Chemical activation of self-passivating metals

Abstract

The invention is entitled Chemical Activation of Self-Passivating Metals. A workpiece made of a self-passivating metal and having one or more surface regions defining a Bayerby layer resulting from a previous metal forming operation is activated for subsequent low temperature gas hardening by exposing the workpiece to vapors generated by heating a non-polymerizing N / C / H compound.

Description

[0001] This application is a divisional application of the Chinese national phase patent application with international application number PCT / US2019 / 035694, international application date June 6, 2019, and invention name “Chemical Activation of Self-Passivating Metals”, which entered the Chinese national phase on December 8, 2020 and has application number 201980038310.3. Background Art

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to U.S. Provisional Patent Application Serial No. 62 / 683,093 filed on June 11, 2018 and U.S. Provisional Patent Application Serial No. 62 / 792,172 filed on January 14, 2019. The entire disclosures of both applications are incorporated herein by reference.

[0004] Conventional carburizing

[0005] Conventional (high temperature) carburizing is a widely used industrial process for increasing the surface hardness ("case hardening") of formed metal articles. In a typical commercial process, a workpiece is contacted with a carbon-containing gas at high temperature (e.g., 1,000°C or higher) so that carbon atoms liberated by decomposition of the gas diffuse into the surface of the workpiece. Hardening occurs by reaction of these diffused carbon atoms with one or more metals in the workpiece, thereby forming different chemical compounds, i.e., carbides, which are then precipitated as discrete, extremely hard, crystalline particles in a metal matrix forming the surface of the workpiece. See Stickels, "Gas Carburizing," pp. 312-324, Vol. 4, ASM Handbook, ASM International.

[0006] Stainless steel is corrosion resistant because a chromium oxide surface coating that forms immediately when the steel is exposed to air blocks water vapor, oxygen, and other chemicals from passing through. Nickel-based, cobalt-based, manganese-based, and other alloys that contain large amounts of chromium (typically 10 weight percent or more) also form these impermeable chromium oxide coatings. Titanium-based alloys exhibit a similar phenomenon in that they also immediately form a titanium dioxide coating when exposed to air, which also blocks water vapor, oxygen, and other chemicals from passing through.

[0007] These alloys are said to be self-passivating not only because they form oxide surface coatings immediately upon exposure to air, but also because these oxide coatings block the permeation of water vapor, oxygen, and other chemicals. These coatings are fundamentally different from the iron oxide coatings (e.g., rust) that form when iron and other low alloy steels are exposed to air. This is because these iron oxide coatings do not block the permeation of water vapor, oxygen, and other chemicals, as can be understood from the fact that if these alloys are not properly protected, they will be completely consumed by rust.

[0008] When stainless steel is conventionally carburized, the chromium content of the steel is locally reduced by forming carbide precipitates which are responsible for case hardening. As a result, there is not enough chromium in the near-surface region immediately surrounding the chromium carbide precipitates to form protective chromium oxide on the surface. Stainless steel is rarely case hardened by conventional (high temperature) carburizing, as the corrosion resistance of the steel is compromised.

[0009] Low temperature carburizing

[0010] In the mid-1980s, a technique for surface hardening of stainless steel was developed, which was to contact the workpiece with a carbon-containing gas at low temperatures (usually below about 500°C). At these temperatures, and assuming that the carburizing duration is not too long, the carbon atoms released by the decomposition of the gas diffuse into the workpiece surface, usually at a depth of 20-50 μm, without forming carbide precipitates. Nevertheless, a very hard surface layer (surface layer) is still obtained. Since carbide precipitates will not be produced, the corrosion resistance of the steel will not be impaired and may even be improved. This technique, which is referred to as "low temperature carburizing", is described in many publications including US5,556,483, US5,593,510, US5,792,282, US6,165,597, EPO 0787817, Japan 9-14019 (Kokai 9-268364) and Japan 9-71853 (Kokai 9-71853).

[0011] Nitriding and carbonitriding

[0012] In addition to carburizing, nitriding and carbonitriding can also be used to case harden various metals. Nitriding works in much the same way as carburizing, except that nitriding uses a nitrogen-containing gas that decomposes to produce nitrogen atoms for case hardening, rather than a carbon-containing gas that decomposes to produce carbon atoms for case hardening.

[0013] However, in the same manner as carburizing, if nitriding is done at higher temperatures and without rapid quenching, hardening occurs by the formation and precipitation of discrete compounds of the diffused atoms (i.e., nitrides). On the other hand, if nitriding is done at lower temperatures without a plasma, hardening occurs due to the stresses exerted on the crystal lattice by the nitrogen atoms that have diffused into the metal, without the formation of these precipitates. As with carburizing, stainless steels are not usually nitrided by conventional (high temperature) or plasma nitriding processes because the inherent corrosion resistance of the steel is lost when the chromium in the steel reacts with the diffused nitrogen atoms to form nitrides.

[0014] In carbonitriding, the workpiece is exposed to nitrogen and a carbon-containing gas, whereby both nitrogen atoms and carbon atoms diffuse into the workpiece for case hardening. In the same manner as carburizing and nitriding, carbonitriding can be accomplished at higher temperatures, in which case case hardening occurs by forming nitride and carbide precipitates, or carbonitriding can be accomplished at lower temperatures, in which case case hardening occurs by the sharp local stress fields generated in the crystal lattice of the metal by nitrogen and carbon atoms dissolved interstitially in the lattice that have diffused into the metal. For convenience, all three processes (i.e., carburizing, nitriding, and carbonitriding) are collectively referred to in this disclosure as "low temperature case hardening" or "low temperature case hardening processes."

[0015] activation

[0016] Because of the lower temperatures involved in low temperature case hardening, carbon and / or nitrogen atoms will not penetrate the protective chromium oxide coating of stainless steel. Therefore, low temperature case hardening of these metals is usually preceded by an activation ("depassivation") step in which the workpiece is exposed to a halogen-containing gas (such as HF, HCl, NF 3 、F 2 or Cl 2 ) at a high temperature (e.g., 200 to 400°C) to make the steel's protective oxide coating permeable to carbon and / or nitrogen atoms.

[0017] WO 2006 / 136166 (US 8,784,576) to Somers et al., the disclosure of which is incorporated herein by reference, describes an improved process for low temperature carburizing of stainless steel, wherein acetylene is used as the active ingredient in the carburizing gas, i.e., as a source compound that provides carbon atoms for the carburizing process. As shown therein, since the acetylene source compound is also sufficiently reactive to depassivate the steel, a separate activation step with a halogen-containing gas is not required. Thus, the carburizing technique of the present disclosure can be considered self-activating.

[0018] WO 2011 / 009463 (US 8,845,823) by Christiansen et al., the disclosure of which is also incorporated herein by reference, describes a similar improved process for carbonitriding of stainless steel, wherein oxygen-containing "N / C compounds" such as urea, formamide, etc. are used as source compounds for providing the nitrogen and carbon atoms required for the carbonitriding process. The disclosed technique can also be considered self-activating, as a separate activation step with a halogen-containing gas is said to be unnecessary.

[0019] Surface preparation and Beilby layer

[0020] Low temperature surface hardening is usually carried out on workpieces with complex shapes. In order to form these shapes, some type of metal forming operation is usually required, such as a cutting step (e.g., sawing, scraping, machining) and / or a forging process step (e.g., forging, drawing, bending, etc.). Due to these steps, structural defects of the crystal structure and contaminants such as lubricants, moisture, oxygen, etc. are often introduced into the near-surface area of ​​the metal. Therefore, in most workpieces with complex shapes, a highly defective surface layer is usually produced, which has an ultrafine grain structure caused by plastic deformation and a significant level of contamination. This layer, which can be up to 2.5 μm thick and is called the Bayerby layer, is formed directly below the protective, coherent chromium oxide layer or other passivation layer of stainless steel and other self-passivating metals.

