JPH02294461A - Carburizing treating method for steel member - Google Patents

Abstract

PURPOSE:To surely improve pitting resistance, wear resistance and toughness by cooling down a steel member to a temp. lower than the A1 transformation point at the specific cooling velocity after executing carburizing treatment to the steel member into the specific surface carbon concn. and successively executing carburizing quenching or carburizing-nitriding quenching. CONSTITUTION:The carburizing treatment is executed to the steel member so that the surface carbon concn. thereof comes to 0.8-1.0%. This carburizing treatment may be executed e.g. by heat-holding at about 900-930 deg.C for about 2-4hr under carburizing gas atmosphere of CO and CO2 series. After that, the steel member is cooled down to a temp. lower than A1 transformation point with 0.5-10 deg.C/min cooling velocity. After that, the steel member is rapidly cooled after holding at about 800-850 deg.C for about 5-20min under the carburizing gas atmosphere of CO and CO2 series or carburizing-nitriding gas atmosphere composed, of adding adequate quantity of NH3 gas to the above series. By this method, without enlarging the variation caused by the heat treatment the pitting resistance, wear resistance and toughness can be surely improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a carburizing method for carburizing and quenching or carbonitriding a steel member.

(Prior art) For steel parts that require pitting resistance and wear resistance, such as automobile gear parts, carburizing and quenching or carbonitriding and quenching are performed to improve these properties. ing.

These hardening processes are performed by heating the steel member in a CO and COZ-based carburizing gas or carbonitriding gas atmosphere and then rapidly cooling it.
However, C is added to steel members to improve hardenability.
It reacts with r, St, Mn, etc. to generate oxides. For this reason, an abnormal structure with poor hardenability is generated on the surface of the steel member, and this abnormal structure becomes an obstacle to improving the pitting resistance and wear resistance of the steel member.

Therefore, recently, as shown in Japanese Patent Application Laid-open No. 62-33757, steel members are carburized in advance, cooled to a temperature below the A1 transformation point, and then reheated. , a method of nitriding has been proposed.

This method aims to improve pitting resistance and wear resistance by improving the abnormal structure caused by the first carburizing treatment by a second nitriding treatment, and also aims to improve toughness by reheating. .

(Problem to be solved by the invention) However, after carburizing a steel member in advance,
If it is cooled to a temperature below the A1 transformation point and then reheated and nitrided, the same level of pitting resistance and
There was a problem that wear resistance and toughness were not improved, and depending on the processing conditions, these improvements were not sufficient.

In view of the above, an object of the present invention is to reliably improve pitting resistance, wear resistance, and toughness by limiting treatment conditions.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present invention aims to improve the surface carbon concentration of a steel member during the carburizing process and the A1 concentration after the carburizing process.
This limits the cooling rate in the process of cooling to a temperature below the transformation point.

Specifically, the solution taken by the present invention is to prepare a steel member with a surface carbon concentration of 0. After carburizing to 8 to 1.0%, cooling at a cooling rate of 0.5 to 10°C/min until reaching a temperature below the A1 transformation point, and then carburizing and quenching or carbonitriding and quenching. It is something to do.

(Function) With the above configuration, the surface carbon concentration of the steel member is 0.8 to 1.
．． Since it is 0%, the necessary amount of C is dissolved in the surface layer to ensure improved pitting resistance and wear resistance, while carbides do not precipitate at grain boundaries in the metal structure after quenching. .

Moreover, since the steel member is cooled to a temperature below the A1 transformation point at a cooling rate of 0.5° C./min or more after carburizing, carbides are not precipitated at grain boundaries during this cooling process.

Furthermore, since the cooling rate in the cooling process is 10° C./min or less, the amount of deformation due to heat treatment does not increase.

(Example) An embodiment of the present invention will be described below.

First, the steel member is heated at a temperature of about 900 to about 930°C in a Co and CO2-based carburizing gas atmosphere so that the surface carbon concentration of the steel member is 0.8 to 1.0%. Carburizing treatment is carried out by holding for about 4 hours. Next, this steel member is cooled at a cooling rate of 0.5 to 10° C./min until it reaches a temperature below the A1 transformation point. After that, this steel member was mixed with CO and CO. In the carburizing gas atmosphere of the system, or with an appropriate amount of N
About 8 in a carbonitriding gas atmosphere with H3 gas added.
After being maintained at a temperature of 0.000 to about 850° C. for about 5 to about 20 minutes, it is rapidly cooled and then carburized and quenched or carbonitrided and quenched.

