WO2008124241A2

WO2008124241A2 - Deep low temperature case hardening

Info

Publication number: WO2008124241A2
Application number: PCT/US2008/056564
Authority: WO
Inventors: Sunniva R. Collins; William H. Glime; Charles W. Hayes; Gerhard H. Schiroky; Peter C. Williams
Original assignee: Swagelok Company
Priority date: 2007-04-05
Filing date: 2008-03-12
Publication date: 2008-10-16
Also published as: WO2008124241A3

Abstract

The depth of the altered surface layer produced by low temperature carburizing a stainless steel workpiece can be significantly increased by introducing fissures or interstices into the workpiece's surface before low temperature carburization is completed.

Description

DEEP LOW TEMPERATURE CASE HARDENING

Related Applications

[0001] The present application claims the benefit of pending United States provisional application serial number 60/921,936 filed on April 5, 2007, for DEEP LOW TEMPERATURE CASE HARDENING, the entire disclosure of which is fully incorporated herein by reference.

Background

[0002] Case hardening is a widely used industrial process for enhancing the surface hardness of shaped metal articles. In a typical commercial process, the workpiece is contacted with a gaseous carbon compound at elevated temperature whereby carbon atoms liberated by decomposition of the carbon compound diffuse into the workpiece' s surface. Hardening occurs through the reaction of these diffused carbon atoms with one or more metals in the workpiece thereby forming distinct chemical compounds, i.e. carbides, followed by precipitation of these carbides as discrete, extremely hard, crystalline particles in the metal forming the workpiece's surface. See, Stickels, "Gas Carburizing", pp 312 to 324, Volume 4, ASM Handbook, © 1991, ASM International.

[0003] Carbide precipitates not only enhance surface hardness, they can also promote corrosion. For this reason, stainless steel is rarely case hardened by conventional gas carburization, since the corrosion resistance of the steel is compromised.

[0004] In the mid 1980's, a technique for case hardening stainless steel was developed in which the workpiece is contacted with a carburizing gas at low temperature, typically below 500° C (932° F). At these temperatures, and provided that carburization does not last too long, carbon atoms diffuse into the workpiece surfaces, typically to a depth of 20-50 μ, without formation of carbide precipitates. Nonetheless, an extraordinarily hard case (surface layer) is obtained, which is believed due to the stress placed on the crystal lattice of [0005] This technique, which is referred to a "low temperature carburization," is described in a number of publications including U.S. 5,556,483, U.S. 5,593,510, U.S. 5,792,282, U.S. 6,165,597, U.S. 6,547,888, EPO 0787817, Japan 9-14019 (Kokai 9-268364) and Japan 9- 71853 (Kokai 9-71853). The disclosures of these documents are incorporated herein by reference.

Summary

[0006] In accordance with this invention, it has been found that much thicker carbon- hardened surfaces free of carbide precipitates can be formed by low temperature carburization in a workpiece made of stainless steel or other alloyed metal if the workpiece is formed or treated, prior to completion of low temperature carburization, so that its surfaces define fissures and/or interstices therein.

[0007] Thus, this invention provides a process for producing a metal product having an altered surface layer, the process comprising subjecting a metal workpiece to a low temperature diffusion-based surface treatment to produce an altered surface layer containing an increased concentration of interstitially diffused atoms in an amount sufficient to alter the properties of the metal forming the surface layer, the low temperature diffusion-based surface treatment being carried out under conditions so that the altered surface layer is free of precipitates of compounds formed from the diffused atoms, wherein the surfaces of the metal workpiece subjected to the low temperature diffusion-based surface treatment define fissures, interstices or both.

[0008] In addition, this invention provides new metal products made by this process.

[0009] hi a particular embodiment, this invention provides a low temperature carburized stainless steel product having an altered surface layer which contains an increased concentration of carbon atoms relative to the base metal from which the product is made, which is harder than the base metal from which the product is made, and which is free of carbide precipitates, the altered surface layer defining fissures, interstices or both so that the increased concentration of carbon atoms extends into the interior of the product to a depth which extends below the fissures and interstices.

