Chemical activation of self-passivating metals

Abstract

A method for treating a workpiece made of a self-passivating metal and having a Beilby layer is disclosed, which includes exposing the workpiece to vapor generated by heating a reagent having a guanidine [HNC(NH)] moiety complexed with HCl to activate the workpiece for low-temperature interstitial surface hardening.

Description

[Technical Field]

[0001] Cross-reference of related applications This application claims priority to the following U.S. provisional patent applications: No. 62 / 922,241 filed on 6 December 2019; No. 63 / 017,259 filed on 29 April 2020; No. 63 / 017,262 filed on 29 April 2020; No. 63 / 017,265 filed on 29 April 2020; No. 63 / 017,271 filed on 29 April 2020; and No. 63 / 076,425 filed on 10 September 2020. The entire disclosure of each of these applications is incorporated herein by reference, and priority to each of these applications is claimed thereunder. [Background technology]

[0002] Conventional carburizing Conventional (high-temperature) carburizing is a widely used industrial process ("skin hardening") to increase the surface hardness of metal molded products. In commercial processes, the workpiece may come into contact with a carbon-containing gas at high temperatures (e.g., above 1,000°C), causing carbon atoms released by the decomposition of the gas to diffuse into the surface of the workpiece. The reaction of these diffused carbon atoms with one or more metals in the workpiece results in hardening, which in turn forms different chemical compounds, namely carbides, followed by the precipitation of these carbides as distinct, very hard, crystalline grains in the metal matrix, forming the surface of the workpiece. See Stickels, “Gas Carburizing”, pp 312 to 324, Volume 4, ASM Handbook, (Copyright) 1991, ASM International.

[0003] Stainless steel is corrosion-resistant because the chromium oxide surface coating that forms immediately upon exposure to air is impermeable to the permeation of water vapor, oxygen, and other chemicals. Nickel-based, cobalt-based, manganese-based, and other alloys containing a significant amount of chromium (potentially 10 wt% or more) also form these impermeable chromium oxide coatings. Titanium alloys exhibit a similar phenomenon, as they also immediately form a titanium dioxide coating upon exposure to air, and these too are impermeable to the permeation of water vapor, oxygen, and other chemicals.

[0004] These alloys are said to be self-passivating not only because they immediately form an oxide surface coating upon exposure to air, but also because these oxide coatings are impermeable to the permeation of water vapor, oxygen, and other chemicals. These coatings are fundamentally different from the iron oxide coating, e.g., rust, that forms on iron and other low-alloy steels when exposed to air. This is because these iron oxide coatings are not impermeable to the permeation of water vapor, oxygen, and other chemicals, as can be recognized by the fact that these alloys can be completely consumed by rust if not adequately protected.

[0005] Traditionally, when stainless steel is carburized, the chromium content of the steel is drastically reduced locally due to the formation of carbide precipitates, which contribute to surface hardening. As a result, there is insufficient chromium in the near-surface region directly surrounding the chromium carbide precipitates to form protective chromium oxide on the surface. Because this impairs the corrosion resistance of the steel, stainless steel is hardly surface-hardened by conventional (high-temperature) carburizing.

[0006] Low-temperature carburizing In the mid-1980s, a technique was developed for case-hardening stainless steel by bringing the workpiece into contact with a carbon-containing gas at low temperatures, for example, below approximately 500°C. At these temperatures, under conditions where carburization does not last very long, carbon atoms released by the decomposition of the gas diffuse into the workpiece surface (to a depth of 20-50 μm) without the formation of carbide precipitates. Nevertheless, a very hard case (surface layer) is obtained. Since no carbide precipitates are formed, the corrosion resistance of the steel is not impaired, but rather improved. This technique is called "low-temperature carburizing" and is described in numerous publications, including US No. 5,556,483, US No. 5,593,510, US No. 5,792,282, US No. 6,165,597, EPO No. 0787817, Japan No. 9-14019 (Publication 9-268364), and Japan No. 9-71853 (Publication 9-71853).

[0007] Nitriding and Carburizing Nitriding In addition to carburizing, nitriding and carbonitriding can be used to surface harden various metals. Nitriding works essentially the same as carburizing, except that nitriding uses a nitrogen-containing gas that decomposes to produce nitrogen atoms for surface hardening, rather than a carbon-containing gas that decomposes to produce carbon atoms for surface hardening.

[0008] However, as with carburizing, when nitriding is achieved at higher temperatures and without rapid quenching, hardening occurs through the formation and precipitation of separate compounds of diffused atoms, i.e., nitrides. On the other hand, when nitriding is achieved at lower temperatures and without plasma, hardening occurs without the formation of these precipitates, due to the stress placed on the metal's crystal lattice by nitrogen atoms diffused into the lattice. As with carburizing, stainless steel is not typically nitrided by conventional (high-temperature) or plasma nitriding because the inherent corrosion resistance of steel is lost when chromium in the steel reacts with diffused nitrogen atoms to form nitrides.

[0009] In carbonitriding, the workpiece is exposed to both nitrogen and carbon-containing gases, thereby diffusing both nitrogen and carbon atoms into the workpiece for surface hardening. Like carburizing and nitriding, carbonitriding can be achieved at higher temperatures (where skin hardening occurs due to the formation of nitride and carbide precipitates) or at lower temperatures, in which case skin hardening occurs due to a sharply localized stress field generated in the metal's crystal lattice by nitrogen and carbon atoms dissolved in the gaps and diffused into the lattice. For convenience, all three of these processes—carburizing, nitriding, and carbonitriding—are collectively referred to in this disclosure as "low-temperature surface hardening" or "low-temperature surface hardening processes."

[0010] activation Because the temperatures involved in low-temperature surface hardening are so low, carbon and / or nitrogen atoms do not penetrate the chromium oxide protective coating of stainless steel. Therefore, before low-temperature surface hardening of these metals, an activation ("passivation") process is usually performed in which the workpiece is brought into contact with a halogen-containing gas such as HF, HCl, NF3, F2, or Cl2 at a high temperature, e.g., 200-400°C, to make the protective oxide coating of the steel permeable to the passage of carbon and / or nitrogen atoms.

[0011] Somers et al., WO2006 / 136166 (US8,784,576) (the disclosure of which is incorporated herein by reference), describes an improved process for low-temperature carburizing of stainless steel in which acetylene is used as an active material component in the carburizing gas, i.e., as a source compound for supplying carbon atoms for the carburizing process. As shown therein, a separate activation step using a halogen-containing gas is not required because the acetylene source compound is sufficiently reactive to similarly depassivate the steel. Thus, the carburizing technique of this disclosure can be considered self-activating.

[0012] Christiansen et al., in publication WO2011 / 009463 (US8,845,823) (the contents of which are also incorporated herein by reference), describe a similar improved process for carbonitriding stainless steel in which oxygen-containing "N / C compounds," such as urea and formamide, are used as source compounds to supply the nitrogen and carbon atoms required for the carbonitriding process. The technique in this disclosure can also be considered self-activating, since it is said that another activation step using a halogen-containing gas is also unnecessary.

[0013] Surface treatment and bail bee layer Low-temperature surface hardening is often performed on workpieces with complex shapes. Developing these shapes typically requires several types of metal forming operations, such as cutting (e.g., sawing, machining) and / or forging (e.g., forging, drawing, bending, etc.). As a result of these processes, structural defects in the crystalline structure, as well as contaminants, such as lubricants, moisture, and oxygen, are often introduced into the near-surface region of the metal. Consequently, in most workpieces with complex shapes, a highly defective surface layer is usually formed, characterized by a plastic deformation-induced ultrafine-grained structure and a significant level of contamination. This layer can be up to 2.5 μm thick, is known as the bailby layer, and forms directly beneath a protective, coherent chromium oxide layer or other passivation layer of stainless steel and other self-passivating metals.

[0014] As shown above, traditional methods for activating stainless steel for low-temperature surface hardening involve contact with halogen-containing gases. These activation techniques are essentially unaffected by this bailby layer.

[0015] However, the same cannot be said for the self-activation techniques described in the above disclosure by Somers et al. and Christiansen et al., where the workpiece is activated by contact with acetylene or "N / C compounds". Rather, from experience, when complex-shaped stainless steel workpieces have not been surface-treated by electropolishing, mechanical polishing, chemical etching, etc. to remove their Beilby layer before surface hardening begins, these disclosed self-activation surface hardening techniques either do not function at all or, if they function somewhat, at best result in uneven and inconsistent results between surface areas.

[0016] See Ge et al., The Effect of Surface Finish on Low-Temperature Acetylene-Based Carburization of 316L Austenitic Stainless Steel, METALLURGICAL AND MATERIALS TRANSACTIONS B, Vol. 458, Dec. 2014, pp 2338-2345, 2104 The Minerals, Metal & Materials Society and ASM International. As stated therein, "[stainless] steel samples with inappropriate surface finishes, for example due to machining, cannot be successfully carburized by a process based on acetylene." In particular, see Figure 10(a) and the related discussion on pages 2339 and 2343. This reveals that the "machining-induced segregation layer" (i.e., the Beilby layer), which was intentionally introduced by etching and then scratching with a sharp blade, although etched, cannot be activated and carburized by acetylene, even though the surrounding portions of the workpiece that were etched but not scratched are readily activated and carburized. Thus, as a practical matter, these self-activation surface hardening techniques cannot be used for complex-shaped stainless steel workpieces unless they are first pretreated to remove their Beilby layer.

[0017] To address this problem, US 10,214,805, which was assigned to the assignee of the present invention, discloses an improved process for the low-temperature nitriding or carbonitriding of workpieces made of self-passivating metals, where the workpiece is contacted with the vapors generated by heating an oxygen-free nitrogen halide salt. As described therein, in addition to supplying the nitrogen and optionally carbon atoms required for nitriding and carbonitriding, these vapors can also activate the workpiece surface for these low-temperature surface hardening processes, even if these surfaces may have had a bayberry layer for a previous metal-forming operation. As a result, this self-activating surface hardening technique can be used directly for these workpieces, even if they define complex shapes for a previous metal-forming operation and even if they have not been pretreated to first remove their bayberry layer.

[0018] Kinetics of Low-Temperature Carburizing As soon as the workpiece is ready for carburizing, it is contacted with a carburizing gas for a time sufficient to diffuse carbon atoms into the workpiece surface at an elevated temperature.

[0019] In low-temperature carburizing, the carburizing gas is maintained at an elevated carburizing temperature that is high enough to promote the diffusion of carbon atoms into the surface of the article but not so high that carbide precipitates form to any significant extent.

[0020] This can be more easily understood by referring to Figure 1, a time-temperature-transformation (TTT) phase diagram of AISI 316 stainless steel [316SS (UNS S31600)], which shows the time and temperature conditions under which carbide precipitates form when steel is carburized using a specific carburizing gas. In particular, Figure 1 shows, for example, that when a workpiece is heated within the envelope defined by curve A, metal carbides of formula M23C6 are formed. Thus, it will be recognized that when a workpiece is heated under time and temperature conditions that are above the lower half of curve A, carbide precipitates will form within the surface of the workpiece. For this reason, low-temperature carburizing is carried out below curve A so that carbide precipitates do not form.

[0021] Figure 1 also shows that, with a constant carburizing gas, the carburizing temperature that promotes carbide precipitate formation also changes as a function of carburizing time. For example, Figure 1 shows that at a carburizing temperature of 1350°F, carbide precipitates begin to form after only one-tenth of an hour (6 minutes). On the other hand, at a carburizing temperature of approximately 975°F, carbide precipitates do not begin to form until carburizing has progressed for 100 hours or so. Because of this phenomenon, low-temperature carburizing is usually carried out at a constant carburizing temperature that is maintained below the temperature at which carbide precipitates form at the end of carburizing. For example, in a low-temperature carburizing process expected to last 100 hours using the alloy and carburizing gas in Figure 1, carburizing would normally be carried out at a constant temperature of 925°F or less, because this safely maintains the workpiece below the temperature at which carbide precipitates form at the end of carburizing (i.e., 975°F). Alternatively, as shown in Figure 1, carburizing is usually carried out along line M, because this keeps the workpiece safe below point Q so that carbide precipitates do not form.

[0022] In low-temperature carburizing processes, it can take 50–100–1000 hours or more to achieve the desired amount of carburization. Therefore, if carburizing is carried out safely below point Q at a constant temperature, it will be observed that at some instant during the initial stages of carburizing, the carburizing temperature at t will be far below curve A. This is also shown in Figure 1, where line segment S represents the difference between the temperature of curve A at the end of carburizing and the carburizing temperature (925°F), while line segment T represents this difference one hour after carburizing has started. As can be seen by comparing line segments S and T, if the carburizing temperature is maintained at a constant 925°F such that it is at least 50°F lower than point Q at the end of carburizing, there will be a difference of 150°F between the actual carburizing temperature and curve A one hour after carburizing has started (1175°F–925°F). Since the carburizing rate depends on temperature, it can be seen that the relatively low carburizing temperature of 925°F during the initial stages of carburizing slows down the overall carburizing process carried out in this manner.