In this case, the surface carbon concentration of the steel member due to carburizing treatment is 0,
The reasons for limiting the content to within the range of 8 to 1.0% and limiting the cooling rate after carburizing to within the range of 0.5 to 10°C/min are as follows.

That is, if the surface carbon concentration of the steel member is less than 0.8%, the pitting resistance and wear resistance cannot be improved, so the surface carbon concentration of the steel member is preferably 0.8% or more.

Moreover, the surface carbon concentration is O.・If a steel member that has been carburized to a carburization rate of 8% or more is cooled at a cooling rate of less than 0.5°C/min, carbides will precipitate at grain boundaries in the metal structure during this cooling process, resulting in improvement in toughness. Therefore, the cooling rate is preferably 0.5° C./min or more.

Furthermore, if the steel member is cooled at a cooling rate of more than 10° C./min after carburizing treatment, the amount of deformation accompanying the heat treatment will increase, so the cooling rate is preferably 10° C./min or less.

Furthermore, if the surface carbon concentration of the steel member is more than 1.0%,
If cooling is performed at a cooling rate of 10°C/min or less after carburizing, carbides will precipitate at grain boundaries in the metal structure after carburizing or carbonitriding, making it impossible to improve toughness. The carbon concentration is preferably 1.0% or less.

Next, regarding the composition of the steel member, it is preferable to mix the following elements within the ranges described below.

That is, C is preferably within the range of 0.10 to 0.30%. The reason for this is that C is an element necessary to ensure the strength of the core of steel members, and if it is less than 0.10%, its effect is insufficient, and if it exceeds 0.30%, the This is because the hardness becomes too hard, resulting in decreased toughness and poor machinability.

The content of Si is preferably within the range of 0.05 to 0.50%. The reason for this is that, like C, Si is an effective element for improving the strength of steel members, and its effect is insufficient if it is less than 0.05%.On the other hand, this St has a strong tendency to form oxides. This is because it promotes the formation of an abnormal surface layer after carburizing treatment and has a carburizing inhibiting effect, and if it exceeds 0.50%, these disadvantages will occur.

Regarding Mn, it is preferably within the range of 0.30 to 1.80%. The reason is that Mn is an element that improves hardenability, and 0
．． This is because the effect is insufficient if it is less than 30%; 1.
This is because if it exceeds 80%, the hardenability becomes excessive and the machinability deteriorates.

Since P and S are impurities, each has a value of 0.030
% or less is preferable.

Regarding Ni, 2.50% or less is preferable. The reason for this is that Ni is an element that improves hardenability and is effective in improving the toughness of the base, but if it exceeds 2.50%, the effect is saturated and the economic efficiency is impaired.

Regarding Cr, it is preferably within the range of 0.30 to 1.50%. The reason is that Cr is a material that improves hardenability, and 0
．． This is because the effect is insufficient if it is less than 30%, and on the other hand, this C' is also a source of carbide generation, and if it exceeds 1.50%, the carbide generation effect becomes too excessive.

The content of Mo is preferably 0.50% or less. The reason for this is that although Mn is an element that improves hardenability, if it exceeds 0.50%, its effect is saturated and economic efficiency is impaired.

Below, we will discuss the processing details of specific examples and comparative examples carried out to evaluate the present invention, as well as the measurement results of the presence or absence of precipitation of grain boundary carbides and the amount of heat treatment deformation in these specific examples and comparative examples, based on the drawings. I will explain.

As a steel member, seven automatic matching transmission gears made of SCM415H material, module: 1.1, number of teeth: 30 were prepared, and each gear was designated as sample material 1 to test material 7. Then, samples 2 to 4 were used as specific examples, sample materials 1 and 5 to 7 were used as comparative examples, and the following treatments were performed on each sample.

First, as shown in the heat treatment pattern diagram of FIG.
Carburization treatment was carried out by holding at a temperature of 00°C for 3 hours, and the surface carbon concentration was 0.85% for test materials 1 and 2, 1.0% for test materials 3 to 5, and 1.0% for test materials 6. and 7 were set at 1.1%.

Next, for sample material 1, 0. 1℃/min, sample material 2
and 3 at a cooling rate of 0.5°C/min, specimens 4 and 6 at a cooling rate of 10°C/min, and specimens 5 and 7 at a cooling rate of 15°C/min. It was cooled until it reached a temperature below the A1 transformation point.

