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
  • C23C8/08—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
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12014—All metal or with adjacent metals having metal particles
  • Y10T428/12028—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, etc.]
  • Y10T428/12063—Nonparticulate metal component
  • Y10T428/12139—Nonmetal particles in particulate component

Definitions

• nitride-strengthened, stainless steel containing as a dispersoid therein particles of metal nitride having a free energy of formation of greater, i.e., more negative, than about-21,000 cal./mole.
• the nitrides are present at small interparticle spacings, e.g., 2 microns, which is important since the strength of the nitrided steel increases as the interparticle spacing decreases.
• nitridestrengthened steel by producing thicker nitrided members, e.g., greater than 10 mils, having an increase in strength similar to that realized from relatively thin members. This was accomplished by developing internally nitrided powders with an interparticle spacing of less than 2 microns, a method for their production and a method for densifying them into thicker members substantially free of pores.
• lt is accordingly an object of this invention to provide a nitride'strengthened steel powder.
• steel powder containing a metal component capable of forming a nitride hereinafter referred to as a nitride former, is subjected to a nitriding atmosphere at an elevated temperature so as to produce a nitride-strengthened steel powder with an interparticle nitride spacing of less than 2 microns, preferably less than 0.5 microns.
• the powder is thereafter hot pressed with or without the addition of dissimilar powder.
• nitride formers are within the purview ofthe invention.
• the requirements of such materials are that the nitrides, i.e., the dispersoids have a sufficiently high free energy of formation to lead to the production of very small particles.
• such nitrides should possess a free energy of formation of greater, i.e., more negative, than about -21,000 cal./mole. They should be present in an amount sufficient to provide an interparticle spacing of less than 2 microns and preferably less than 0.5 microns, the volume percent being dependent on the interparticle spacing and being larger for a smaller interparticle spacing at a constant particle size.
• the nitride used should possess a very low solubility in the steel treated so as to possess a reduced tendency to coarsen at elevated temperatures such as those to which the dispersion-strengthened article will be subjected in use.
• Extensive evaluation has indicated that the preferred and by far most superior nitride former to be employed is titanium. Titanium has a relatively high solubility in stainless steel and its nitride has a very high free energy of formation. Other nitride formers are available, but none as good as titanium. For lower temperature applications, strengthening may be achieved by use of nitride formers such as aluminum, vanadium, and columbium.
• Such dispersion-strengthened materials could be used at temperatures where coarsening is not too rapid, but these materials would not have the high temperature capabilities of titanium-dispersoid strengthened steel. Still other nitride formers such as boron, zirconium, cerium, hafnium, thorium, etc., are not soluble to a high extent in steel. In the preferred embodiment in which a titanium nitride dispersoid is formed, steel containing 0.5 to 3 percent titanium is preferred. Less than about 0.5 percent titanium results in a product having satisfactory room temperature properties but the particles are such that they tend to grow at elevated temperatures and the resulting interparticle spacing might be greater than 2 microns. When more than about 3 percent titanium is present in the steel, additional improvement in properties may be obtained but such would be disproportionately less than obtainable with 0.5 to 3 percent titanium.
• the nitriding atmosphere can comprise nitrogen, ammonia, mixtures of the two and mixtures of them with other compatible gases.
• compatible gases refers to nonoxidizing or inert gases such as hydrogen or argon.
• Ammonia is the preferred atmosphere and should be substantially free of moisture and oxygen. When using nitrogen, it is preferable to use it in a pressurized condition, i.e., at pressures above atmospheric, so as to obtain a nitriding rate comparable to that obtained from ammonia. The presence of small amounts of moisture or oxygen severely affects the nitriding rate in nitrogen.
• Nitriding as described herein is both time and temperature dependent as are all diffusion processes. Temperatures generally range from 1,600 F. to just short of the melting point. However, temperatures as low as l,400 F. can be used for finer powders, e.g., 1 mil. A preferred temperature range is 1,7002,200 F. This range balances the advantage of nucleating more nitride particles at lower temperatures with the accompanying disadvantage of slower nitriding times and increased dispersoid growth. Time can only be determined from the temperature and powder size, but should not be longer than that necessary for sufficient nitrogen diffusion so as to avoid unnecessary particle growth. It could be in excess of the period at which the material is exposed to the nitriding atmosphere.
