DE1292464B - Diffusion-coated ferrous metal body and process for its production - Google Patents

Description

The main purpose of metal coatings is surface protection. Overdone Metals are common materials whose surfaces are resistant to corrosion, Oxidation and abrasion are protected. Many of these metal coatings are electroplated, produced by hot dipping or by spraying on metal. These coatings is considered as a class in common that the pad is one of the metal base represents a distinctly different secondary element and is essentially about mechanical Ties. Usually, such coatings have the disadvantage that the adhesive forces not sufficient to unite the coating and the base metal in the event of deformation Maintain bond. These coatings are accordingly made general only after the base metal has been brought into the desired shape.

From British patent specification 411982 it is known to iron or Steel is exposed to a mixture at temperatures between 650 and 1150 ° C Aluminum and / or magnesium, an inorganic carbide such as carborundum, a To subject ammonium salt and a chloride of chromium. This is where the carbide split into its components, and these combine with the iron, so that the iron is carbonized. But it has been found that a steering of the carbon in the coatings is important in order to achieve good corrosion behavior. the The carbon concentration in the coating should be lower than in the base.

In the earlier patent 1241679 a method for diffusing in of metals in an iron surface described in which to produce a Surface alloy layer made of iron and an element such as chromium, nickel, manganese or cobalt, the diffusion metals are introduced into a non-carbonizing melt, in which as a carrier metals of group II a with a melting point below 900 ° C are included, with the diffusion treatment at a temperature between about 800 ° C and the melting point of the object. After this Process is obtained by diffusion-coated iron surfaces in which the carbon content in the coating is lower than in the coated iron body.

The invention relates to a diffusion-coated ferrous metal body made from a base material made from non-stabilized ferrous metal with a diffusion coating made of a ferritic iron-chromium alloy in which the carbon concentration is lower than in the base material. The diffusion-coated according to the invention Ferrous metal bodies with a surface alloy layer of iron and chromium produced by immersion in a non-carbonizing melt, which acts as a carrier Contains caleium, barium, strontium and / or magnesium and chromium as a diffusion element, the diffusion-coated body obtained after being removed from the melt is deterred.

The products according to the invention can be used with very different Establish chromium concentrations, but, preferably, the chromium concentration will be measured on the coating surface to at least 12 percent by weight to the surface to give the property of a stainless steel. In a particularly preferred way is the chromium concentration of the products according to the invention on the surface of the diffusion coating more than 28 percent by weight. It has been shown that the Products according to the invention, their chromium concentration on the surface of the diffusion coating Exceeds 28 percent by weight, against a known, insidious i type of corrosion, namely the corrosion with crater formation are remarkably resistant.

The ferrous metal body can, for example, be made of cast iron, mild steel or stainless steel. By inducing one or more diffusion elements in addition to an iron body, the process can also be used to convert chromium, that is present in an iron body that has already been treated, to be removed from the body or to deplete it of it, and in this way the surface alloy composition of the body to change.

Metals suitable as transmitters for the process are calcium, barium, Strontium and / or magnesium. The transmitters are preferably used in amounts of at least 10 percent by weight, based on the total weight of the melt, usually each Used alone, but also work satisfactorily in combination with one another. Calcium is preferred; it has been shown to be relatively lower Vapor pressure at the working temperatures, the metal diffusion into that of the treatment subject to iron solids favored. The transformers can be partly with different Diluents can be replaced in order to reduce the amount of carrier required and to modify the transmission properties of the diffusion elements. Examples such diluents are copper, lead, tin and calcium nitride.

The strongest thermodynamic tendency to liquid-solid transition results when the transformer is saturated with chromium in the bath melt.

The liquid-solid transfer leads to the incorporation of chromium the surface of the pad. The diffusion of the element further inwards at the high temperatures at which the material to be treated and Bath takes place, then leads to washing of the coating.

The metal transfer bath contains the metal transfer agent (s), chromium and optionally diluents. Since chromium has limited solubility in the transformer is, the solid diffusion element can be incorporated in excess of both Solid as well as liquid phase. The medium for moving the diffusion elements into the iron body is from the carrier located in the liquid phase of the bath educated. If the solid phase content of the transfer bath is too high, the undesired one occurs Appearance that particles are embedded in the coating surface. The bathroom part, that for a given bath composition to avoid the above problem is to be present as a liquid phase, can easily be used for each bath composition determine. Usually more than half the amount by weight of the bath becomes the liquid phase educated.