DETAILED DESCRIPTION

[0010] According to this invention, low temperature carburized workpieces formed from stainless steel and other austenitic metals are provided with much thicker carbide-free hardened surface layers than possible in the past by forming or treating the workpiece prior to completion of low temperature carburization so that its surfaces to be subjected to low temperature carburization define fissures and/or interstices therein.

Low Temperature Carburization

[0011] The primary focus of this invention is on the low temperature carburization of iron-, nickel- and cobalt-based alloys, especially stainless steel. In this process, which is extensively described in the above-noted U.S. 5,556,483, U.S. 5,593,510, U.S. 5,792,282, U.S. 6,165,597, U.S. 6,547,888, EPO 0787817, Japan 9-14019 (Kokai 9-268364) and Japan 9-71853 (Kokai 9-71853), elemental carbon diffuses into the metal matrix forming the workpiece without formation of carbide precipitates.

[0012] Low temperature carburization normally produces an outermost oxide surface layer on the workpiece being treated about 20-30 nm thick. See, Japan 9-71853 (Kokai 9-71853). Depending on the carburizing conditions, this outermost oxide surface layer may also be covered with soot. In addition, under this oxide surface layer, an extremely thin outer surface layer of the metal may contain a small amount of carbide precipitates, especially if the low temperature carburization conditions are too severe. See, U.S. 5,556,483, U.S. 5,593,510 and U.S. 5,792,282. hi order for the workpiece to exhibit an attractive metallic appearance, this soot and outermost oxide surface layer must be removed, hi addition, this extremely thin outermost metal surface layer must also be removed in order for the workpiece to exhibit good corrosion resistance, at least if this outermost metal surface layer contains carbide precipitates. Therefore, as a practical matter, these unwanted by-products of the low temperature carburization process (i.e., the soot, oxide surface layer, and thin outermost metal layer containing carbide precipitates, if any) are removed before the workpiece is used. Accordingly, in the context of this disclosure, reference to a workpiece surface layer which is "free of carbide precipitates" or which is made "without formation of carbide precipitates" refers to the carbon-hardened surface layer remaining after the unwanted by-products of the low temperature carburization process (i.e., the soot, oxide surface layer, and thin outermost metal layer containing carbide precipitates, if any) are removed.

[0013] In this invention, low temperature carburization is carried out in the same way as done in the past by contacting the workpiece with a carburizing gas at an elevated temperature for a time sufficient to produce in the primary surface layer of the workpiece (i.e. the surface layer of the workpiece after the unwanted by-products of the low temperature carburization process are removed) an elevated amount of elemental carbon without formation of carbide precipitates. For this purpose, the carburization temperature will normally be no greater than about 500° C, although higher temperatures can be used by following the modified approach described in commonly assigned U.S. 6,547,888. Moreover, carburization will normally last 20-50 hours, although longer or shorter processing times can be used. As a result, a primary surface layer typically about 20-5 Oμ thick and normally containing about 2-15 atomic %, more typically about 5-10 atomic % or even 9-12 atomic % atomic carbon will be obtained.

[0014] Incidentally, because low temperature carburization is a diffusion-based process, the concentration of carbon in the workpiece surfaces decreases from a maximum at or very near the outermost surface of the workpiece down to an equilibrium value (which is the carbon concentration in the "native" or untreated metal from which the workpiece is made) in accordance with Fick's law. Thus, it will be understood that the above reference to a carbon concentration of about 2-15 atomic %, for example, means that this is the carbon concentration at or near the outermost edge of the primary surface layer of the workpiece.

Alloys

[0015] The present invention will normally be carried on workpieces made from iron or nickel-based alloys. Such materials are well known and described for example in the above-noted US Patent No. 5,792,282, U.S. Patent No. 6,093,303, U.S. Patent No. 6,547,888, EPO 0787817 and Japanese Patent Document 9-14019 (Kokai 9-268364).

[0016] Particular alloys of interest are steels, especially steels containing 5 to 50, preferably 10 to 40, wt.% Ni. Preferred alloys contain 10 to 40 wt.% Ni and 10 to 35 wt.% Cr. More preferred are the stainless steels, especially the AISI 300 series steels. Of special interest are AISI 301, 303, 304, 309, 310, 316, 316L, 317, 317L, 321, 347, CF8M, CF3M, 254SMO, A286 and AL6XN stainless steels. The AISI 400 series stainless steels and especially Alloy 410, Alloy 416 and Alloy 440C are also of special interest.

