, to obtain the plated black heart malleable cast iron member in which plating bath float is suppressed regardless of bubbles, and in which plating defects such as bare spots are suppressed, without pickling which is conventionally performed. As a result, it is found that a black heart malleable cast iron member having a surface suitable for the formation of a plating layer can be obtained by performing graphitization, which is essential to produce a plated black heart malleable cast iron member, in a specific atmosphere, without performing any pickling treatment, and then by performing a modest particle projection treatment and immersion into a flux under specific immersion conditions. Furthermore, it is also found that plating bath float is sufficiently suppressed in the obtained black heart malleable cast iron member during the immersion in the plating bath, allowing a plating layer to be formed adequately during plating formation. The details are described below.
The black heart malleable cast iron member with the plating layer formed thereon is herein referred to as a “plated black heart malleable cast iron member”. In particular, a cast iron part of the plated black heart malleable cast iron member in contact with the plating layer may sometimes be referred to as a “cast iron surface”.

The main material constituting the black heart malleable cast iron member in the present invention is a black heart malleable cast iron. The contents of elements in the black heart malleable cast iron are preferably set to be as follows: 2.0% by mass or more and 3.4% by mass or less of carbon; and 0.5% by mass or more and 2.0% by mass or less of silicon, with the balance being iron and inevitable impurities. When the carbon content is 2.0% by mass or more, a casting operation becomes easier because of good fluidity of a molten metal, thus making it possible to reduce a failure rate due to the flow of the molten metal. When the carbon content is 3.4% by mass or less, the precipitation of graphite can be prevented during a casting process and a cooling process thereafter. When the silicon content is 0.5% by mass or more, the effect of promoting graphitization by the presence of silicon can be obtained, thus enabling the graphitization to be completed in a short time. When the silicon content is 2.0% by mass or less, the precipitation of graphite can be prevented during the casting process and the cooling process thereafter.
Further, the black heart malleable cast iron in the embodiment of the present invention more preferably contains 0.005% by mass or more and 0.020% by mass or less in total of one or two elements selected from the group consisting of bismuth and aluminum. When the total content of bismuth and aluminum is 0.005% by mass or more, the precipitation of graphite can be prevented during the casting process and the cooling process thereafter. When the total content of bismuth and aluminum is 0.020% by mass or less, the graphitization is not significantly inhibited. In addition to these elements, the black heart malleable cast iron in the embodiment of the present invention may contain 0.5% by mass or less of manganese.

In a preferred embodiment of the present invention, it is preferable that the black heart malleable cast iron member before the graphitization is preheated at a temperature of 275° C. or higher and 425° C. or lower. The term “preheating” as used in the present invention refers to a heat treatment in a low-temperature range that is performed on the black heart malleable cast iron member after casting and before the graphitization. By performing the preheating, graphite obtained after the graphitization can be present while being dispersed at positions of crystal grain boundaries of ferrite, which makes the crystal grain size of the ferrite finer than that of a conventional black heart malleable cast iron. In addition, the time required for graphitization can also be shortened. The effect of such preheating is demonstrated more remarkably when the black heart malleable cast iron member contains one or two elements selected from the group consisting of bismuth and aluminum.
Since the present invention relates to the formation of the plating layer, the alloy composition of the black heart malleable cast iron member in the method for producing a plated black heart malleable cast iron member according to the present invention is not limited to a specific alloy composition. The alloy composition in the present invention may be any alloy composition as long as it does not deviate significantly from the range of the alloy compositions mentioned above, which are generally considered as the alloy composition of black heart malleable cast iron. Similarly, the crystal grain size of ferrite after the graphitization is not particularly limited in the present invention. Therefore, the preheating mentioned above is not an essential process in the present invention, and thus it is obviously permissible in the present invention to form a plating layer on the surface of graphitized black heart malleable cast iron without preheating.

In the method for producing a plated black heart malleable cast iron member according to the present invention, heat treatment called “graphitization” is performed, which involves heating and holding the black heart malleable cast iron member obtained after casting, preferably the black heart malleable cast iron member obtained after the above-mentioned preheating, for example, at a temperature of 720° C. or higher. The graphitization is a step inherent in a production method of a black heart malleable cast iron. In the graphitization process, cementite is decomposed, for example, by heating the black heart malleable cast iron member to a temperature exceeding 720° C. corresponding to the A1 transformation point to precipitate graphite, and a matrix made of austenite is cooled to be transformed into ferrite, which can impart the toughness to the black heart malleable cast iron member. It is preferable that the graphitization is classified into first graphitization, which is performed first, and second graphitization, which is performed after the first graphitization.
It is preferable that the first graphitization is a process of decomposing cementite in austenite in a range of temperatures exceeding 900° C. to precipitate graphite. In the first graphitization, carbon separated by decomposing cementite contributes to the formation of graphite. The temperature at which the first graphitization is performed is more preferably 920° C. or higher and 980° C. or lower. The holding time required for the first graphitization varies depending on the size of the black heart malleable cast iron member to be graphitized. When the above-mentioned preheating is performed, the holding time of the first graphitization is preferably set at 30 minutes or more and 3 hours or less, and more preferably 2 hours or less.
It is preferable that the second graphitization is a process of transforming from austenite into ferrite and decomposing cementite in ferrite and/or pearlite in a range of temperatures lower than the temperature at which the first graphitization is performed to precipitate graphite. The second graphitization is preferably performed while gradually decreasing the graphitization temperature from a second graphitization start temperature to a second graphitization completion temperature. Thus, the graphite can be precipitated by gradually reducing the solid solubility of carbon in austenite, ensuring that the transformation from austenite into ferrite proceeds.
The second graphitization start temperature is preferably 720° C. or higher and 800° C. or lower. The second graphitization completion temperature is preferably 680° C. or higher and 780° C. or lower, and more preferably 720° C. or lower and is lower than the second graphitization start temperature. The time required from the start of the second graphitization to the completion thereof also varies depending on the size of the black heart malleable cast iron member to be graphitized. When the above-mentioned preheating is performed, the time of the second graphitization is preferably set at 30 minutes or more and 3 hours or less, and more preferably 2 hours or less. It is preferable that, when shifting from the first graphitization to the second graphitization, the graphitization temperature is lowered from the temperature of the first graphitization to the start temperature of the second graphitization. It is noted that the production method of the embodiment of the present invention does not include lowering the graphitization temperature from the temperature of the first graphitization to a temperature lower than the second graphitization start temperature, for example, the room temperature or the like, and then raising this temperature up to the second graphitization start temperature. The time required to lower the graphitization temperature while shifting from the first graphitization to the second graphitization is not particularly limited.

<Non-Oxidizing Atmosphere>

In the method for producing a plated black heart malleable cast iron member according to the embodiment of the present invention, the graphitization of the black heart malleable cast iron member is performed in the non-oxidizing and decarburizing atmosphere. The term “non-oxidizing atmosphere” as used in the present invention means not only a reducing atmosphere in a strict sense, i.e., an atmosphere that has a partial pressure of oxygen lower than the equilibrium partial pressure of oxygen in chemical formula 1 at the graphitization temperature to be described later, but also an atmosphere in which an iron oxide is not formed to the extent that it interrupts with the formation of a plating layer, through the reaction of iron contained in the black heart malleable cast iron member with gas constituting the atmosphere. That is, the term “non-oxidizing atmosphere” as used in the present invention is based on the wider concept that also includes an atmosphere which does not form an oxide layer with a thickness enough to interfere with the formation of the plating layer. Specifically, it is preferable that the term “non-oxidizing atmosphere” as used herein is that the partial pressure of oxygen in the atmosphere for the graphitization is ten times or less as high as the equilibrium partial pressure of oxygen in chemical formula 1 to be described in detail below. Thus, according to a preferred embodiment, when the equilibrium partial pressure of oxygen in chemical formula 1 at the graphitization temperature is determined, the non-oxidizing atmosphere in the present invention includes a situation in which the partial pressure of oxygen in the graphitization atmosphere is ten times or less as high as the equilibrium partial pressure of oxygen in chemical formula 1, and even a situation in which the partial pressure of oxygen in the graphitization atmosphere is equal to or lower than the determined equilibrium partial pressure of oxygen to perform the graphitization. The partial pressure of oxygen in the graphitization atmosphere is more preferably six times or less, still more preferably three times or less, and yet more preferably equal to or less than the above-mentioned equilibrium partial pressure of the oxygen in chemical formula 1.
A chemical formula representing the typical reaction among iron oxidation reactions is represented by chemical formula 1.
[Chemical Formula 3]
2Fe(S)+O₂(g)=2FeO(s) (1)
Where Fe(s) represents solid iron, O₂(g) represents gaseous oxygen, and FeO(s) represents solid ferrous oxide (wustite). There are known several ion oxidation reactions other than that represented by chemical formula 1, but the oxidation reaction that has the lowest standard Gibbs energy at the graphitization temperature is the reaction represented by chemical formula 1. Therefore, in the atmosphere where the iron oxidation reaction represented by chemical formula 1 is less likely to proceed, any iron oxidation reactions represented by other chemical formulas are less likely to proceed either.
In the non-oxidizing atmosphere where the graphitization is performed, the equilibrium partial pressure of oxygen in chemical formula 1 at the graphitization temperature may be determined, and then the partial pressure of oxygen in the atmosphere may be preferably ten times or less as high as the above-mentioned equilibrium partial pressure of oxygen in the chemical formula 1 as mentioned above. In particular, the partial pressure of oxygen in the atmosphere is preferably equal to or lower than the determined equilibrium partial pressure of oxygen. With such an arrangement, the reaction represented by chemical formula 1 maintains its chemical equilibrium or proceeds from the right to the left of the chemical formula, thereby more sufficiently interrupting with the formation of an iron oxide. The value of the equilibrium partial pressure of oxygen in chemical formula 1 at the graphitization temperature can be determined by calculation using the value of the standard Gibbs energy of chemical formula 1 mentioned in the literature. Table 1 shows an example of the calculation of equilibrium partial pressures of oxygen in chemical formula 1 during the first graphitization (980° C.) and second graphitization (760° C.). For this calculation, reference was made to the value of the standard Gibbs energy mentioned in “NIST-JANAF, Thermochemical Tables”, (U.S.A.), written by M. W. Chase., 4th edition, American Institute of Physics, Aug. 1, 1998

	TABLE 1

	Equilibrium partial pressure of oxygen (atm)

	First graphitization	Second graphitization
Chemical formula	(980° C.)	(760° C.)