The metal transfer bath can be made in a number of ways will. One can put the transmitter and one or more diffusion element (s) together heat to working temperature. You can also use one or more transmission element (s) Manufacture in specified concentrations and add to the molten transmitter, being the mixture then heated to working temperature. You can add chrome to refresh the bath at intervals or continuously Use in dosed quantities to facilitate extensive coating work. The addition of chromium can take place in practically any form, the addition in the form of a fine powder is preferred. The addition can also be made in a non-elementary form, such as with a chromium-nickel or iron-chromium alloy as the source of chromium. Furthermore can one use connections of the diffusion element that go from the transmitter to the metal be reduced; Cr03 or Cr2O3 e.g. B. can easily be reduced to chrome.

The working temperature of the bath is chosen so that the diffusion rate of the elements is favorably influenced and the transmitter (or the transmitter combination) is kept in the molten state. At temperatures below 800 ° C, the diffusion rate is generally too slow. A working temperature of about 1000 to 1200 ° C. is preferred, especially when calcium is used as a transmitter. In practice, the maximum temperature can be seen as the normal boiling point of the transformer, but in any case the working temperature must be below the normal melting point of the iron solid to be treated.

The thickness of the diffusion of a given diffusion element The coating obtained depends on the length of time the iron body remains in the transfer bath influences and can be chosen very differently. For example, you can use a chromium-saturated calcium bath operated at 1100 ° C a coating in one minute of 0.0076 mm.

A special pre-treatment of the ferrous metal body before immersion in the metal transfer bath is not necessary. Good coatings are with the process according to the invention even in the presence of scale or thin C) oil films on the Surface of the backing metal has been obtained. On the other hand, it is natural expedient to work with clean surfaces of the ferrous metal, and to achieve For optimal results, the metal bodies are preferably used for conventional degreasing subject. It is also desirable to have all seams for best results or to remove sharp irregularities from the surface of the backing metal, otherwise the action of mechanical forces can later lead to their removal, leaving the uncoated backing metal unprotected from corrosive environments were. Furthermore, liquid can be used in re-entrant angles in the event of scratches or seams Corrosive substances are retained, leading to an unusually sharp attack being able to lead. The backing metal is preferably polished by polishing for these reasons pre-treated to remove deep scratches or rough edges. surprisingly are with iron backing metals, which are made using the well-known basic oxygen refining slightly better results than available with backing metals, which are refined according to the older technology of the stove-freshening process.

An excellent feature of the method according to the invention is the powerful cleaning action of the transferring agent, especially when calcium is used, on the coatings, which results simultaneously with the coating formation on ferrous metal and which effectively limits the concentrations of carbon and nitrogen in the coatings to maximum levels that are below the values achievable so far. The concentration of other interstitial impurities, such as oxygen, sulfur, phosphorus, etc., is also reduced.

The diffusion process can be used in an advantageous manner to achieve Insert intricately shaped bodies by shaping or machining Iron parts subjected to the diffusion coating treatment long enough to be on them to obtain a coating of the desired thickness (usually 0.08 to 0.25 mm). Man can pierce the coating and remove the iron backing by dissolving it. on in this way, entangled parts of light weight can be made. The in substantially unlimited deposition power of the metal transfer baths (i.e. the ability to Covering small recesses or cavities on the inside) allows the uniform cover intricate shapes.

According to the invention are also new products with a ferrous metal base available that have a diffusion coating of an alloy of ferritic iron and Have chromium and have physical and metallurgical properties that one has long sought to achieve and the too significant improvements the corrosion resistance, the appearance and the deformability compared to before known products obtained by chromium diffusion.

As mentioned, the control has the carbon concentration of the chromium diffusion coating on an iron base due to the strong tendency of the carbon in the underlay detail, at the temperatures necessary for the formation of a diffusion coating to migrate into the chromium-rich coating and in this in the form of carbides des Concentrating iron and chromium presented a particular problem. Because of this difficulty is encountered in the previously known manner on a non-stabilized Iron base bodies produced chromium diffusion coatings with a higher carbon concentration than received in the metal backing. With such products, even if the carbon concentration of the substrate is low, achieving a maximum Corrosion resistance of a chromium-rich alloy coating illusory, and for products in which the carbon concentration of the substrate has a value such as greater than 0.01 percent by weight, in terms of strength and economy is practical and desirable, this concentration relationship finds which one the carbon concentration in the coating exceeds that of the base, theirs Reflected in the emergence of properties by which such products are known by many important uses are excluded. For example, according to the stand the technique by chromium diffusion coating of mild steel of the usual commercial grades manufactured objects no real entrance in the field of decorative, You can find glossy articles because they have iron-chromium-carbides on the coating surface have or form carbides at the grain boundaries, which increase the corrosion resistance and the degree to which such products undergo substantial deformation without Tolerate cracking of the chrome diffusion coating, greatly reduce it.

