Classifications

• • 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
  • C22C38/18—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

Definitions

• Hoffman ABSTRACT An austenitic stainless steel having excellent galling resistance by reason of a silicon-containing surface oxide film and a high work hardening rate, good wear resistance, good corrosion resistance in chloridecontaining environments, and excellent oxidation resistance, containing 10 to 25 percent chromium, 3 to 15 percent nickel, 6 to 16 percent manganese, 2 to 7 percent silicon, 0.001 to 0.25 percent carbon, 0.001 to 0.4 percent nitrogen, and balance iron except for incidental impurities. Up to 4 percent molybdenum, up to 4 percent copper, 0.09 percent maximum phosphorus, up to 0.25 percent maximum sulfur and up to 0.50 percent maximum selenium may be present.
• the steel is readily workable on ordinary equipment into plate, sheet, strip, bar, rod and like wrought products.
• This invention relates to an austenitic stainless steel having excellent galling resistance in conventional wrought form, good wear resistance, good corrosion resistance in chloride-containing environments, excellent high temperature oxidation resistance, and a high work hardening rate.
• the alloy of this invention can be readily worked with conventional equipment into plate, sheet, strip, bar, rod and the like, and retains a substantially austenitic structure throughout a wide temperature range.
• the steel of the invention is adapted to applications in which moving metal-to-metal contact and corrosive attack are encountered in combination.
• the steel has particular utility for fabrication into roller chains, link belts on conveyors, valves subjected to elevated temperature, woven metal belts for continuous heat treating furnaces, fasteners, pins and bushings.
• Galling may be defined as the development of a condition on a rubbing surface of one or both contacting metal parts wherein excessive friction between minute high spots on the surfaces results in localized welding of the metals at these spots. With continued surface movement this results in the formation of even more weld junctions which eventually sever in one of the base metal surfaces. The result is a build-up of metal on one surface, usually at the end of a deep surface groove. Galling is thus associated primarily with moving metal-to-metal contactand results in sudden catastrophic failure by seizure of the metal parts.
• wear is synonymous with abrasion and can result from metal-to-metal contact or metal to nonmetal contact, e.g. the abrasion of steel mining equipment by rocks and similar mineral deposits.
• Such wear is characterized by relatively uniform loss of metal from the surface, as. contrasted to localized grooving with consequent metal build-up, as a result of rubbing a much harder metallic surface against a softer metallic surface.
• galling and wear can perhaps best be illustrated by the fact that galling can be eliminated by mating or coupling a very hard metallic surface with a much softer metallic surface, whereas wear or abrasion in metal-to-metal contact would be increased by mating a very hard surface with a much softer one.
• the austenitic AISI Type 304 is suited to a variety of uses involving welding and fabrication, but the galling and wear resistance of this steel are poor, and the metal is likely to fail when subjected to such conditions.
• a precipitation-hardening stainless steel, sold under the registered trademark ARMCO 17-4 PH (about 16.5 percent chromium, about 4.0 percent nickel, about 4.0 percent copper, about 1.0 percent manganese, about 1.0 percent silicon, up to 0.07 percent carbon, 0.35 percent columbium, and remainder iron), while possessing high strength and hardness in the hardened condition, exhibits only fair galling and wear resistance.
• the broad composition ranges are about 10 percent to about 22 percent chromium, about 14 percent to about 25 percent nickel, about 5 percent to about 12 percent silicon, one or more of the elements molybdenum up to about 10 percent, tungsten up to about 8 percent, vanadium up to about 5 percent, columbium up to about 5 percent and titanium up to about 5 percent, these additional elements being in sum total of about 3 percent to about 12 percent.
• Carbon is present up to about 0.15 percent and nitrogen up to about 0.05 percent.
• silicon is stated to form silicides of molybdenum, tungsten and the like, in finely dispersed form in the matrix of the precipitationhardened steel. These silicides are of extreme hardness, thereby providing good wear resistance.
• a prior art steel presently considered to have the best resistance to wear and galling is the straight chromium AISI Type 440C, containing about 16 percent to 18 percent chromium, about 1 percent maximum manganese, about 1 percent maximum silicon, about 0.75 percent maximum molybdenum, about 0.95 percent to 1.20 percent carbon, and remainder iron.
• This steel is hardenable by heat treatment but has poor corrosion resistance and poor fonnability. It is difficult to roll into plate, strip, sheet, bar or rod, and articles of ultimate use cannot be readily fabricated from plate, sheet, strip, bar or rod form.
• composition comprises from about 15.5 to about 20 percent chromium, from about 1 1 percent to about 14 percent manganese, from about 1.1 percent to about 3.75 percent nickel, from about 0.01 percent to about 0.12 percent carbon, from about 0.20 percent to about 0.38 percent nitrogen, up to about 1 percent silicon, up to about 0.06 percent phosphorus, up to about 0.04 percent sulfur, and remainder substantially iron.