[0021] As mentioned above, the traditional method used to activate stainless steel for low temperature surface hardening is by contact with a halogen-containing gas. These activation techniques are essentially unaffected by this Bayerby layer.

[0022] However, the same cannot be said for the self-activation techniques described in the above-mentioned Somers et al. and Christiansen et al. disclosures, in which the workpiece is activated by contact with acetylene or an "N / C compound." Rather, experience has shown that if stainless steel workpieces having complex shapes are not surface treated to remove their Bayerby layer by electropolishing, mechanical polishing, chemical etching, etc. before surface hardening begins, the self-activating surface hardening techniques of these disclosures do not work at all, or, if they do work, the results produced are spotty at best and inconsistent between surface areas.

[0023] See Ge et al., The Effect of Surface Finish on Low-Temperature Acetylene-Based Carburization of 316L Austenitic Stainless Steel, METALLURGICAL ANDMATERIALS TRANSACTIONS B, Vol. 458, December 2014, pp. 2338-2345, TheMinerals, Metal & Materials Society and ASM International. As stated therein, "[Stainless steel] samples having inadequate surface finish due to, for example, machining cannot be successfully carburized by acetylene-based processes." In particular, see FIG. 10(a) and the related discussion on pages 2339 and 2343, which clearly show that a "machining-induced distribution layer" (i.e., a Bayerby layer) that has been intentionally introduced by etching and then scraping with a sharp blade cannot be activated and carburized with acetylene, even though surrounding portions of the workpiece that have been etched but not scratched will be readily activated and carburized. Thus, in practice, these self-activating surface hardening techniques cannot be used for stainless steel workpieces having complex shapes unless the workpieces are first pretreated to remove their Bayerby layers.

[0024] To address this problem, commonly assigned US 10,214,805 discloses an improved process for low temperature nitriding or carbonitriding of workpieces made of self-passivating metals, wherein the workpieces are contacted with vapors generated by heating an oxygen-free halogenated nitrogen salt. As described herein, in addition to providing the nitrogen atoms and optionally carbon atoms required for nitriding and carbonitriding, these vapors are also capable of activating the surfaces of the workpieces for these low temperature surface hardening processes, even though these surfaces may have a Bayerby layer as a result of previous metal forming operations. Thus, this self-activating surface hardening technique can be directly applied to these workpieces, even though these surfaces define complex shapes as a result of previous metal forming operations and even though these surfaces have not been pretreated to remove their Bayerby layer. Summary of the invention

[0025] In accordance with the present invention, it has now been discovered that additional classes of compounds, namely organic compounds (a) containing at least one carbon atom, (b) containing at least one nitrogen atom, (c) containing only carbon atoms, nitrogen atoms, hydrogen atoms and optionally halide atoms, (d) being solid or liquid at room temperature (25°C) and atmospheric pressure, and (e) having a molecular weight of ≤5,000 Daltons (hereinafter referred to as "non-polymeric N / C / H compounds"), will also produce materials capable of both supplying nitrogen and carbon atoms for low temperature carbonitriding and activating self-passivating metal surfaces for use in this and other low temperature surface hardening processes, even though these surfaces may carry a Bayerby layer as a result of previous metal forming operations.

[0026] In particular, it has been found according to the invention that a low temperature surface hardening process can be made self-activating if the source compound for supplying nitrogen atoms for nitriding (and carbon atoms for carbonitriding) is a non-polymeric N / C / H compound, i.e. the workpiece being surface hardened is made of a self-passivating metal with a Bayerby layer from a previous metal forming operation.

[0027] Thus, in one embodiment, the present invention provides a method for activating a workpiece for low temperature carburizing, carbonitriding or nitriding, the workpiece being made of a self-passivating metal and having one or more surface regions including a Bayerby layer resulting from a previous metal forming operation, the method comprising: contacting the workpiece with vapors produced by heating a non-polymeric N / C / H compound to a temperature sufficiently high to convert the non-polymeric N / C / H compound to vapor, contacting the workpiece with these vapors at an activation temperature below the temperature at which nitride and / or carbide precipitates form.

[0028] Additionally, in another embodiment, the present invention provides a method for simultaneously activating and carbonitriding a workpiece, the workpiece being made of a self-passivating metal and having one or more surface regions defining a Bayerby layer resulting from a previous metal forming operation, the method comprising: contacting the workpiece with vapors produced by heating a non-polymeric N / C / H compound to a temperature sufficiently high to convert the non-polymeric N / C / H compound to vapor, contacting the workpiece with these vapors at a carbonitriding temperature sufficiently high to diffuse nitrogen and carbon atoms into the surface of the workpiece but below the temperature at which nitride precipitates or carbide precipitates are formed, thereby carbonitriding the workpiece without forming nitride or carbide precipitates. DETAILED DESCRIPTION

[0029] Definitions and terminology

[0030] As mentioned above, the fundamental difference between conventional (high temperature) case hardening and the newer low temperature case hardening process first developed in the mid-1980s is that in conventional (high temperature) case hardening, hardening occurs due to the formation of carbide and / or nitride precipitates on the surface of the metal being hardened. In contrast, in low temperature case hardening, hardening occurs due to stresses exerted on the crystal lattice of the metal at the surface of the metal due to carbon and / or nitrogen atoms that have diffused into these surfaces. Because the carbide and / or nitride precipitates responsible for the surface hardening in conventional (high temperature) case hardening are not present in stainless steel case hardened by low temperature carburizing, and further because low temperature case hardening does not adversely affect the corrosion resistance of the stainless steel, the original idea was that case hardening only occurs in low temperature carburizing due to the acute local stress field generated by interstitially dissolved carbon and / or nitrogen atoms that have diffused into the (austenite) crystal structure of the steel.

[0031] However, more recent and more sophisticated analytical work has shown that when alloys consisting of some or all of the alloy volume consists of a ferrite phase, certain types of previously unknown nitride and / or carbide precipitates may form in these ferrite phases in minor amounts. Specifically, recent analytical work has shown that in AISI 400 series stainless steels, which typically exhibit a ferrite phase structure, previously unknown nitrides and / or carbides may precipitate in minor amounts when the alloys are cryogenically surface hardened. Similarly, recent analytical work has shown that in duplex stainless steels, which contain both ferrite and austenite phases, small amounts of previously unknown nitrides and / or carbides may precipitate in the ferrite phase of these steels when the duplex stainless steels are cryogenically surface hardened. While the exact nature of these previously unknown, newly discovered nitride and / or carbide precipitates remains unknown, it is known that the chromium content of the ferrite matrix immediately surrounding these "pair equilibrium" precipitates is not reduced. As a result, the corrosion resistance of these stainless steels remains unchanged because the chromium responsible for the corrosion resistance remains evenly distributed throughout the metal.

[0032] Thus, for purposes of this disclosure, it will be understood that when referring to a workpiece surface layer that is "substantially free of nitride and / or carbide precipitates" or to a workpiece that is case hardened "without forming nitride and / or carbide precipitates" or to "a temperature below that at which nitride and / or carbide precipitates form," this reference is referencing the type of nitrides and / or carbides responsible for case hardening in conventional (high temperature) case hardening processes, said precipitates containing sufficient chromium to cause the metal matrix immediately surrounding these precipitates to lose its corrosion resistance due to the reduction in its chromium content. This reference is not referring to previously unknown newly discovered nitride and / or carbide precipitates that may form in small amounts in the ferrite phase of AISI 400 stainless steel, duplex stainless steel, and other similar alloys.