[0023] Adjustment of carburizing temperature As discussed in U.S. Patent No. 6,547,888, this limitation can be largely eliminated by initiating the carburizing process with a higher carburizing temperature than typically used in the past, and then gradually decreasing this temperature as the carburizing progresses, so that the carburizing temperature safely reaches below the envelope defined by the curve in the workpiece phase diagram at the end of the carburizing process.

[0024] This approach is shown in Figure 2. 1 This is similar to curve M in Figure 1, except that it is shown by curve X, which indicates that the carburizing temperature decreases from an initial high value to a lower final value throughout the carburizing process. Specifically, curve X shows that carburizing is started at an initial carburizing temperature of 1125°F, which is about 50°F lower than the temperature at which carbide precipitates begin to form 30 minutes after the start of the carburizing process (point W in Figure 2), and then the carburizing temperature decreases as carburizing progresses, reaching a final carburizing temperature of 925°F at the end of the carburizing process, which is the same end temperature used in the conventional process shown in Figure 1.

[0025] The carburizing temperature at any time t during the carburizing process is maintained within a predetermined amount of temperature (e.g., 50°F, 75°F, 100°F, 150°F, or even 200°F) at which carbides begin to form at that time. In other words, the carburizing temperature is maintained below curve A, within a predetermined amount of temperature (e.g., a temperature buffer), throughout the carburizing process. By this means, the carburizing temperature is maintained considerably higher than in conventional single, low-temperature runs, which are even lower than the temperature at which carbide precipitates begin to form. The net effect of this approach is an increase in the overall carburizing rate, because the carburizing temperature is higher than it would otherwise be for most of the carburizing process. At any time t during carburizing, the instantaneous rate of carburizing is temperature-dependent, and in this approach, this instantaneous rate is increased by increasing the instantaneous carburizing temperature. The net effect is a higher overall carburizing rate, which in turn leads to a shorter overall time to complete the carburizing process. 1 Please note that Figure 2 is the same TTT diagram as Figure 1.

[0026] Naturally, when operating at the higher carburizing temperatures described above, it is still necessary to ensure that carbide precipitates do not form to a substantial extent during carburizing. Therefore, the carburizing temperature is set not only so as not to drop below a minimum predetermined amount at any given time t, as described above, but also so as not to exceed a maximum value that is too close to curve A. In other words, to ensure that carbide precipitates do not form, the carburizing temperature must still be maintained below curve A, by a sufficient amount (e.g., 25°F or 50°F) at any given time t. In practical implementation, this means that the carburizing temperature is set within the range below curve A, with its maximum being a sufficient distance below curve A (e.g., 25°F or 50°F), and its minimum being further below curve A, by the predetermined amount mentioned above (i.e., 50°F, 75°F, 100°F, 150°F, or 200°F). Therefore, the carburizing temperature can be set to lie below curve A, within some suitable range (e.g., 25°F to 200°F or 50°F to 100°F).

[0027] Figure 3 2 Curve Y in [reference] shows an alternative method, which can be implemented in the same way as described above, except that the carburizing temperature is decreased stepwise rather than continuously. Gradual reduction can often be simpler, particularly from an equipment standpoint. Since the carburizing process can take several hours to many hours, the number of increments can vary from a small number, such as 3 to 5, to a large number, such as 10, 15, 20, 25, or even more.

[0028] The need for faster surface treatment Many of the methods described above may require a considerable amount of time for curing. Many conventional methods require hours, or even days, to achieve a useful hardness level and a substantial carburized layer depth on the order of tens of microns. Therefore, developing a method that achieves the curing levels and depths of prior art methods in less time and at less cost would be advantageous. 2Please note that Figure 3 is the same TTT diagram as Figures 1 and 2. [Overview of the project]

[0029] A method for processing a workpiece made from a self-passivated metal and having a bailby layer is disclosed. The method involves activating the workpiece for low-temperature interstitial surface hardening by exposing it to vapor produced by heating a reagent having a guanidine [HNC(NH2)2] moiety and complexed with HCl.

[0030] A method for producing surface-hardened parts in the manufacturing of continuous conveyor belts is disclosed. The method includes purging the atmosphere of the continuous conveyor belt with gas while maintaining the atmosphere at a temperature of 600°C or less; placing untreated parts on the continuous conveyor belt; applying a reagent by vapor, solvent, or together with a vehicle that carries the reagent to activate the parts, for example, by coating the untreated parts (the reagent has a guanidine [HNC(NH2)2] moiety and is complexed with HCl); activating the workpiece for low-temperature intermittent surface hardening by exposing the workpiece to vapor generated by heating the reagent; and performing low-temperature intermittent surface hardening on the parts for a period of less than 2 hours.

[0031] A method is disclosed for processing a workpiece made from a self-passivated metal and having a bailby layer. The method involves activating the workpiece for low-temperature intermittent surface hardening by exposing the workpiece to vapors produced by heating one or more nonpolymer N / C / H compounds at an exposure temperature lower than the temperature at which nitride and / or carbide precipitates form within the workpiece. The one or more N / C / H compounds are (a) solid or liquid at 25°C and atmospheric pressure, (b) have a molecular weight of ≤ 5,000 daltons, and (c) are not complexed with a hydrohalide acid or can be complexed. If the nonpolymer N / C / H compound is not complexed, any halogen atom replaces one or more unstable hydrogen atoms of the nonpolymer N / C / H compound. If the nonpolymer N / C / H compound is complexed, any halogen atom forms part of the hydrohalide complexing acid. [Brief explanation of the drawing]

[0032] [Figure 1] This is a time-temperature-transformation (TTT) phase diagram for AISI 316 stainless steel [316SS (UNS S31600)]. [Figure 2] Figure 1 shows several temperature gradient (gradient) protocols superimposed on the TTT. [Figure 3] Figure 1 shows an additional temperature gradient protocol superimposed on the TTT. [Figure 4] The following shows exemplary breads used in some of the examples. [Figure 5] Table 1 shows the hardness depth profiles measured by the Vickers test for steel treated with two different reagents, DmbgHCl and GuHCl. [Figure 6(a)] This is the Auger depth profile of a case-hardened stainless steel (316SS (UNS S31600)) Pan 1, showing the overlap of carbon and nitrogen concentrations within the surface layer in the presence of dimethyl biguanide HCl (DmbgHCl). [Figure 6(b)]This is the Auger depth profile of a case-hardened stainless steel (316SS (UNS S31600)) Pan 1, showing the overlap of carbon and nitrogen concentrations within the surface layer in the presence of guanidine HCl (GuHCl). [Figure 7] An example of a temperature rise protocol overlaid on a TTT phase diagram for 316SS (UNS S31600) is shown. [Figure 8] Figure 7 shows an example of a temperature reduction protocol superimposed on the TTT phase diagram. [Figure 9] This shows an optical image of the surface of a treated 316L stainless steel ferrule. [Modes for carrying out the invention]

[0033] Definitions and technical terms As shown above, the fundamental difference between traditional (high-temperature) surface hardening and the modern low-temperature surface hardening process first developed in the mid-1980s is that in traditional (high-temperature) surface hardening, hardening occurs as a result of the formation of carbide and / or nitride precipitates on the surface of the metal being hardened. In contrast, in low-temperature surface hardening, hardening occurs as a result of the stress exerted on the metal's crystal lattice on the metal's surface, as a result of carbon and / or nitrogen atoms diffused into these surfaces. Since carbide and / or nitride precipitates, which contribute to surface hardening in traditional (high-temperature) surface hardening, are not found in stainless steel surface hardened by low-temperature carburizing, and furthermore, low-temperature surface hardening does not adversely affect the corrosion resistance of stainless steel, the original idea was that in low-temperature carburizing, surface hardening occurs solely as a result of sharply localized stress fields generated by carbon and / or nitrogen atoms dissolved in the gaps and diffused into the (austenite) crystal structure of the steel.

[0034] However, recent, more sophisticated analytical work has revealed that when low-temperature surface hardening is performed on alloys where some or all of the alloy volume consists of ferrite phase, small amounts of several types of previously unknown nitride and / or carbide precipitates may form in these ferrite phases. Specifically, recent analytical work suggests that in AISI 400 series stainless steels, which generally exhibit a ferrite phase structure, small amounts of previously unknown nitride and / or carbide precipitates may occur when the alloy is low-temperature surface hardened. Similarly, recent analytical work suggests that in duplex stainless steels containing both ferrite and austenite phases, small amounts of previously unknown nitride and / or carbide precipitates may occur in the ferrite phase of these steels when they are low-temperature surface hardened. The exact properties of these previously unknown, newly discovered nitride and / or carbide precipitates remain unknown, but it is known that the ferrite matrix directly surrounding these “para-equilibrium” precipitates does not experience a drastic reduction in its chromium content. As a result, the corrosion resistance of these stainless steels remains unimpaired because the chromium, which is responsible for corrosion resistance, remains uniformly distributed throughout the metal.

[0035] Therefore, for the purposes of this disclosure, where a workpiece surface layer "essentially free of nitride and / or carbide precipitates," or a workpiece surface-hardened "without the formation of nitride and / or carbide precipitates," or "a temperature below the temperature at which nitride and / or carbide precipitates form," this reference will be understood to indicate a type of nitride and / or carbide precipitate that contributes to surface hardening in traditional (high-temperature) surface hardening processes, and which contains a sufficient amount of chromium, and thus the metal matrix directly surrounding these precipitates loses its corrosion resistance as a result of the drastic reduction in its chromium content. This reference does not refer to previously unknown, newly discovered nitride and / or carbide precipitates disclosed herein that may form in small amounts in the ferrite phase of AISI 400 stainless steel, duplex stainless steel, and other similar alloys.

[0036] Furthermore, for the purposes of this disclosure, it should be understood that "carburizing and nitriding," "nitriding and carburizing," and "nitrocarburizing" refer to the same process.

[0037] In addition, “passivation” in this disclosure, in relation to the alloys treated by this invention, is understood to refer to a type of alloy that, upon exposure to air, rapidly forms a protective oxide coating that is impermeable to the permeation of water vapor, oxygen, and other chemicals. Therefore, metals such as iron and low-alloy steels that can form an iron oxide coating upon exposure to air are not considered to be “passivated” within the scope of this term, because these coatings are not impermeable to the permeation of water vapor, oxygen, and other chemicals.

[0038] alloy This invention can be implemented for any metal or metal alloy that self-passivates in the sense that it forms a coherent protective chromium-rich oxide layer upon exposure to air (which is impermeable to the passage of nitrogen and carbon atoms). These metals and alloys are well known and have been described in previous patents relating to low-temperature surface hardening processes, for example, US No. 5,792,282, US No. 6,093,303, US No. 6,547,888, EPO No. 0787817, and Japanese Patent Document 9-14019 (Publication 9-268364).

[0039] Of particular interest are stainless steels, i.e., steels containing 5-50, preferably 10-40 wt% Ni and enough chromium to form a protective layer of chromium oxide on the surface when the steel is exposed to air. This includes alloys containing about 10% or more chromium. Preferred stainless steels contain 10-40 wt% Ni and 10-35 wt% Cr. 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 are more preferred. AISI 400 series stainless steels and, in particular, Alloy 410, Alloy 416 and Alloy 440C are also of particular interest.

[0040] Other types of alloys that can be processed by this invention are nickel-based, cobalt-based, and manganese-based alloys that contain enough chromium, for example about 10% or more, to form a coherent protective chromium oxide coating when the steel is exposed to air. Examples of such nickel-based alloys include Alloy600, Alloy625, Alloy825, AlloyC-22, AlloyC-276, Alloy20Cb, and Alloy718. Examples of such cobalt-based alloys include MP35N and BiodurCMM. Examples of such manganese-based alloys include AISI 201, AISI 203EZ, and Biodur108.

[0041] Another type of alloy to which this invention can be implemented is titanium alloys. As is well understood in metallurgy, these alloys form a coherent protective titanium oxide coating upon exposure to air, and they are also impermeable to the passage of nitrogen and carbon atoms. Specific examples of such titanium alloys include Grade 2, Grade 4, and Ti6-4 (Grade 5). Similarly, alloys based on other self-passivating metals such as zinc, copper, and aluminum can also be activated (depassivated) by the techniques of this invention.

[0042] The specific phase of the metal treated by this invention is not important in the sense that this invention can be implemented for any phase structure of metal, including but not limited to austenite, ferrite, martensite, and duplex metals (e.g., austenite / ferrite).