Next, as shown in the heat treatment pattern diagram in Figure 2, each sample material 1 to 7 was held at a temperature of 870°C for 10 minutes to refine the crystal grains, and then N of
The temperature was maintained at 820°C for 15 minutes in a carbonitriding gas atmosphere containing H3 gas to reduce the surface nitrogen concentration to 0.
．． After reducing it to 2%, it was placed in a salt bath and carbonitrided and quenched.

Furthermore, as shown in the heat treatment pattern diagram of FIG. 3, each of the test materials 1 to 7 was held at a temperature of 170''C for 90 minutes, and then air-cooled and tempered.

The results are shown in Table 1, and the amount of deformation in the tooth trace direction measured after carburizing treatment was as follows: Sample material 1:-3
μm1 Test material 2: -3 μm, Test material 3: -4 μm, Test material 4: -7 μm1 Test material 5: -20 μm1 Test material 6: -
8 μm1 Sample material 7:-22 μm, and regarding the presence or absence of grain boundary carbide precipitation after tempering, test materials 1, 6, and 7 have precipitated grain boundary carbides, and sample materials 2 to 5 have grain boundary carbides precipitated. No boundary carbide precipitated.

(Hereafter, blank space) Sample material 1 has a surface carbon concentration of 0.85%, which is appropriate, but the cooling rate is as low as 0.1°C/min. ), grain boundary carbides were precipitated in the metal structure.

Sample material 5 had a surface carbon concentration of 1.0%, which was appropriate, but the cooling rate was as high as 15° C./min, so the amount of heat treatment deformation was large.

Sample material 6 had a surface carbon concentration of 1.1%, which was too high, so despite the appropriate cooling rate, the metallographic structure remained unchanged, as shown in the micrograph in Figure 5 (400x magnification). Many grain boundary carbides were precipitated inside.

Since sample material 7 had a high surface carbon concentration of 1.1% and a high cooling rate of 15° C./min, grain boundary carbides precipitated in the metal structure and the heat treatment deformed the melon to a large extent.

On the other hand, sample material 2 has a surface carbon concentration of 0.85%,
Cooling rate is 0. At 5°C/min, sample material 3 had a surface carbon concentration of 1.0% and a cooling rate of 0.0%. 5° C./min, each of which was appropriate, so that grain boundary carbides did not precipitate in the metal structure and the amount of heat treatment deformation was small.

In addition, since sample material 4 also has an appropriate surface carbon concentration of 1.0% and a cooling rate of 10°C/min, grains are present in the metal structure, as shown in the micrograph in Figure 6 (400x magnification). No interface carbides were precipitated, and the amount of deformation due to heat treatment was small.

Below, the results of a pitting resistance test conducted using an actual machine to evaluate the present invention will be explained.

This pitting resistance test was conducted at a surface pressure of 190 kgf/
m m 2, number of rotations: 2 0 0 O r. p. I
．． The number of pitting cycles was measured for sample materials 1 and 3 as comparative examples and sample material 6 as a specific example under the test conditions of , lubricating oil = ATF, oil temperature: 90 ° C. The results are as follows. That is, sample material 1 7 3.2X10' cycle EG material 1:
5. 8 x 10' t Ike & sample material 1: 3
．． There were 7 x 10' cycles.

This test result also shows that the number of cycles until pitting occurs in the specific example is greater than that in the comparative example, which supports the effectiveness of the present invention.

(Effects of the Invention) As explained above, according to the carburizing method for steel members according to the present invention, the steel members can be treated with a surface carbon concentration of 0.8 to 1.
．． Carburized to 0%, and the subsequent cooling rate was set to 0.5-10℃/min, ensuring pitting resistance, wear resistance, and toughness without increasing the amount of deformation caused by heat treatment. can be improved.

[Brief explanation of the drawing]

Figures 1 to 3 are heat treatment pattern diagrams for the test materials, with Figure 1 depicting carburizing treatment, Figure 2 depicting carbonitriding and quenching treatment, Figure 3 depicting tempering treatment, and Figure 4. - Figures 6 are micrographs showing the metallographic structure of each sample material, where Figure 4 is sample material 1, Figure 5 is sample material 3, and Figure 6 is sample material 6.
belongs to. Figure Figure Figure

Claims (1)

[Claims] (1) Steel members whose surface carbon concentration is 0.8 to 1.0%
After carburizing, the cooling rate is 0.5-10℃/
1. A method for carburizing a steel member, comprising cooling the steel member until it reaches a temperature equal to or lower than the A_1 transformation point in minutes, followed by carburizing and quenching or carbonitriding and quenching.