(1) 2Fe + O₂= 2FeO	3.4 × 10⁻¹⁶	5.1 × 10⁻²¹
(2) 2C + O₂= 2CO	2.6 × 10⁻¹⁹	2.8 × 10⁻²¹

The partial pressure of oxygen in the graphitization atmosphere needs to be recognized in order to know whether the partial pressure of oxygen in the graphitization atmosphere is equal to or lower than the equilibrium partial pressure of oxygen in chemical formula 1 shown in Table 1 and how many times the partial pressure of oxygen in the graphitization atmosphere is as high as the equilibrium partial pressure of oxygen in chemical formula 1. As a method for measuring the partial pressure of oxygen in the atmosphere, for example, there is a method for directly measuring the partial pressure of oxygen in the atmosphere using a zirconia oxygen concentration meter, a quadrupole mass spectrometer, or the like. However, when measuring extremely low partial pressures of oxygen, such as those shown in Table 1, these direct methods may not achieve sufficient measurement accuracy.
When using converted gas as atmosphere gas for the graphitization, the ratio of the partial pressure of carbon monoxide to that of carbon dioxide in the atmosphere or the ratio of the partial pressure of hydrogen to that of water vapor in the atmosphere can be measured, thereby indirectly calculating the partial pressure of oxygen that is equilibrated with these gases, for example, as described in Patent Document 3. This calculation is performed on the assumption that chemical equilibrium is established in a reaction (2CO+O₂=2CO₂) in which carbon monoxide and oxygen react to generate carbon dioxide or in a reaction (2H₂+O₂=2H₂O) in which hydrogen and oxygen react to generate water vapor in a heat treatment furnace.
In the embodiment of the present invention, as a method for making the graphitization atmosphere non-oxidizing, a well-known method capable of reducing the partial pressure of oxygen can be used. Specific methods include, for example, a method for maintaining the interior of a heat treatment furnace in a high vacuum, a method for filling the interior of the heat treatment furnace with non-oxidizing gas, and the like, but are not limited thereto.
In a preferred embodiment of the present invention, the non-oxidizing atmosphere contains converted gas that is generated by combustion of a mixed gas of combustion gas and air. Since the converted gas can be generated at relatively low cost, the production cost required for the graphitization can be reduced, compared to the case where other non-oxidizing atmospheres are used. Combustion gases that can be used to generate the converted gas include propane gas, butane gas, a mixed gas of the above gases, liquefied petroleum gas, liquefied natural gas, and the like.
A gas generator can be used to generate the converted gas. Increasing the mixing ratio of the air in the combustion gas generates a complete combustion type gas with high CO₂and N₂contents. In contrast, decreasing the mixing ratio of the air generates an incompletely combustion type gas with high CO and H₂contents. The water vapor contained in the converted gas can be partially removed by a refrigeration dehydrator.
When using the converted gas to form a non-oxidizing atmosphere, if the partial pressure of oxygen in the heat treatment furnace, which has been known by any of the above-mentioned methods, is much higher than the equilibrium partial pressure of oxygen in chemical formula 1 shown in Table 1, the partial pressure of oxygen can be lowered either by reducing the mixing ratio of the air mixed in the combustion gas to increase the ratio of the CO gas and the H₂gas, or by decreasing the cooling temperature of the refrigeration dehydrator to lower the dew point of the converted gas. Alternatively, both these methods may be used.
It is noted that in the embodiment of the present invention, the graphitization is performed in the non-oxidizing and decarburizing atmosphere as mentioned later. That is, although the graphitization atmosphere is also the decarburizing atmosphere, it is not so important to make the graphitization atmosphere non-oxidizing, compared to making the graphitization atmosphere decarburizing. In other words, even when an oxide layer is slightly formed on the surface of the black heart malleable cast iron member during the graphitization, the oxide layer does not pose any problem as long as it does not significantly interfere with the formation of a plating layer. Therefore, the term “non-oxidizing atmosphere” as used in the present invention is based on the wider concept as mentioned above.
In a preferred embodiment of the present invention, the second graphitization is performed in the reducing atmosphere, i.e., an atmosphere where the partial pressure of oxygen is lower than the equilibrium partial pressure of oxygen in chemical formula 1 mentioned above. Even when an oxide is formed on the surface of the black heart malleable cast iron member during the first graphitization, the oxide formed once is reduced by performing the second graphitization under the reducing atmosphere, so that the thickness of the oxide can be reduced not to interfere with the formation of the plating layer.

In the method for producing a plated black heart malleable cast iron member according to the present invention, the graphitization atmosphere for the black heart malleable cast iron member is also a decarburizing atmosphere. The term “decarburizing atmosphere” as used in the present invention refers to an atmosphere in which carbon contained in the black heart malleable cast iron member is oxidized by oxygen gas in the atmosphere to become carbon monoxide, and the carbon monoxide gas is detached outward from the surface of the black heart malleable cast iron member, thereby promoting the removal of carbon. This chemical reaction can be represented by chemical formula 2 below.
[Chemical Formula 4]
2C(s)+O₂(g)=2CO(g) (2)
where C(s) represents solid carbon, O₂(g) represents gaseous oxygen, and CO(g) represents gaseous carbon monoxide. The oxidation reactions of carbon include a reaction (C+O₂=CO₂) in which carbon reacts with oxygen to generate carbon dioxide, as well as the reaction represented by chemical formula 2. However, the reaction represented by chemical formula 2 that has a lower standard Gibbs energy proceeds preferentially in a range of temperatures of 720° C. or higher at which the graphitization is performed.
To perform the graphitization under the decarburizing atmosphere, the equilibrium partial pressure of oxygen in chemical formula 2 at the graphitization temperature may be determined, and then the graphitization may be performed in a state where the partial pressure of oxygen in the graphitization atmosphere is higher than the equilibrium partial pressure of oxygen. With such an arrangement, the reaction represented by chemical formula 2 proceeds from the left to the right of the chemical formula 2, whereby carbon contained in the black heart malleable cast iron reacts with oxygen to generate carbon monoxide, which is detached outward to promote decarburizing. As in the case of the above-mentioned chemical formula 1, the value of the equilibrium partial pressure of oxygen in chemical formula 2 at the graphitization temperature can be determined by calculation using the value of the standard Gibbs energy of chemical formula 2 mentioned in the literature. Table 1 shows an example of the calculation of the equilibrium partial pressures of oxygen in chemical formula 2 during the first graphitization (980° C.) and second graphitization (760° C.).
To know whether the partial pressure of oxygen in the graphitization atmosphere is higher than the equilibrium partial pressure of oxygen in chemical formula 2 shown in Table 1, it is necessary to measure the partial pressure of oxygen in the atmosphere. Because the method for measuring the partial pressure of oxygen in the atmosphere has been described above, a description thereof is omitted. When the determined partial pressure of oxygen in the atmosphere is higher than the equilibrium partial pressure of oxygen in chemical formula 2 shown in Table 1, the graphitization can be performed in the decarburizing atmosphere as it is. When the partial pressure of oxygen in the heat treatment furnace is equal to or lower than the equilibrium partial pressure of oxygen in chemical formula 2 in the case of using the converted gas in the atmosphere, the partial pressure of oxygen can be adjusted to be higher than the equilibrium partial pressure of oxygen in chemical formula 2, for example, either by a method of increasing the mixing ratio of the air in the converted gas generator or increasing the dew point of the converted gas. It is noted that the method for adjusting a partial pressure of oxygen is not limited thereto.
In the embodiment of the present invention, since the graphitization is performed in a decarburizing atmosphere, no graphite is formed on the surface of the black heart malleable cast iron member during the graphitization process. Thus, according to the production method of the present invention, the black heart malleable cast iron member that hardly has any graphite formed on its surface can be produced after the graphitization and before formation of the plating layer. Thereafter, the plating layer with excellent adhesiveness can be formed on the surface of the black heart malleable cast iron member.
In the embodiment of the present invention, the graphitization including both the first graphitization and the second graphitization may be performed in the non-oxidizing and decarburizing atmosphere, or otherwise at least the first graphitization is preferably performed in the non-oxidizing and decarburizing atmosphere. In the latter case, it is considered that the second graphitization is performed in an atmosphere which is not a decarburizing atmosphere. However, since the second graphitization is performed at a temperature lower than that in the first graphitization, the rate of precipitation of graphite on the surface of the black heart malleable cast iron member during the second graphitization is slower than that during the first graphitization. Therefore, by performing at least the first graphitization in the decarburizing atmosphere, the effects of the present invention can be obtained.
In this way, the method for producing a plated black heart malleable cast iron member according to the present invention includes the step of performing the graphitization in the non-oxidizing and decarburizing atmosphere. For example, to achieve the non-oxidizing and decarburizing atmosphere during the first graphitization (at 980° C.), the partial pressure of oxygen in the furnace is set to be higher than 2.6×10⁻¹⁹atm, which is the equilibrium partial pressure of oxygen in chemical formula 2 shown in Table 1, and ten times or less as high as 3.4×10⁻¹⁶atm, which is the equilibrium partial pressure of oxygen in chemical formula 1 shown in Table 1, as an example.
As mentioned above, the method for producing a plated black heart malleable cast iron member according to the present invention enables preparation of the surface of the black heart malleable cast iron member that is suitable for the formation of the plating layer by using the graphitization process which is essential for production. In particular, since the graphitization is performed in the decarburizing atmosphere, graphite, which is one of the substances that causes bare spots, is hardly formed on the surface of the black heart malleable cast iron member. Further, since the graphitization is performed in the non-oxidizing atmosphere, there is almost no oxide layer, or if there is, it is extremely thin. Therefore, it is possible to obtain the surface of the black heart malleable cast iron member suitable for plating formation.