[0033] Likewise, it should be understood that for the purposes of this disclosure, "carbonitriding" and "nitrocarburizing" or "nitrocarburization" refer to the same process.

[0034] Additionally, "self-passivating" as used in this disclosure in conjunction with reference to alloys treated by the present invention should be understood to refer to alloy types that rapidly form a protective oxide coating upon exposure to air that is a barrier to the permeation of water vapor, oxygen, and other chemicals. Thus, metals that may form an iron oxide coating upon exposure to air, such as iron and low alloy steel, are not considered "self-passivating" within the meaning of this term because these coatings do not block the permeation of water vapor, oxygen, and other chemicals.

[0035] alloy

[0036] The present invention can be carried out on any metal or metal alloy that is self-passivating in the sense of forming a coherent protective chromium-rich oxide layer that blocks the passage of nitrogen and carbon atoms when exposed to air. These metals and alloys are well known and described, for example, in earlier patents for low temperature surface hardening processes, examples of which include US5,792,282, US6,093,303, US6,547,888, EPO 0787817 and Japanese Patent Document 9-14019 (Kokai 9-268364).

[0037] Alloys of particular interest are stainless steels, i.e. steels that typically contain about 5 to 50 wt. %, preferably 10 to 40 wt. %, Ni and sufficient chromium to form a protective layer of chromium oxide (typically 10% or more) on the surface when the steel is exposed to air. Preferred stainless steels contain 10 to 40 wt. % Ni and 10 to 35 wt. % Cr. More preferred are AISI 300 series steels, such as AISI 301, 303, 304, 309, 310, 316, 316L, 317, 317L, 321, 347, CF8M, CF3M, 254SMO, A286 and AL6XN stainless steels. AISI 400 series stainless steels, especially 410 alloy, 416 alloy and 440C alloy, are also of particular interest.

[0038] Other types of alloys that can be treated by the present invention are nickel-based, cobalt-based, and manganese-based alloys that also contain sufficient chromium to form a coherent protective chromium oxide protective coating when the steel is exposed to air, typically about 10% or more. Examples of such nickel-based alloys include Alloy 600, Alloy 625, Alloy 825, Alloy C-22, Alloy C-276, Alloy 20Cb, and Alloy 718, to name a few. Examples of such cobalt-based alloys include MP35N and Biodur CMM. Examples of such manganese-based alloys include AISI 201, AISI 203EZ, and Biodur 108.

[0039] Another type of alloy on which the present invention may be applied is a titanium-based alloy. As is well known in metallurgy, these alloys, when exposed to air, form a coherent protective titanium dioxide coating that also blocks the passage of nitrogen and carbon atoms. Specific examples of such titanium-based alloys include Ti 6-4 (grade 5), Ti 2, Ti 4, and Ti 6-4 (grade 5). In the same manner, alloys based on other self-passivating metals such as zinc, copper, and aluminum may also be activated (depassivated) by the techniques of the present invention.

[0040] The specific phase of the metal treated in accordance with the present invention is not critical in the sense that the present invention may be practiced on metals having any phase structure including, but not limited to, austenite, ferrite, martensite, bimetallic (eg, austenite / ferrite).

[0041] Activation with non-polymeric N / C / H compounds

[0042] According to the invention, a workpiece made of a self-passivating metal and having a Bayerby layer on at least one surface region thereof is activated (i.e., depassivated) for low-temperature surface hardening by contacting the workpiece with vapors generated by heating a non-polymeric N / C / H compound.

[0043] As described above, the non-polymeric N / C / H compounds of the present invention can be described as any compound that (a) contains at least one carbon atom, (b) contains at least one nitrogen atom, (c) contains only carbon, nitrogen, hydrogen and optionally halogen atoms, (d) is solid or liquid at room temperature (25° C.) and atmospheric pressure, and (e) has a molecular weight of ≤5,000 Daltons. Non-polymeric N / C / H compounds with a molecular weight of ≤2,000 Daltons, ≤1,000 Daltons or even ≤500 Daltons are of greater interest. Non-polymeric N / C / H compounds containing a total of 5-50 C+N atoms, more typically 6-30 C+N atoms, 6-25 C+N atoms, 6-20 C+N atoms, 6-15 C+N atoms or even 6-12 C+N atoms are of greater interest.

[0044] Specific types of non-polymeric N / C / H compounds useful in the present invention include primary amines, secondary amines, tertiary amines, azo compounds, heterocyclic compounds, ammonium compounds, azides and nitriles. Among them, compounds containing 6-30 C+N atoms are ideal. Compounds containing 6-30 C+N atoms, alternating C=N bonds and one or more primary amine groups are of particular interest. Examples include melamine, aminobenzimidazole, adenine, benzimidazole, guanidine, pyrazole, cyanamide, dicyandiamide, imidazole, 2,4-diamino-6-phenyl-1,3,5-triazine (benzoguanamine), 6-methyl-1,3,5-triazine-2,4-diamine (acetoguanamine), 3-amino-5,6-dimethyl-1,2,4-triazine, 3-amino-1,2,4-triazine, 2-(aminomethyl)pyridine, 4-(aminomethyl)pyridine, 2-amino-6-methylpyridine and 1H-1,2,3-triazolo(4,5-b)pyridine, 1,10-phenanthroline, 2,2'-bipyridine and (2-(2-pyridyl)benzimidazole).

[0045] The three triazine isomers and various aromatic primary amines containing 6-30 C+N atoms, such as 4-methylaniline (p-toluidine), 2-methylaniline (o-toluidine), 3-methylaniline (m-toluidine), 2-aminobiphenyl, 3-aminobiphenyl, 4-aminobiphenyl, 1-naphthylamine, 2-naphthylamine, 2-aminoimidazole and 5-aminoimidazole-4-carbonitrile, are also of interest. Also of interest are aromatic diamines containing 6-30 C+N atoms, such as 4,4'-methylene-bis(2-methylaniline), benzidine, 4,4'-diaminodiphenylmethane, 1,5-diaminonaphthalene, 1,8-diaminonaphthalene and 2,3-diaminonaphthalene. Hexamethylenetetramine, benzotriazole and ethylenediamine are also of interest.

[0046] Another class of compounds of interest, including some of the above compounds, is compounds that form nitrogen-based chelating ligands, i.e., polydentate ligands containing two or more nitrogen atoms arranged to form independent coordination bonds with a single central metal atom. Compounds that form bidentate chelating ligands of this type are of particular interest. Examples include o-phenanthroline, 2,2'-bipyridine, aminobenzimidazole, and guanidine chloride (guanidine chloride is discussed further below).

[0047] Another interesting class of non-polymeric N / C / H compounds are graphitic carbon nitrides described in WO 2016 / 027042, the disclosure of which is incorporated herein in its entirety. 3 N 4 This material includes one-atom-thick stacked layers or sheets of carbon nitride in which there are three carbon atoms for every four nitrogen atoms. Solids containing as few as three such layers and as many as 1000 or more are possible. Although carbon nitride is made in the absence of other elements, doping with other elements is contemplated.