[0043] Activation by nonpolymer N / C / H compounds According to this invention, a workpiece made from a self-passivated metal and having a bailby layer on at least one surface region thereof is activated (i.e., depassivated) for low-temperature surface hardening by contacting the workpiece with vapors produced by heating (thermal decomposition) a reagent containing a nonpolymer N / C / H compound. Mixtures of different nonpolymer N / H / C compounds can also be used for this purpose. In addition to causing the depassivation of the workpiece, as will be further described below, the nonpolymer N / H / C compounds of this invention can also supply nitrogen and carbon atoms for simultaneous surface hardening of the workpiece, e.g., carburizing, nitriding, and / or carbonitriding. Since different nonpolymer N / C / H compounds supply these nitrogen and carbon atoms in different amounts and degrees, mixtures of these compounds can be used to adjust a particular nonpolymer N / C / H compound to be used for specific operating conditions desired for simultaneous surface hardening.

[0044] The nonpolymer N / C / H compounds of this 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. This includes nonpolymer N / C / H compounds having molecular weights of ≤2,000 daltons, ≤1,000 daltons or even ≤500 daltons. This includes nonpolymer N / C / H compounds containing a total of 4–50 C+N atoms, 5–50 C+N atoms, 6–30 C+N atoms, 6–25 C+N atoms, 6–20 C+N atoms, 6–15 C+N atoms, and even 6–12 C+N atoms.

[0045] Specific classes of nonpolymer N / C / H compounds that can be used in this invention include primary amines, secondary amines, tertiary amines, azo compounds, heterocyclic compounds, ammonium compounds, azides, and nitriles. Among these, those containing 4-50 C+N atoms are preferred. These include those containing 4-50 C+N atoms, alternating C=N bonds, and one or more primary amine groups. Examples include melamine, aminobenzimidazole, adenine, benzimidazole, guanidine, biguanide, triguanide, 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'-bipyridyl, and (2-(2-pyridyl)benzimidazole). Specific triguanides include 1,3-bis(diaminomethylidene)guanidine and N-carbamidomidoylimidodicarbonimido acid diamide.

[0046] The three triazine isomers, as well as various aromatic primary amines containing 4-50 C+N atoms, such as 4-methylbenzeneamine (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-carbonitride, are also included. Aromatic diamines containing 4-50 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, are also of interest. Hexamethylenetetramine, benzotriazole, and ethylenediamine are also included.

[0047] Another class of compounds to be included, which includes some of the above compounds, is one that forms nitrogen-based chelate ligands, i.e., polydentate ligands containing two or more nitrogen atoms arranged to form separate coordinate bonds with a single central metal atom. Compounds that form this type of bidentate chelate ligand are included. Examples include o-phenanthroline, 2,2'-bipyridine, aminobenzimidazole, and guanidium chloride (guanidium chloride is described further below).

[0048] Another type of nonpolymer N / C / H compound that can be included is used to produce carbon nitride and / or carbon nitride intermediates(s) as described in WO2016 / 027042 (the entirety of which is incorporated herein by reference). The intermediate species can be involved in or contribute to the low-temperature activation and curing of the workpiece. Precursors may include melamine and GuHCl and can form a variety of carbon nitride species. These species have the empirical formula C3N4 and contain stacked layers or sheets of 1 atomic thickness, the layers being formed from carbon nitride, in this case with 3 carbon atoms for every 4 nitrogen atoms. Solids can contain as few as 3 such layers and as many as 1000 or more layers. The carbon nitride is prepared in the absence of other elements, although doping with other elements is also attempted.

[0049] Another subgroup of nonpolymer N / C / H compounds described above is one that contains 20 or fewer C+N atoms and at least two N atoms.

[0050] In some cases, at least two of the N atoms in these compounds are not primary amines bonded directly to a 6-carbon aromatic ring or via an intermediate aliphatic moiety. In other words, one or more of the N atoms in these particular nonpolymer N / C / H compounds can be primary amines bonded to a 6-carbon aromatic ring, but at least two of the N atoms in these compounds must be primary amines bonded to a different form, such as a secondary or tertiary amine or something other than a 6-carbon aromatic ring.

[0051] In nonpolymer N / C / H compounds of this subgroup (i.e., nonpolymer N / C / H compounds containing 20 or fewer C+N atoms and at least two N atoms), the N atoms can be bonded to each other directly, as happens in the azole moiety, for example, but more generally, they are bonded to each other by one or more intermediate carbon atoms.

[0052] This subgroup of nonpolymer N / C / H compounds includes those containing 15 or fewer C+N atoms, and those containing at least 3 N atoms. It also includes those containing 15 or fewer C+N atoms and at least 3N atoms.

[0053] Nonpolymer N / C / H compounds in this subgroup can be considered to have a relatively high degree of nitrogen substitution. In this situation, a relatively high degree of nitrogen substitution would be considered to mean that the N / C atomic ratio of the compound is at least 0.2. This includes compounds with N / C atomic ratios of 0.33 or higher, 0.5 or higher, 0.66 or higher, 1 or higher, 1.33 or higher, or even 2 or more. This also includes nonpolymer N / C / H compounds with N / C atomic ratios of 0.25-4, 0.3-3, 0.33-2, and even 0.5-1.33.

[0054] This subgroup of nonpolymer N / C / H compounds containing 10 or fewer C+N atoms includes, in particular, those with N / C atom ratios of 0.33-2 and more specifically, 0.5-1.33.

[0055] Nonpolymer N / C / H compounds of this subgroup containing 8 or fewer C+N atoms, particularly those with an N / C atom ratio of 0.5-2 or even 0.66-1.5, especially triguanide reagents, are of particular interest.

[0056] To achieve this relatively high degree of nitrogen substitution, nonpolymer N / C / H compounds in this subgroup can contain one or more nitrogen-rich moieties, such as imine moieties [C=NR], cyano moieties [-CN], and azo moieties [RN=NR]. These moieties can be part of a 5- or 6-membered heterocycle containing one or more additional N atoms, such as when the imine moiety forms part of an imidazole or triazine group, or when the azole moiety forms part of a triazine or triazole group.

[0057] These parts can also be considered independent in the sense that they are not part of a larger heterocyclic group. If so, two or more of these parts can be bonded to each other by intermediate N atoms, for example, when multiple imine parts are bonded to each other by intermediate N atoms, as in 1,1-dimethyl biguanide hydrochloride, or when a cyano group is bonded to an imine part by an intermediate N atom, as in 2-cyanoguanidine. Alternatively, they can simply be pendanted from the rest of the molecule, as in 5-aminoimidazole-4-carbonitride, or they can be directly attached to primary amines, as in 1,1-dimethyl biguanide hydrochloride, formamidine hydrochloride, acetamidine hydrochloride, 2-cyanoguanidine, cyanamide, and cyanoguanidine monohydrochloride.

[0058] As shown above, if the nonpolymer N / C / H compounds of this subgroup contain one or more primary amines, these primary amines are preferably not bonded to the carbon atoms of the 6-carbon aromatic ring. Rather, they are preferably bonded to something else, for example, to the carbon atoms of the imine moiety [C=NR], such as those resulting in 1,1-dimethyl biguanide hydrochloride, formamidine hydrochloride, acetamidine hydrochloride, 2-cyanoguanidine, cyanamide, and cyanoguanidine monohydrochloride. Alternatively, the primary amines can be directly or indirectly bonded to a heterocyclic moiety containing at least one, preferably at least two additional N atoms, such as those resulting in 2-aminobenzimidazole, 2-aminomethylbenzimidazole dihydrochloride, 5-aminoimidazole-4-carbonitride, and 3-amino-1,2,4-triazine.

[0059] In nonpolymer N / C / H compounds of this subgroup containing one or more secondary amines, the secondary amine can be part of a heterocycle containing an additional 0, 1, or 2 nitrogen atoms. An example of such a compound in which the secondary amine is part of a heterocycle without an additional nitrogen atom is 1-(4-piperidyl)-1H-1,2,3-benzotriazole hydrochloride. Examples of such compounds in which the heterocycle contains one additional nitrogen atom are 2-aminobenzimidazole, 2-aminomethylbenzimidazole dihydrochloride, imidazole hydrochloride, and 5-aminoimidazole-4-carbonitride. An example of such a compound in which the secondary amine is part of a heterocycle containing two additional nitrogen atoms is benzotriazole. Alternatively, the secondary amine can be attached to a cyano moiety, such as in 2-cyanoguanidine and cyanoguanidine monohydrochloride.

[0060] In nonpolymer N / C / H compounds of this subgroup containing one or more tertiary amines, the tertiary amine can be part of a heterocycle containing one or two additional N atoms, an example being 1-(4-piperidyl)-1H-1,2,3-benzotriazole hydrochloride.

[0061] In some embodiments of the invention, the nonpolymer N / C / H compound used contains only N, C, and H atoms. In other words, the specific nonpolymer N / C / H compound used does not contain halogens. In other embodiments of the invention, the nonpolymer N / C / H compound contains, associates with, or can complex with one or more optional halogen atoms.

[0062] One way this can be done is by including a hydrohalic acid, such as HCl, in the compound in the form of an aggregate or complex. If so, this nonpolymer N / C / H compound is referred to in this disclosure as “complexed.” On the other hand, if a nonpolymer N / C / H compound is not complexed with such an acid, it is referred to in this disclosure as “uncomplexed.” If neither “complexed” nor “uncomplexed” is used, it will be understood that the terms in question refer to both complexed and uncomplexed nonpolymer N / C / H compounds.

[0063] Another way in which arbitrary halogen atoms can be included in the nonpolymer N / C / H compounds of this invention is by replacing some or all of its unstable hydrogen atoms with halogen atoms, preferably Cl, F, or both. For simplicity of explanation, noncomplexed nonpolymer N / C / H compounds of this subgroup containing one or more halogen atoms replacing the unstable H atoms are referred to herein as “halogen-substituted,” while noncomplexed nonpolymer N / C / H compounds of this invention that do not contain such halogen atoms are referred to herein as “unsubstituted.”

[0064] In those embodiments of this invention, in which the nonpolymer N / C / H compound used contains an arbitrary halogen atom, all nonpolymer N / C / H compounds used can contain an arbitrary halogen atom. In addition, both types of halogen-containing nonpolymer N / C / H compounds can be used, namely, complexed nonpolymer N / C / H compounds in which the halogen atom is part of a complexed hydrohalic acid, and non-complexed nonpolymer N / C / H compounds in which the halogen atom replaces an unstable H atom.

[0065] As shown above, the nonpolymer N / C / H compounds of this invention can, if desired, be complexed with suitable hydrohalic acids, such as HCl (e.g., HF, HBr, and HI). In this context, “complexed” is understood to mean the type of association that occurs when a simple hydrohalic acid such as HCl is combined with a nitrogen-rich organic compound such as 2-aminobenzimidazole. When both are dissolved in water, HCl can dissociate, but 2-aminobenzimidazole does not. In addition, when the water evaporates, the resulting solid consists of a mixture of these individual compounds on an atomic basis—for example, a complex. It exclusively consists of a salt in which the Cl- anion from HCl is ionically bonded to the N atom in 2-aminobenzimidazole, which becomes positive by accepting an H+ cation derived from HCl.

[0066] When water evaporates from an aqueous mixture of ammonia and HCl, the H+ cation from HCl combines with the N atom from ammonia to form a positively charged ammonium cation. As the water continues to evaporate, the Cl- anion from HCl forms ionic bonds with these positively charged ammonium cations. As a result, a novel compound, the salt ammonium chloride, is formed. The same thing does not necessarily happen when the nonpolymer N / C / H compounds of this invention are complexed with HCl or other hydrohalic acids, because, due to the specific chemical structure of these compounds, the nitrogen atoms in these compounds are less likely to form ionic salt bonds.

[0067] For example, nonpolymer N / C / H compounds in which the N atom exists in the form of a secondary or tertiary amine can form complexes using bonds other than exclusive ionic bonds, because most of these N atoms are less likely to accept H+ cations and become positively charged to the extent necessary to form ionic salt bonds. Therefore, in some embodiments of this invention, the complexed nonpolymer N / C / H compound preferably contains at least two nitrogen atoms in the form of a secondary and / or tertiary amine.

[0068] Similarly, it is also evident that nonpolymer N / C / H compounds in which at least one N atom is present in the imine moiety (C=NR) also form complexes. This is especially true when the carbon atom of the imine moiety is directly bonded to a nitrogen atom, such as in imidazole rings, guanidine and its derivatives, and acid amidine compounds, for example, formamidine hydrochloride and acetamide hydrochloride. Therefore, in other embodiments, the acid-complexed nonpolymer N / C / H compounds of this invention preferably comprise one, two, three, or even four imine moieties (C=NR). These include compounds in which one or more carbon atoms of these imine moieties are directly bonded to an N atom.