According to the present invention, it is possible on unstabilized Ferrous metal to achieve a chromium diffusion coating with the above problems be eliminated. Specifically, a body can be made from a non-stabilized iron base obtained with a diffusion coating made of an iron-chromium alloy, in which the The carbon concentration of the coating is lower than that of the base. Further bodies can be achieved that have a non-stabilized iron base with a strength-wise carbon content that meets the practical requirements, preferably of more than 0.01 percent by weight, and a diffusion coating of an iron-chromium alloy with a chromium content of the coating surface of at least 12 percent by weight, the surface the coating is free of iron-chromium carbides. When the method according to the invention with a subsequent, rapid deterrent under appropriately controlled conditions is carried out, bodies of the type described above can be achieved in which the Iron-chromium alloy coating is characterized by grain boundary precipitates to be essentially free of iron-chromium carbides, so that the maximum benefits a corrosion-resistant coating of an alloy of ferritic iron and Chromium are achievable.

The term "non-stabilized ferrous metal base" includes Ferrous metal substrates that contain no more than about 0.2 percent by weight titanium, niobium, Tantalum, zirconium or vanadium or combinations thereof and no more than about 2 percent by weight chromium, manganese, molybdenum or tungsten or combinations thereof contain.

In the examples, the carbon content of the coating and the carbon concentration of the entire body are determined analytically and from this the carbon content of the base is calculated. The whole body carbon concentration is obtained by analyzing the body consisting of the coating and the base obtained in the process. To determine the carbon concentration of the coating, the coating is separated from the body obtained and the coating material isolated in this way is analyzed. For separation, an incision is made along one edge of the body obtained in the process to expose the base material, and then the body is immersed in boiling alloous nitric acid for 1 to 4 hours. The acid dissolves the substrate, leaving behind the chromium diffusion coating, which is determined on carbon. The thickness of the body is measured in the form (Ta) obtained in the process according to the invention and, after the substrate has been removed, the thickness of the coating (TJ, whereupon these values and the values for the carbon concentration of the coating (C i) and the carbon concentration are measured of the whole body (Cb); obtained in the above manner, the carbon concentration of the substrate (C,) is calculated in all cases according to the following formula: Examples 2 to 5 illustrate the manufacture of new products according to the invention from an unstabilized iron base and a diffusion coating made from an iron-chromium alloy, the carbon concentration of the coating being below that of the base.

The products according to the invention, in which the microstructure of the Chromium diffusion layer is directed so that a grain boundary precipitation of carbides Avoided are obtainable according to the invention .. if you put the process in conjunction with a subsequent, rapid deterrent and thereby the period carefully between removing the body from the bath melt and quenching limited. When the body removed from the bath takes significant time to the heavily elevated temperatures are left which they melt through the treatment in the bath received, a redistribution of the carbon in the body can occur to grain boundary precipitates of carbides in the coating as well as an increase the carbon concentration in the coating over the carbon concentration of the substrate underneath.

In general, the coated body should be within about 3 to 40 Introduced into the quenching medium seconds after removal from the coating bath but the expert in the field of heat treatment has it at hand: the optimal conditions using the microstructure marking described here and determine carbon analysis. The cooling limit conditions at which a Grain boundary precipitation is avoided and it is ensured that the carbon concentration the cover is held under that of the underlying surface, will in any given case depend on the thickness of the body being treated as well the effective thermal conductivity of the environment in which the transfer from the Bath is done in the medium, and the quenching medium is determined.

The following examples serve to further illustrate the invention on the basis of preferred working methods. The amounts of the various Unless otherwise specified, ingredients are expressed as percentages by weight. The thicknesses mentioned of the coatings formed on the iron bodies are metallographic or by measuring the thickness of the film obtained after removing the support certainly. The compositions of the surface of the coating are according to the X-ray fluorescence technique certainly. The following first group of examples includes the manufacture of iron alloy coatings explained using a single diffusion element in the diffusion bath. Example 1 The diffusion process advantageously allows, by alteration of the surface properties of austenitic stainless steel is stress corrosion cracking to prevent. Stress corrosion cracking generally occurs under residual stress or when stress is applied and austenitic stainless steel cracks can be observed when the steel is put under tension and on its surface Allowing chloride-containing solutions to act. You can get through previous residual stresses Cancel heat treatment, but cracking in corrosive conditions can result from other stresses that occur during quenching after heat treatment to be introduced.