[0048] In some embodiments of the present invention, the non-polymeric N / C / H compounds used will contain only N, C and H atoms. In other words, the specific non-polymeric N / C / H compounds used will be halogen-free. However, in other embodiments of the present invention, some or all of the unstable hydrogen atoms in the non-polymeric N / C / H compounds can be replaced by halogen atoms, preferably by Cl, F or both. In this regard, for simplicity of description, the non-polymeric N / C / H compounds of the present invention containing one or more halogen atoms are referred to herein as "halogen-substituted", while the halogen-free non-polymeric N / C / H compounds of the present invention are referred to herein as "unsubstituted".

[0049] In those embodiments of the present invention wherein using halogen-substituted non-polymeric N / C / H compounds, all non-polymeric N / C / H compounds used can be substituted by halogen. However, more generally, there will also be an additional amount of unsubstituted non-polymeric N / C / H compounds. In these embodiments, based on the total amount of non-polymeric N / C / H compounds used, i.e. based on the total amount of halogen-substituted and unsubstituted non-polymeric N / C / H compounds, the amount of halogen-substituted non-polymeric N / C / H compounds will generally be ≥1 weight %. More generally, on this same basis, the amount of halogen-substituted non-polymeric N / C / H compounds used will be ≥2 weight %, ≥3.5 weight %, ≥5 weight %, ≥7.5 weight %, ≥10 weight %, ≥12.5 weight %, ≥15 weight % or even ≥20 weight %. Similarly, on this same basis, the amount of halogen-substituted non-polymeric N / C / H compound used will typically also be ≤ 75 wt%, more typically ≤ 60 wt%, ≤ 50 wt%, ≤ 40 wt%, ≤ 30 wt% or even ≤ 25 wt%.

[0050] According to the present invention, it has surprisingly been found that, in addition to supplying nitrogen and carbon atoms for case hardening, the vapors produced by heating the non-polymeric N / C / H compounds into vapors are so potent that they readily activate the surface of self-passivated metals despite the presence of a significant Bayerby layer. Even more surprisingly, it has also been found that workpieces activated in this manner can be case hardened in a shorter period of time than has been possible in the past. For example, while an earlier activation process followed by 24-48 hours of low temperature case hardening may be required to achieve a suitable condition, the activation of the present invention followed by low temperature case hardening can achieve comparable conditions in as little as two hours.

[0051] Although not wishing to be bound by any theory, it is believed that the vapor of such non-polymeric N / C / H compound decomposes by pyrolysis prior to and / or upon contact with the workpiece surface, thereby producing ionic and / or free radical decomposition species effective to activate the workpiece surface. In addition, such decomposition also produces nitrogen and carbon atoms, which diffuse into the workpiece surface, thereby surface hardening the workpiece by low temperature carbonitriding.

[0052] Thus, it will be appreciated that when non-polymeric N / C / H compounds are used for activation according to the present invention, activation and at least partial surface hardening will occur simultaneously, which may make it unnecessary to include additional nitrogen- and / or carbon-containing compounds in the system for enhanced surface hardening processes. However, this is not to say that such additional compounds cannot or should not be included.

[0053] In this regard, it should be appreciated that the extent to which a workpiece is surface hardened when activated according to the present invention depends on a variety of different factors, including the nature of the particular alloy being treated, the particular non-polymeric N / C / H compound being used, and the temperature at which activation occurs. Generally speaking, activation according to the present invention occurs at a temperature slightly below the temperature typically associated with low temperature surface hardening. In addition, different alloys may differ from one another in the temperatures at which they are activated and surface hardened. In addition, different non-polymeric N / C / H compounds contain greater or lesser relative amounts of nitrogen and carbon atoms.

[0054] In this case, in some embodiments of the invention, a particular alloy may become fully case hardened while being activated solely due to the nitrogen and carbon atoms released from the non-polymeric N / C / H compound. If this is the case, it may not be necessary to enhance the case hardening process by including additional nitrogen and / or carbon containing compounds or compounds in the system for supplying additional nitrogen and / or carbon atoms.

[0055] However, in other embodiments of the present invention, a particular alloy may not become completely surface hardened only due to the nitrogen atoms and carbon atoms released by the non-polymeric N / C / H compound during activation. If this is the case, additional nitrogen-containing and / or carbon-containing compounds may be included in the system for supplying additional nitrogen atoms and / or carbon atoms to enhance the surface hardening process. If this is the case, these additional nitrogen-containing and / or carbon-containing compounds can be supplied to the depassivation (activation) furnace at the same time as the depassivation (activation) begins or at any time before the depassivation (activation) is completed. Typically, this additional nitrogen-containing and / or carbon-containing compound will be different from the non-polymeric N / C / H compound used for surface hardening, but if desired, this additional nitrogen-containing and / or carbon-containing compound can also be the same compound.

[0056] In addition or as an alternative to enhanced surface hardening during activation in this way, enhanced surface hardening can be postponed until activation has been completed by supplying additional nitrogen- and / or carbon-containing compounds only after activation is complete. If this is the case, the enhanced surface hardening can be carried out in the same reactor as used for activation or in a different reactor.

[0057] According to the invention, the temperature to which the workpiece is subjected during activation should be high enough to effect activation but not so high as to form nitride and / or carbide precipitates.

[0058] In this regard, it is well understood that in a low temperature surface hardening process, if the workpiece is exposed to excessive temperatures, unwanted nitride and / or carbide precipitates will form. In addition, it is also understood that the maximum surface hardening temperature that a workpiece can withstand without forming these nitride and / or carbide precipitates depends on many variables, including the specific type of low temperature surface hardening process being performed (e.g., carburizing, nitriding, or carbonitriding), the specific alloy being surface hardened (e.g., nickel-based alloys and iron-based alloys), and the concentration of nitrogen and / or carbon atoms diffused in the workpiece surface. See, for example, commonly assigned US6,547,888. Therefore, it is also well understood that when performing a low temperature surface hardening process, care must be taken to avoid excessive surface hardening temperatures to avoid the formation of nitride and / or carbide precipitates.

[0059] Thus, in the same manner, when performing the activation process of the present invention, care should also be taken to ensure that the temperature to which the workpiece is exposed during activation is not so high as to form unwanted nitride and / or carbide precipitates. Typically, this means that the maximum temperature to which the workpiece is exposed during activation and simultaneous and / or subsequent surface hardening should not exceed about 500°C, preferably 475°C or even 450°C, depending on the specific alloy being treated. Thus, for example, when activating and surface hardening nickel-based alloys, the maximum treatment temperature can typically be as high as about 500°C, as these alloys typically do not form nitride and / or carbide precipitates before reaching higher temperatures. On the other hand, when activating and surface hardening iron-based alloys such as stainless steel, the maximum treatment temperature should ideally be limited to about 475°C, preferably 450°C, as these alloys tend to become sensitive to the formation of nitride and / or carbide precipitates at higher temperatures.

[0060] As far as the minimum processing temperature is concerned, there is no practical lower limit other than the fact that the temperature of both the non-polymeric N / C / H compound and the workpiece itself must be high enough for the workpiece to become activated due to the generated vapor. Typically, this means that the non-polymeric N / C / H compound will be heated to a temperature of ≥100°C, although more typically the non-polymeric N / C / H compound will be heated to a temperature of ≥150°C, ≥200°C, ≥250°C or even ≥300°C. Activation temperatures of ≥350°C, ≥400°C or even ≥450°C are contemplated.