• the steel of the present invention consists essentially of about percent to about 25 percent chromium, about 3 percent to about nickel, about 6 percent to about 16 percent manganese, about 2 percent to about 7 percent silicon, about 0.001 percent to about 0.25 percent carbon, about 0.001 percent to about 0.4 percent nitrogen, up to about 4 percent molybdenum, up to about 4 percent copper, a maximum of about 0.09 percent phosphorus, a maximum of about 0.25 percent sulfur, a maximum of about 0.50 percent selenium, and balance substantially iron except for incidental impurities, all percentages being by weight.
• the elements chromium, nickel, manganese, silicon, and nitrogen, and the balance therebetween, are critical in every sense. Omission of one of the elements, or departure of any of these critical elements from the ranges set forth above results in loss of one or more of the desired properties.
• Nickel is varied directly in proportion to the silicon content, for reasons set forth hereafter.
• the silicon content of the steel of the invention is of particular criticality. Although not wishing to be bound by theory, it is believed that silicon within the range of 2 percent to 7 percent, and more particularly within the range of 3 percent to 5 percent by weight, performs a dual function. First, it appears to modify the composition of the surface oxide film of the steel, making it more stable and adherent. Secondly, silicon exerts a significant influence on the work hardening rate of the steel. An increase in silicon within the limits set forth above results in an increase in the work hardening rate.
• silicon does not form a silicide of molybdenum, tungsten, vanadium, columbium and/or titanium which silicide is relied upon to impart wear resistance in the steel of that patent.
• the silicon present in the surface oxide film is believed to be dispersed as a substitutional atom in the oxide lattice providing a low shear strength oxide film which is tightly adherent to the surface.
• another oxide film rapidly forms at ordinary temperatures, so that the surface, is in effect, self-healing.”
• compositions are modified by addition of sulfur in amounts of about 0.15 percent to 0.25 percent, and/or selenium in amounts of about 0.25 percent to 0.50 percent.
• At least 10 percent chromium is required for corrosion resistance. More than 25 percent chromium results in extreme difficulties in processing, and disturbs the austenitic balance of the alloy. For many applications a maximum of 19 percent, or even 17 percent, chromium should be observed in order to insure a substantially fully austenitic structure.
• Nickel is an austenite former, and at least 3 percent nickel is required in order to assure an austenitic structure. Preferably 4 percent, and more preferably 6 percent, nickel is added for this purpose. Since silicon is a ferrite former, nickel is added in direct proportion to the silicon content, e.g. when silicon is low, nickel is low. A'maximum of 15 percent, or still better, 13 percent by weight nickel must be observed since hot workability of the steel is adversely affected with nickel in amounts exceeding about 13 percent and certainly above 15 percent. It is of course also evident that large amounts of nickel greatly increase the cost of the alloy. Preferably a maximum of 12 percent nickel is observed for a preferred maximum silicon content of 5 percent, while a maximum of 10 percent nickel is preferred for the more preferred maximum silicon content of 4.2 percent.
• silicon is essential in an amount of at least 2 percent for its effect in making the surface oxide layer more stable and adherent.
• an increase in the silicon content increases the work hardening rate of the steel of the invention.
• this effect is somewhat mitigated due to the necessity to increase the nickel directly in proportion to the increased silicon content (to offset the. ferrite-forming potential of silicon), and an increase in the nickel content tends to lower slightly the work hardening rate of the steel.
• the net effect is an increase in the work hardening rate as the silicon content is increased. At least 3 percent silicon is preferred for these reasons, and the more preferred minimum is 3.7 percent silicon.
• silicon is a ferrite former, more than 7 percent silicon cannot be tolerated, at the nickel levels herein contemplated, in order to insure a substantially austenitic structure. Moreover, a silicon content in excess of 7 percent adversely affects hot workability, and for best cold formability the silicon content should not exceed 5 percent. For optimum properties the maximum silicon content is about 4 percent.
• manganese is a weak austenite former, it is present primarily for its effect in stabilizing the austenitic structure of the steel and in keeping nitrogen in solid solution. For these purposes, at least about 6 percent manganese is essential. More than about 16 percent manganese would upset the composition balance and would lower the general corrosion resistance of the steel. Preferably a maximum of 13 percent, and even more preferably a maximum of 8.5 percent, are observed with the chromium, nickel and silicon ranges set forth above.
• Nitrogen is present, the minimum being about 0.001 percent, and a purposeful addition is preferably made for its effects as an austenite former and in strengthening and work hardening the steel. Low nitrogen levels have no noticeable benefit, while a maximum of 0.4 percent nitrogen must be observed in order to avoid exceeding the solubility limits of nitrogen in the steel. Optimum benefits are realized with nitrogen present in the range of 0.03 percent to 0.3 percent, or even better within the range 0.10 percent 0.20 percent.