In a preferred embodiment of the present invention, the black heart malleable cast iron member after the graphitization and before the particle projection treatment has, on its surface, a ferrite layer with a thickness exceeding 100 μm. The term ferrite layer as used herein refers to a layered microstructure made of ferrite, called α(alfa) phase in an iron-carbon binary phase diagram, that hardly contains carbon. In a preferred embodiment, as the decarburizing proceeds on the surface of the black heart malleable cast iron member, consequently austenite with less carbon content is formed and eventually becomes the ferrite layer with a thickness exceeding 100 μm when cooled after completion of the graphitization. After the formation of the ferrite layer, graphite is not present at the surface of the black heart malleable cast iron member as well as in the inside of the vicinity of the ferrite surface layer. This arrangement is preferable because it can form the more robust plating layer with excellent adhesiveness.
Although the white heart malleable cast iron is subjected to decarburizing in a decarburizing atmosphere, the black heart malleable cast iron and the pearlite malleable cast iron are not normally subjected to the graphitization in a decarburizing atmosphere. However, in the present invention, the graphitization is performed in the decarburizing atmosphere for the purpose of enabling the formation of the plating layer with excellent adhesiveness. Thus, even when the ferrite layer is formed on the surface of the black heart malleable cast iron member, the ferrite layer barely affects the mechanical properties of the black heart malleable cast iron member as long as the thickness of the ferrite layer is not so large.
In the embodiment of the present invention, when a ferrite layer is formed on the surface of the black heart malleable cast iron member, a thin oxide layer of iron may be formed on the surface of the ferrite layer. Even when the oxide layer is formed, the oxide layer can be removed through the particle projection treatment and the flux treatment as post-steps if its thickness is small. The formation of the thin oxide layer is preferable because it can prevent the decarburizing of the surface of the black heart malleable cast iron member from proceeding excessively. The allowable thickness of the oxide layer which can be formed on the surface of the ferrite layer is preferably 20 μm or less, and more preferably 10 μm or less.

In the embodiment of the present invention, silicon oxide is present on the surface of the black heart malleable cast iron member after the completion of the graphitization. As mentioned above, silicon is one of the elements that constitute the black heart malleable cast iron. Silicon is an element that is oxidized more easily than iron and carbon. Thus, even when the graphitization is performed in the non-oxidizing atmosphere in the present invention, it is inevitable that silicon contained in the black heart malleable cast iron is oxidized to form silicon oxide. The silicon oxide formed during the graphitization process is mainly present on the surface of the black heart malleable cast iron member. When the above-mentioned ferrite layer is formed on the surface of the black heart malleable cast iron member, the silicon oxide is present on the surface of the ferrite layer. As mentioned above, the formation of the oxides on the surface of the black heart malleable cast iron member causes the bare spots. However, the present invention can prevent the occurrence of bare spots by performing the particle projection treatment mentioned below, on the silicon oxide present on the surface of the black heart malleable cast iron member.

In the embodiment of the present invention, the particle projection treatment is performed on the surface of the black heart malleable cast iron member after the above graphitization and before the immersion in a flux such that silicon oxide remains on the surface. The particle projection treatment in the embodiment of the present invention is a treatment to the extent that a crack or strain energy is introduced into the surface of the black heart malleable cast iron member, but not a treatment with a strong destructive force that removes an oxide film from the surface of the member to be subjected to the plating as in the conventional technology. It is not preferable to provide the surface of the member with high energy that is enough to remove the oxide film made of silicon oxide or the like because such a high energy makes the formation speed of the plating layer higher than necessary during the formation of the plating layer mentioned below, making it difficult to control the thickness of the plating layer. Therefore, the particle projection treatment is performed to the extent that the silicon oxide remains on the surface of the black heart malleable cast iron. The area of the surface of the black heart malleable cast iron member that is subjected to the particle projection treatment is not necessary the entire surface, but may be a part of the surface of the member.
The particle projection treatment is a treatment of spraying projectile particles at high speed against the surface of the black heart malleable cast iron member, and is classified into mechanical and pneumatic types. The mechanical type treatment includes a method of using the centrifugal force of a vane wheel (impeller) to project projectile particles (media) against a member (workpiece) which is a material to be treated. Specifically, examples of the above-mentioned mechanical type treatment include shot processes, such as shot blasting and shot peening, and sand processes, such as sand blasting. Examples of the pneumatic type treatment include a method in which projectile particles are projected by compressed air (air blasting). To perform the homogeneous particle projection treatment, the number of projection positions provided by the above-mentioned impeller or compressed air is preferably two or more. The above-mentioned material to be treated (workpiece) may be moved, for example, be agitated or rotated during the treatment, or may be fixed.
The material, grain size, hardness, etc., of the projectile particles (media) are not limited. Examples of the material for the projectile particles can include steel, cast steel, stainless steel, alumina, ceramic, glass, silica sand, and the like. Steel balls, grits, and sand are preferred as the material for the projectile particles, in that order. Examples of the shape or form of the projectile particle include a spherical shape, a cut wire having a short columnar shape obtained by simply cutting a metal wire, and a grit with acute-angled corners. The preferred range of the grain size of the projectile particles depends on the size and shape of the material to be treated and the material for the projectile particles. For example, the preferred range of grain size of the projectile particles is 5 to 10 mm when steel balls are used as the projectile particles. By using projectile particles with a grain size of 5 mm or more, a sufficient impact can be applied onto the surface of the material to be treated. In addition, by using projectile particles with a grain size of 10 mm or less, the particles can also be projected even onto recesses of the material to be treated having a complicated shape, and at the same time, the thickness of the plating layer can be prevented from becoming extremely large due to excessive impact. The more preferred range of the grain size of the steel ball used for the projectile particle is 6 to 8 mm.
As one specific example of the particle projection treatment, there is a proposed method in which steel balls with a diameter of, for example, 6 mm to 8 mm are used as the projectile particles, and a large number of the above projectile particles are struck onto the surface of the black heart malleable cast iron member using an impeller while agitating the black heart malleable cast iron member.
In the embodiment of the present invention, since the particle projection treatment is performed on the surface of the black heart malleable cast iron member such that silicon oxide remains on the surface, the black heart malleable cast iron member has the silicon oxide on its surface after the particle projection treatment and before a flux treatment mentioned below. The purpose of the above-mentioned treatment in the present invention is not the removal of the silicon oxide, and thus the silicon oxide remains on the surface of the black heart malleable cast iron member after the above-mentioned treatment. Here, a large amount of the silicon oxide may remain thereon. For example, as shown in Examples below, the area ratio of silicon oxide occupying the surface of the black heart malleable cast iron member after the particle projection treatment may be, for example, 50% or more, even 70% or more, and still even 90% or more, relative to the amount of silicon oxide present on the surface of the black heart malleable cast iron member before the particle projection treatment. Whether silicon oxide remains on the surface of the black heart malleable cast iron member after the particle projection treatment can be confirmed by imaging element mapping images of silicon and oxygen on the surface or cross-section of samples or the like, as illustrated in Examples below.
The surface of the black heart malleable cast iron member, after the particle projection treatment, of the present invention has a work-affected layer subjected to the particle projection treatment. That is, the plated black heart malleable cast iron member obtained as the final product has the work-affected layer on the cast iron surface of the black heart malleable cast iron member.
In the embodiment of the present invention, the projection time, projection speed, projection angle, projection amount, and the like are not particularly limited as long as the silicon oxide remains on the surface of the black heart malleable cast iron member after the above-mentioned treatment. They can be set appropriately according to the size of the material (workpiece) to be treated (when the material to be treated is, for example, a pipe joint, the nominal size is ⅛ to 8 inches). From the viewpoint of differentiating this treatment from a conventional removal treatment of silicon oxide, the projection time can be, for example, 20 minutes or less, preferably 10 minutes or less, and for example, 3.0 minutes or more. The embodiment of the present invention differs from Patent Document 2, which performs shot blasting for a long period of time of 30 to 40 minutes instead of pickling, in that it performs the modest particle projection treatment in this way.
The reason why plating bath float and bare spots are suppressed by the modest particle projection treatment as mentioned above is not yet understood sufficiently, but it can be considered as follows. Silicon oxide present on the surface of the black heart malleable cast iron member can be considered as the substance that causes bare spots and plating bath float. The above-mentioned particle projection treatment deforms and flattens the ferrite layer present on the surface of the black heart malleable cast iron member and causes cracks in the silicon oxide. Because of this, it is considered that stress is introduced into the ferrite layer and silicon oxide on the surface of the black heart malleable cast iron member, which promotes the reaction with a plating solution. In addition, the plating solution can easily reach the silicon oxide as well, which are present while being buried in the ferrite layer. These effects are considered to make it easier for the above silicon oxide to be released when immersed in the plating bath.