[0061] According to the present invention, specific alloy becomes activated to carry out the required time of low temperature surface hardening and also depends on many factors, including the property of the alloy being activated, the specific non-polymeric N / C / H compound being used and the temperature of activation. Generally speaking, activation can be completed in as short as 1 second to as long as 3 hours. However, more generally, most alloys will become fully activated in 1 to 150 minutes, 5 to 120 minutes, 10 to 90 minutes, 20 to 75 minutes or even 30 to 60 minutes. The time period that specific alloy becomes fully activated by the process of the present invention can be easily determined by carrying out routine experiments one by one. In addition, in those cases where activation and surface hardening occur simultaneously, whether or not other nitrogen and / or carbon compounds are included in the system for strengthening surface hardening, the shortest time of activation will usually depend on the shortest time required for completing the surface hardening process.

[0062] With respect to pressure, the activation process of the present invention can be carried out at atmospheric pressure, above atmospheric pressure, or below atmospheric pressure including hard vacuum (i.e., at a total pressure of 1 Torr (133 Pa (Pascals) or less)) and soft vacuum (i.e., at a total pressure of about 3.5 to 100 Torr (about 500 to about 13,000 Pa (Pascals))).

[0063] The amount of non-polymeric N / C / H compound used to activate a particular workpiece also depends on many factors, including the nature of the alloy being activated, the surface area of ​​the workpiece being treated, and the specific non-polymeric N / C / H compound being used. The amount can be readily determined by routine experimentation using the following working examples as a guide.

[0064] Finally, it should be noted that an important feature of the present invention is that its non-polymeric N / C / H compounds are oxygen-free. The reason is to avoid the generation of escaped oxygen atoms when these compounds react, which would otherwise occur if these compounds contained oxygen atoms. As mentioned above, it is believed that the activation according to the present invention occurs due to the ions and / or free radical decomposition species generated when the non-polymeric N / C / H compounds of the present invention decompose. It is believed that any such escaped oxygen atoms will react with these ions and / or free radical decomposition species and thus disable these ions and / or free radical decomposition species. Indeed, this explains why the process described in the patent of Christiansen et al. above encounters difficulties when the workpiece being processed has a Bayerby layer, because the N / C compounds actually used there contain a large amount of oxygen. This problem is avoided according to the present invention because the non-polymeric N / C / H compounds being used are oxygen-free.

[0065] In some respects, the activation process of the present invention appears similar to the activation process described in US Pat. No. 8,414,710 to Minemura et al., in which decomposition products produced by heating certain amino resins were used to "depassivate" certain iron-based alloys. However, the iron-based alloys described therein are not truly "self-passivating" as that term is understood in the art. This is because the amount of chromium contained in the iron-based alloys (5% by weight or less) is too small for the alloy to form a protective chromium oxide coating, which provides corrosion resistance to iron-based alloys, and is typically 10% by weight or more. Moreover, the patent itself makes it clear that the "passivating" film it refers to is composed of iron oxide (i.e., rust), which is well known to be a barrier to the transmission of water vapor, oxygen, and other chemicals.

[0066] Furthermore, the amino resin activating compounds used in Minemura et al. are condensation polymers having high molecular weights. Generally speaking, these materials will not pyrolyze at the low temperatures required for the activation process of the present invention, which are necessary to avoid the formation of nitride and / or carbide precipitates. In fact, the lowest activation temperature described in this reference is 600°C, which is significantly higher than the temperature at which nitride and / or carbide precipitates begin to form (usually around 500°C).

[0067] Therefore, Minemura et al. has no real relevance to the present invention, not only because the alloys it describes are not "self-passivating" as that term is understood in the art, but also because the temperatures required to cause pyrolysis of its amino resin activating compound will also cause nitride and / or carbide precipitates to form.

[0068] Low temperature heat curing

[0069] As described above, in addition to activating the surface of the self-passivating metal for low temperature nitriding or carbonitriding, the vapor generated by heating the non-polymeric N / C / H compound of the present invention also supplies nitrogen and carbon atoms, which achieve at least partial thermal hardening of the workpiece by means of these thermal hardening processes even if no additional reagents are included in the reaction system.

[0070] However, if desired, the rate at which low temperature thermal hardening occurs can be increased by including additional nitrogen-containing and / or carbon-containing reagents in the reaction system, particularly by contacting the workpiece with additional nitrogen-containing compounds that are capable of decomposing to produce nitrogen atoms for nitriding, additional carbon-containing compounds that are capable of decomposing to produce carbon atoms for carburizing, additional compounds containing carbon atoms and nitrogen atoms that are capable of decomposing to produce carbon atoms and nitrogen atoms for carbonitriding, or any combination of these compounds.

[0071] These additional nitrogen- and / or carbon-containing compounds can be added to the reaction system at any time. For example, the compounds can be added after activation of the workpiece has been completed or while activation is occurring. Finally, the compounds can also be added before activation begins, although it is believed that low temperature surface hardening will be more effective if the compounds are added while and / or after activation.

[0072] In general, at least when using halogen-containing gases, the temperature at which self-passivating alloys will be activated (depassivated) is generally slightly lower than the temperature used for subsequent low temperature surface hardening of these alloys. For example, activation of AISI 316 stainless steel with HCl gas is generally carried out at about 300-350° C., while low temperature carburizing of this alloy is generally carried out at about 425-450° C. The same relationship applies to the activation process of the present invention, as the temperature at which a particular alloy will be activated as a result of this process will generally be lower than the temperature generally used for surface hardening of the alloy by low temperature nitriding, carbonitriding or carburizing.

[0073] For this reason, when carrying out a combined activation and enhanced surface hardening process according to the present invention, it may be desirable to select a reaction temperature that is intermediate to the temperature that is optimal for each process so that the overall combined process can be optimized. This can be readily done by routine experimentation, with the understanding that care should be taken to avoid temperatures that are so high as to form unwanted nitride and / or carbide precipitates, as described above.

[0074] In a particularly interesting method, activation and heat hardening are accomplished according to the present invention in a closed system as described, for example, in commonly assigned US 10,214,805, i.e., in a reaction vessel that is completely sealed to prevent any material from entering or leaving during the entire process of the activation and heat hardening process. In order to ensure that activation and heat hardening are properly performed, it is desirable that a sufficient amount of vapor of the non-polymeric N / C / H compound is in contact with the surface of the workpiece, especially those surface areas with a significant Bayerby layer. Because the non-polymeric N / C / H compound used for activation and heat hardening according to the present invention will generally be a granular solid, a simple method of ensuring that contact is properly accomplished is to coat or otherwise cover these surfaces with such a granular solid, and then seal the reaction vessel before heating of the workpiece and the non-polymeric N / C / H compound begins. The non-polymeric N / C / H compound may also be dissolved or dispersed in a suitable liquid and then applied to the workpiece in this manner.

[0075] These methods are particularly convenient when large production runs containing many small workpieces, such as ferrules and catheter fittings, are being heat hardened simultaneously in the same reaction vessel.

[0076] The present method of activation and heat hardening in a closed system as described above is similar in some respects to the technique disclosed in Bessen, US Pat. No. 3,232,797, in which a thin steel strip is coated with a guanidine compound including guanidine chloride and then heated to decompose the guanidine compound and nitride the steel strip. However, in the case of the thin steel strip being nitrided, there is no self-passivation in the sense of forming a firmly adherent, coherent protective oxide coating that blocks the passage of nitrogen and carbon atoms. Thus, the technique described there has little relevance to the present invention, in which stainless steel and other self-passivating metals that block the passage of nitrogen and carbon atoms by contact with vapors of non-polymeric N / C / H compounds as part of a low temperature heat hardening process are transparent to these atoms.

