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JP-7855510-B2 - Chemical activation of self-passivating metals

JP7855510B2JP 7855510 B2JP7855510 B2JP 7855510B2JP-7855510-B2

Inventors

  • イリング，シプリアン，アデア ウイリアム
  • ウイリアム，ピーター，シー．
  • セムコウ，クリスティナ
  • ジョンズ，トッド

Assignees

  • スウェージロック カンパニー

Dates

Publication Date
20260508
Application Date
20201204
Priority Date
20191206

Claims (16)

  1. A method for processing a workpiece made from a self-passivated metal and having a bailby layer, A method comprising carbonitriding the workpiece, wherein the carbonitriding involves exposing the workpiece to vapor generated by heating a reagent having a guanidine [HNC( NH₂ ) ₂ ] portion and complexed with HCl for a period of 1 to 5 minutes , thereby activating the workpiece for low-temperature intermittent surface hardening .
  2. The reaction vessel containing the workpiece is further maintained at a temperature of 700°C or lower during the exposure. 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 method according to claim 2, wherein the treated surface layer comprises fine carbide precipitates; and the nitrogen in the treated surface layer is mainly present as at least one of invading nitrogen and fine nitride precipitates.
  4. The method according to claim 3, wherein the formation of the fine carbide precipitates does not substantially reduce the corrosion resistance provided by the surface passivation layer in the workpiece; and the surface passivation layer comprises 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 reagent comprises 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: An outer sublayer rich in invading nitrogen; and an inner sublayer 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 occurs 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 the at least one component containing a self-passivating metal and having a bailby layer on the continuous conveyor belt; and carbonitriding the at least one component, wherein the carbonitriding comprises exposing the at least one component to vapor produced by heating a reagent having a guanidine [HNC( NH₂ ) ₂ ] portion and complexed with HCl 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. The method according to claim 1, further comprising exposing the workpiece to vapor, applying a heating protocol that slopes from a lower temperature to a higher temperature during the exposure to enhance the decomposition of the reagent and/or surface harden the workpiece.
  13. The method according to claim 12, wherein the temperature lower than the aforementioned is approximately 450°C or higher, and the temperature higher than the aforementioned 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; The temperature is gradually increased from approximately 480°C to approximately 500°C; and the temperature is maintained at 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; The temperature is gradually increased from approximately 480°C to approximately 470°C; and the temperature is maintained at 470°C for approximately 30 minutes.

Description

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 full disclosures of each of these applications are incorporated herein by reference, and priority to each of these applications is claimed thereunder. 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. 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, also immediately forming a titanium dioxide coating upon exposure to air, which is also impermeable to the permeation of water vapor, oxygen, and other chemicals. These alloys are said to be self-passivated 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. 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 rarely surface-hardened by conventional (high-temperature) carburizing. Low-Temperature Carburizing In the mid-1980s, a technique was developed for surface 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 carburizing 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 carbide precipitates are not formed, the corrosion resistance of the steel is not impaired, and in fact, it is improved. This technique is called "low-temperature carburizing" and is recognized under U.S. Patent Nos. 5,556,483, U.S. Patent Nos. 5,593,510, U.S. Patent Nos. 5,792,282, and U.S. This is described in numerous publications, including No. 6,165,597, EPO 0787817, Japan 9-14019 (Publication 9-268364), and Japan 9-71853 (Publication 9-71853). Nitriding and Carburitriding In addition to carburizing, nitriding and carburitriding 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. However, similar to 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 nitroge