Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/04—Treatment of selected surface areas, e.g. using masks
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/28—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases more than one element being applied in one step
  • C23C8/30—Carbo-nitriding
  • C23C8/32—Carbo-nitriding of ferrous surfaces

Definitions

• the present invention relates to a surface hardened austenitic stainless steel part for improving wear resistance and its manufacturing method.
• Austenitic stainless steels have been utilized in various fields, required for corrosion resistance such as food machines, machines for chemical fields, plants and auto-motive engines and others. Among them there are many applications, also required for wear resistance, like sliding machine parts such as shafts and the like, valves and the like, gears and the like and so on. To improve this wear resistance, hardening by heat treatment such as quenching or the like, or surface-hardening such as carburizing, carbonitriding or the like is often applied to machine structural carbon steels, alloy steels and tool steels and the like.
• austenitic stainless steel In a case of austenitic stainless steel it is not hardened by quenching. Thus since sufficient hardness cannot be obtained by carburizing in the austenitic stainless steel, when it is necessary to harden the surface of the stainless steel particularly, wet plating such as hard chromium plating or the like, coating of a hard layer by PVD (Physical Vapor Deposition) or nitriding of the surface or the like is performed.
• PVD Physical Vapor Deposition
• coating of a film by plating or PVD or the like has a drawback in adhesive properties of the film with the base material. Particularly, when a surface pressure is increased, the coating cannot be applied stably.
• a surface nitriding treatment is a method in which nitrogen is penetrated through a surface to harden it.
• Stainless steel containing much Cr (Chromium) is generally difficult to nitride for the sake of the presence of an oxide film.
• the nitriding of the stainless steel becomes easy by use of a hydrochloric acid treatment process, a halide treatment process, or ion-nitriding treatment process or the like. These processes are often utilized as a surface-hardening method of austenitic stainless steel.
• the surface hardening of the austenitic stainless steel is needed for improving its fatigue strength. However, it is further needed for the sake of improving its wear resistance.
• the improvement of the wear resistance suppresses wear on a sliding surface of a sliding part to improve durability thereof. Additionally the improvement of the wear resistance has such effects that wear loss in a tool for polishing or cutting is reduced and scratching of a surface of a stainless steel part is suppressed.
• Nitriding treatment is not necessary to quench unlike the carburizing treatment. Accordingly, in the nitriding treatment the surface can be hardened by a comparatively low temperature treatment. However, there is the most suitable temperature for increasing the surface hardness, and if the thickness of a hardened layer is increased at the temperature, it takes long time for nitriding. Otherwise, if the temperature is increased the thickness of the hardened layer can be increased. However, the obtained surface hardness is decreased.
• the method using the nitriding is important as a surface-hardening method of the austenitic stainless steel, sufficiently satisfactory hardening method cannot be necessarily obtained.
• the object of the present invention is to provide an austenitic stainless steel part excellent in wear resistance, which is used in a sliding portion or the like, has high surface hardness and has sufficient hardness in a portion just below the surface, and its manufacturing method.
• the present inventors have studied variously to improve the performance of austenitic stainless steel surface-hardened by gas nitriding.
• the surface nitriding process of the stainless steel includes ion nitriding, the nitriding is performed under a reduced pressure and the treatment speed is slow and even the shape to be nitrided can be limited.
• gas nitriding can be treated in high amounts, and it has been considered that the gas nitriding is suitable for mass production.
• the surface hardness of the steel by nitriding generally reaches about 1200 to 1300 HV. However, although a harder compound layer can be formed on the surface of the steel by treatment conditions, it is generally considered that the compound layer is so brittle that it cannot be utilized. In examinations using these austenitic stainless steels, steel having a surface hardness of 1350 HV or more at Vickers hardness was found.
• This hard surface layer is not a brittle compound layer, which has been obtained in machine structural steels, ferritic stainless steels and the like, but it has sufficient toughness. Thus, after preparing test pieces for wear tests, wear resistances were checked. As a result, it has been recognized that the wear resistances are extremely excellent.