[0017] Particular nickel-based alloys which can be low temperature carburized in accordance with this invention include Alloy 600, Alloy 625, Alloy 825, Alloy C-22, Alloy C-276, Alloy 20 Cb and Alloy 718, to name a few examples. [0018] In addition to iron- and nickel-based alloys, low temperature carburization in accordance with the present invention can also be practiced on cobalt-based alloys as well as manganese-based alloys. Examples of such cobalt-based alloys include MP35N and Biodur CMM, while examples of such manganese-based alloys include AISI 201, AISI 203EZ and Biodur l08.

[0019] The particular phase of the metal being processed in accordance with the present invention is unimportant, as the invention can be practiced on metals of any phase structure including, but not limited to, austenite, ferrite, martensite, duplex metals (e.g., austenite/ferrite), etc.

Surface Fissures and/or Interstices

[0020] hi prior low temperature carburization processes, the concentration of carbon atoms falls off from a relatively high value at or near the workpiece surface to an equilibrium or "native" value at depth which can be as little as 5 μ from the workpiece' s outer surface, but is more typically on the order of 20-50 μ from the workpiece's outer surface. Greater depths of diffused carbon, e.g., as deep as 75 μ or even 100 μ are possible, however.

[0021] In accordance with this invention, the depth to which carbon penetrates into the workpiece's surfaces is substantially increased over and above these values by forming or treating the workpiece to have fissures and/or interstices in its surfaces prior to carburization. Although not wishing to be bound to any theory, it is believed this improved result is made possible because the carbon-containing compound which decomposes to yield elemental carbon for diffusion migrates into the workpiece's interior via these fissures and interstices before decomposing. Accordingly, the diffusion path of at least some of the diffusing carbon atoms starts at the bottom of these fissures and interstices and ends up at a depth which extends below these fissures and interstices. As a result, these diffusing carbon atoms extend much deeper into workpiece's surface than possible in earlier processes in which the diffusing carbon atoms are liberated only at the workpiece's outermost surface.

[0022] For example, where a fissure or interstice extends to a depth of 200 μ into the workpiece's interior, elemental carbon is liberated for diffusion at this depth of 200 μ rather than at the workpiece's outermost surface. Thus, the diffusion travel path of this liberated carbon atom starts at a depth which is already 200 μ into the workpiece's interior and hence ends up an additional 5 μ, 20-50 μ, 75 μ or even 100 μ deeper into the workpiece's interior, depending on the conditions of employed in the low temperature carburization reaction. The net effect is that the final depth of the precipitate- free carburized surface layer extends to a depth of 205 μ, 220-250 μ, 275 μ or even 300 μ rather than the 5 μ, 20-50 μ, 75 μ or 100 μ levels achieved in normal low temperature carburization.

[0023] Surface fissures and/or interstices can be incorporated into the workpiece surfaces according to this invention in a variety of different ways. For example, a previously formed workpiece can be treated to impart such fissures and/or interstices into its surfaces before the low temperature carburization process is completed. This can be done, for example, mechanically such as by subjecting the workpiece to mechanical impacts (e.g., blows, shocks, hits, strikes, collisions) sufficient to mechanically degrade (i.e., break) the surface layer of the workpiece. For example, the workpiece can be tumbled in a ball mill. Normally, this will be done before the low temperature carburization process is started. However, this can also be done at the same time the low temperature carburization process is being carried out. In addition, where multiple low temperature carburization steps are used, this can also be done after a preliminary low temperature carburization step is completed, before or during the final low temperature carburization.

[0024] Alternatively or additionally, incorporating surface fissures and/or interstices into a workpiece can be done by forming the workpiece so that it inherently defines these fissures and/or interstices. For example, the workpiece can be made by powder metallurgy techniques wherein a powder of the metal from which the workpiece is to be made is compacted under high pressure, normally followed or accompanied by heating to elevated temperature to sinter the powder particles together thereby forming a coherent, self- supporting structure. Normally, the metal powder will be formed into a "green compact," i.e. a self-supporting preform of the workpiece product to be made, which is then subsequently fired for sintering its metal particles together.