The method for producing a plated black heart malleable cast iron member according to the present invention has the following features: graphitization in the non-oxidizing and decarburizing atmosphere; no pickling treatment before the plating formation treatment; and the above-mentioned modest particle projection treatment on the black heart malleable cast iron member. In addition, the method of the present invention has the feature that the black heart malleable cast iron member is immersed in a flux for a predetermined time or more as mentioned above.
As the flux used in the embodiment of the present invention, a well-known weakly acidic chloride aqueous solution suitable for the flux can be used. In general, the flux has functions of forming a thin film on the surface of a member to be plated to thereby improve the wettability with a molten metal and preventing rusting until the hot-dip plating is performed, and as a result, exhibits effects of making the thickness of the plating layer formed on the surface of the member to be plated uniform or improving the adhesiveness of the plating layer to the surface of the member. Thus, the step of immersing the member to be plated, into the flux in the hot-dip plating cannot be omitted. The immersion of the black heart malleable cast iron member into the flux in the present invention also exhibits a unique function of removing the thin oxide layer formed during the graphitization, in addition to the above-mentioned functions.
In the embodiment of the present invention, the immersion in the flux serves to demonstrate the new function of removing the oxide layer formed on the surface of the black heart malleable cast iron member during the casting and graphitizing processes, which can omit the step of removing the oxide by the pickling in the prior art. The flux made of the chloride aqueous solution can be repeatedly used, which eliminates the need to discard an acidic solution when performing pickling. The chemical reaction between the black heart malleable cast iron member and the weakly acidic chloride aqueous solution used in the flux is milder than a chemical reaction between the black heart malleable cast iron member and a strongly acidic solution used in the conventional pickling, and also generates less gas during its treatment. Therefore, the production method according to the present invention can significantly reduce a burden on the environment, compared to the conventional production methods.
When the flux is made of a chloride aqueous solution, the chloride concentration in the chloride aqueous solution is preferably 10% by mass or more and 50% by mass or less. When the chloride concentration is 10% by mass or more, the effect of removing the oxide layer becomes remarkable. The effect of removing the oxide layer does not change so much even when the chloride concentration increases to exceed 50% by mass. When the chloride concentration is 50% by mass or less, the chloride consumed in initial make-up of a flux bath can be saved. In addition, the formed flux film does not become too thick and thus is easily dried. A more preferred concentration of the chloride aqueous solution is 20% by mass or more and 40% by mass or less.
In a preferred embodiment of the present invention, the chloride contained in the flux is one or more of zinc chloride, ammonium chloride, and potassium chloride. The flux is more preferably an aqueous solution containing zinc chloride and ammonium chloride. The ratio of the ammonium chloride content to the zinc chloride content in the flux is preferably 2 or more and 4 or less to 1 in molar ratio. Among them, the flux in which the ratio of the ammonium chloride content to the zinc chloride content is 3 to 1 in molar ratio, that is, the flux in which the ratio of the ammonium chloride content to the zinc chloride content is 54% to 46% in mass ratio is more preferable because it can be easily dried.
When the flux is an aqueous solution containing zinc chloride and aluminum chloride, the temperature of the flux is preferably 60° C. or higher and 95° C. or lower. When the temperature of the flux is 60° C. or higher, the effect of removing the oxide layer becomes remarkable. When the temperature of the flux is 95° C. or lower, boiling of the flux can be prevented, so that the black heart malleable cast iron member can be immersed in the flux more safely, and the oxide layer can also be removed more stably. When the temperature of the flux is 90° C. or higher, hydrolysis of ammonium chloride proceeds to stabilize the concentration of the flux, so that the effect of removing the oxide layer is also enhanced. Thus, the temperature of the flux is more preferably 90° C. or higher.
A preferred time for immersing the black heart malleable cast iron member in the flux depends on conditions, such as the composition, concentration, and temperature of the flux, the degree of deterioration of the flux, the size of the black heart malleable cast iron member, and the thickness of the oxide layer formed on the surface of the black heart malleable cast iron member. The times of immersion in the flux is 3.0 minutes or more, and preferably 5.0 minutes or more and 60 minutes or less. The immersion time of 5.0 minutes or more is preferable because of its remarkable effect of removing the oxide layer. The effect of removing the oxide layer is not significantly affected when the immersion time exceeds 60 minutes. Therefore, the immersion time of 60 minutes or less enables prevention of excessive dissolution of the black heart malleable cast iron member, which can prolong the flux. The immersion time in the flux is more preferably 10 minutes or more and 50 minutes or less, and still more preferably 15 minutes or more and 40 minutes or less. However, when the thickness of the oxide layer formed on the surface of the black heart malleable cast iron member is very thick, the black heart malleable cast iron member may be immersed in the flux for more than 60 minutes.
Repeated immersion of the black heart malleable cast iron member in the flux causes the flux to turn green. This is presumed to be because iron is dissolved in the flux to form iron(II) chloride (ferrous chloride). Further continued use of the flux causes the flux to turn red-brown. This is presumed to be because iron(II) chloride is oxidized to form iron(III) chloride (ferric chloride). Still further continued use of the flux causes further oxidation to form and precipitate iron(III) hydroxide. Since the attachment of iron(III) hydroxide on the surface of the black heart malleable cast iron member causes bare spots, iron(III) hydroxide is preferably removed from the flux by filtration. The concentration of the flux is managed to be within a preferred range while removing the iron(III) hydroxide by filtration, thereby making it possible to continuously use the flux once in the bath for a long period of time.
The concentration of the flux can be managed by well-known means, such as analysis of the specific gravity or pH value of the flux, or chemical components contained in the flux. For example, when using as the flux, a chloride aqueous solution in which the ratio of the ammonium chloride content to the zinc chloride content is 3 to 1 in molar ratio, the concentration of the chloride aqueous solution can be adjusted within a preferred range from 10% by mass or more to 50% by mass or less by adjusting the dissolution amount of solutes so that the specific gravity of the chloride aqueous solution measured at 90° C. becomes 1.05 or more and 1.30 or less. By adjusting the dissolution amount of solutes so that the specific gravity of the chloride aqueous solution measured at 90° C. becomes 1.10 or more and 1.20 or less, the concentration of the chloride aqueous solution can be adjusted within a more preferable range from 20% by mass or more to 40% by mass or less. Even when the concentration of the flux is reduced by continuously using the flux, the concentration of the flux can be managed not to deviate from the preferred range by adding the solutes so that the specific gravity of the flux falls within the above-mentioned range. The specific gravity of the flux can be measured using, for example, a floating balance. The preferred pH range of the flux used in the present invention is 3.0 or more and 6.0 or less.