• this compound layer In the case of a machine structural steel or a ferritic stainless steel, a compound layer often appears on the surface layer depending on treatment conditions. It is considered that this compound layer was formed by increasing the content of active nitrogen on the surface layer, produced by decomposition of NH 3 due to the advancing of nitriding, and by nitriding Fe, Cr or the like with the increased nitrogen.
• the steel cannot hold a hard and brittle compound layer sufficiently and the hard compound layer is easily broken by small stress.
• brittleness is prominent and the hard compound layer is not made full use of.
• Ni is said to be an element, which generally prevents nitriding, and since Ni is small, penetration of nitrogen and penetration of carbon become easy. Therefore, it is assumed that the nitrogen content near the surface of the steel during nitriding is further increased as compared with a case where the austenitic stainless steel contains a small amount of Mn and a large amount of Ni.
• the solubility content of nitrogen is high since the austenitic steel has an austenitic phase, and the penetration of nitrogen is actively performed since Ni content is low. Additionally, a compound layer is easy to be formed by the penetration of carbon. Further, the hardness of the portion just below the compound is significantly increased since large amounts of fine carbide and nitride having high solid solubility content are formed.
• the compound layer is held by the lower layer having sufficient strength, the brittleness of the compound layer is compensated, resulting in that the compound layer has become a surface-strengthened layer excellent in wear resistance.
• a steel, which is manufactured to a part of the present invention, is an austenitic stainless steel containing 3 to 20 mass % of Mn.
• the reason for Mn content of 3 mass % or more is that in a case of an austenitic steel when Mn is decreased, Ni content is increased and surface hardening hardness due to nitriding is not significantly increased.
• the Mn content is set to at most 20 mass %.
• Such stainless steels include for example SUS 201, SUS 202, SUS 304J2, SUH 35, SUH 36 and the like in the JIS standard.
• the compositions other than Mn are not particularly limited if they are in ranges, which belong to the austenitic stainless steels. Nevertheless, it is preferable that the Ni content is, if possible, smaller than the Mn content since there is a possibility that sufficient surface hardness of the steel cannot be obtained.
• the hardness of a carbonitrized surface is set to 1350 HV or more. This is because sufficiently high wear resistance cannot be obtained when the hardness thereof is lower than 1350 HV.
• the depth of a hardened layer whose hardness is 1000 HV or more is set to 10 ⁇ m or more.
• the austenitic stainless steel part surface hardened in the above-mentioned conditions cannot be obtained by only nitriding. Thus it is necessary to manufacture the part by carbonitriding.
• As the surface nitriding of the austenitic stainless steel there is a method in which the surface of the steel is heated in an atmosphere containing halogen gas or halide gas to activate the surface, and nitriding gas containing NH 3 is introduced into a furnace and the heated surface of the steel is nitrided.
• carbonitriding is performed according to the method of using the halogen or halide.
• an austenitic steel is heated at 200 to 550 °C for 10 minutes to 3 hours in atmosphere containing 0.5 to 20 volume % of halogen gas or halide gas such as F 2 , Cl 2 , HCl or NF 3 and the balance of nitrogen, hydrogen or inactive gas to activate the surface of the steel.
• halogen gas or halide gas such as F 2 , Cl 2 , HCl or NF 3
• the surface of the steel After the surface of the steel was activated, the surface is carbonitrided by heating at a temperature range of 430 to 600 °C for 20 minutes or more in a mixed gas atmosphere containing NH 3 for nitriding and CO or CH 4 for carbonizing.
• These carbonitriding atmospheric gases include 10 to 95 volume % of NH 3 and 5 to 30 volume % of one of CO or CH 4 , or both of them.
• the reason for 10 volume % or more of NH 3 is that if NH 3 is less than 10 volume %, nitriding cannot be performed sufficiently and a hardened layer cannot be obtained.
• 100 volume % NH 3 may be used for the purpose of nitriding. However, since it is necessary to use a carbonizing gas, NH 3 is contained by at most 95 volume %.
• the gas content is set to at most 30 volume %.
• the carbonitriding atmospheric gas may contain NH 3 and CO or CH 4 by enough contents for nitriding and for carbonizing the surface of the steel, and other components may be inactive gas, hydrogen, nitrogen or other hydrocarbon gases and the like, which are not limited. Further, if the above-mentioned composition ranges are satisfied as in a case where NH 3 is mixed to RX gas, carbonitriding gas may be prepared by mixing carburizing gas, which has been used, with NH 3 .