[0025] Powder metallurgy techniques are well known, and any powder metallurgy technique can be used in this invention for forming the workpiece to be low temperature carburized. As is well known, the product densities of articles made by powder metallurgy teclmiques can be as high as 100% of theoretical. Normally, however, product densities less than 100% of theoretical are achieved. In any event, this invention is effective on any workpiece made by powder metallurgy techniques having a product density of less than 100% of theoretical. Product densities of about 50 to 99% of theoretical, more typically about 60 to 97% of theoretical, or even about 70 to 95% of theoretical, are desirable. Product densities of about 99% or less of theoretical are contemplated, as are product densities of about 98% or less, about 97% or less, about 95% or less, about 93% or less, and even about 90% or less of theoretical.

[0026] Still other ways for forming surface fissures and/or interstices in the workpiece surface, as well as optional modifications of conventional low temperature carburization for taking advantage of these fissures and/or interstices, include:

[0027] powder metallurgy techniques as described above using two or more different metal powders. The powder metallurgy workpiece may be fully dense or have some porosity when made. Before or after sintering, one of the powders is selectively etched out or removed from the surface region, wholly or partially. The source compound for the gas phase low temperature case hardening process (i.e., the gas phase carbon compound which decomposes to yield elemental carbon) propagates into the fissures and/or interstices and provide a deepened hardening into the powder metallurgy workpiece. The resultant case depths are substantially greater than those achieved in the same metal made without selective removal, wholly or partially, of one of the metal powders;

[0028] forming the workpiece from two or more metal layers plied together by compression such as by stamping or the like. As a result, the metal layers inherently define gas- permeable channels at the interfaces between adjacent metal layers. Additional channels can be formed at these interfaces by purposely designing the metal layers to have such channels, either prior to or as a result of the stamping process. The gas phase source compound of the low temperature case hardening procedure propagates into the body of the workpiece through these channels. The workpiece obtained has increased specific strength and specific stiffness compared to an otherwise identical un-layered workpiece;

[0029] forming the workpiece from two or more metal layers secured together by brazing. During low temperature carburization, not only is the hardness of the workpiece increased but, in addition, further brazing of the metal layers together occurs as a result of the heat used in the low temperature carburization process;

[0030] forming the workpiece from two or more metal layers plied together by compression or by brazing, as described above, with the individual metal layers being subjected to a preliminary low temperature carburization procedure before being joined together, with the main temperature carburization procedure being is applied again to the metal layer composite after it is formed;

[0031] forming the workpiece from two or more metal layers plied together by compression or by brazing, as described above, with or without the individual metal layers being subjected to a preliminary low temperature carburization procedure before being joined together, with the metal layer composite being formed in the shape of a coil. The channels in the interfaces between adjacent layers in the coil allow the gas phase source compound of the low temperature case hardening procedure to propagate into the body of the workpiece through these channels;

[0032] forming slits or grooves into and generally normal to the surface of the workpiece. Such slits or grooves may be formed in any way, e.g., mechanically, chemically, by laser abrasion, etc., and can be arranged in any manner such as being aligned in a row, randomly oriented, etc. Low temperature carburization is applied by gas diffusion into the slits or grooves, treating the metal of the object deep under the surface. While the workpiece contains less material as a whole due to formation of the slits or grooves, its overall strength is enhanced due to the low temperature carburization treatment.

[0033] forming the workpiece from two or more metal layers plied together by compression, as described above, with a carbon bearing material being sandwiched between the metal layers. Examples of carbon bearing materials useful for this purpose include graphite, carbon nanofibers and organic compounds capable of decomposing and yielding elemental carbon at the conditions encountered in the subsequent low temperature carburization step. The carbon bearing material not only acts as a binder between the metal layers, but also participates in the subsequent low temperature carburization procedure by yielding elemental carbon for diffusion into the workpiece metal.