Since stress corrosion cracking is known to set in on the surface of an austenitic steel, a practical way of solving this problem is to use the diffusion coating process to change the surface concentration of nickel and chromium without changing the bulk of the steel underneath. In a series of tests, Type 304 stainless steel sections are treated using calcium-chromium baths at 1050-1100 ° C and quenched after removal from the bath. After the treatment, the sections are bent into a U-shape and immersed in boiling 42% aqueous magnesium chloride solutions; this test represents the most severe, customary, short-term test for stress corrosion cracking. The results (Table I) clearly show that after only 10 minutes of treatment in the various baths, the resistance of the sections to this type of cracking is markedly increased. Table I. Treatment final surface concentration time in boiling, 42o / oiger, Bath time o / o aqueous M9C12 solution until cracking Minutes Cr I Ni hours Control attempt ................... - 18 8 25 1. Ca-Cr .......................... 10 50 2 180 2. Ca-Cr. . . . . . . . . . . . . . . . . . . ... . . . . i / 4 26 4 90 60 58 2 130 180 62 2 881 The diffusion baths 1 and 2 contain 600 g calcium and 60 g chromium and are operated at about 1060 ° C. As the table shows, the nickel concentration in the steel body decreases. Since these baths do not contain any nickel, there is no equilibrium distribution of the nickel in the baths and the treatment bodies and thus some of the nickel diffuses from the surface of the treatment body into the bath.

Example 2 An iron container with a bath of 1800 g calcium and 72 g of powdered chrome (- 325 mesh) is heated to 1140 ° C. In the bathroom will a section of Al-killed steel (type 1008) treated for 9 minutes and the coated one Section then removed and, starting from a temperature of approximately 1000 ° C, quickly put off. A coating of 0.025 mm is obtained on the steel base, which analytically results in 38% Cr on the surface. The analysis gives a carbon concentration of the coating or the whole body of 93 and 401 ppm, and the carbon concentration of the base is calculated to be 407 ppm.

Example 3 An iron container with a bath of 2300 g calcium and 115 g of powdered chrome (- 325 mesh) is heated to 1140 ° C. In the bathroom that is moved with a mechanical stirrer, a section is made of steel of the SAE type 1070 of 0.51 mm thickness entered for 5 minutes, then the coated section from the Removed bath and, starting from approx. 1000 ° C, quickly quenched. On the steel base a coating of 0.017 mm is obtained. The analysis for carbon shows for the Coating 240 ppm and for the whole body 1860 ppm. The carbon concentration of the document is calculated to be 1973 ppm.

Example 4 An iron container with a bath of 2000 g calcium and 100 g of powdered chromium (-100 mesh) is heated to 1100 ° C. In the bathroom that agitated by a mechanical stirrer, a section of edge steel becomes 45 minutes of type SAE 1008 with a thickness of 1.52 mm, the coated section is then removed and, starting from a temperature of approximately 1000 ° C, rapidly cooled. You get a coating of 0.043 mm on the base metal, the analytical on the surface 4311 / o chromium yields. Analysis for carbon gives 110 ppm for the coating and 336 ppm for the whole body. The carbon concentration of the base calculated at 350 ppm.