[0069] According to this invention, it has been found that vapors generated by heating and / or thermally decomposing a reagent containing a non-polymer N / C / H compound, either composited with a hydrogen halide or not, readily activate the surface of self-passivated metals, even in the presence of a significant bailby layer. In addition, very often, these vapors also supply nitrogen and carbon atoms for simultaneous surface hardening of the workpiece. Even more surprisingly, it has also been found that surface hardening carried out in this manner can be achieved in a much shorter period than conventionally possible. For example, while previous processes for activation followed by low-temperature surface hardening may take 24-48 hours to achieve a favorable case, the inventive process for activation and low-temperature surface hardening can achieve an equivalent case in less than 2 hours, in as little as 1 minute, regardless of whether the surface hardening occurs simultaneously with or after activation.

[0070] While we do not wish to be bound by any particular theory, it is thought that the vapor of this nonpolymer N / C / H compound decomposes by heating and / or thermal decomposition either before and / or as a result of contact with the workpiece surface, yielding ionic and / or free radical decomposition species that effectively activate the workpiece surface. In addition, this decomposition also produces nitrogen and carbon atoms, which diffuse into the workpiece surface, thereby being cured by low-temperature carburizing and nitriding.

[0071] Therefore, it will be recognized that when nonpolymer N / C / H compounds are used for activation according to this invention, activation and at least some surface hardening occur almost simultaneously, thus eliminating the need to include additional nitrogen and / or carbon-containing compounds in the system to enhance the surface hardening process. However, this does not mean that such additional compounds cannot or should not be included.

[0072] In this regard, it should be recognized that the degree to which a workpiece is surface-hardened when activated by this invention depends on a variety of different factors, including the properties of the particular alloy being treated, the particular nonpolymer N / C / H compound used, and the temperature at which activation occurs. Generally speaking, activation by this invention can usually occur at temperatures somewhat lower than those associated with low-temperature surface hardening. Activation by this invention can also occur at higher temperatures, e.g., above 600°C. In addition, different alloys can differ from one another in terms of the temperatures at which they are activated and surface-hardened. Furthermore, different nonpolymer N / C / H compounds contain more or less relative amounts of nitrogen and carbon atoms.

[0073] Therefore, in some embodiments of the invention, certain alloys can be activated and simultaneously fully surface-hardened simply as a result of nitrogen and carbon atoms released from non-polymer N / C / H compounds. If so, it may be unnecessary to enhance the surface hardening process by including one or more additional nitrogen and / or carbon-containing compounds in the system to supply additional nitrogen and / or carbon atoms.

[0074] However, in other embodiments of the invention, certain alloys may not be fully surface-hardened simply as a result of nitrogen and carbon atoms released by the non-polymer N / C / H compound during activation. If so, additional nitrogen and / or carbon-containing compounds can be included in the system to supply additional nitrogen and / or carbon atoms to enhance the surface hardening process. Examples include nitrogen, hydrogen, methane, ethane, ethylene, acetylene, ammonia, methylamine, and mixtures thereof. If so, these additional nitrogen 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. It should be understood that this additional nitrogen and / or carbon-containing compound may be different from, but may be the same as, the non-polymer N / C / H compound used for surface hardening, if desired.

[0075] In addition to enhancing surface hardening during activation in this manner, and / or alternatively, enhancing surface hardening can be postponed until after the activation has finished and is completed by supplying additional nitrogen and / or carbon-containing compounds. If so, the enhanced surface hardening can be carried out in the same reactor used for activation or in a different reactor.

[0076] The amount of nonpolymer N / C / H compound used to activate a particular workpiece also depends on many factors, including the properties of the alloy being activated, the surface area of ​​the workpiece being treated, and the specific nonpolymer N / C / H compound used. This can be easily determined by routine experiments, using the following examples for reference.

[0077] In addition, any reagent described herein may be used in conjunction with the reagent disclosed in U.S. Patent No. 10,214,805.

[0078] Finally, it should be noted that a key feature of this invention is that its nonpolymer N / C / H compound is oxygen-free. The reason is to avoid the transient generation of oxygen atoms in the reaction of these compounds, which would otherwise occur if these compounds contained oxygen atoms. As shown above, according to this invention, activation is thought to occur due to ions and / or free radical decomposition species generated when the nonpolymer N / C / H compound of this invention decomposes. Any such transient oxygen atoms are thought to react with these ions and / or free radical decomposition species and become inactive. In fact, this explains why the process described in the above-mentioned patent of Christiansen et al. is difficult when the workpiece being treated has a Bailby layer, because the N / C compound actually used there contains a considerable amount of oxygen. This problem is avoided by this invention, because the nonpolymer N / C / H compound used is oxygen-free.

[0079] Any preferred form of any reagent described herein may be used in conjunction with this disclosure. This includes powders, liquids, gases, and combinations thereof. "Reagent" herein includes any substance, including nonpolymer N / C / H compounds or other compounds used for the activation and / or curing of metals.

[0080] Low temperature heat curing As shown above, in addition to activating the surface of self-passivating metals for low-temperature nitriding or carburitriding, the vapors produced by heating the nonpolymer N / C / H compounds of this invention can also supply nitrogen and carbon atoms to achieve at least some degree of thermal curing of the workpiece through these thermosetting processes, even without the inclusion of additional reagents in the reaction system.

[0081] However, if desired, the rate at which low-temperature thermal curing occurs can be increased by including additional nitrogen and / or carbon-containing reagents in the reaction system—in particular by contacting the workpiece with an additional nitrogen-containing compound that can decompose to produce nitrogen atoms for nitriding, an additional carbon-containing compound that can decompose to produce carbon atoms for carburizing, an additional compound containing both carbon and nitrogen atoms that can decompose to produce both carbon and nitrogen atoms for carbonitride, or any combination thereof.

[0082] These additional nitrogen and / or carbon-containing compounds can be added to the reaction system at any time. For example, they can be added after the activation of the workpiece is complete, or simultaneously with the activation. Finally, they can also be added before the activation begins, but low-temperature surface hardening is considered to be more effective when they are added simultaneously with and / or after the activation.

[0083] Activation and thermosetting can be achieved by this invention in a closed system, for example, as described in US Patent No. 10,214,805, assigned to the assignee of the invention, i.e., in a reaction vessel completely sealed to prevent any inflow or outflow of material throughout the entire activation and thermosetting process. To ensure that activation and thermosetting are carried out properly, it is desirable that a sufficient amount of vapor of the nonpolymer N / C / H compound comes into contact with the surface of the workpiece, in particular, with respect to the surface areas having a substantial bailby layer. Since the nonpolymer N / C / H compound used for both activation and thermosetting according to this invention is often in the form of fine particulate solids, a simple way to ensure that this contact is carried out properly is to coat or otherwise cover these surfaces with this fine particulate solid, and then seal the reaction vessel before heating of the workpiece and the nonpolymer N / C / H compound begins. The nonpolymer N / C / H compound can also be dissolved or dispersed in a suitable liquid and then coated onto the workpiece.

[0084] These approaches are particularly advantageous when a large batch containing many small workpieces, such as ferrules and fittings for conduits, is to be thermally cured simultaneously in the same reactor.

[0085] The approach of this invention, in which activation and thermosetting are carried out in the closed system described above, is in some respects similar to the technique disclosed in Bessen's US Patent No. 3,232,797, in which a thin steel strip is coated with a guanidium compound containing guanidium chloride, and then heated to decompose the guanidium compound and nitride the steel strip. However, the thin steel strip nitrided therein does not self-passivate in the sense that a strongly bonded, coherent protective oxide coating (which is impermeable to the passage of nitrogen and carbon atoms) is formed. Therefore, the technique described therein is of little relevance to this invention. In this invention, stainless steel and other self-passivating metals that are impermeable to the passage of nitrogen and carbon atoms are made permeable to these atoms by contact with vapor of a non-polymer N / C / H compound as part of a low-temperature thermosetting process.

[0086] Rapid curing using guanidine HCl reagent According to this disclosure, the applicant has determined that certain reagent classes of nonpolymer N / C / H compounds, including a guanidine [HNC(NH2)2] moiety or functionally complexed with HCl, exhibit unexpectedly superior results, including providing suitable activation and simultaneous surface hardening for steel in as little as one minute, as opposed to 2-48 hours.

[0087] In particular, the results indicate that at least three reagents belong to this system, 1,1-dimethyl biguanide HCl (hereinafter referred to as "DmbgHCl"): [ka] and guanidine HCl (hereinafter referred to as "GuHCl"): [ka] Furthermore, biguanide HCl (BgHCl) was shown to successfully induce very rapid surface hardening under low-temperature conditions. For example, 8 mg of these reagents (tested separately) were able to achieve a hardened carburized layer depth of 20–24 μm after 2 hours of low-temperature (500°C) treatment. As described in more detail below, this result is considerably faster than when other reagents were used using a similar method. The hardened layer was formed on the wall of a cylindrical crucible pan made of 316SS (UNS S31600) stainless steel. An image of exemplary pan 1 is shown in Figure 4. The pan has a diameter of approximately 0.5 cm and a height of approximately 0.5 cm. The pan was machined from a round bar stock using standard metal cutting tools. No other significant surface preparations were performed. The machined surface of pan 1 likely has a bailby layer. The tests were performed using a Netzsch simultaneous thermal analysis (STA) instrument. 3 Bread 1 was skin-baked using the following modification, following the procedure disclosed in U.S. Patent No. 10,214,805: 3 The Netzsch simultaneous thermal analysis (STA) instrument is described in more detail in “Fourier Transform of Infrared (FT-IR) Spectrometers Coupled to Thermal Analysis: Concepts, Instruments and Applications from RT to 2000℃, Analyzing and Testing” NGB-FTIR-EN-0220-NWS, attached as Annex A. [Table 1]

[0088] As shown in Table 1, the applicants have found that these reagents can unexpectedly shorten the exposure treatment time from 2 hours to 1 minute while having the same curing effect. The hardness depth profiles measured by the Vickers test are shown in Figure 5 for steel treated with two different reagents, DmbgHCl and GuHCl, according to Table 1. These are for 316SS (UNS S31600) stainless steel crucible pans 1 treated at 500 °C for 2 hours according to Table 1. There were two pans 1 treated with each reagent, DmbgHCl and GuHCl. All samples show improved hardness in the surface region (about 20 μm case-hardened layer depth). 4 For example, pan 1 in Figure 4. 5 Tests have shown that moisture reacts with various reagents used in US Patent No. 10,214,805, causing chemical changes. 6 The activating reagent was placed in the pan immediately before curing and heat dehydrated.

[0089] The functionality with a guanidine [HNC(NH2)2] moiety or HCl complexation is a common chemical structure in all of DmbgHCl, GuHCl, and BgHCl. Other tested reagents lacking the guanidine moiety have not been shown to produce a case-hardened layer depth of about 20 μm in less than 2 hours under similar conditions.

[0090] Other compounds containing guanidine with HCl, such as biguanide HCl (BgHCl) and melamine HCl (MeHCl), are also suitable. Other suitable guanidine-containing compounds include triguanide 7More specifically, suitable examples of guanides, biguanides, biguanidines, and triguanides include chlorhexidine and chlorhexidine salts, analogs, and derivatives, such as chlorhexidine acetate, chlorhexidine gluconate, and chlorhexidine hydrochloride, picroxidine, alexidine, and polyhexanides. Other examples of guanides, biguanides, biguanidines, and triguanides that can be used in the present invention include chlorproguanil hydrochloride, proguanil hydrochloride (currently used as an antimalarial drug), metformin hydrochloride, phenformin, and buformin hydrochloride (currently used as an antidiabetic drug).

[0091] The results described herein discuss the use of guanidine partial-containing compounds complexed with HCl, but these results can also be obtained using guanidine partial reagents that are not complexed with HCl. Reagent complexing with any hydrogen halide can achieve similar results. Guanidine partial reagents without HCl complexing can also be mixed with other reagents having HCl complexing, e.g., other reagents described in U.S. Patent No. 10,214,805. An important criterion may be whether the reagent or mixture of reagents has a liquid phase while decomposing in the low-temperature carburizing and nitriding temperature range (e.g., 450–500C). The extent to which the reagent evaporates without decomposing before reaching that temperature range is an important consideration.