[0034] powder metallurgy techniques as described above (whether using a single metal powder or multiple different metal powders), with a carbon bearing material being used as a binder for forming the green compact to be sintered. The carbon bearing material not only acts as a binder for making the green compact, but also participates in the subsequent low temperature carburization procedure by yielding elemental carbon for diffusion into the workpiece metal. Examples of carbon bearing materials useful for this purpose include hydrocarbon waxes, other solid hydrocarbonaceous materials, and conventional binder materials filled with graphite or other carbonaceous materials. Product Workpiece

[0035] As indicated above, the metal products produced by earlier low temperature diffusion-based processes include an altered surface layer containing an increased concentration of interstitially diffused atoms in an amount sufficient to alter the properties of the metal forming the surface layer. Nonetheless, this altered surface layer is still free of precipitates of compounds formed from the diffused atoms. Thus, for example, when a stainless steel workpiece is low temperature carburized, an altered surface layer is obtained which is not only free of chromium carbide precipitates but, in addition, exhibits greater hardness due to the presence of the diffused carbon atoms.

[0036] This same result is also achieved in this invention. However, this invention departs from earlier work in that, in this invention, the surface of the workpiece which is subjected to the low temperature diffusion-based process defines fissures, interstices or both. In other words, the workpiece is formed and/or treated to intentionally introduce fissures, interstices or both into its surfaces to be hardened such that these surfaces define more than the incidental or de minimis amounts of cracks and fissures inherently present in the surfaces of conventionally made workpieces. Preferably, enough fissures and/or interstices are introduced so that a majority of the metal between adjacent fissures and/or interstices receives diffused carbon atoms as a result of the low temperature carburization process. Alternatively, enough fissures and/or interstices are introduced so that most, or even essentially all, of the metal between adjacent fissures and/or interstices receives diffused carbon atoms as a result of the low temperature carburization process.

[0037] Intentionally introducing fissures, interstices or both into the surfaces to be hardened allows the source compounds supplying the diffusing atoms to migrate into the depths of these fissures and interstices before these diffusing atoms are liberated. This, in turn, allows the travel path of the diffusing atoms to start deep inside these fissures and interstices, at a significant depth below the outermost surface of the workpiece, rather than at the workpiece' s outermost surface as in earlier practice. The result is that the distance these diffusing atoms travel below the outermost surface of the workpiece during the diffusion process is significantly greater than in earlier practice. The overall result is that much thicker altered surface layers can be achieved.

[0038] For example, altered surface layers 3, 5, 10 and even 20 times thicker than previously obtainable are possible according to this invention. Thus, altered surfaces layers >100 μ thick are possible in accordance with this invention. Altered surface layers having thicknesses of >120 μ, >135 μ, >150 μ, >175 μ, ≥200 μ, >200 μ, >300 μ,≥400 μ,≥500 μ and even > 1,000 μ, are contemplated. Moreover, when layered articles are low temperature carburized, the thickness of the carburized surface can be unlimited, since all of the metal layers can be carburized.

Other Low Temperature Diffusion-Based Surface Treatments

[0039] Although this invention concentrates on low temperature carburization of iron-, nickel- and cobalt-based alloys, other analogous diffusion-based surface treatments can also be used.

[0040] In low temperature carburization, as indicated above, atomic carbon diffuses interstitially into the workpiece surfaces, i.e., carbon atoms travel through the spaces between the metal atoms without significant substitutional diffusion of the metal atoms. Because the processing temperature is low, these carbon atoms form a solid solution with the metal atoms of the workpiece surfaces. They do not react with these metal atoms to form other compounds. Low temperature carburization is therefore different from normal carburization carried out at higher temperatures in which the carbon atoms react to form carbide precipitates, i.e., specific metal compounds such as M₂₃C₆ (e.g., Cr₂₃C₆ Or chromium carbide), M₅C₂ and the like, arranged in the form of discrete phases separate and apart from the metal matrix in which they are contained.

[0041] Other analogous processes are known for altering the surface characteristics of a metal workpiece by interstitial diffusion of atoms into the workpiece surfaces at relatively low temperatures to form solid solutions with the metal atoms therein without formation of new compounds in separate phases. Examples include nitriding of iron, chromium and/or nickel based alloys, carbo-nitriding of iron, chromium and/or nickel based alloys, and nitriding of titanium-based alloys, to name a few. For convenience, all of these processes will be referred to collectively as "low temperature interstitial diffusion based surface treatments" or "low temperature" treatments.