<Hot-Dip Plating>

The method for producing a plated black heart malleable cast iron member according to the present invention has a step of performing hot-dip plating on the black heart malleable cast iron member taken out of the flux. The plating layer is formed on the surface of the black heart malleable cast iron member by the hot-dip plating. In the production method according to the present invention, graphite is hardly formed on the surface of the black heart malleable cast iron member after the graphitization and before the formation of the plating layer, and the plating layer with excellent adhesiveness can be formed on its surface thereafter through the particle projection treatment and the flux treatment. As the plating layer of the present invention, a plating layer made of a metal or an alloy can be used. Specifically, a metal such as zinc, tin, or aluminum, or an alloy thereof can be used, but the plating layer is not limited thereto. The hot-dip galvanizing is preferably performed.
In a preferred embodiment of the present invention, the step of performing the hot-dip plating includes a step of performing the hot-dip galvanizing. Zinc is preferable because it has a high ionization tendency and the function of sacrificial corrosion protection. When the first plating is hot-dip galvanizing, a zinc layer (η(eta) layer) is formed on the outermost surface of the plated black heart malleable cast iron member, and iron-zinc alloy layers (δ(delta) 1 layer and ζ(zeta) layer) are formed between the zinc layer and the surface of the black heart malleable cast iron member. These layers are firmly adhered to each other, whereby the plating layer with good adhesiveness is formed as a whole.
In the embodiment of the present invention, by performing graphitization in a decarburizing atmosphere, as mentioned above, the ferrite layer can be generated on the surface of the black heart malleable cast iron member after the graphitization and before the formation of the plating layer. The same applies to the case where the ferrite layer is formed, and in this case, ferrite and zinc react with each other to form an alloy layer. After performing hot-dip plating (for example, after the formation of the hot-dip galvanized layer), the ferrite layer may remain inside the plating layer, or alternatively the ferrite layer may disappear.
When the step of performing hot-dip plating includes a step of performing hot-dip galvanizing, the temperature of a galvanizing bath used for hot-dip galvanizing is preferably 450° C. or higher and 550° C. or lower. When the temperature of the galvanizing bath is 450° C. or higher, the solidification of zinc in the galvanizing bath can be prevented. When the temperature of the galvanizing bath is 550° C. or lower, an excessive reaction between the galvanized layer and the surface of the black heart malleable cast iron member can be prevented. A more preferred temperature of the galvanizing bath is 480° C. or higher and 520° C. or lower.
In a preferred embodiment of the present invention, when the step of performing the hot-dip plating includes the step of performing the hot-dip galvanizing, the galvanizing bath used for the hot-dip galvanizing may contain aluminum. When aluminum is molted in the galvanizing bath, the formation of a zinc oxide film on the surface of a molten plating solution in the galvanizing bath is suppressed, thereby making the liquid surface of the plating solution clean. The formed galvanized layer also increases its gloss and improves its appearance.
The method for producing a plated black heart malleable cast iron member according to the present invention can form the plating layer by the hot-dip plating without causing bare spots even when omitting pickling. The reason for this is not necessarily clear, but is presumed to be as follows. The first reason is that there are few substances that cause bare spots on the surface of the black heart malleable cast iron member after the graphitization and before the hot-dip plating. Graphite which is one of the substances that cause bare spots is hardly formed because the graphitization is performed in the decarburizing atmosphere. An oxide layer is hardly formed or is formed extremely thinly if it is present, because the graphitization is also performed in the non-oxidizing atmosphere.
It is considered that, even if the oxide layer partially remains, most of the oxide layer is removed when being immersed in the flux after through the above-mentioned particle projection treatment. When the immersion time in the flux is short, gas generated during the hot-dip plating may be attached as air bubbles on the surface of the member to be plated, causing the above-mentioned plating bath float. The details of the reason for this is not clear, but this is presumed to be because if the immersion time in the flux is insufficient, the substance that would cause the generation of gas still remains on the surface of the black heart malleable cast iron member. However, in the embodiment of the present invention, the plating bath float hardly occurs if the immersion time in the flux is set sufficiently long.
The second reason is that the oxide layer thinly formed on the surface of the black heart malleable cast iron member is peeled off the surface of the black heart malleable cast iron member during the hot-dip plating process, and thereby the oxide layer becomes harmless. When the flux is made of an aqueous solution containing zinc chloride and ammonium chloride, iron oxides on the surface of the black heart malleable cast iron member may chemically react with ammonium chloride to form a black product. The product is normally less likely to be peeled off and thus becomes one of the substances that cause bare spots. However, in the embodiment of the present invention, a phenomenon is observed in which during the hot-dip plating, the black product is peeled off the surface of the black heart malleable cast iron member to float on the surface of the plating solution in the plating bath. From this fact, it is presumed that in the embodiment of the present invention, in a case where the above-mentioned black product is formed, this black product is peeled off during the hot-dip plating process, so that no bare spots occur even when omitting pickling.

The plated black heart malleable cast iron member according to the present invention can be produced by performing hot-dip plating without a heat treatment, after immersing the black heart malleable cast iron member in the flux. As one implementable embodiment of the present invention, the production method may further include the step of heating the black heart malleable cast iron member before performing the hot-dip plating, after taking out the black heart malleable cast iron member from the flux, as mentioned below.
Preheating the black heart malleable cast iron member before the hot-dip plating tends to suppress the occurrence of bare spots. The heating temperature of the black heart malleable cast iron member depends on the size and shape of the black heart malleable cast iron member. When heating the black heart malleable cast iron member, the heating temperature is preferably 90° C. or higher. The heating temperature is more preferably 100° C. or higher and 250° C. or lower. When the heating temperature is 100° C. or higher, the flux can be dried sufficiently, and simultaneously the detoxification by the reaction between the flux and the oxide layer on the surface of the black heart malleable cast iron member is promoted. When the heating temperature is 250° C. or lower, flux peeling and additional oxidation of the surface of the black heart malleable cast iron member can be prevented with no decomposition of the flux due to the increase in the temperature. Further, the more preferred heating temperature is 150° C. or higher and 200° C. or lower.
When heating, known heating means such as a heat treatment furnace can be used. For example, the black heart malleable cast iron member taken out from the flux may be inserted into the heat treatment furnace heated to a predetermined temperature in advance, and then is taken out from the heat treatment furnace when its temperature reaches a desired temperature. Subsequently, the hot-dip plating may be performed on the black heart malleable cast iron member before its temperature drops significantly. In this case, the temperature of the black heart malleable cast iron member is not necessarily a temperature at which the entire black heart malleable cast iron member is heated uniformly, but it suffices that the temperature of at least a part of the surface of the black heart malleable cast iron member on which the flux film is formed reaches the predetermined temperature. However, if a part of the surface of the black heart malleable cast iron member on which the hot-dip plating is to be performed does not reach the predetermined temperature, there is a risk of the occurrence of bare spots on the part of the surface. Therefore, it is preferable that the temperature of the entire surface subjected to the hot-dip plating reaches the above-mentioned preferred temperature range.
The time required for the heating depends on the size and shape of the black heart malleable cast iron member. For example, when a large-sized black heart malleable cast iron member is to be subjected to the hot-dip plating, it is more preferable to heat the member by taking more time to heat according to the heat capacity of the member until the temperature at the center of the member reaches the preferred temperature range. This suppresses a decrease in the temperature of the surface of the black heart malleable cast iron member during the hot-dip plating, and thus can prevent the occurrence of bare spots.
The above-mentioned heating may be performed to sufficiently peel iron oxides on the surface of the black heart malleable cast iron by transforming them into a black product for the purpose of sufficiently suppressing the occurrence of bare spots as mentioned in detail below. On the other hand, bubbles are more likely to be generated on the surface of the black heart malleable cast iron member by the heating mentioned above. When the bubbles are generated, plating bath float easily occurs depending on the shape of the black heart malleable cast iron member. Therefore, one of the embodiments of the present invention includes not performing the above-mentioned heat treatment depending on the shape of the black heart malleable cast iron member from the viewpoint of sufficiently suppressing the plating bath float mentioned above.
The above-mentioned phenomenon in which the black product is peeled off the surface of the black heart malleable cast iron member to float on the surface of the plating solution in the plating bath during the hot-dip plating tends to be remarkably observed especially when the production method further includes the step of heating the black heart malleable cast iron member taken out of the flux before performing the hot-dip plating. The detailed reason for this is not clear, but is presumed to be related to the fact that the black heart malleable cast iron member heated within the preferred temperature range is immersed in the hot-dip plating bath, and the surface temperature of the black heart malleable cast iron member immediately after the immersion is higher than that in the case of immersion without heating. That is, in the case of the immersion without heating, when the flux on the surface of the black heart malleable cast iron member is decomposed in contact with the molten metal, the temperature of the reaction between a decomposition product of the flux and the iron oxide on the surface of the black heart malleable cast iron becomes low, whereby the reaction rate therebetween becomes low. Consequently, it is considered that the entire iron oxides cannot be completely changed into the black product and thus is less likely to be peeled off. In contrast, when immersing the black heart malleable cast iron member in the hot-dip plating bath after the heating, it is considered that the reaction between the decomposition product of the flux and the iron oxide is completed shortly because the reaction temperature as well as the reaction rate are high, thus changing the entire iron oxides into the black product, which is easily peeled off the surface of the black heart malleable cast iron member.
The plated black heart malleable cast iron member of the present invention contains a silicon oxide in the above-mentioned hot-dip galvanized layer. It is presumed that this silicon oxide is incorporated into the hot-dip galvanized layer after being detached from the surface of the black heart malleable cast iron during the hot-dip galvanizing process, as will be mentioned in detail in Examples below. Since the silicon oxide contained in the hot-dip galvanized layer is detached from the surface of the black heart malleable cast iron member, this silicon oxide does not cause bare spots. In addition, the plated black heart malleable cast iron member of the present invention is subjected to the particle projection treatment in the producing process. Thus, the black heart malleable cast iron member has the work-affected layer on the cast iron surface thereof.

A pipe joint is exemplified as the plated black heart malleable cast iron member of the present invention. That is, the present invention may include a method for producing a plated black heart malleable cast iron member when the plated black heart malleable cast iron member is a pipe joint. The plated black heart malleable cast iron member according to the present invention has excellent adhesiveness of the plating layer formed on its surface, and thus can be suitably used for pipe joints that require high corrosion resistance. When using the plated black heart malleable cast iron according to the present invention in the pipe joint, a male or female thread used for the connection of the pipe joint can be provided on the end of the pipe joint by machining after the hot-dip plating is applied.
The plated black heart malleable cast iron member and the pipe joint according to the present invention only need to be provided with the hot-dip galvanized layer, and other layers may be applied on the hot-dip galvanized layer by painting with a thermosetting resin, lining with a thermosetting resin, chemical conversion coating, sputtering of metal, thermal spraying, or the like.