[0092] Surface layer with overlapping carbon and nitrogen The surface layer formed in the above test contains two distinct sublayers characterized by low-temperature carburizing and nitriding. The outer sublayer is rich in invading nitrogen. The inner sublayer is rich in invading carbon. The hardness depth profile shows that the carburized layer depth represented by these two layers after 2 hours of treatment with DmbgHCl and GuHCl (e.g., a hardened carburized layer depth of 20-24 μm) is similar to the carburized layer depth achieved by 2-day treatment using the conventional method and reagents described in U.S. Patent No. 10,214,805. The applicants have also discovered a method for hardening stainless steel by forming a carbon-containing surface layer that includes an overlap of nitrogen concentrations within its surface layer. The applicants believe that this overlap of nitrogen and carbon concentrations may be due to the formation of fine carbide precipitates that do not exhibit adverse effects on the properties of coarser-grained precipitates that deplete chromium atoms from the nearby base metal (which, in turn, adversely affect the chromium oxide passivation layer). Therefore, the fine precipitates can also preserve the corrosion-resistant, chromium oxide passivation layer on the stainless steel (for example, extracting less than 20% of the chromium from that layer). Low-temperature intermittent hardening, such as that described in U.S. Patent No. 10,214,805. 8Under these conditions, crude carbide and nitride precipitates are unlikely to form. The temperature is likely too low for the substitutional diffusion of chromium and other metal atoms necessary for crude carbide precipitation. In fact, as described in more detail above, avoiding harmful crude carbide and nitride precipitates is one reason for performing hardening under these conditions. Under these same conditions, overlap of invading nitrogen and carbon concentrations is also unlikely. See, for example, Xiaoting Gu et al., “Numerical Simulations of Carbon and Nitrogen Composition Depth Profiles in Nitrocarburized Austenitic Stainless Steels,” Metal. and Mater. Transactions A, 45A, (2014), 4268-4279 (hereafter, “Gu et al.”), incorporated herein by reference. Gu et al. summarizes the thermodynamics behind the physical separation of invading carbon and nitrogen concentrations that occurs during low-temperature carbonizing and nitriding. See, for example, Gu et al. 4268 (abbreviation) and 4277. Therefore, Gu et al. strongly suggest the opposite regarding the overlap of interstitial carbon and nitrogen concentrations. Ibid. However, Gu et al. leave open the possibility of an overlap of nitrogen and carbon concentrations, in which case the elements are not purely interstitial but are fixed in compounds such as nitride or carbide precipitates. 7 The basic structure of triguanide is as follows: [ka]

[0093] Despite the fact that the overlap of coarse nitride and carbide precipitates and intrusion of carbon and nitrogen is essentially contrary to thermodynamics, the applicants have recently unexpectedly discovered overlaps of carbon and nitrogen concentrations within the case-hardened layers of stainless steel. The applicants believe that these concentration overlaps are due to the formation of fine carbide and / or nitride precipitates. 8 For example, carbonitriding is performed at a temperature of 450-500°C.

[0094] Figures 6(a) and 6(b) are Auger depth profiles of case-hardened stainless steel (316SS (UNS S31600)) pan 1, showing the overlap of carbon and nitrogen concentrations in the surface layer in the presence of dimethyl biguanide HCl (DmbgHCl) and guanidine HCl (GuHCl) reagents, respectively. The x-axis in Figures 6(a) and 6(b) represents the depth from the surface in microns. These two scans are from the floors of two 316SS crucible pan 1 (see Figure 4) treated at 470°C for 5 hours according to Table 2 below. They show regions with only the nitrogen and carbon results of interest. Figure 6(a) shows more nitrogen separation in the shallower parts (1-2 μm from the surface) of the hardened carburized layer depth. Carbon has a greater presence in the deeper parts. Figure 6(b) shows not only its nitrogen-carbon separation but also a second peak of carbon coexisting with nitrogen near the surface.

[0095] Therefore, Figures 6(a) and 6(b) show a significant concentration of carbon near the surface, consistent with nitrogen. Figures 6(a) and 6(b) also show that the surface nitrogen concentration is approximately 8–10 atomic percent. The carbon concentration is 5–7 atomic percent. Therefore, Figures 6(a) and 6(b) indicate that at least some of the carbon is more likely to exist as carbide precipitates rather than interstitial. The applicants surmise that such precipitates are probably finely milled, as coarse-grained precipitates are unexpected under these low-temperature conditions, as described above. See Gu et al. and the discussion in U.S. Patent No. 10,214,805 above. Such a surface layer may have a carbon concentration of at least 5–15 atomic percent and a nitrogen concentration of at least 5–15 atomic percent.

[0096] To prepare the samples for Figures 6(a) and 6(b), pan 1 was skin-baked according to the procedure disclosed in U.S. Patent No. 10,214,805, with the following modifications: 9 For example, bread 1 in Figure 4. [Table 2]

[0097] As shown in Table 2, the applicants found that these reagents could unexpectedly reduce the exposure time from 2 hours to 1 minute while maintaining a comparable curing effect. In summary, the above results suggest that the carbon surface concentration, consistent with the nitrogen surface concentration in Figures 6(a) and 6(b), is due to finely precipitated metal carbides. Apart from what is shown in Figures 6(a) and 6(b) and Table 2, there is other evidence supporting this hypothesis. For example, when the surface hardness of the carbide-rich portions of the cured material is measured, it is harder than the hardness of interstitial atomic quenching alone without such precipitates. In addition, visual inspection of the quenched layer structure by instantaneous preparation does not show the lath structure typical of the formation of coarser metal carbides and nitrides. All of this data is consistent with the precipitation of fine metal carbides during low-temperature reagent-induced quenching described in Table 2. 10 Tests have shown that moisture reacts with various reagents used in U.S. Patent No. 10,214,805, causing chemical changes. 11 The activating reagent was placed in the pan just before curing and then heat-dehydrated. 12 This distribution was also considered to be that of the visually cut specimens. 13In contrast to the sample from U.S. Patent No. 10,214,805, no visible distribution was observed in the cut sample. Note that when the reagent (1-2 mg instead of 8 mg) was insufficient for the sample in this experiment, a visible distribution similar to that shown in U.S. Patent No. 10,214,805 was observed. This suggests that the main difference between the reagent in this application and that in U.S. Patent No. 10,214,805 is the overall chemical potency of the reagent (i.e., under immediate conditions, the reagent is more potent than under the conditions in U.S. Patent No. 10,214,805).

[0098] Finely milled carbides in 316SS are expected to exhibit minimal loss of corrosion resistance compared to coarser carbides. One reason for this is that minimal chromium migration is expected under the low-temperature conditions of fine carbide formation. This suggests less chromium depletion in the chromium oxide passivation layer that provides corrosion resistance to the stainless steel. This is all consistent with the relatively small size of fine carbides (e.g., relatively small volume and mass compared to coarse carbides). Due to their small size, fine carbides can be formed using relatively less chromium compared to coarse precipitates. In addition, fine precipitates are not expected to exhibit the detrimental effects on steel properties observed in the case of coarse precipitates. These fine precipitates may coexist with invading elemental impurities, such as invading nitrogen. Furthermore, fine nitride precipitates may be present.

[0099] remote curing As described in the referenced literature, rapid hardening activated by a reagent in stainless steel (e.g., 316SS stainless steel (UNS S31600)) can be carried out when the reagent, in particular the guanide-type reagent complexed with HCl of this disclosure, and the workpiece are in relatively close proximity, for example, separated by a distance of 0.1 μm or less. Often, during the activation and hardening process, the reagent comes into direct proximity to, or even further into contact with, a portion of the steel. Some process designers further assume that such close proximity is necessary for rapid hardening.

[0100] Processes requiring reagents and workpieces to be in close proximity are difficult to scale up for industrial processes. For example, it is difficult to use a single reagent to activate and cure multiple workpieces. Proximity restrictions make continuous process processing (e.g., by conveyor belt) difficult, if not impossible. Moreover, proximity requirements limit the number of workpieces that can be processed by each individual reagent (e.g., one workpiece per reagent at any given time), making it impossible to use reagents efficiently. In other words, under such conditions, a larger quantity of reagent may be required to process each individual workpiece.

[0101] Therefore, it would be advantageous to develop a low-temperature curing process that can separate the reagent from the steel. Such a process would, among other things, enable industrial scale-up and more efficient use of the reagent. In addition, more "remote" curing can avoid problems arising from processing with reagents / workpieces in closer proximity, such as a reduction in pitting or defects on the workpiece surface caused by proximity to or contact with the reagent.

[0102] The applicants have found that the procedure of the present disclosure can be used to remotely harden steel surfaces, particularly when using guanide-type reagents complexed with HCl of the present disclosure. Specifically, it has been found that the same or similar surface hardening effect described herein can be achieved when the target surface for hardening is separated from the activating reagent by a distance of 8 inches (20 cm) or more. Recent results have shown that rapid, low-temperature, reagent-activated hardening can be equally effective, even when the reagent and workpiece are separated by these distances, as they are in close proximity.

[0103] In this study, a skin-baked layer was formed on the wall of a cylindrical crucible pan made of 316SS (UNS S31600) stainless steel. An image of exemplary pan 1 is shown in Figure 4. The pan has a diameter of approximately 0.5 cm and a height of approximately 0.5 cm. The pan was machined from a round bar stock using standard metal cutting tools. No other significant surface preparations were present. The machined surface of pan 1 likely has a bailby layer. The test was performed using a Netzsch simultaneous thermal analysis (STA) instrument. 14 .

[0104] In these experiments, bread 1 was skin-baked using the procedure disclosed in U.S. Patent No. 10,214,805, with the following modifications: 14 The Netzsch simultaneous thermal analysis (STA) instrument is described in more detail in “Fourier Transform of Infrared (FT-IR) Spectrometers Coupled to Thermal Analysis: Concepts, Instruments and Applications from RT to 2000℃, Analyzing and Testing” NGB-FTIR-EN-0220-NWS, attached as Annex A. 15 For example, bread 1 in Figure 4. [Table 3]

[0105] As shown in Table 3, the applicants found that these reagents could unexpectedly reduce the exposure time from 2 hours to 1 minute while maintaining an equivalent curing effect.

[0106] As shown in Figure 4, pan 1 has a hole 1a on its upper surface. In the experimental setup, hole 1a is exposed to atmospheric pressure nitrogen purge gas. The gas cell is located approximately 8 inches (20 cm) above pan 1. Vapors released from the reagents involved in the process travel to the gas cell containing the analyzer. As will be discussed below, the applicants believe that vapors traveling at least this distance, i.e., 8 inches (20 cm), harden the target as quickly and effectively as if the reagents were placed just adjacent to or in contact with the steel. The applicants demonstrated 0.5 cm remote hardening within the crucible pan and lid.

[0107] These results indicate that a 316SS metal surface, not in direct contact with the reagent and at a distance of 8 inches (20 cm) from the reagent, can be effectively activated and skin-hardened by the reagent. Specifically, crucible pans and lids from pan 1 treated at 500°C for 2–5 hours show cases of 28–32 μm at a reagent / treatment surface distance of 0.5 cm. Similar results were obtained for both DmbgHCl, BgHCl, and GuHCl reagents. Furthermore, the applicants found that the vapors generated by the decomposition of the reagent could travel at least 8 inches (20 cm). This carburized layer depth during this period corresponds to contact hardening as described in U.S. Patent No. 10,214,805 and other references cited herein. Therefore, activation and skin-hardening treatments are considered to be just as effective at these distances as they are at close range, including direct contact. 16 Tests have shown that moisture reacts with various reagents used in U.S. Patent No. 10,214,805, causing chemical changes. 17 The activating reagent was placed in the pan just before curing and then heat-dehydrated.

[0108] Based on this data and related observations, the applicants conclude that vapors from the decomposition reagent are carried to surfaces not in contact with the reagent (e.g., crucible pan and lid), remotely activating and / or curing those surfaces. The applicants are currently analyzing the composition and properties of these vapors. They have found that their potency is directly related to the amount of reagent; for example, less remote activation / curing is observed when the reagent is deficient in the reaction system (less reagent is used).

[0109] In one variation of the above process, the reagent and metal catalyst may be mixed together in powder form to improve reactivity. More specifically, the metal catalyst may include 316SS or other alloy metal powders, which are mixed with the reagent. When the reagent was mixed with a metal catalyst such as 316SS powder in a ceramic crucible pan, greater reagent reactivity was observed compared to the reagent alone in the ceramic crucible pan.

[0110] The above developments have considerable economic benefits. They imply that reagents can treat multiple, remote surfaces in parallel (e.g., simultaneously) with considerable efficacy, as if each were treated sequentially in direct or close contact with the reagent. For example, remote, rapid curing treatments of 1-2 hours, or even 1 minute, could be used in the continuous conveyor belt manufacturing of cured parts. A single reagent (e.g., DmbgHCl, GuHCl, or BgHCl) can be broken down at a distance from the workpiece (e.g., ferrules) as they move along the belt, effectively treating each of them simultaneously. This would greatly improve the yield and speed of curing workpieces. The efficiency of reagent use would also be improved. The amount of reagent required per workpiece would be less under such a batch processing regime than if each workpiece were treated sequentially in separate reaction vessels.

