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
  • C23C8/06—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
• • 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
  • C23C8/08—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 only one element being applied
  • C23C8/24—Nitriding
  • C23C8/26—Nitriding of ferrous surfaces

Definitions

• the first stage is carried out at a temperature of about 650C with the residual ammonia content of the throughflowing furnace gases of at least 10% while the second stage is carried out with a residual ammonia content of 45 to 60% at a temperature of about 570C for a period of two to four hours.
• the present invention relates to a method of or a process for the nitriding of low-alloy or unalloyed steel, e.g. so-called soft steel, to increase the wear resistance, abrasion resistance, surface hardness and' strength of at least the surface and edge zones of the steel article. More particularly, the invention relates to improvements in the nitriding of such steel whereby disadvantages encountered heretofore are obviated.
• the nitriding of steel has long been recognized as a means for increasing the hardness and improving other properties of surface zones of a steel body.
• the nitriding process involves a diffusion of nitrogen, generally from an environment containing nitrogen in an available form, e.g. an ammonia-containing gas stream. This process yields a hard wear-resistant case and is advantageous where the surface is to be exposed to wear, abrasion or the like.
• nitriding has been carried out heretofore at a temperature below the 01-7 transformation of the iron-nitrogen system, i.e. under a temperature of 585C.
• the most important advantage of a treatment below the transformation temperature is a general freedom from distortion in the ultimate product.
• the nitrogen diffuses into the steel only relatively slowly and, since the solubility of the a for nitrogen is slight at best, it is necessary to provide surface-hardening times with conventional nitriding techniques of extremely long duration, e.g. 50 hours or above.
• Another object of this invention is to provide a nitriding system for reducing the duration of the nitriding treatment and providing an improved product.
• the sec- 0nd stage carried out after cooling of the article, also makes use of an ammonia-containing atmosphere and is effected at a temperature below the a-ytransition non-alloyed or low-alloy steels), as distinct from the higher-alloy nitriding steels, are generally nitrided only for limited periods because they suffer with prolonged exposure to the nitriding environment and temperature a substantial loss in strength.
• lowalloy steels and especially those containing no or only temperature preferably at a temperature of 570C.
• the first stage results in a rapid and considerable diffusion of nitrogen into the steel body whereas the slow cooling through and below the my transformation temperature results in a saturation of the diffusion zone with nitrogen and carbon, at least at the junction of this zone with the remainder of the body.
• the first temperature stage is carried out in the realm of the maximum solubility of nitrogen in the y mixed crystal phase (about 65 0C).
• the thickness of this layer is a function of the nitrogen level of the environment and hence the supply of nitrogen to the metal surface, and a function of the treatment time at this temperature. The thickness can range from several microns to several tenths of a millimeter.
• the separating or intermediate layer has a higher strength than the core lattice of the steel body and ensures an effective transition of the surface layer or case to the core lattice even under high specific loading, as tests have demonstrated using rolling processes.
• the nontransformed region beneath the intermediate pr separating layer comprises a mixed crystals and has, at a temperature of 650C, a diffusion coefficient which is abouta factor of 5 greater than the diffusion coefficient at temperatures below the a-y transformation point (about 585C) so that greater penetration depths for nitrogen per unit time may be noted.
• the slow cooling to the second stage 585C in which the temperature during the cooling interval lies for the greater part thereof at or below the a-y transformation temperature (585C) reduces the tendency toward transformation stresses. Furthermore, it ensures a desirable grain structure in the surface zones of the steel body inasmuch as time is provided for diffusion equalization during cooling and during the retention of the body at the second stage temperature. Any distortion is thus within the limits of conventional nitriding processes.
• the second nitriding stage is carried out, as indicated previously, at temperatures below the 01-3 transforma.
• Each of nitriding temperature stages is associated with a characteristic dissociation degree of the ammonia and in the first temperature stage (operating at about 650C) the degree of dissociation should be at least 0.8. This is achieved by feeding ammoniacontaining gas into the system through the furnace with the ammonia level adjusted so that the atmosphere upon treatment of the steel body has a residual ammonia content in excess of The upper limit of the residual ammonia content, a function of the size of the furnace, is determined by that which is necessary to obtain uniform nitriding of the body at the speed at which the ammonia gases flow through the system. ln the second temperature stage (operating at about 570C) the dissociation degree should range between 0.25 and 0.35, corresponding to a residual ammonia content of 45 to 60%.
• Nitriding is carried out in the first stage for a period of at least 2 hours and preferably between 2 and 8 hours or more while the second-stage nitriding is carried out for'2 to 4 hours.
• the cooling of the body between the first and second stages is carried out at a low rate, as noted earlier, and preferably over a period of hours although this time it is not critical. It has been found to be practical to allow the body to cool naturally in contact with the furnace gases which are not circulated or cooled, especially during this period. The heat loss from the body is thus transferred out of the system through the furnace walls.
• the cooling of the body from the second-stage treatment may be as rapid as is desired and may be effected by gas or liquid quenching.
• cooling is effected from the second-stage temperature to ambient by circulating gas at ambient temperature through the furnace.
• this gas is nitrogen.
• the quantity of the carbon carrier methylamine in the second stage may be between 5 and 20 volume percent of the requisite ammonia level.
• the process according to the invention is preferably used for the nitriding of structural elements of unalloyed or low-alloy structural steel which may be used in the same manner as case-hardened and tempered steels and have a higher wear resistance and strength retention.
• the starting material may be tempered or normalized, e.g. in a soaking pit or the like. Because of the deep nitrogen diffusion and the higher surface strength of the steel body it is possible to avoid tempering the steel prior to nitriding. It is moreover desirable to use a steel in which the core structure and strength is improved by heating to temperatures of 650C for periods of the order of those stated for the first stage.
• the total SPECIFIC EXAMPLE A steel body having the following consitiuents in per cent by weight carbon 0.25, manganese 0.8, silicon 0.1, aluminum less than 0.05, chromium up to 1, molybdenum up to 0.5, nickel up to 0.5, the balance iron, in the form of a bar is heated to 650 C. and maintained thereat for 4 hours in a mantle furnace with a stream of ammonia flowing at a rate such that the residual ammonia content is 25 per cent by volume.
• the steel bar is permitted to cool in the furnace to 570 C. for 2 hours and is maintained thereat for a further 2 hours and is treated with a nitriding atmosphere containing 50 per cent residual ammonia with about 15 per cent of the ammonia content methylamine. Treatment is terminated by normally cooling the bar to room temperature and flushing the furnace with nitrogen. An effective nitride case of 0.6 to 0.8 mm anda white layer of 0.04 mm thickness was provided on the bar.
• a process for nitriding a body of low-alloy or nonalloyed steel which comprises treating said body in a first stage for a period of about 2 hours to 8 hours at a temperature of about 650 C, said temperature being above the 01-7 transformation temperature in the ironnitrogen system, in an ammonia-containing gas and sufficient to effect nitrogen diffusion into at least a surface 6 zone of said body; thereafter slowly cooling said body; bon carrier is a compound of nitrogen and carbon. and subsequently treating said body with an ammonia- 4.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)

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Citations (2)

• 1973
  • 1973-04-10 IT IT49347/73A patent/IT983006B/it active
  • 1973-04-20 FR FR7314522A patent/FR2182991B3/fr not_active Expired
  • 1973-04-26 US US354731A patent/US3870572A/en not_active Expired - Lifetime
  • 1973-05-01 JP JP48047639A patent/JPS4948528A/ja active Pending

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