EXAMPLES

Example 1

Molten metal containing 3.1% by mass of carbon, 1.5% by mass of silicon, and 0.4% by mass of manganese, with the balance being iron and inevitable impurities, was prepared. Then, 700 kg of the prepared molten metal was poured into a ladle, to which 210 g (0.030% by mass) of bismuth was then added and stirred, and immediately thereafter the resulting molten metal was poured into a mold to cast a plurality of pipe joints having an elbow shape of the sizes shown in Table 3. The content of bismuth, which had a high vapor pressure, in the pipe joint was less than or equal to 0.020% by mass. After the cast pipe joint was taken out of the mold, it was subjected to modest shot blasting for the purpose of removing casting sand adhering to the surface of the joint. The maximum wall thickness of the resulting pipe joint was approximately 8 mm, and the mass per pipe joint was approximately 900 g.
Next, the resulting pipe joint was preheated in an atmospheric atmosphere at a temperature of 275° C. or higher and 425° C. or lower, followed by graphitization. The graphitization was performed by heat treatment in two stages, namely, first graphitization where the casting was held at 980° C. for 90 minutes, and second graphitization where the temperature was decreased from 760° C. to 720° C. over 90 minutes. The temperature decreasing time required for the transition from the end temperature of the first graphitization to the start temperature of the second graphitization was 90 minutes.
The graphitization was performed using a heat treatment furnace in which the atmosphere was controlled. The heat treatment furnace was supplied with the converted gas generated by an exothermic converted gas generator. The converted gas was generated by mixing of air into a combustion gas composed of a mixture of 30% by volume of propane gas and 70% by volume of butane gas and then by combustion of the mixed gas. The mixing ratio of the air in the mixed gas composed of the combustion gas and the air was set to be between 95.4% by volume and 95.6% by volume.
The generated converted gas was passed through a refrigeration dehydrator set at a temperature of 2° C. to remove part of water vapor, and then supplied to the heat treatment furnace. The total pressure of the converted gas supplied into the heat treatment furnace was the atmospheric pressure. The gas in the heat treatment furnace in each of the first graphitization and the second graphitization was sampled from an outlet of the heat treatment furnace. Then, the concentration of the sampled gas was measured using an infrared absorption CO concentration meter and a CO₂concentration meter, and the dew point of the sampled gas was measured using a dew point meter. Table 2 shows the measured volume percentages of CO and CO₂in the heat treatment furnace, the measured dew points of the sampled gas, and estimated values of the partial pressures of oxygen in the heat treatment furnace determined by the equilibrium calculations. The dew point corresponds to the amount of moisture contained in the gas. The remaining gas not mentioned in Table 2 was hydrogen and nitrogen.

	TABLE 2

	First graphitization (980° C.)	Second graphitization (760° C.)

		Partial				Partial
		pressure of				pressure of
		oxygen in the	Dew			oxygen in the	Dew
CO	CO₂	heat treatment	point	CO	CO₂	heat treatment	point
(vol %)	(vol %)	furnace (atm)	(° C.)	(vol %)	(vol %)	furnace (atm)	(° C.)

First	12.1	5.4	9.8 × 10⁻¹⁶	18.3	11.0	6.0	1.6 × 10⁻²⁰	18.3
Example

When comparing the estimated value of the partial pressure of oxygen in the furnace shown in Table 2 with the equilibrium partial pressure of oxygen shown in Table 1, the partial pressure of oxygen in the furnace during the first graphitization was a value of the same order of 10 to the power of minus 16 as the equilibrium partial pressure of oxygen in chemical formula 1 of 3.4×10⁻¹⁶atm, but was several thousand times as high as the equilibrium partial pressure of oxygen in chemical formula 2 of 2.6×10⁻¹⁹atm. From this fact, it is presumed that the atmosphere of the first graphitization was non-oxidizing and strongly decarburizing.
Then, with regard to the second graphitization, the partial pressure of oxygen in the furnace during the second graphitization was a value of the order of 10 to the power of minus 20 that did not exceed a value of ten times as high as the equilibrium partial pressure of oxygen in chemical formula 1 of 5.1×10⁻²¹atm, but higher than the equilibrium partial pressure of oxygen in chemical formula 2 of 2.8×10⁻²¹atm. From this fact, it is presumed that the atmosphere of the second graphitization was non-oxidizing and decarburizing.
The color of the surface of the pipe joint after the completion of the graphitization was light gray. In the interior of the pipe joint far from its surface, the typical microstructure of black heart malleable cast iron was formed, and this microstructure was made of a ferrite matrix and lump graphite included in the matrix. A ferrite layer of about 200 μm in thickness, which was made of only the ferrite phase, was formed near the surface of the pipe joint. A thin oxide layer of about 20 μm in thickness was formed on the top surface of the ferrite layer.
As Comparative Examples, in experiments Nos. 2 to 5 shown in Table 3, the pipe joints were pickled by being immersed in a 10% hydrochloric acid solution for the period of time listed in Table 3 after the above-mentioned graphitization and before the immersion in the flux bath.
In experiments Nos. 6 and 7 shown in Table 3, the pipe joints were subjected to shot blasting as the particle projection treatment under the following conditions after the graphitization. Meanwhile, in the experiments No. 1 to 5 shown in Table 3, the immersion in the flux mentioned below was performed without shot blasting. The shot blasting was performed under the following conditions. Specifically, by using an apron shot blasting apparatus, a workpiece (material to be treated) was placed in a recess of a rubber loop apron, and then steel balls with a diameter of about 6 mm were projected against the workpiece from projectile means (rotating impellers) installed in two upper locations of the apron, while changing the direction of the workpiece by rotating the apron. In one treatment, the amount of introduction of the workpiece was 400 kg, and its treatment time was set to 10 minutes.
Next, a flux raw material containing 46% by mass of zinc chloride and 54% by mass of ammonium chloride was dissolved in tap water, so that its concentration was adjusted to have the specific gravity of 1.25 at 50° C., and the resulting solution was heated to 90° C., thereby preparing a flux bath. Then, the pipe joint was immersed in the flux bath for the period of time shown in Table 3. The pipe joint taken out of the flux was inserted into a furnace chamber of a muffle furnace heated to 300° C. in an atmospheric atmosphere and heated for 5 minutes. At this time, the pipe joint was heated so that the temperature of its surface was estimated to be 150° C. or higher and 200° C. or lower.
Thereafter, the pipe joint was taken out of the muffle furnace and immediately immersed in a hot-dip galvanizing bath. After one minute elapsed, the pipe joint was taken out of the bath, rinsed with water, dried, and cooled to produce a pipe joint of the black heart malleable cast iron that had a plating layer on its surface. The components of the molten metal used in the hot-dip galvanizing bath contained 0.03% by mass of A1 with the balance being Zn. The temperature of the hot-dip galvanizing bath for all pipe joints was 500° C. or higher and 520° C. or lower. Then, the presence or absence of plating bath float during the immersion in the plating bath was examined. The results are shown in Table 3.

TABLE 3

Presence		Presence
or	Flux	or

Type of pipe joint

Size

Pickling

absence

immersion

absence

Experiment		Product	(nominal)	Test	time	of shot	time	of plating
No.	Shape	name	inches	(pieces)	(minutes)	blasting	(minutes)	bath float

1	Elbow	L	¾	5	—	Absence	15	Presence
2	Different	RT	¾ × ½	4	10	Absence	15	Presence
3	diameter T			1	30	Absence	15	Presence
4	(different			1	60	Absence	15	Presence
	only in
	branch
	diameter)
5	Socket with	RS	1 × ¾	1	10	Absence	15	Presence
	different
	diameter
6	Elbow	L	¾	6	—	Presence	15	Absence
7	Elbow	L	¾	3	—	Presence	10	Absence

As shown in Table 3, in experiment No. 1 where neither pickling nor shot-blasting was performed, plating bath float occurred in the plating bath. Also, in experiments Nos. 2 to 5 where pickling was performed but shot blasting was not, plating bath float occurred in the plating bath. The experiments Nos. 2 to 4 were Comparative Examples in which the pipe joint with the same shape was used and the pickling time was changed, resulting in the occurrence of plating bath float even though the pickling time was increased. As a result of the plating bath float, these pipe joints were more likely to have bare spots.
In contrast, in experiments Nos. 6 and 7, shot blasting was performed under the above-mentioned conditions, followed by the flux treatment and the immersion in the plating bath, so that no plating bath float occurred in the plating bath. As a result, no bare spots occurred.

Example 2

In both the experiment No. 1 (without shot blasting) in Table 3 and the experiment No. 6 (with shot blasting) in Table 3, the pipe joint having an elbow shape with a nominal diameter of ¾ inches was used to examine the number of occurring bare spots by varying the immersion time in the hot-dip galvanizing bath as shown in Table 4. It is noted that the conditions were the same as in Example 1, except for the immersion time in the plating bath.
The outer appearance of the plating layer of the obtained pipe joint was visually observed to determine the presence or absence of so-called “bare spots” where a galvanized layer was not formed. Three pipe joints were prepared for each condition, and the number of pipe joints with bare spots among the three pipe joints was determined. The results are shown in Table 4. It is noted that in each of Examples shown in Table 4, the plating thickness was measured at 10 locations where plating was formed, and it was 70 μm or more.

TABLE 4

Experiment	Immersion	Bare spots (pieces)

No.	time (s)	With shot blasting	Without shot blasting

8	25	0	1
9	30	0	1
10	35	0	1
11	40	0	2
12	45	0	1
13	50	0	1
14	55	0	1

As can be seen from Table 4, no bare spots occurred in the case of performing modest shot blasting, whereas bare spots occurred in the case of not performing shot blasting even when the immersion time in the plating bath was increased. Although not shown in Table 4, it is found that the immersion time required to reach the target film thickness (for example, 70 μm) in the case of performing shot blasting became shorter than that in the case of not performing shot blasting. The reason for these differences is thought to be that as the surface subjected to the shot blasting treatment is activated, the plating is formed more easily in the case of performing shot blasting than in the case of not performing shot blasting even when these cases are the same in terms of the immersion time.
From the results of this example, it can be seen that the method for producing a plated black heart malleable cast iron member according to the present invention enables the formation of an adequate plating layer with suppressed and preferably no bare spots even when pickling after graphitization, which is conventionally required, is omitted. This is thought to be because the graphitization essential to produce a plated black heart malleable cast iron member is performed in the specific atmosphere, and the moderate particle projection treatment is then performed, followed by immersion in the flux under the specific immersion conditions, whereby the black heart malleable cast iron member with the surface suitable for the formation of a plating layer is obtained, and plating bath float is sufficiently suppressed during the immersion in the plating bath, so that the plating layer is adequately formed during the plating formation.