[0111] The applicants recognized yet another advantage of this process. The remote curing described herein avoids some of the problems that arise from keeping the reagent and the treated surface in close proximity. In particular, direct exposure to the reagent can cause pitting or other undesirable surface effects. These problems resulting from remote activation and curing were not observed.

[0112] Reagent azeotrope In addition to the configurations described above, reagents can be combined to form a variety of azeotropic mixtures. An azeotropic mixture is a mixture of liquids having a constant boiling point and composition through evaporation. The evaporation temperature of an azeotropic mixture may be approximately equal to or higher than the boiling point of either of the two liquids in the mixture in its pure form. Reagent azeotropic mixtures can be used in connection with this disclosure, and reagents can be combined for convenience to enhance or improve their reagent properties for use in activation and curing.

[0113] For example, melamine can be combined with guanide reagents (e.g., any of the guanide reagents described above) in an azeotropic mixture to facilitate the use of melamine in certain curing processes. Melamine, a cyclic tri-guanide (without HCl complexation), due to its chemical properties, assists in the rapid activation and curing of the alloys discussed herein. However, in its pure form, melamine may be unsuitable for activation and curing applications. This is because pure melamine evaporates at temperatures too low to facilitate curing by some of the processes disclosed herein. By combining melamine with a appropriately selected liquid in an azeotropic mixture, its evaporation temperature can be effectively increased. For example, by mixing melamine with another guanide-like reagent, the mixture can have a higher azeotropic mixture evaporation temperature. This makes the melamine portion of the mixture more useful in inducing curing at a suitable temperature. Examples of guanide-like reagents that can be used for azeotropic mixtures containing melamine include biguanide HCl, dimethyl biguanide HCl, and guanidine HCl. The weight proportions may vary. Examples of melamine to guanide-like weight ratios in azeotropic mixtures include 5% to 95%, 10% to 90%, 25% to 75%, or 50% to 50%. Other compounds may also be included in the reagent or azeotropic mixture as needed. For example, a mixture of melamine and guanide-like reagents may further contain additional reagents or other compounds that can enhance certain properties of the reagent mixture.

[0114] While the combination of melamine with guanide-like reagents has been discussed above as an exemplary azeotropic mixture, it should be understood that any suitable combination of reagents explicitly described herein or included by reference is possible. Melamine can be combined with other reagents. Furthermore, as described above, mixtures of three or more reagents are also possible, for example, to promote the formation of an azeotropic mixture.

[0115] A method for preparing a reagent mixture for an azeotrope may involve fusing or melting reagents together at a temperature lower than the boiling point of each individual reagent. The melting point of the resulting mixture or azeotrope may be lower than the melting point of any of the mixed reagents (if pure). Alternatively, a reagent mixture for such an azeotrope can be prepared by suspending two or more reagents in a solvent or a finely distilled petroleum distillate (e.g., paint). The solvent can then be removed, leaving the reagent mixture. For example, one method of removing the solvent is to evaporate it on a metal or ceramic surface, leaving a dry 2-reagent mixture.

[0116] The above development has considerable economic benefits. A rapid, 1-2 hour curing process can be used in the continuous conveyor belt manufacturing of cured workpieces under nitrogen (or other atmosphere) purging. The reagent (e.g., DmbgHCl and GuHCl) can be applied directly by spraying, or suspended or mixed with a liquid or solid vehicle, which can then be applied directly onto the workpiece (e.g., ferrules) as they move along the belt by conventional coating methods such as spraying, dipping, or steaming. Alternatively, the workpiece can be pre-treated with some form of reagent (coated with a water or oil-based coating, powder coated, etc.). This will greatly improve the yield and speed of cured parts production.

[0117] tracer According to yet another feature of this invention, the processing reagents used in this invention—nonpolymer N / C / H compounds—can be enhanced with specific, rare isotopes of C, N, H, and / or other elements and can function as tracer compounds for diagnostic purposes. For example, the nonpolymer N / C / H compound can be seeded at low concentrations with the same or different nonpolymer N / C / H compounds made using rare isotopes of N, C, or H, or with completely different compounds made using such rare isotopes. By using mass spectrometry or other suitable analytical techniques to sense these tracers, quality control of the low-temperature surface hardening process of this invention at production scale can be easily determined.

[0118] For this purpose, the treatment reagent can be enhanced with at least one of the following halide isotopes: ammonium chloride-(15N), ammonium chloride-(15N,D4), ammonium chloride-(D4), guanidine-(13C) hydrochloride, guanidine-(15N3) hydrochloride, guanidine-(13C,15N3) hydrochloride, guanidine-(D5) deuteriochloride, and any of their isomers. Alternatively, or in addition, the treatment reagent can be enhanced with at least one of the following non-halide isotopes: adenine-( 15 N2), p-toluidine-(phenyl- 13 C6), melamine-( 13 C3), melamine-(triamine- 15 N3), hexamethylenetetramine-(13C6,15N4), benzidine-(ring-D8), triazine(D3), and melamine-(D6), and any of their isomers.

[0119] Optional companion gas In addition to the gases described above, the gas atmosphere in which activation is achieved by this invention also includes one or more other companion gases—that is, gases different from the gas compounds described above. For example, this gas atmosphere may include an inert gas such as argon, as shown in the following examples. In addition, other gases that do not significantly adversely affect the activation process of the invention may also be included, such as hydrogen, nitrogen, and unsaturated hydrocarbons such as acetylene and ethylene.

[0120] Exposure of workpiece to atmospheric oxygen In yet another embodiment of this invention, the workpiece is exposed to atmospheric oxygen between activation and surface hardening, that is, after the activation of the workpiece is substantially complete but before the low-temperature surface hardening is substantially complete.

[0121] As previously shown, the traditional method for activating stainless steel and other self-passivating metals for low-temperature carburizing and / or carbonitriding is by contacting the workpiece with a halogen-containing gas. In this regard, in some of the early studies in this area described in US Nos. 5,556,483, US Nos. 5,593,510 and US Nos. 5,792,282, the halogen-containing gas used for activation was limited to a highly corrosive and expensive fluorine-containing gas. This was because, if other halogen-containing gases, particularly chlorine-containing gases, were used, the workpiece would repassivate immediately upon exposure to atmospheric oxygen between activation and thermosetting. Therefore, in these early studies, only those activated workpieces containing a significant amount of fluorine atoms could be exposed to the atmosphere without immediately repassivating.

[0122] Another feature of this invention is that the trade-off between the undesirable corrosion and costs associated with the use of fluorinated activators and the undesirable need to avoid repassivation when chlorinated activators are used is broken, because it has been found that activated workpieces produced by this invention do not readily repassivate even when exposed to atmospheric oxygen for more than 24 hours, even if they do not contain fluorine atoms.

[0123] Temperature gradient protocol overview The applicants developed a low-temperature curing method that is effective on a timescale of hours rather than days (in contrast to the methods shown and discussed above, particularly in relation to Figure 1-3). Therefore, the applicants needed to develop new methods for temperature control or gradient during curing to facilitate these faster curing processes. In particular, the applicants developed a temperature gradient procedure that optimizes activation and / or curing while still avoiding the formation of harmful precipitates under these unprecedented timescales.

[0124] Development of rapid low-temperature curing As described above, the results indicate that at least DmbgHCl, GuHCl, and BgHCl successfully induced very rapid surface hardening under low-temperature conditions. Specifically, 8 mg of any of the reagents tested separately was able to achieve a hardened carburized layer depth of 20–24 μm after 2 hours of low-temperature (500°C) treatment. As is clear from the above discussion, this is much faster than the treatment discussed in relation to Figure 1-3.

[0125] In these studies, a skin-baked layer was formed on the wall of a cylindrical crucible pan made of 316SS (UNS S31600) stainless steel. An image of exemplary pan 1 is shown in Figure 4. The pan has a diameter of approximately 0.5 cm and a height of approximately 0.5 cm. The pan is machined from a round bar stock using standard metal cutting tools. No other significant surface preparations were present. The machined surface of pan 1 likely has a bailby layer. The tests were performed using a Netzsch simultaneous thermal analysis (STA) instrument.18

[0126] Bread 1 was skin-baked using the following modification, following the procedure disclosed in U.S. Patent No. 10,214,805: 18 The Netzsch simultaneous thermal analysis (STA) instrument is described in more detail in “Fourier Transform of Infrared (FT-IR) Spectrometers Coupled to Thermal Analysis: Concepts, Instruments and Applications from RT to 2000℃, Analyzing and Testing” NGB-FTIR-EN-0220-NWS, attached as Annex A. 19 For example, bread 1 in Figure 4. [Table 4]

[0127] Functionality with a guanidine [HNC(NH2)2] moiety or HCl complexation is a common chemical structure for both DmbgHCl, BgHCl, and GuHCl. Other reagents tested that lack a guanidine moiety did not show the ability to produce a carburized hardened layer depth of approximately 20 μm in less than 2 hours under similar conditions. As shown in Table 4, the applicants found that these reagents could unexpectedly reduce the exposure time from 2 hours to 1 minute while maintaining a comparable hardening effect.

[0128] Suitable guanides, biguanides, biguanidines and triguanides for use in this aspect of the present disclosure 22Examples include chlorhexidine and chlorhexidine salts, analogs and derivatives, such as chlorhexidine acetate, chlorhexidine gluconate and chlorhexidine hydrochloride, picroxidine, alexidine and polyhexanides. Other preferred examples include chlorproguanil hydrochloride, proguanil hydrochloride (currently used as an antimalarial drug), metformin hydrochloride, phenformin and buformin hydrochloride (currently used as antidiabetic drugs). 20 Tests have shown that moisture reacts with various reagents used in U.S. Patent No. 10,214,805, causing chemical changes. 21 The activating reagent was placed in the pan just before curing and then heat-dehydrated. 22 The basic structure of triguanide is as follows: [ka]

[0129] The results described herein discuss the use of guanidine partial-containing compounds with HCl complexing, but these results can also be obtained using guanidine partial reagents without HCl complexing. Reagent complexing with any hydrogen halide can achieve similar results. Guanidine partial reagents without HCl complexing can also be mixed with other reagents with HCl complexing, e.g., other reagents described in U.S. Patent No. 10,214,805. An important criterion may be whether the reagent or mixture of reagents has a liquid phase while decomposing in the low-temperature carburizing and nitriding temperature range (e.g., 450–500C). The extent to which the reagent evaporates without decomposing before reaching that temperature range is an important consideration.

[0130] Temperature treatment during rapid curing The applicants share the objectives of the above-mentioned studies, particularly U.S. study 6,547,888, with regard to determining temperature treatment protocols for accelerating or promoting low-temperature curing. Because the above-mentioned advances in reagent technology have accelerated processing times from days to hours, the applicants have developed entirely new protocols. In particular, their objective is to use temperature profiles to optimize the intensity of reagent vapor at critical points during processing.

[0131] Temperature rise protocol Unlike the temperature control protocols in the references cited above, which focus on decreasing temperature to avoid precipitate formation, the applicants developed a temperature-increasing protocol. One purpose of the increase is, among other things, to accelerate the generation of products (for either activation or curing) from the thermal decomposition of the reagent. In particular, the applicants consider that the activation of the workpiece for nitriding and / or carburizing can be the rate-limiting step to curing. Therefore, it is not necessary to employ higher heating temperatures until this rate-limiting step is overcome and the activation becomes substantial. Before that, additional heating will not effectively assist curing. They developed a heating protocol that starts at a relatively low temperature while the activation process progresses. As soon as the activation is substantially sufficient to cure the workpiece to nitrogen and carbon, the protocol provides an intensive, "pulsed" heating step. This intensive pulse decomposes the reagent and provides carbon and nitrogen for curing in the appropriate time.

[0132] An exemplary temperature rise protocol is shown in Figure 7. Figure 7 is a TTT phase diagram for 316SS (UNS S31600) reproduced from Figure 2 of U.S. Patent Application Publication No. 2010 / 0116377. The newly proposed temperature rise protocol is shown in Figure 7 as an annotated line 7a. The region in the TTT diagram where precipitates are formed is indicated as 7b. The precipitate region 7b is enclosed by the curve QQ. It should be understood that the temperature gradient 7a in Figure 7 is merely suggestive of a favorable temperature rise protocol. The specific temperatures and times shown in Figure 7 and associated with the temperature gradient 7a are not meant to be precise or accurate. Rather, they are meant to indicate the physical and chemical changes desired by the temperature rise protocol of this disclosure.