[0077] Optional co-activating compound - oxygen-free nitrogen halide salt

[0078] According to another feature of the invention, it has been found that the rate at which activation and simultaneous nitriding or carbonitriding occurs can be significantly enhanced by including one or more oxygen-free nitrogen halide salts in the reaction system, as described in the commonly assigned US 10,214,805 mentioned above. Moreover, "included in the reaction system" means that the oxygen-free nitrogen halide salt is also vaporized by heating, so that the vapor so generated also contacts the surface of the workpiece being activated.

[0079] As described in US 10,214,805, these salts can be generally described as including any compound that (1) includes a halide anion that provides the oxygen-free nitrogen halide salt with a solubility in water of at least 5 mol / L at room temperature, (2) contains at least one nitrogen atom, (3) does not contain oxygen, and (4) evaporates when heated to 350° C. at atmospheric pressure.

[0080] Specific examples of such salts include ammonium chloride, ammonium fluoride, guanidine chloride, guanidine fluoride, pyridinium chloride, pyridinium fluoride, benzyltriethylammonium chloride, methylammonium chloride, allylamine hydrochloride, p-toluidine hydrochloride, benzylamine hydrochloride, benzyltetramine, tetrahydrochloride, methylpyrazolediamine dihydrochloride, butenylamine hydrochloride, benzidine dihydrochloride, benzyltriamine dihydrochloride, imidazole hydrochloride, 2-(aminomethyl)benzimidazole dihydrochloride, 1,1-dimethylguanidine dihydrochloride, 2-guanidine-4-methylquinazoline hydrochloride, 1,3-diaminopropane dihydrochloride, and any isomers thereof. Mixtures of these compounds may also be used.

[0081] The amount of this oxygen-free nitrogen halide salt included in the reaction system can vary widely, and substantially any amount can be used. For example, based on the combined weight of this oxygen-free nitrogen halide salt and the non-polymeric N / C / H compound of the present invention, the amount of the oxygen-free nitrogen halide salt can vary between 0.5 wt % and 99.5 wt %. Concentrations of about 0.1 to 50 wt %, more typically 0.5 to 25 wt %, 1 to 10 wt %, or even 2 to 5 wt % of this oxygen-free nitrogen halide salt are more common.

[0082] Optional co-activating compound - N / C compound

[0083] As mentioned above, WO 2011 / 009463 (US 8,845,823) to Christiansen et al. teaches that stainless steel and other self-passivating metals can be depassivated by exposing the metal to vapors produced by pyrolysis of "N / C compounds." Although the patent broadly suggests that any compound containing a nitrogen / carbon bond can be used for this purpose, the only specific compounds fairly described contain oxygen. Furthermore, there is no indication of the need to remove any Bayerby layer that may be present on the surface of the workpiece before activation begins.

[0084] In any case, according to the optional features of the present invention, if necessary, the activation process of the present invention can also be enhanced by including one or more of these oxygen-containing N / C compounds in the reaction system during the activation process. If this is the case, the amount of this optional N / C compound used will generally be ≤50 weight % based on the combined weight of all nitrogen-containing compounds (i.e., non-polymeric N / C / H compounds of the present invention, and optional N / C compounds discussed herein, and optional oxygen-free nitrogen halides just discussed above) in the system that participate in the activation process. This is because, as described above, the presence of oxygen will hinder the activation of the active species produced when the non-polymeric N / C / H compounds of the present invention are heated to decomposition. More generally, on this same basis, the amount of this optional N / C compound used will be ≤40 weight %, ≤30 weight %, ≤25 weight %, ≤20 weight %, ≤15 weight %, ≤10 weight %, ≤5 weight %, ≤2 weight %, ≤1 weight %, ≤0.5 weight % or even ≤0.1 weight %.

[0085] Tracer

[0086] According to yet another feature of the invention, the treating agent used in the invention - the non-polymeric N / C / H compound - can be enriched with specific uncommon isotopes of C, N, H and / or other elements to be used as a tracer compound for diagnostic purposes. For example, the non-polymeric N / C / H compound can be seeded at low concentrations with the same or different non-polymeric N / C / H compound made with rare isotopes of N, C or H, or with an entirely different compound made with such rare isotopes. By sensing these tracers using mass spectrometry or other suitable analytical techniques, quality control of the low temperature surface hardening process of the invention on a production scale can be readily determined.

[0087] To this end, the treating agent may be enriched in at least one of the following halide isotopes: ammonium chloride-(15N), ammonium chloride-(15N, D4), ammonium chloride-(D4), guanidine hydrochloride-(13C), guanidine hydrochloride-(15N3), guanidine hydrochloride-(13C, 15N3), guanidine-(D5), deuterium chloride and any isomers thereof. Alternatively or additionally, the treating agent may be enriched in at least one of the following non-halide isotopes: adenine-( 15 N 2 ), p-toluidine-(phenyl- 13 C 6 ), melamine-( 13 C 3 ), melamine-(triamine- 15 N 3 ), hexamethylenetetramine-(13C6, 15N4), benzidine-(cyclo-D8), triazine (D3) and melamine-(D 6 ) and any isomers thereof.

[0088] Optional accompanying gas

[0089] In addition to the gases mentioned above, the gaseous atmosphere in which activation is performed according to the present invention may also include one or more other accompanying gases, i.e., gases different from the gaseous compounds mentioned above. For example, this gaseous atmosphere may include an inert gas, such as argon as shown in the following working examples. In addition, other gases that do not adversely affect the activation process of the present invention in any significant way may also be included, examples of which include, for example, hydrogen, nitrogen, and unsaturated hydrocarbons such as acetylene and ethylene.

[0090] Expose the workpiece to atmospheric oxygen

[0091] In yet another embodiment of the invention, the workpiece is exposed to atmospheric oxygen between activation and case hardening, ie, after activation of the workpiece has been substantially completed but before low temperature case hardening has been substantially completed.

[0092] As previously mentioned, the traditional way to activate stainless steel and other self-passivating metals for low temperature carburizing and / or carbonitriding is to contact the workpiece with a halogen-containing gas. In this regard, in some early work in this field as described in the aforementioned US5,556,483, US5,593,510 and US5,792,282, the halogen-containing gas used for activation was limited to corrosive and expensive fluorine-containing gases. This is because when other halogen-containing gases, especially chlorine-containing gases, are used, once the workpiece is exposed to atmospheric oxygen between activation and thermal hardening, the workpiece will repassivate. Therefore, in this early work, only those activated workpieces containing a large amount of fluorine atoms can be exposed to the atmosphere without immediate repassivation.

[0093] In accordance with another feature of the present invention, the compromise between the undesirable corrosion and expense associated with the use of fluorine-based activators and the undesirable need to avoid repassivation when using chlorine-based activators has been broken because it has been found that even if the activated workpiece produced by the present invention does not contain fluorine atoms, the activated workpiece is not susceptible to repassivation when exposed to atmospheric oxygen for 24 hours or more.

[0094] Working Example

[0095] In order to more thoroughly describe the present invention, the following working examples are provided.

[0096] Example 1

[0097] A machined workpiece made of A1-6XN alloy (which is a super austenitic stainless steel characterized by an increased nickel content) is placed in a laboratory reactor together with powdered 2-aminobenzimidazole (as an activating compound) arranged in direct contact with the workpiece. The reactor is purged with dry Ar gas and then heated to 327°C and maintained for 60 minutes, after which the reactor is heated to 452°C and maintained for 120 minutes.

[0098] After removal from the reactor and cooling to room temperature, the workpiece was inspected and found to have a conformationally homogeneous outer shell (ie, surface coating) exhibiting a near-surface hardness of 630 HV.