Example 3

For the purpose of examining the effect of the particle projection treatment in the present invention, the metal microstructures near the surfaces of samples obtained after each step was performed were observed using a scanning electron microscope.

FIG. 1A is an example of a backscattered electron image of a cross-section near the surface of a black heart malleable cast iron member after the graphitization and before the particle projection treatment under the same conditions as those of Example 1. The light gray phase shown in FIG. 1A indicates a ferrite matrix. Lump graphite was not seen in the matrix. This is thought to be because the graphitization was performed in the decarburizing atmosphere, thus promoting the chemical reaction shown in Chemical Equation 2 above on the surface of the sample, resulting in the loss of graphite. The ferrite layer made of such a ferrite matrix was present with a thickness of approximately 200 μm in the depth direction from near its surface shown in FIG. 1A.
In an area from the top surface of the sample to a depth of approximately 10 μm shown in FIG. 1A, there were phases of a shape close to a spherical shape, shown in dark gray, which were distributed in the ferrite matrix. The size of the phase of the shape close to the spherical shape was larger than approximately 1 μm. In an area which was even deeper from the top surface of the sample than the area at the depth of approximately 10 μm, there were elongated phases shown in dark gray, as well as finely distributed phases, each being located between the elongated phases, all of which were seen in the ferrite matrix. The width of this elongated phase was finer than 1 μm, and the size of the finely dispersed phase above was even smaller than that width. The thickness of the area where these phases were present was approximately 20 μm.
FIG. 1B is an element mapping image of silicon in the same area as that in FIG. 1A. FIG. 1C is an element mapping image of oxygen in the same area as that in FIG. 1A. The positions where silicon and oxygen were distributed were very consistent with the positions where the above-mentioned dark gray phases were distributed in FIG. 1A. According to an element mapping image of iron (not shown) in the same area as that of FIG. 1A, iron was deficient in parts of the area where silicon and oxygen were concentrated. From these facts, it is considered that the dark gray phase is not due to an oxide of iron but to a phase of silicon oxide. Silicon is an element contained in the black heart malleable cast iron member. The elongated phase is considered to be a silicon oxide phase formed along the crystal grain boundary of the ferrite. The finely distributed phase is considered to be a silicon oxide phase formed in crystal grains of ferrite.
FIG. 2 is an example of a backscattered electron image of the surface of a black heart malleable cast iron member after the graphitization and the previous steps were performed under the same conditions as those of Example 1, before the particle projection treatment. According to an element mapping image (not shown) obtained by photographing the same area as that in FIG. 2 , it is considered that the light or dark gray areas in FIG. 2 indicate silicon oxide, while the white areas and white particles indicate ferrite made of iron, which is a heavy element.

FIG. 3 is an example of a backscattered electron image of a cross-section near the surface of a black heart malleable cast iron member after the shot blasting as the particle projection treatment and the previous steps were performed under the same conditions as those of Example 1, before the immersion in the flux. As in FIG. 1A, the light gray phase indicates a ferrite matrix, and the dark gray phase indicates a silicon oxide phase. A flattened microstructure was seen on the surface of the sample, under which voids was seen. Further, a silicon oxide phase having the shape close to the spherical shape and observed in FIG. 1A was not seen here. The thickness of the area where the silicon oxide phases were distributed was thinner than that in FIG. 1A. It should be noted that the magnification is different between FIG. 1A and FIG. 3 above.
FIG. 4 is an example of a backscattered electron image of the surface of a black heart malleable cast iron member after the shot blasting as the particle projection treatment was performed under the same conditions as those of Example 1 before the immersion in the flux. The white particles of ferrite observed in FIG. 2 above were hardly recognized in FIG. 4 , and instead flat ferrite indicated by white areas was seen. It is recognized that cracks are caused in ferrite, and that many phases of silicon oxide, shown in dark gray, are distributed in a granular form in cracked parts of the ferrite. The granular silicon oxide is also present on the surface of the flat part of the ferrite.
From the observation of the photographs in FIGS. 1 to 4 above, it is considered that the areas containing phases of ferrite and silicon oxide with the shape close to a spherical shape, which were distributed on the top surface of the black heart malleable cast iron member after the graphitization and before the particle projection treatment, were partially removed or crushed in depth by the shot blasting and deformed plastically. In other words, the area close to the surface shown in FIG. 3 is an example of the “work-affected layer” in the present invention.
From the observation of the photographs in FIGS. 1 to 4 above, it is recognized that the silicon oxide phase is not completely removed by the shot blasting, and most of it remains. In particular, it is recognized that a large number of finely dispersed silicon oxide phases between the elongated phases seen in FIG. 1A also remain in deep positions away from the surface in FIG. 3 . That is, in the embodiment of the present invention, the particle projection process is performed in such a way that silicon oxide remains on the surface, as shown in the photographs illustrated above. In contrast, in the prior art, when the oxide phase and the work-affected layer are removed by pickling, the whole silicon oxide is also removed. Therefore, the photographs, such as those shown in FIGS. 3 and 4 above, in which silicon oxide is present on the surface of the black heart malleable cast iron member before the immersion in the flux as a pretreatment for the hot-dip plating, can be indirect evidence indicating that pickling is not performed.

FIG. 5 is an example of a backscattered electron image of a cross-section of the black heart malleable cast iron member containing a plating layer in its full thickness after hot-dip plating was performed under the same conditions as those of Example 1, except that heating after the immersion in the flux was not performed. A dark gray area from the bottom up to ¼ of the whole area of FIG. 5 indicates the cross-section of the black heart malleable cast iron member, and a light gray area above that area indicates the cross-section of the hot-dip galvanized layer. The thickness of the hot-dip galvanized layer was approximately 70 μm. The boundary between these two areas was flat with no gap. According to an element mapping image of iron (not shown) in the same area as that in FIG. 5 , it is found that a relatively large amount of iron was present in an area from the bottom to ⅓ of the plating layer, and almost no iron was present in an area of ⅔ of the plating layer above it. From this, it is considered that the hot-dip galvanized layer has at least two areas consisting of an area that is located near the boundary with the black hear malleable cast iron surface and made of solid solution of iron and zinc, and an area that is located outside of the above-mentioned area and made of a phase with a small amount of iron solid-soluted in pure zinc.
Black phases were distributed in the area close to the surface of the black heart malleable cast iron member shown in FIG. 5 and in the central area of the hot-dip galvanized layer in the thickness direction. According to an element mapping image of silicon (not shown) and an element mapping image of oxygen (not shown) in the same area as that in FIG. 5 , the positions where silicon and oxygen were distributed were very consistent with the positions of the above-mentioned black phases in FIG. 5 . According to an element mapping image of zinc (not shown) in the same area as that of FIG. 5 , zinc was deficient in parts of the area where silicon and oxygen were concentrated. From these facts, it is considered that the black phase is not due to an oxide of zinc but to a phase of silicon oxide.
FIG. 6 is an example of a backscattered electron image showing near the boundary between the hot-dip galvanized layer and the cast iron surface of the black heart malleable cast iron member subjected to the hot-dip plating under the same conditions as those of Example 1 except that heating after the immersion in the flux was not performed. A dark gray area at the lower side of FIG. 6 indicates the cross-section of the black heart malleable cast iron member, and a light gray area above that area indicates the cross-section of the hot-dip galvanized layer. A black silicon oxide phase was present in the vicinity of the black heart malleable cast iron surface in contact with the hot-dip galvanized layer, and this phase has a microstructure due to the work-affected layer illustrated in FIG. 3 above. In the hot-dip galvanized layer, a silicon oxide phase was present at the position located at the upper side of FIG. 6 and slightly apart from the boundary with the black heart malleable cast iron surface. In contrast, almost no silicon oxide phase was present at the position near the boundary with the black heart malleable cast iron surface.
The following can be inferred from the observation of the photographs in FIGS. 5 and 6 above. Since the flux and the plating bath did not contain any compounds containing silicon, the black phases in FIGS. 5 and 6 , which seem to be phases of silicon oxide in the hot-dip galvanized layer, are inferred to be formed as follows: silicon oxide present in the work-affected layer of the black heart malleable cast iron member shown in FIG. 3 was peeled off from the surface of the black heart malleable cast iron member during the hot-dip galvanizing treatment and was then incorporated into the hot-dip galvanized layer.
During the hot-dip galvanizing treatment, the work-affected layer reacts violently with the molten zinc because it has a significant residual stress and contains a large amount of voids. In this reaction process, it may be considered that the oxide layer containing silicon oxide is detached from the surface of the black heart malleable cast iron member and broken apart, and thus remains dispersed in the hot-dip galvanized layer as illustrated in FIGS. 5 and 6 . Therefore, the presence of the silicon oxide phase dispersed in the hot-dip galvanized layer can be the indirect evidence indicating that the particle projection treatment, such as shot blasting, is performed on the surface of the black heart malleable cast iron member after the graphitization without pickling so as to maintain the silicon oxide.
As mentioned above, the distribution of the silicon oxide phases is less observed near the boundary with the black heart malleable cast iron surface. Although the reason for this is not clear, it may be thought that when an area of the hot-dip galvanized layer which is made of a solid solution of iron and zinc is formed, a silicon oxide is not incorporated into the solid solution, but may be discharged into the molten zinc, and then when an area made of less iron and more zinc is solidified, it is solidified while containing the discharged silicon oxide. The fact that there is almost no silicon oxide phase in a location of the plating layer close to the boundary with the black heart malleable cast iron surface is considered to mean that bare spots are prevented during the formation of the plating layer.
The disclosure herein includes the following aspects mentioned in Japanese Patent Application No. 2019-053581 that serves as the basis of priority claim.