[0133] As shown in Figure 7, the initial step is to heat the reagent at 470°C for 30 minutes. This step can facilitate the activation of the workpiece. Subsequently, this initial heating is increased to 480°C for 15 minutes. Finally, in the last 15 minutes of the first hour of the heat treatment, the heating is increased to 500°C. Such a temperature increase provides a relatively short-term (e.g., 15 minutes) "pulse" or relatively large increase at the maximum temperature of 500°C during heating within the first hour of the heat treatment. One purpose of the pulse is to provide enough heat to decompose the reagent and supply nitrogen and carbon to the curing process after the initial heating has sufficiently activated the workpiece. Again, these specific times and temperatures are illustrative only. They illustrate a pulsed heating protocol that can enhance or increase the decomposition power of the reagent to activate the workpiece during the first hour of treatment. It should be understood that these specific time and temperature modifications are still within the framework of this disclosure, as long as these or similar results are similarly obtained. Another exemplary variation of protocol 5a is as follows: 0.5 hours at 500°C, 0.25 hours at 510°C, and 0.25 hours at 530°C. More generally, the rising protocols disclosed herein can vary the temperature from at least 450°C to 550°C, but a wider temperature range is possible. The difference, or stepwise change, in temperature can be at least 100°C or less.

[0134] The temperature protocol 7a in Figure 7 is a step protocol. This can be advantageous in terms of implementation considerations, as described above in relation to Figure 3 (for example, when considering limitations of experimental or manufacturing heating equipment). However, the step configuration of 7a is illustrative and non-limiting. It should be understood that the same effects described herein can be achieved using smooth or partially smooth temperature protocols, which remain within the scope of this disclosure.

[0135] The heating protocol 7a can achieve multiple objectives simultaneously. First, it can provide the reagent with as much heat as possible to promote hardening and / or activation of the surface being treated. Second, it can avoid the formation of carbide or nitride precipitates by entering region 7b in Figure 7. Third, protocol 7a can address the heat capacity problem by giving sufficient time to “preheat” the reagent to obtain a reagent bulk temperature sufficient to slope through a peak (e.g., 500°C at 1 hour in Figure 7). As soon as the peak is reached, the heating is eased (Figure 7, after 1 hour). In this way, the heating protocol 7a can optimize the intensity of the pulse or surge of reagent-derived vapor causing hardening of the workpiece at critical points in the process (e.g., 45 minutes–1 hour in the heat treatment shown in Figure 7). As described above, such a heat treatment can “open” or activate the workpiece to nitrogen and carbon during hardening, and / or accelerate the actual hardening by carburizing and / or carbonitriding.

[0136] Heating protocol 7a also, or alternatively, facilitates the initial loading of interstitial carbon and nitrogen atoms in the workpiece at a lower temperature, and then proceeds to a higher temperature. This may result in the formation of fine carbides as disclosed herein, and not coarse carbides (or nitrides). The initial loading is thought to inhibit the formation of coarse carbides and nitrides.

[0137] Temperature reduction protocol In addition to the rising heat treatments discussed above, the applicants have also developed a temperature-decreasing treatment for rapid curing on the order of hours, rather than days. The purpose of the temperature-decreasing treatment is to maintain a high temperature of the workpiece without the precipitation of carbides or nitrides during activation and curing. As mentioned above, higher temperatures drive the kinetics of both the activation and curing processes, as well as the decomposition of reagents.

[0138] An exemplary temperature rise protocol is shown in Figure 8. Figure 8 is a TTT diagram for the same 316SS (UNS S31600) as in Figure 7. The newly proposed temperature fall protocol is shown in Figure 8 as an annotated line 8a. The region in the TTT diagram where precipitates are formed is shown as 7b, as in Figure 7. Again, the precipitate region 7b is enclosed by the curve QQ. It should be understood that protocol 8a in Figure 8 merely suggests a favorable temperature fall protocol. The specific temperatures and times shown in Figure 8 and associated with the temperature gradient 8a are not meant to be precise or accurate. Rather, they are meant to indicate the physical and chemical changes desired by the temperature fall protocol of this disclosure.

[0139] The temperature protocol 8a in Figure 8 is a step protocol. This can be advantageous in terms of implementation considerations (e.g., limitations on experimental or manufacturing heating equipment), as described above in relation to Figure 3. However, the step configuration of 8a is illustrative and non-limiting. It should be understood that the same effects described herein can be achieved using smooth or partially smooth temperature protocols, which remain within the scope of this disclosure.

[0140] As shown in Figure 8, the initial step is to heat the reagent at 500°C for 15 minutes. Subsequently, this initial heating is reduced to 480°C for 15 minutes. Finally, during the last 30 minutes of the first hour of heat treatment, the heating is reduced to 470°C. Such a temperature reduction avoids curve QQ in the TTT diagram of Figure 8, and thus avoids the precipitation region 7b. In other words, temperature protocol 8a provides increased heating of the reagent and workpiece during activation and curing while avoiding precipitate formation. This increased heating can conveniently increase the kinetics of reagent decomposition, activation, and / or curing. Again, these specific times and temperatures are illustrative only. They illustrate a decreasing heating protocol that can increase the kinetics of decomposition, activation, and / or curing. It should be understood that modifications of these specific times and temperatures are still within the framework of this disclosure, as long as these or similar results are similarly obtained. Another exemplary variation of protocol 6a is as follows: 0.25 hours at 530°C, 0.25 hours at 510°C, and 0.5 hours at 500°C. More generally, the descent protocols disclosed herein can vary the temperature from at least 450°C to 550°C, but a wider temperature range is possible. The difference, or stepwise change, in temperature can be at least 100°C or less.

[0141] Rapid protocol for a 15-20 μm cured layer in 60 seconds In addition to the above, the applicants developed a curing protocol that generates a 15-20 μm hardened layer in approximately 60 seconds of reagent treatment. Samples were prepared from 1 / 16” back ferrules made of 316SS steel. In the curing process, the samples were exposed to vapors formed by heating the following reagents: biguanide HCl, 1,1-dimethyl biguanide HCl, and GuHCl. Both reagents generated a hardened carburized layer depth of 15-20 μm in the ferrule samples.

[0142] The temperature protocol was as follows: First, the sample was linearly heated from room temperature to approximately 600°C. The heating was carried out at a rate of 25°C / min. As soon as 600°C was reached, the temperature was maintained for 60 seconds, during which time the sample was exposed to reagent vapor. Subsequently, the sample was cooled to room temperature at a rate of 20°C / min.

[0143] Figure 9 shows an optical image of a cross-section of the surface of the 316L stainless steel ferrule 910 treated as described above. The protocol generated a relatively uniform case-hardened layer 920 around the periphery of the ferrule sample. ASTM G61 periodic electrokinetic polarization (CPP) testing showed that the treated ferrule 910 became fully passivated at approximately 900 mV, indicating relatively high corrosion resistance. These results suggest that the hardened outer layer contains one or more of the following: dispersions of fine metal carbide precipitates, dispersions of fine metal nitride precipitates, coarse metal carbide precipitates suspended in the corrosion-resistant solid solution-treated metal phase, and coarse metal nitride precipitates suspended in the corrosion-resistant solid solution-treated metal phase. If the precipitates were not dispersed and not suspended in the corrosion-resistant solid solution-treated metal phase, CPP testing would reveal pitting corrosion at mV values ​​lower than 900 mV.

[0144] combination of heating protocols Although heating protocols 7a and 8a are presented separately above, it should be understood that they can be implemented in combination. For example, it may be advantageous to implement the heating pulse of protocol 7a either following or before protocol 8a. Other combinations and variations are possible and all are included within the scope of this disclosure.

[0145] influence The above developments have considerable economic benefits. Heating protocols 5a and 6a, as well as the deformations discussed above, can further reduce the curing time to less than the 2 hours reported above for guanidine-based reagents (etc.). Curing times of less than 1 hour are possible. Rapid, 1-2 hour or less surface curing treatments can be used in the continuous conveyor belt manufacturing of cured workpieces under nitrogen (or other atmosphere) purging. Reagents (e.g., DmbgHCl and GuHCl) can be sprayed directly onto workpieces (e.g., ferrules) as they move along the belt. Alternatively, workpieces can be pre-treated with some form of reagent (coated with a water or oil-based coating, powder coated, etc.). This will greatly improve the yield and speed of cured parts production.

[0146] The temperature to which the workpiece is subjected during activation and / or curing according to this invention must be high enough to achieve activation, but not high enough to form nitride and / or carbide precipitates.

[0147] In this regard, it is well understood that in low-temperature surface hardening processes, exposure of the workpiece to excessively high temperatures can lead to the formation of undesirable nitride and / or carbide precipitates. Furthermore, it is understood that the maximum surface hardening temperature a workpiece can withstand without forming these nitride and / or carbide precipitates depends on several variables, including the specific type of low-temperature surface hardening process performed (e.g., carburizing, nitriding, or carbonitriding), the specific alloy being surface hardened (e.g., nickel-to-iron alloys), and the concentrations of nitrogen and / or carbon atoms diffused into the workpiece surface. See, for example, US Patent No. 6,547,888, assigned to the assignee of the present invention. Therefore, it is also well understood that in implementing low-temperature surface hardening processes, care must be taken to avoid excessively high surface hardening temperatures in order to avoid the formation of nitride and / or carbide precipitates.

[0148] Similarly, in carrying out the activation and / or hardening process of the invention, care must be taken to ensure that the temperature to which the workpiece is exposed during activation is not so high as to form undesirable nitride and / or carbide precipitates. Generally, this means that the maximum temperature to which the workpiece is exposed during activation and / or subsequent surface hardening should not exceed about 700°C, possibly 600°C, preferably 500°C, or in other examples even 450°C, depending on the particular alloy being treated. For example, when a nickel-based alloy is activated and surface hardened, the maximum process temperature could be as high as about 700°C, because these alloys are unlikely to form nitride and / or carbide precipitates until they reach higher temperatures. On the other hand, when an iron-based alloy, such as stainless steel, is activated and surface hardened, the maximum process temperature should preferably be limited to about 475°C, preferably 450°C, because these alloys tend to be more susceptible to the formation of nitride and / or carbide precipitates at higher temperatures.

[0149] In terms of minimum process temperature (i.e., activation and / or curing), there is no practical lower limit other than the fact that the temperatures of both the nonpolymer N / C / H compound and the workpiece itself must be high enough for the workpiece to be activated as a result of the vapors produced. Typically, this means that the nonpolymer N / C / H compound is heated to a temperature of ≥100°C, but more preferably 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 intended.

[0150] This invention explains that the time it takes for a particular alloy to be activated for low-temperature surface hardening and / or surface hardened also depends on many factors, including the properties of the alloy being activated, the specific nonpolymer N / C / H compound used, and the temperature at which activation occurs. Generally speaking, activation and / or hardening can be achieved in as little as one second to as long as three hours. However, alloys can be sufficiently activated in 1 to 150 minutes, 1 to 120 minutes, 1 to 90 minutes, 1 to 75 minutes, 1 to 60 minutes, for example, 5 to 120 minutes, 10 to 90 minutes, 20 to 75 minutes, or even 30 to 60 minutes. Hardening can occur simultaneously with or after activation. In either case, hardening can occur on a similar timescale to activation. The time it takes for a particular alloy to be sufficiently activated by the process of the invention can be determined on a case-by-case basis. Furthermore, when activation and surface hardening occur simultaneously, the shortest time for activation is usually determined by the shortest time required to complete the surface hardening process, regardless of whether additional nitrogen and / or carbon compounds are included in the system to enhance surface hardening.

[0151] With respect to pressure, the activation and / or curing process of the invention can be carried out at atmospheric pressure, above atmospheric pressure, or under reduced pressure (including a tight vacuum, i.e., a total pressure of 1 Torre (133 Pa (Pascals) or less, and a mild vacuum, i.e., a total pressure of about 3.5 to 100 Torre (about 500 to about 13,000 Pa (Pascals))).

[0152] Further Examples To illustrate this invention more fully, the following embodiments are provided.

[0153] Example 1 The machined workpiece, made from A1-6XN alloy, is a super-austenitic stainless steel characterized by an increased nickel content. This workpiece was placed in a laboratory reactor along with powdered 2-aminobenzimidazole as an activating compound, arranged in direct contact with the workpiece. The reactor was purged with dry Ar gas and then heated to 327°C, held for 60 minutes, and subsequently heated to 452°C, held for 120 minutes.

[0154] After removing the workpiece from the reactor and cooling to room temperature, it was examined and found to have a uniform case (i.e., surface coating) with a three-dimensional structure exhibiting a hardness near the surface of 630 HV.