[0042] All such low temperature interstitial diffusion-based surface treatments can be used in accordance with the present invention. That is to say, each of these low temperature interstitial diffusion-based surface treatments can be applied to metal workpieces using the technology of this invention to make new products with thicker altered surface layers than available in the past. [0043] Thus, it will be appreciated that, although this invention is described here in terms of low temperature carburization for convenience, this invention also applies to such other analogous processes as well.

[0044] Although only a few embodiments of this technology have been described above, it should be appreciated that many modifications can be made. All such modifications are intended to be included within the scope of this disclosure, which is to be limited only by the following claims.

Claims

Claims:

1. A metal product produced by subjecting a metal workpiece to a low temperature diffusion-based surface treatment to produce an altered surface layer containing an increased concentration of interstitially diffused atoms in an amount sufficient to alter the properties of the metal forming the surface layer, the low temperature diffusion-based surface treatment being carried out under conditions so that the altered surface layer is free of precipitates of compounds of the diffused atoms, wherein the surfaces of the metal workpiece subjected to the low temperature diffusion-based surface treatment define fissures, interstices or both.

2. The metal product of claim 1, wherein the thickness of the altered surface layer of the metal product is thicker than the altered surface layer of an otherwise identical product made in the same way from an otherwise identical metal workpiece not defining such surface fissures, interstices or both.

3. The metal product of claim 2, wherein the metal product is produced by the low temperature carburization of a stainless steel workpiece to produce an altered surface layer containing interstitially diffused carbon atoms, the altered surface layer being harder than the base metal from which the product is formed and free of carbide precipitates.

4. The metal product of claim 3, wherein the thickness of the altered surface layer is >150 μ.

5. The metal product of claim 4, wherein the surface of the metal workpiece subjected to low temperature carburization defines mechanically-induced fissures.

6. The metal product of claim 4, wherein the surface of the metal workpiece subjected to low temperature carburization defines interstices as a result of being made by powder metallurgy techniques.

7. The metal workpiece of claim 4, wherein the metal workpiece is made from multiple metal layers plied together by compression, brazing or both, interstices being defined in the interfaces between adjacent metal layers.

8. A low temperature carburized stainless steel product having an altered surface layer which (a) contains an increased concentration of carbon atoms relative to the base metal from which the product is made, (b) is harder than the base metal from which the product is made, and (c) is free of carbide precipitates, the altered surface layer defining fissures, interstices or both so that the increased concentration of carbon atoms extends into the interior of the product to a depth which extends below the fissures and interstices.

9. A process for producing metal product comprising subjecting a metal workpiece to a low temperature diffusion-based surface treatment to produce an altered surface layer containing an increased concentration of interstitially diffused atoms in an amount sufficient to alter the properties of the metal forming the surface layer, the low temperature diffusion-based surface treatment being carried out under conditions so that the altered surface layer is free of precipitates of compounds of the diffused atoms, wherein the surface of the metal workpiece subjected to the low temperature diffusion-based surface treatment defines fissures, interstices or both.

10. The process of claim 9, wherein the thickness of the altered surface layer in the metal product is thicker than the altered surface layer of an otherwise identical product made in the same way from an otherwise identical metal workpiece not defining such surface fissures, interstices or both.

11. The process of claim 10, wherein the metal product is produced by the low temperature carburization of a stainless steel workpiece to produce an altered surface layer containing interstitially diffused carbon atoms, the altered surface layer being harder than the base metal from which the product is formed and free of carbide precipitates.

12. The process of claim 11, wherein the thickness of the altered surface layer is ≥ 150 μ.

13. The process of claim 12, wherein the surface of the metal workpiece subjected to low temperature carburization defines mechanically-induced fissures.

14. The process of claim 12, wherein the surface of the metal workpiece subjected to low temperature carburization defines interstices as a result of being made by powder metallurgy techniques.

15. The process of claim 12, wherein the metal workpiece is made from multiple metal layers plied together by compression, brazing or both, interstices being defined in the interfaces between adjacent metal layers.

16. The process of claim 9, wherein the workpiece is treated after it is formed to introduce fissures, interstices or both into its surfaces to be low temperature carburized in a subsequent low temperature diffusion-based surface treatment, the workpiece being subjected to a preliminary low temperature diffusion-based surface treatment before being treated to introduce fissures, interstices or both into its surfaces.