First Aspect

A method for producing a plated black heart malleable cast iron member including a black heart malleable cast iron member and a plating layer formed on a surface of the black heart malleable cast iron member, the method including the steps of:

- performing graphitization in a non-oxidizing and decarburizing atmosphere;
- performing a particle projection treatment on a surface of the black heart malleable cast iron member obtained after the graphitization;
- immersing the black heart malleable cast iron member obtained after the particle projection treatment in a flux for 3.0 minutes or more;
- heating the black heart malleable cast iron member to 90° C. or higher after taking out of the flux; and
- performing hot-dip plating on the heated black heart malleable cast iron member.

Second Aspect

The method for producing a plated black heart malleable cast iron member according to the first aspect, wherein the non-oxidizing and decarburizing atmosphere is an atmosphere in which a partial pressure of oxygen is ten times or less as high as an equilibrium partial pressure of oxygen in chemical formula 1 below and higher than an equilibrium partial pressure of oxygen in chemical formula 2 below.
[Chemical Formula 5]
2Fe(S)+O₂(g)=2FeO(s) (1)
[Chemical Formula 6]
2C(s)+O₂(g)=2CO(g) (2)

Third Aspect

The method for producing a plated black heart malleable cast iron member according to the first or second aspect, wherein the plated black heart malleable cast iron member to be immersed in the flux has a silicon oxide on its surface.

Fourth Aspect

The method for producing a plated black heart malleable cast iron member according to any one of the first to third aspects, wherein the particle projection treatment is one of shot blasting, shot peening, sand blasting, and air blasting.

Fifth Aspect

The method for producing a plated black heart malleable cast iron member according to any one of the first to fourth aspects, wherein a period of time for performing the particle projection treatment is 3.0 minutes or more and 20 minutes or less.

Sixth Aspect

The method for producing a plated black heart malleable cast iron member according to any one of the first to fifth aspects, further including the step of, before the step of performing the graphitization, preheating the black heart malleable cast iron member at a temperature of 275° C. or higher and 425° C. or lower.

Seventh Aspect

The method for producing a plated black heart malleable cast iron member according to any one of the first to sixth aspects, wherein the step of performing the graphitization includes first graphitization that includes heating the black heart malleable cast iron member at a temperature exceeding 900° C. and second graphitization that is performed on the black heart malleable cast iron member at a start temperature of 720° C. or higher and 800° C. or lower and at a completion temperature of 680° C. or higher and 780° C. or lower.

Eighth Aspect

The method for producing a plated black heart malleable cast iron member according to the seventh aspect, wherein at least the first graphitization in the step of performing the graphitization is performed in the non-oxidizing and decarburizing atmosphere.

Ninth Aspect

The method for producing a plated black heart malleable cast iron member according to any one of the first to eighth aspects, wherein the non-oxidizing and decarburizing atmosphere contains a converted gas generated by combustion of a mixed gas of combustion gas and air.

Tenth Aspect

The method for producing a plated black heart malleable cast iron member according to any one of the first to ninth aspects, further including the step of, after taking out of the flux, heating the black heart malleable cast iron member to a temperature of 100° C. or higher and 250° C. or lower in the step of heating the black heart malleable cast iron member.

Eleventh Aspect

The method for producing a plated black heart malleable cast iron member according to any one of the first to tenth aspects, wherein the flux is an aqueous solution containing a weakly acidic chloride.

Twelfth Aspect

The method for producing a plated black heart malleable cast iron member according to any one of the first to eleventh aspects, wherein the flux is an aqueous solution containing zinc chloride and ammonium chloride.

Thirteenth Aspect

The method for producing a plated black heart malleable cast iron member according to any one of the first to twelfth aspects, wherein the step of performing the hot-dip plating includes a step of performing hot-dip galvanizing.

Fourteenth Aspect

The method for producing a plated black heart malleable cast iron member according to any one of the first to thirteenth aspects, wherein the black heart malleable cast iron member is a pipe joint.

Fifteenth Aspect

A plated black heart malleable cast iron member including a black heart malleable cast iron member and a plating layer formed on a surface of the black heart malleable cast iron member, wherein the plating layer is a hot-dip galvanized layer,

Sixteenth Aspect

The plated black heart malleable cast iron member according to the fifteenth aspect, wherein the black heart malleable cast iron member is a pipe joint.
The present application claims priority to Japanese Patent Application No. 2019-053581, the disclosures of which are incorporated herein by reference in its entirety.

Claims

1. A method for producing a plated black heart malleable cast iron member comprising a black heart malleable cast iron member and a plating layer formed on a surface of the black heart malleable cast iron member, the method comprising the steps of:

performing graphitization in a non-oxidizing and decarburizing atmosphere;

performing a particle projection treatment on a surface of the black heart malleable cast iron member obtained after the graphitization such that silicon oxide remains on the surface;

immersing the black heart malleable cast iron member obtained after the particle projection treatment in a flux for 3.0 minutes or more; and

performing hot-dip plating on the black heart malleable cast iron member obtained after the immersion in the flux.

2. The method for producing the plated black heart malleable cast iron member according to claim 1, wherein the non-oxidizing and decarburizing atmosphere is an atmosphere in which a partial pressure of oxygen is ten times or less as high as an equilibrium partial pressure of oxygen in chemical formula 1 below and higher than an equilibrium partial pressure of oxygen in chemical formula 2 below.

[Chemical Formula 1]

2Fe(S)+O₂(g)=2FeO(s) (1)

[Chemical Formula 2]

2C(s)+O₂(g)=2CO(g) (2)

3. The method for producing the plated black heart malleable cast iron member according to claim 1, wherein the particle projection treatment is one of shot blasting, shot peening, sand blasting, and air blasting.

4. The method for producing the plated black heart malleable cast iron member according to claim 1, wherein a period of time for performing the particle projection treatment is 3.0 minutes or more and 20 minutes or less.

5. The method for producing the plated black heart malleable cast iron member according to claim 1, further comprising the step of, before the step of performing the graphitization, preheating the black heart malleable cast iron member at a temperature of 275° C. or higher and 425° C. or lower.

6. The method for producing the plated black heart malleable cast iron member according to claim 1, wherein the step of performing the graphitization includes first graphitization that includes heating the black heart malleable cast iron member at a temperature exceeding 900° C. and second graphitization that is performed on the black heart malleable cast iron member at a start temperature of 720° C. or higher and 800° C. or lower and at a completion temperature of 680° C. or higher and 780° C. or lower.

7. The method for producing the plated black heart malleable cast iron member according to claim 6, wherein at least the first graphitization in the step of performing the graphitization is performed in the non-oxidizing and decarburizing atmosphere.

8. The method for producing the plated black heart malleable cast iron member according to claim 1, wherein the non-oxidizing and decarburizing atmosphere contains a converted gas generated by combustion of a mixed gas of combustion gas and air.

9. The method for producing the plated black heart malleable cast iron member according to claim 1, further comprising the step of, after taking the black heart malleable cast iron member out of the flux, heating the black heart malleable cast iron member to a temperature of 90° C. or higher.

10. The method for producing the plated black heart malleable cast iron member according to claim 1, wherein the flux is an aqueous solution containing a weakly acidic chloride.

11. The method for producing the plated black heart malleable cast iron member according to claim 1, wherein the flux is an aqueous solution containing zinc chloride and ammonium chloride.

12. The method for producing the plated black heart malleable cast iron member according to claim 1, wherein the step of performing the hot-dip plating includes a step of performing hot-dip galvanizing.

13. The method for producing the plated black heart malleable cast iron member according to claim 1, wherein the black heart malleable cast iron member is a pipe joint.

14. A plated black heart malleable cast iron member comprising a black heart malleable cast iron member and a plating layer formed on a surface of the black heart malleable cast iron member, wherein the plating layer is a hot-dip galvanized layer,

the black heart malleable cast iron member has a work-affected layer on a cast iron surface thereof, and

the hot-dip galvanized layer contains a silicon oxide.

15. The plated black heart malleable cast iron member according to claim 14, which is a pipe joint.

16. The method for producing a plated black heart malleable cast iron member according to claim 2, wherein a period of time for performing the particle projection treatment is 3.0 minutes or more and 20 minutes or less.

17. The method for producing a plated black heart malleable cast iron member according to claim 2, wherein the step of performing the graphitization includes first graphitization that includes heating the black heart malleable cast iron member at a temperature exceeding 900° C. and second graphitization that is performed on the black heart malleable cast iron member at a start temperature of 720° C. or higher and 800° C. or lower and at a completion temperature of 680° C. or higher and 780° C. or lower.

18. The method for producing a plated black heart malleable cast iron member according to claim 1, wherein an area ratio of silicon oxide occupying the surface of the black heart malleable cast iron member after the particle projection treatment is 50% or more relative to an amount of silicon oxide present on the surface of the black heart malleable cast iron member before the particle projection treatment.

19. The method for producing a plated black heart malleable cast iron member according to claim 2, wherein an area ratio of silicon oxide occupying the surface of the black heart malleable cast iron member after the particle projection treatment is 50% or more relative to an amount of silicon oxide present on the surface of the black heart malleable cast iron member before the particle projection treatment.