• the nitriding atmosphere could be removed with nitrogen diffusion being only a fraction of the way through the material, e.g., half way.
• Nitriding would then be completed by the dissolving of unstable nitrides such as chromium nitrides which release the nitrogen necessary to complete nitridation. This completion could occur at any temperature within the nitriding range and if desirable, could be performed simultaneously with the removal of excess nitrogen, an operation described below.
• the unstable nitrides form during the early stages of nitridation when there is an over abundance of nitrogen It is necessary that the powders be distributed.
• Distribution can be accomplished in any number of ways which includes aligning the powders in a thin layer or imparting motion to them, e.g., tumbling, during nitridation.
• a presently preferred method employs an inclined rotary drum with ribs within the drum to constantly pick the powder up and drop it freely. To impart increased motion the drum could have hammers attached to its outer surface.
• Other means which impart motion include fluidized beds, free falling and vibratory columns.
• the steel powder being nitrided could be ferritic or austenitic. Austenitic powder is preferred since it has greater strength at high temperatures. Moreover, nitrides in austenitic powder grow at a slower rate than do nitrides in ferritic powder.
• the powder used should be clean, i.e., it should have a low surface oxygen content. Powder with a thick oxide film, e.g., blue or black film, has its nitriding rate severely restricted and yields larger interparticle spacings than do clean powders. Nitriding of mil clean powder should proceed faster than the nitrogen. Degassing was performed in a vacuum at 2,100 F. Powders C and E were degassed for 26 and 24 hours respectively and the other powders for four hours.
• the protective cans were evacuated and sealed.
• the canned nitriding of 5 mil foil, other parameters remaining the same 5 powders were then extruded from 2,100 F. at a ratio of 23/1. due to geometrical considerations. Finally, the can was removed by pickling in acid.
• chromium nitrides In order to avoid the formation of an excess number of Found below in Table 11 are the 0.2 percent yield strengths chromium nitrides at the grain boundaries, it is desirable, but n the ul im e tensile strengths for the members produced not always necessary, to degas and remove excess nitrogen, from the processed powders as well as their interparticle the amount over that necessary to react with the nitride lspersoid spacings. former. Chromium nitride formation removes chromium from TABLE II solid solution thus reducing the materials corrosion and oxmm A to: idation resistance. Moreover, chromium nitrides will soften on 0.2% Ys U. T. S. particle mi ifiami O at 2 000 F.
• the shaped members could consist solely of 20.4 26.7 0.38-0.53 .486 densified powder or could consist of powder pressed onto a 20 g'g 15-8 g2 substrate, such as a strip or sheet of corrosion or oxidation regm 7 V 7 um ,A'Wm w i V Sistahi alloy, 47 Pf Ni, 22 P 20 F From the table it is seen that all the members exhibited good 9 1 M0 and residuals, chosen so as to Produce articles properties, an ultimate tensile strength in excess of 8 k.s.i. at tailored for Specific application5- h yi Certain 25 2,000 F. and interparticle spacings of less than 2 microns.
• hiirided POWder can be member A although they were both produced from the same miXed With the Powder Ofa Precipitation hardemng alloy, -3 size powder under identical conditions. This is because A con- 4 p r n 19 p r r. 11 p n 10 p r M tained more nickel than B and C and, since nickel lowers the 3 percent Ti, 1.5 percent Aland residuals.
• H powder was of a larger size than G
• the hot pressing is carried out in anonoxidizing atmosphere powder, thereby requiring more time for nitridation and alat temperatures in excess of 1,600 F. and preferably within lowing for increased dispersoid growth.
• the interparticle the range of l,800-2,200 F. At temperatures below 1,600 spacing for member H is between 0.77 and 1.54 microns F., the material becomes too stiff to adequately densify and whereas the interparticle spacing for member G is between within the 1,8002,200 F. range, the advantages and disad- 0.38 and 0.42 microns. vantages of high temperatures are best balanced.
• High tem- Members B and D were further tested to evaluate their high peratures advantageously give the powder greater plasticity temperature rupture strength characteristics.
• Table 111 are the testing temperatures as well as the rupture soid growth. Heat for pressing can be retained from nitriding stress and rupture time. or degassing operations. Pressure is dependent upon the d r TABLE 111 egree of compression necessary to densify the material so as to substantially eliminate pores. Pores are detrimental to the material insofar as they, like notches, are areas of pronounced stress concentration which increases the materials tendency f' Temperature Stress Time to fail. Extrusion is the preferred method of applying pressure.