• the carbonitriding time is less than 20 minutes, there is a possibility that a surface compound layer cannot be obtained, and a surface hardness of 1350 HV or more cannot be obtained. If the carbonitriding time is 20 minutes or more, the limitation of the time is not needed. Then the carbonitriding time is increased, the thickness in a hardened layer of 1000 HV or more can be increased. However, wear resistance is not improved more than a certain level and it is preferable that the carbonitriding time is within at most 50 hours since corrosion resistance can be deteriorated.
• stainless steel parts which are required for wear resistance and to which the present invention is effectively applied, are described as follows.
• sliding mechanical parts including an engine valve, a compressor shaft, a compressor vane, a piston ring, a bearing ball, a micro motor shaft, a motor shaft, and the like.
• fluid wear resistant parts include a filter mesh, a nozzle, a valve, a piping joint, a reducer, a pump and the like.
• fastening parts include a bolt, a nut, a screw, a tapping screw, and the like.
• tools and the like include a dresser, a cutting saw, a wire saw, a saw, a drill and the like.
• the stainless parts can also be applied to an extrusion mold, a die cast mold, an injection die and the like.
• Stainless steels having compositions shown in Table 1 were used. First, they were cut to prepare disk-shaped test pieces of 35 mm in diameter and 10 mm in thickness. In a case where the test piece is used as a rotating test piece for an Amsler wear test, the circumferential surface of the disk is further polished in a mirror surface to remove edges. The obtained test pieces were heated at 300 °C and the heating was maintained in an atmosphere containing NF 3 to perform nitriding or carbonitriding thereby hardening the surfaces of the test pieces. The atmospheric gases, temperatures and treatment time during surface-nitriding are shown in Table 2.
• the surface hardness was measured at Vickers hardness (HV0.1) of test force of 0.9806 N, and the hardness distribution in cross-section was measured at Vickers hardness (HV0.05) of test force of 0.4903 N.
• HV0.1 Vickers hardness
• HV0.05 Vickers hardness
• Amsler wear test was performed as follows. In two cylindrical rolling abrasion testers circumferential surfaces of the above-mentioned test pieces were pressed against cylindrical metal surfaces (made of SKH52) of 35 mm and 50 mm in diameter at 150 kg, and the abrasion testers were rolled in the same direction in sliding portions. Then the sliding speed was set to 0.12 m/sec and specific abrasion wear [mg/(m ⁇ sec)] was obtained without lubrication.
• the results of the abrasion test are shown in Table 2 on the next page.
• test piece of test No. 6 is the austenitic stainless steel of the high Mn content
• the surface hardening treatment is not carbonitriding. Accordingly, the surface hardness of the test piece of test No. 6 is not sufficient and is brittle.
• a carbonitriding temperature is low
• a hardened depth of 1000 HV or more is 7 ⁇ m, which is shallow.
• the surface hardness is less than 300 HV All cases of test Nos. 4 to 6 exhibit large specific abrasion wear and worse results as compared with the cases of test Nos. 1 to 3 according to the present invention.
• the surface carbonitrided stainless steel part of the present invention according to the surface carbonitrided stainless steel part of the present invention and its manufacturing method, by setting a Vickers hardness of the surface to 1350 HV or more and setting a depth of a hardened layer having 1000 HV or more from the surface of the steel to 10 ⁇ m or more, when the part according to the present invention is applied to a part required for sliding and wear resistance particularly, the service life can be improved significantly. Further, since the manufacturing method is performed by only heating in a gas atmosphere, a large number of parts can be simultaneously treated. Thus the stainless steel parts of the present invention can be adopted to sliding mechanical parts, fluide wear resistant parts, fastening parts and tools and the like as stainless steel parts required for wear resistance. Therefore, the present invention can be applied to wide fields.

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• 2002
  • 2002-10-04 JP JP2002291799A patent/JP3961390B2/ja not_active Expired - Fee Related
• 2003
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  • 2003-10-06 WO PCT/JP2003/012806 patent/WO2004031434A1/ja active Application Filing
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