[0099] Example 2

[0100] Example 1 was repeated, except that the activating compound consisted of a mixture of guanidine hydrochloride and 2-aminobenzimidazole in a mass ratio of 0.01 to 0.99. In other words, the amount of guanidine hydrochloride was 1 wt % based on the total amount of non-polymeric N / C / H compound used. In addition, the reactor was heated to 452° C. and maintained for 360 minutes instead of 120 minutes.

[0101] The workpiece was found to exhibit a near-surface hardness of 660 HV.

[0102] Example 3

[0103] Example 2 was repeated, except that the workpiece was made of AISI 316 stainless steel and the activating compound consisted of a mixture of guanidine hydrochloride and 2-aminobenzimidazole. In the first run, the mass ratio of guanidine hydrochloride to 2-aminobenzimidazole was 0.01 to 0.99 (1% by weight of guanidine hydrochloride, based on the total amount of non-polymeric N / C / H compound used), while in the second run, this mass ratio was 0.10 to 0.90 (10% by weight of guanidine hydrochloride, based on the total amount of non-polymeric N / C / H compound used).

[0104] The workpiece produced in the first run exhibited a near-surface hardness of 550 HV, while the workpiece produced in the second run exhibited a near-surface hardness of 1000 HV. In addition, the case-hardened surface of the workpiece produced in the second run exhibited excellent case depth and complete conformality throughout its surface compared to the case-hardened surface of the workpiece produced in the first run.

[0105] Example 4

[0106] Example 3 was repeated, except that the activating compound used was a mixture of guanidine hydrochloride and 2-aminobenzimidazole in a mass ratio of 0.50:0.50 (50 wt. % guanidine hydrochloride based on the total amount of non-polymeric N / C / H compounds used).

[0107] The hardened surface or "skin" of the workpiece obtained exhibited a near-surface hardness of 900 HV with almost complete conformality throughout its surface, but with some pitting.

[0108] Although only a few embodiments of the present invention have been described above, it should be appreciated that many modifications may be made without departing from the spirit and scope of the present invention. All such modifications should be included within the spirit and scope of the present invention, which is limited only by the claims.

Claims

1. A method for treating a metal workpiece, the metal workpiece being made of a self-passivating metal and having one or more surface regions comprising a Bayerby layer, the method comprising: include: A non-polymeric halogen-free N / C / H compound is heated to a processing temperature to generate vapor, the processing temperature being below a temperature at which nitride and / or carbide precipitates are formed, wherein the non-polymeric halogen-free N / C / H compound: (a) at least one of the following: melamine, aminobenzimidazole, adenine, benzimidazole, guanidine, cyanamide, dicyandiamide, 2,4-diamino-6-phenyl-1,3,5-triazine, 6-methyl-1,3,5-triazine-2,4-diamine, 2-(aminomethyl)pyridine, 4-(aminomethyl)pyridine, 2-amino-6-methylpyridine, 1,10-phenanthroline, 2,2'-bipyridine, (2-(2-pyridyl) benzimidazole), 4-methylaniline, 2-methylaniline, 3-methylaniline, 2-aminobiphenyl, 3-aminobiphenyl, 4-aminobiphenyl, 1-naphthylamine, 2-naphthylamine, 2-aminoimidazole, 5-aminoimidazole-4-carbonitrile, 4,4'-methylene-bis(2-methylaniline), benzidine, 4,4'-diaminodiphenylmethane, 1,5-diaminonaphthalene, 1,8-diaminonaphthalene, 2,3-diaminonaphthalene, hexamethylenetetramine and ethylenediamine, (b) is solid or liquid at 25°C and atmospheric pressure, and (c) has a molecular weight of ≤ 5,000 Daltons, and The workpiece is exposed to the vapor.

2. The method of claim 1, wherein the treatment temperature is ≤ 500°C.

3. The method of claim 2, wherein the treatment temperature is ≤ 475°C.

4. The method of claim 2, wherein the non-polymeric halogen-free N / C / H compound has a molecular weight of ≤ 500 Daltons.

5. The method of claim 2, wherein at least one of the following: The non-polymeric halogen-free N / C / H compound contains 5-50 C+N atoms; The non-polymeric halogen-free N / C / H compound comprises 6-30 C+N atoms, alternating C=N bonds and one or more primary amine groups; and The non-polymeric halogen-free N / C / H compound is an aromatic amine containing 6 to 30 C+N atoms.

6. The method of claim 2, wherein the non-polymeric halogen-free N / C / H compound is unsubstituted with respect to containing only C, N and H atoms.

7. The method of claim 1, wherein the self-passivating metal comprises at least one of: Titanium-based alloys, and An alloy comprising at least 10 wt. % Cr and at least one of iron, nickel, cobalt and manganese.

8. The method of claim 7, wherein the self-passivating metal is a titanium-based alloy.

9. The method of claim 7, wherein the self-passivating metal is an alloy comprising at least 10 wt. % Cr and at least one of iron, nickel, cobalt, and manganese.

10. The method of claim 1, wherein the self-passivating metal is a stainless steel comprising 10 to 40 wt% Ni and 10 to 35 wt% Cr.

11. The method of claim 7, further comprising: include: The workpiece is subjected to at least one of the following by contacting the workpiece with a gas different from the vapor: Low temperature carburizing Low temperature nitriding Low temperature carbonitriding To form a hardened surface layer on the workpiece surface without nitride or carbide precipitates, the gas comprises at least one of: Compounds capable of decomposing to produce nitrogen atoms for nitriding, Compounds capable of decomposing to produce carbon atoms for carburization and A compound that can decompose to produce nitrogen and carbon atoms for carbonitriding.

12. The method of claim 11, wherein the workpiece is contacted with an additional gas only after the workpiece has been exposed to the non-polymeric, halogen-free N / C / H compound.

13. The method of claim 11, further comprising exposing the workpiece to oxygen after the workpiece has been exposed to the non-polymeric halogen-free N / C / H compound, and further wherein the workpiece is free of fluorine atoms.

14. The method of claim 13, in: Exposing the workpiece to the non-polymeric halogen-free N / C / H compound is performed in a depassivation furnace, Low temperature carburizing, low temperature nitriding and / or low temperature carbonitriding is performed in a heat treatment furnace, and The workpiece is exposed to oxygen while being transferred between the depassivation furnace and the heat treatment furnace.

15. A method for processing a workpiece, in: The artifacts: is a corrosion resistant self-passivating metal comprising at least one of: (i) a stainless steel comprising 5-50 wt. % Ni and at least 10 wt. % Cr, and (ii) an alloy comprising at least 10 wt. % Cr and at least one of iron, nickel, cobalt and manganese, and a titanium-based alloy, having one or more surface regions with a Bayerby layer, A protective coating having at least one of chromium oxide or titanium oxide; and The method comprises: The non-polymeric halogen-free N / C / H compound is heated to a processing temperature high enough to convert the non-polymeric halogen-free N / C / H compound into a vapor, the processing temperature being less than 500° C. and less than the temperature at which nitride and / or carbide precipitates are formed, wherein the non-polymeric halogen-free N / C / H compound: (a) at least one of the following: melamine, aminobenzimidazole, adenine, benzimidazole, guanidine, cyanamide, dicyandiamide, 2,4-diamino-6-phenyl-1,3,5-triazine, 6-methyl-1,3,5-triazine-2,4-diamine, 2-(aminomethyl)pyridine, 4-(aminomethyl)pyridine, 2-amino-6-methylpyridine, 1,10-phenanthroline, 2,2'-bipyridine, (2-(2-pyridyl) benzimidazole), 4-methylaniline, 2-methylaniline, 3-methylaniline, 2-aminobiphenyl, 3-aminobiphenyl, 4-aminobiphenyl, 1-naphthylamine, 2-naphthylamine, 2-aminoimidazole, 5-aminoimidazole-4-carbonitrile, 4,4'-methylene-bis(2-methylaniline), benzidine, 4,4'-diaminodiphenylmethane, 1,5-diaminonaphthalene, 1,8-diaminonaphthalene, 2,3-diaminonaphthalene, hexamethylenetetramine and ethylenediamine, (b) is solid or liquid at 25°C and atmospheric pressure, and (c) has a molecular weight of ≤ 5,000 Daltons, and contacting the workpiece with the vapor to: depassivating the workpiece; and The workpiece is simultaneously surface hardened by diffusing at least one of carbon and nitrogen atoms into the surface of the workpiece without forming carbide and / or nitride precipitates.