• Powders having the composition and B 2100 5 49 size shown below in Table I were internally nitrided at the 3 28 35 temperatures shown in Table I for a period of about 10 D 3 a: minutes. All the powders contain titanium which is regarded D 1700 20 196 as the superior nitride former, as noted above.
• Nitriding was performed in a flowing ammdnia atmosphere From the table it is seen that the members exhibit good high with the powders moving along the inclined rotary drum temperatures rupture strength characteristics. described above. Subsequently, the powders were placed in The examples set out above should be construed as exemcans with open ends and degassed so as to remove excess plary only and in no way limiting. Although the members ranged faith 0116 to 0.488 inch in diameter, they could have been larger or smaller. The final size of the members is limited only by the capabilities of the pressing and handling equipment.
• a composition consisting essentially of steel powder containing as a dispersoid therein particles of metal nitride having a free energy of formation of greater than about -2l,000 cal/mole, said nitride particles being present at an interparticle spacing ofless than about 2 microns.
• composition according to claim 1 wherein said dispersoid is a nitride ofa metal from the group consisting of titanium, aluminum, vanadium and columbium.
• composition according to claim 1 wherein said dispersoid is titanium nitride and said powder is austenitic steel.
• a hot pressed steel article comprising densified powder, said densified powder comprising steel powder containing as a dispersoid therein particles of metal nitride having a free energy of formation of greater than about 2l,000 cal/mole, said nitride particles being present at an interparticle spacing of less than about 2 microns.
• said dispersoid is a nitride of a metal from the group consisting of titanium, aluminum, vanadium and columbium.
• said densified powder comprises said steel powder and powder of a precipitation hardening alloy.
• An article according to claim 5 having an ultimate tensile strength at 2,000 F. in excess of8 k.s.i.
• a method of strengthening iron alloy powder by internal nitridation which comprises distributing steel powder containing a metal component capable of forming a nitride having a free energy of formation of greater than about 2l,000 cal/mole, said metal component being present in an amount sufficient to provide, after nitriding, nitride particles as a dispersoid in said steel with an interparticle spacing of less than about 2 microns; and internally nitriding said distributed powder in a nitriding atmosphere, said nitriding atmosphere substantially surrounding said powder thereby creating a nitrogen potential at the surface of said powder.
• nitriding atmosphere is selected from the group consisting of ammonia substantially free of moisture and oxygen, nitrogen and mixtures thereof with each and with compatible gases.
• a method according to claim 13 wherein said metal component is from the group consisting of titanium. aluminum, vanadium and columbium 21.
• a method according to claim 13 wherein said metal component is titanium and said steel powder is austenitic steel.
• a method of forming a hot pressed article comprising densified powder which comprises: heating in a nonoxidizing atmosphere steel powder containing as a dispersoid therein particles of metal nitride having a free energy of formation of greater than about -2l,000 cal./mole, said nitride particles being present at an interparticle spacing of less than about 2 microns; and densifying said powder into a coherent substantially non-porous body.
• a method according to claim 23 wherein said dispersoid is a nitride of a metal from the group consisting of titanium. aluminum, vanadium and columbium.
• a method of forming a hot pressed article comprising densified internally nitrided iron alloy powder which comprises: distributing austenitic steel powder containing titanium in an amount sufficient to provide, after nitriding, titanium nitride particles as a dispersoid in said steel with an interparticle spacing of less than about 2 microns; internally nitriding said distributed powder in a nitriding atmosphere, said nitriding atmosphere substantially surrounding said powder thereby creating a nitrogen potential at the surface of said powder; and densifying said powder into a coherent substantially nonporous body.

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• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
• Powder Metallurgy (AREA)

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• 1969
  • 1969-03-07 US US805361A patent/US3650729A/en not_active Expired - Lifetime
• 1970
  • 1970-03-07 JP JP45019250A patent/JPS5212124B1/ja active Pending
  • 1970-03-09 DE DE19702011065 patent/DE2011065A1/de not_active Ceased
  • 1970-03-09 SE SE7003097A patent/SE373605B/xx unknown
  • 1970-03-09 GB GB01164/70A patent/GB1263008A/en not_active Expired

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