EXAMPLE 5 A variety of other samples are coated in a manner similar to Example 4, and the carbon concentration of the chromium diffusion coating and the chromium diffusion coating plus the substrate are then determined analytically, and the carbon concentration of the base is calculated from the analysis values. The results (Table II) show that with the most varied of carbon concentrations on the substrate, the carbon concentration of the coating is always below that of the substrate. Table 1I Steel thickness chrome on the coating carbon concentration test type of steel thick coating surface total ratio mm mm% body I base coated C c : Cs 1 1070 0.508 0.017 1860 1973 240 0.12 2 1008 2.286 0.028 38.2 363 368 67 0.18 3 1070 1.626 0.043 43.2 820 850 152 0.18 4 1008 2.286 0.025 37.5 401 407 93 0.23 5 1008 1.524 0.043 43.0 336 350 110 0.31 6 1070 0.508 0.025 634 676 248 0.37 7 1008 0.508 0.030 39.5 368 394 182 0.46 8 1008 0.508 0.028 34.0 125 133 65 0.49 9 1008 2.286 0.025 419 424 298 0.70 As Examples 2 to 5 show, the new products according to the invention can easily be prepared using calcium as a carrier. Since a sufficient amount of carbon in terms of strength should remain in the base and in view of the fact that the calcium baths have a strong initial carbonization effect, coating times that are too long should be avoided. The table above provides a guideline for the amount of carbon that remains in the base depending on the initial concentration, the thickness of the base and the coating thickness. The limit values of the coating time and coating temperature at which a given carbon content is retained in a metal base of a given type and thickness and the carbon concentration of the coating is kept below that of the base can be determined in a simple experiment. The body obtained from the coating bath is quickly quenched. In order to achieve optimal advantages, the coating process is carried out at over 1000 ° C; - so that the treated body can be quenched from a temperature of at least over 900 ° C. "Rapid quenching" accordingly means a rapid transition of the body, e.g. B. in a period of the order of 3 to 40 seconds, to understand from the bath melt in a coolant that quickly dissipates the heat. A suitable coolant is oil. Most quench oils are satisfactory in this regard, but it is useful to verify by testing any given oil that it does not char in contact with the hot metal surface and thus introduce additional carbon into the coated. Immersing the treated body in a fluidized bed or placing it in a rapid gas flow, such as a helium flow, has also proven to be suitable in order to successfully dissipate the heat quickly after coating. Water is also a satisfactory coolant if careful care is taken to avoid igniting the hydrogen produced when water and calcium come together.

To explain the products according to the invention, which is a non-stabilized Iron pad with a carbon content of over 0.01 percent by weight and one Diffusion coating made of an alloy, free of iron-chromium carbides, on the surface made of ferritic iron and chromium are to be checked for the presence of Iron-chromium carbides on the coating surface X-ray diagrams in the example 2 to 5 samples obtained and the results compared with those obtained in a corresponding investigation of chromium diffusion coatings which, according to prior art chrome diffusion technicians on similar, non-stabilized steels have been manufactured.

An examination of the samples obtained in Examples 2 to 5 is based on The radiographic route has shown peaks, all of which are space-centered cubic Ferrite carriers could be assigned. Peaks corresponding to an iron-chromium carbide could not be determined.

An examination of the corresponding prior art samples resulted in peaks to be assigned to iron-chromium-carbide in each case. Samples that by coatings were made of steels with a lower initial carbon content, showed relatively weaker diagrams of the iron-chromium carbides, but in all Cases and in all other investigations of state-of-the-art products, their carbon content in the unstabilized base is 0.01 percent by weight exceeded, there was clearly an iron-chromium carbide on the coating surface identify.

The original ferritic iron alloy diffusion coating can in the form of a very thin self-supporting film, which as such possesses distinctly unique properties by taking advantage of the inventive Products in the manner described above removes the metal base. You get in this way films made from the ferritic iron-chromium alloy with a chromium content of at least 12%, the thickness of which is of the order of about 0.0126 to 0.076 mm or, if desired, above and across the film has a chromium concentration gradient own. Such films stand out over the films made by after the stand iron bodies coated by chromium diffusion can be obtained using technology, characterized by the fact that the film on the surface with the highest chromium concentration is free of iron-chromium carbides. Furthermore, such films are those of the special preferred products according to the invention are obtained, which with respect to the Corrosion resistance is of particular concern of grain boundary precipitates free of iron-chromium carbides.

Claims (5)

Claims: 1. Diffusion-coated ferrous metal body, characterized through a base material made of non-stabilized ferrous metal with a diffusion coating made of a ferritic iron-chromium alloy in which the carbon concentration is lower than in the base material. 2. Procedure for Fabrication of a body according to claim 1 having a surface alloy layer of iron and chromium, by immersion in a non-carbonizing melt, which acts as a carrier Contains calcium, barium, strontium and / or magnesium and chromium as a diffusion element, characterized in that the diffusion-coated body thus obtained after Removal from the melt is quenched. 3. The method according to claim 1, characterized characterized in that the body to be treated is in a melt of a temperature is immersed between 1000 and 1200 ° C. 4. The method according to claim 2 or 3, characterized in that a melt is used in which a metal carrier is contained in an amount of at least 10 percent by weight. 5. Procedure according to Claims 2 to 4, characterized in that the body of a temperature is quenched above 900 ° C.