[0155] Example 2 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 used was 1 wt%, based on the total amount of nonpolymer N / C / H compound used. In addition, the reactor was heated to 452°C and held at this temperature for 360 minutes instead of 120 minutes.

[0156] The workpiece was found to exhibit a near-surface hardness of 660 HV.

[0157] Example 3 Example 2 was repeated, except that the workpiece was made from 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 wt% guanidine hydrochloride based on the total amount of nonpolymer N / C / H compound used), and in the second run, this mass ratio was 0.10 to 0.90 (10 wt% guanidine hydrochloride based on the total amount of nonpolymer N / C / H compound used).

[0158] Workpieces produced in the first run exhibited a surface hardness of nearly 550 HV, while workpieces produced in the second run exhibited a surface hardness of nearly 1000 HV. In addition, the surface-hardened surface of the workpiece produced in the second run showed superior hardened layer depth and complete conformability across its entire surface compared to the surface-hardened surface of the workpiece produced in the first run.

[0159] Example 4 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 nonpolymer N / C / H compounds used).

[0160] The resulting hardened surface or "case" of the workpiece exhibited a surface hardness close to 900 HV and had almost perfect conformability across its entire surface, although some pitting was present.

[0161] Embodiment The following is a non-exclusive list of exemplary embodiments according to the aspects of this disclosure. 1. A method for processing a workpiece made from a self-passivated metal and having a bailby layer, A method comprising activating a workpiece for low-temperature intermittent surface hardening by exposing the workpiece to vapor generated by heating a reagent having a guanidine [HNC(NH2)2] moiety and complexed with HCl. 2. The method of Embodiment 1, wherein exposure of the workpiece surface hardens the workpiece in addition to activating it. 3. Further includes maintaining the reaction vessel containing the workpiece at a temperature of 700°C or lower during exposure; A method of any one of Embodiments 1-2, wherein the workpiece forms a surface layer having a carbon concentration of 5-15 atomic percent and a nitrogen concentration of 5-15 atomic percent, but substantially free of crude carbide or crude nitride precipitates. 4. Forming a surface layer includes forming fine carbide precipitates within the surface layer; and The method of Embodiment 3, wherein nitrogen in the surface layer is mainly present as at least one of interpenetrating nitrogen and fine nitride precipitates. 5. The formation of fine carbide precipitates does not substantially reduce the corrosion resistance provided by the surface passivation layer within the workpiece; and The method of Embodiment 4, wherein the surface passivation layer contains chromium oxide. 6. One of the methods of Embodiments 1-5, comprising at least one of the following: Exposure will be administered for a period of 2 hours or less; Exposure will be administered for a period of 2 minutes or less; Maintain the reaction vessel containing the workpiece at a temperature of 700°C or lower during exposure; The reagent comprises at least one of dimethyl biguanide HCl, guanidine HCl, biguanide HCl, and melamine HCl; and Low-temperature interstitial surface hardening occurs simultaneously with exposure. 7. The skin-burned layer is less than 30 μm thick. The outer sublayer is rich in invading nitrogen; and Inner sublayer rich in invading carbon A method of any one of embodiments 1-6, including the method of any one of embodiments 1-6. 8. The method of Embodiment 7, wherein the skin-burned layer is less than 20 μm thick. 9. A low-temperature intermittent surface hardening method comprising at least one of carburizing, nitriding, and carbonitriding, any one of Embodiments 1-8. 10. Any one of the methods of Embodiments 1-9, wherein the reagent comprises at least one oxygen-free nitrogen halide salt and a nonpolymer N / C / H compound. 11. Exposure occurs using a workpiece in a reaction vessel at a distance of 8 inches (20 cm) or more from the reagent, using any one of the methods of Embodiments 1-10. 12. A method for producing case-baked parts in the manufacturing of a continuous conveyor belt, Purging the atmosphere of a continuous conveyor belt with gas; While maintaining the atmosphere at a temperature below 700°C: Placing unprocessed parts on a continuous conveyor belt; Exposing the workpiece to vapors generated by heating a reagent having a guanidine [HNC(NH2)2] moiety and complexed with HCl; and Maintaining exposure to reagent vapors for a period of less than two hours. Includes, This method allows the parts to be activated and surface-hardened by exposure to steam. 13. While maintaining the atmosphere at a temperature below 700°C: Placing multiple additional unprocessed parts on a continuous conveyor belt; Exposing additional parts to steam while they are on a continuous conveyor belt to activate the additional parts; and Perform low-temperature surface curing on additional parts over a period of less than 2 hours. The method of Embodiment 12 further includes the method of Embodiment 12. 14. A mixture of a first reagent and a second reagent for activating and / or hardening an alloy, wherein the mixture forms an azeotrope of the first and second reagents, and at least one of the reagents contains a guanide-containing reagent. 15. The mixture of Embodiment 14 having an evaporation point lower than that of the first reagent. 16. The mixture of Embodiment 15, wherein at least one of the first and second reagents contains melamine. 17. A mixture of Embodiment 16, wherein at least one of the first and second reagents comprises at least one of biguanide HCl, dimethyl biguanide HCl, and guanidine HCl. 18. Any one of the mixtures from Embodiments 14-17, wherein the weight ratio of the first reagent to the second reagent in the mixture is one of 5:95%, 10:90%, 25:75%, and 50:50%. 19. The mixture is formed by fusing or melting the first and second reagents at temperatures below the boiling point of the first reagent and the boiling point of the second reagent; and The mixture further comprises a petroleum distillate, which is evaporated, leaving a dry mixture of the first and second reagents, one of the mixtures from Embodiments 14 to 18. 20. Any one of the methods of Embodiments 1-13, further comprising applying a heating protocol during exposure that gradients from lower to higher temperatures to enhance the decomposition of the reagent and / or surface harden the workpiece. 21. The method of Embodiment 20, wherein the lower temperature is approximately 450°C or higher, and the higher temperature is approximately 550°C or lower. 22. The heating protocol is as follows, according to the method of Embodiment 20: Maintain a temperature of approximately 470°C for about 30 minutes; The temperature is gradually increased from approximately 470°C to approximately 480°C; Maintain a temperature of 480°C for approximately 15 minutes; To gradient the temperature from approximately 480°C to approximately 500°C; and Maintain a temperature of 500°C for approximately 15 minutes. 23. A method of Embodiment 20, which involves pulsing the temperature to create a gradient from a lower temperature to a higher temperature. 24. The heating protocol is as follows, according to the method of Embodiment 20: Maintain a temperature of approximately 500°C for about 15 minutes; The temperature is gradually increased from approximately 500°C to approximately 480°C; Maintain a temperature of 480°C for approximately 15 minutes; To gradient the temperature from approximately 480°C to approximately 470°C; and Maintain a temperature of 470°C for approximately 30 minutes. 25. A method for processing a workpiece made from a self-passivated metal and having a bailby layer, The workpiece is exposed to vapors generated by heating one or more nonpolymer N / C / H compounds at an exposure temperature below the temperature at which coarse nitride and / or coarse carbide precipitates form within the workpiece, thereby activating the workpiece for low-temperature intermittent surface hardening. Includes, One or more N / C / H compounds, (a) solid or liquid at 25°C and atmospheric pressure; (b) Having a molecular weight of ≤ 5,000 Daltons; and (c) Not complexed with hydrohalic acid, or can be complexed with hydrohalic acid, and further, (i) If the nonpolymer N / C / H compound is not complexed, any halogen atom can replace one or more unstable hydrogen atoms in the nonpolymer N / C / H compound. (ii) A method in which, when a nonpolymer N / C / H compound is complexed, any halogen atom forms part of the hydrogen halide complex acid. 26. The method of Embodiment 25, comprising at least one of the following: The exposure temperature is 500-700°C; Nonpolymer N / C / H compounds have a molecular weight of ≤500 daltons; and The exposure time is less than one hour. 27. The self-passivating metal is any one of the methods described in Embodiments 25-26, comprising at least one of the following: Titanium alloys; Iron-based, nickel-based, cobalt-based or manganese-based alloys containing at least 10 wt% Cr; and Stainless steel containing 10-40 wt% Ni and 10-35 wt% C. 28. Any one of the embodiments 25-27, wherein the exposure temperature is approximately 600°C or less. 29. Any one of the methods in Embodiments 25-28, wherein the exposure temperature is approximately 550°C or less. 30. A workpiece manufactured by one of the methods of Embodiments 1-11 and 20-28. 31. A workpiece manufactured by any one of the methods of Embodiments 12 and 13. 32. A workpiece manufactured by using any one of the mixtures from Embodiments 14-19.

[0162] Although only a few embodiments of this invention have been described above, it should be recognized that many modifications are possible without departing from the spirit and scope of this invention. All such modifications are intended to fall within the spirit and scope of this invention, limited only by the claims below.

Claims

1. A method for processing a workpiece made from a self-passivated metal and having a bailby layer, The process includes carbonitriding the workpiece, wherein the carbonitriding process involves carbonitriding the workpiece with guanidine [HNC(NH ２ ) ２ A method comprising exposing the workpiece to vapor generated by heating a reagent having a portion and complexed with HCl for a period of 1 to 5 minutes, thereby activating the workpiece for low-temperature intermittent surface hardening.

2. Maintain the reaction vessel containing the workpiece at a temperature of 700°C or lower during the exposure. It further includes, The method according to claim 1, wherein exposing the workpiece to the vapor causes a treated surface layer to form on the workpiece having a carbon concentration of 5 to 15 atomic percent and substantially free of crude carbide or crude nitride precipitates.

3. The treated surface layer comprises fine carbide precipitates; and The method according to claim 2, wherein the nitrogen in the treated surface layer mainly exists as at least one of intrusion nitrogen and fine nitride precipitates.

4. The formation of the aforementioned fine carbide precipitates does not substantially reduce the corrosion resistance provided by the surface passivation layer within the workpiece; and The method according to claim 3, wherein the surface passivation layer contains chromium oxide.

5. The method according to claim 1, comprising at least one of the following: Exposing the workpiece to the vapor further includes maintaining the reaction vessel containing the workpiece at a temperature of 700°C or less during the exposure; and The aforementioned reagent contains guanidine HCl.

6. The method according to claim 1, further comprising exposing the workpiece to the steam to form a skin-baked layer having a thickness of less than 30 μm and comprising the following: The outer sublayer is rich in invading nitrogen; and The inner sublayer is rich in invading carbon.

7. The method according to claim 6, wherein the surface-baked layer has a thickness of less than 20 μm.

8. The method according to claim 1, wherein the reagent comprises at least one of an oxygen-free nitrogen halide salt and a nonpolymer N / C / H compound.

9. The method according to claim 1, wherein the exposure of the workpiece to the vapor is performed using the workpiece in a reaction vessel at a distance of 8 inches or more from the reagent.

10. A method for skin-coating at least one component in the manufacturing of a continuous conveyor belt, Purging the atmosphere of the continuous conveyor belt with gas; While maintaining the aforementioned atmosphere at a temperature of 700°C or lower: Placing at least one of the components, which contains a self-passivating metal and has a bailby layer, on the continuous conveyor belt; and The method involves carbonitriding the at least one component, wherein the carbonitriding involves carbonitriding the at least one component with guanidine [HNC(NH ２ ) ２ Carbonitriding is performed by exposing a reagent having a ] portion and complexed with HCl to vapor generated by heating for a period of 1 to 5 minutes; A method comprising, wherein at least one component is activated and surface-hardened from exposure to the vapor.

11. The method according to claim 10, wherein the at least one component includes a plurality of components.

12. Exposing the workpiece to vapor enhances the decomposition of the reagent and / or surface hardens the workpiece by applying a heating protocol that progresses from lower to higher temperatures during the exposure. The method according to claim 1, further comprising:

13. The method according to claim 12, wherein the temperature lower than the above is approximately 450°C or higher, and the temperature higher than the above is approximately 550°C or lower.

14. The heating protocol is as follows, according to the method of claim 12: Maintain a temperature of approximately 470°C for about 30 minutes; The temperature is gradually increased from approximately 470°C to approximately 480°C; Maintain a temperature of 480°C for approximately 15 minutes; To gradient the temperature from approximately 480°C to approximately 500°C; and Maintain a temperature of 500°C for approximately 15 minutes.

15. The method according to claim 12, wherein applying the heating protocol includes pulsing from a lower temperature to a higher temperature.

16. Exposing the workpiece to steam further comprises the method according to claim 1: Maintain a temperature of approximately 500°C for about 15 minutes; The temperature is gradually increased from approximately 500°C to approximately 480°C; Maintain a temperature of 480°C for approximately 15 minutes; To gradient the temperature from approximately 480°C to approximately 470°C; and Maintain a temperature of 470°C for approximately 30 minutes.

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