16. A method for treating a metal workpiece made of a self-passivating metal including a nickel-based alloy and having one or more surface regions including a Bayerby layer, the method include: A non-polymeric halogen-free N / C / H compound is heated to a processing temperature to generate vapor, the processing temperature being below a temperature at which nitride and / or carbide precipitates are formed, wherein the non-polymeric halogen-free N / C / H compound: (a) at least one of the following: melamine, aminobenzimidazole, adenine, benzimidazole, guanidine, cyanamide, dicyandiamide, 2,4-diamino-6-phenyl-1,3,5-triazine, 6-methyl-1,3,5-triazine-2,4-diamine, 2-(aminomethyl)pyridine, 4-(aminomethyl)pyridine, 2-amino-6-methylpyridine, 1,10-phenanthroline, 2,2'-bipyridine, (2-(2-pyridyl) benzimidazole), 4-methylaniline, 2-methylaniline, 3-methylaniline, 2-aminobiphenyl, 3-aminobiphenyl, 4-aminobiphenyl, 1-naphthylamine, 2-naphthylamine, 2-aminoimidazole, 5-aminoimidazole-4-carbonitrile, 4,4'-methylene-bis(2-methylaniline), benzidine, 4,4'-diaminodiphenylmethane, 1,5-diaminonaphthalene, 1,8-diaminonaphthalene, 2,3-diaminonaphthalene, hexamethylenetetramine and ethylenediamine, (b) is solid or liquid at 25°C and atmospheric pressure, and (c) has a molecular weight of ≤ 5,000 Daltons, and The workpiece is exposed to the vapor.

17. The method of claim 16, wherein the nickel-based alloy comprises at least one of the following alloys designated by the Unified Numbering System (UNS): UNS N06600, UNS N06625, UNS N08825, UNS N06022, UNS N10276, UNSN08020, and UNS N07718.

18. The method of claim 16, wherein the treatment temperature is ≤ 475°C.

19. The method of claim 16, wherein the non-polymeric halogen-free N / C / H compound has a molecular weight of ≤ 500 Daltons.

20. The method of claim 16, wherein at least one of the following: The non-polymeric halogen-free N / C / H compound contains 5-50 C+N atoms; The non-polymeric halogen-free N / C / H compound comprises 6-30 C+N atoms, alternating C=N bonds and one or more primary amine groups; and The non-polymeric halogen-free N / C / H compound is an aromatic amine containing 6 to 30 C+N atoms.

21. The method of claim 16, wherein the non-polymeric halogen-free N / C / H compound comprises only C, N, and H atoms.

22. The method of claim 16, wherein the self-passivating metal comprises titanium.

23. The method of claim 16, wherein the self-passivating metal comprises at least 10 wt. % Cr.

24. The method of claim 16, wherein the self-passivating metal comprises at least one of iron, cobalt, and manganese.

25. The method of claim 16, wherein the workpiece is exposed to atmospheric oxygen at least one of prior to hardening and after exposure to the vapor.

26. The method of claim 16, further comprising: include: The workpiece is subjected to at least one of the following by contacting the workpiece with a gas different from the vapor: Low temperature carburizing Low temperature nitriding Low temperature carbonitriding To form a hardened surface layer on the workpiece surface without nitride or carbide precipitates, the gas comprises at least one of: Compounds capable of decomposing to produce nitrogen atoms for nitriding, Compounds capable of decomposing to produce carbon atoms for carburization and A compound that can decompose to produce nitrogen and carbon atoms for carbonitriding.

27. The method of claim 26, wherein the workpiece is contacted with an additional gas only after the workpiece has been exposed to the non-polymeric halogen-free N / C / H compound.

28. The method of claim 26, further comprising exposing the workpiece to oxygen after the workpiece has been exposed to the non-polymeric halogen-free N / C / H compound.

29. The method of claim 28, in: Exposing the workpiece to the non-polymeric halogen-free N / C / H compound is performed in a depassivation furnace, Low temperature carburizing, low temperature nitriding and / or low temperature carbonitriding is performed in a heat treatment furnace, and The workpiece is exposed to oxygen while being transferred between the depassivation furnace and the heat treatment furnace.

30. A workpiece processed according to the method of any one of claims 16 to 29.

31. A method for processing a workpiece, in: The workpiece: Is a corrosion resistant self-passivating metal including nickel-based alloys, having one or more surface regions with a Bayerby layer, A protective coating having at least one of chromium oxide or titanium oxide; and The method comprises: The non-polymeric halogen-free N / C / H compound is heated to a processing temperature high enough to convert the non-polymeric halogen-free N / C / H compound into a vapor, the processing temperature being less than 500° C. and less than the temperature at which nitride and / or carbide precipitates are formed, wherein the non-polymeric halogen-free N / C / H compound: (a) at least one of the following: melamine, aminobenzimidazole, adenine, benzimidazole, guanidine, cyanamide, dicyandiamide, 2,4-diamino-6-phenyl-1,3,5-triazine, 6-methyl-1,3,5-triazine-2,4-diamine, 2-(aminomethyl)pyridine, 4-(aminomethyl)pyridine, 2-amino-6-methylpyridine, 1,10-phenanthroline, 2,2'-bipyridine, (2-(2-pyridyl) benzimidazole), 4-methylaniline, 2-methylaniline, 3-methylaniline, 2-aminobiphenyl, 3-aminobiphenyl, 4-aminobiphenyl, 1-naphthylamine, 2-naphthylamine, 2-aminoimidazole, 5-aminoimidazole-4-carbonitrile, 4,4'-methylene-bis(2-methylaniline), benzidine, 4,4'-diaminodiphenylmethane, 1,5-diaminonaphthalene, 1,8-diaminonaphthalene, 2,3-diaminonaphthalene, hexamethylenetetramine and ethylenediamine, (b) is solid or liquid at 25°C and atmospheric pressure, and (c) has a molecular weight of ≤ 5,000 Daltons, and contacting the workpiece with the vapor to: depassivating the workpiece; and The workpiece is simultaneously surface hardened by diffusing at least one of carbon and nitrogen atoms into the surface of the workpiece without forming carbide and / or nitride precipitates.

32. The method of claim 31, wherein the nickel-based alloy comprises at least one of the following alloys designated by the Unified Numbering System (UNS): UNS N06600, UNS N06625, UNS N08825, UNS N06022, UNS N10276, UNSN08020, and UNS N07718.

33. The method of claim 31 , wherein the self-passivating metal comprises titanium.

34. The method of claim 31 , wherein the self-passivating metal comprises at least 10 wt. % Cr.

35. The method of claim 31, wherein at least one of the following: The self-passivating metal comprises at least one of iron, cobalt and manganese; and The workpiece is exposed to atmospheric oxygen at least one of prior to hardening and after exposure to the vapor.