• Molybdenum and/or copper may be present up to a maximum of 4 percent each for improving high temperature properties and corrosion resistance. Where such improved properties are not needed, a preferred maximum of 0.75 percent, and more preferred maximum of 0.5 percent, for each element are observed.
• Carbon is of course present as an impurity, and ordinarily will amount to at least about 0.001 percent. Carbon should be restricted to a maximum of about 0.25 percent, preferably about 0.12 percent, and even more preferably about 0.10 percent maximum, since excessive carbon adversely affects corrosion resistance and weldability.
• Phosphorus is held to 0.09 percent maximum for welding and hot working reasons. Sulfur may be added up to 0.25 percent maximum (and/or selenium up to 0.50 percent maximum) for good machinability.
• While the steel of the present invention exhibits good wear resistance, its outstanding and principal property is its resistance to galling.
• EXAMPLE I An exemplary heat has been prepared consisting essentially of 16 percent chromium, 7.4 percent nickel, 8 percent manganese, 4 percent silicon, 0.09 percent carbon, 0.14 percent nitrogen, 0.010 percent phosphorus, 0.014 percent sulfur, 0.02 percent molybdenum, 0.04 percent copper, and balance iron. The heat was melted in an induction furnace, cast into an ingot, hot rolled on a conventional rolling mill to intermediate size and hot rolled to final 1 inch diameter, annealed at 1850 F for 5% hour and water quenched.
• Example I The annealed bar stock of Example I was subjected to galling and wear resistance tests. Test results on galling resistance are summarized in Table I. For purposes of comparison a number of prior art alloys were tested under the same conditions and reported in Table I below.
• EXAMPLE 2 Another exemplary alloy of the invention contained 16 percent chromium, 4.0 percent nickel, 13 percent manganese, 4.0 percentsilicon, 0.05 percent carbon,
• AISI 440C (555) 36 A151 430 (156) v. A181 430 (156) 4 S.N. 238,862 (235) v. SN. 238,362 (235) 22 SN. 238,862 (235) v. AISI 304 (140) 6 AISI 4337 (509) v. AISI 4337 (509) 3 Steel of the present invention "No galling; exceeded limits of test machine.
• the test method utilized in obtaining the data of Table I involved rotation of a polished cylindrical section or button for one revolution under pressure against a polished block surface in a standard Brinell hardness machine.
• a button specimen was prepared by drilling a countersunk hole to accommodate most of the exposed Brinell hardness ball, the specimen then being mounted in bakelite and polished to a 600 grit finish in a Buehler Automet unit to obtain a relatively flat test surface, with the edges slightly rounded. The button was then broken out of the bakelite and the edges were hand deburred.
• a block specimen was ground parallel on two sides and hand polished to a 3/0 emery grit finish, equivalent to a 600 grit finish.
• Both the button and block specimens were degreased by wetting with acetone, and the hardness ball was lubricated just prior to testing.
• the button was hand-rotated slowly at a predetermined load for one revolution and examined for galling at 10X magnification. If galling was not observed (i.e., absence of metal build-up, usually at the end of a groove), a new button and block area couple was tested at successively higher loads until galling was first observed. Confirmation was obtained by testing one more couple or combination at a higher load. Since light loads did not cause full area contact due to the rounded button edges, the actual contact area was measured at 10X to convert to galling stress.
• the button specimen is the first alloy mentioned in each couple and the second alloy is the block specimen. Double asterisks beside the galling stress indicate that the test was terminated at that point because the limits of the test equipment were exceeded.
• Espy (test specimen analyzing 18.0 percent chromium, 1.60 percent nickel, 12.0 percent manganese, 0.10 percent carbon, 0.34 percent nitrogen, and remainder iron) galls when rotated against itself at a stress of only 22 ksi, although the Brinell hardness (235) was about the same as that of the steel of the invention.
• the corrosion resistance of the steel of the invention was compared to that of AISI Type 304, which is generally considered to have corrosion resistance adequate for most applications. These comparisons are set forth in Table III below.
• This invention therefore provides an austenitic stainless steel having excellent galling resistance, good wear resistance, good corrosion resistance against chloridecontaining environments, especially pitting environments, and excellent high temperature oxidation resistance. Moreover, the steel can easily be worked with standard equipment into plate, sheet, strip, bar or rod, and such wrought products can be fabricated readily into ultimate useful products.
• wrought products of the present steel are sufficiently soft and ductile to permit ready fabrication into chains, valves, woven metal belts, fasteners of various types, and other articles of ultimate use wherein metal-to-metal contact under stress or load would be encountered.
• the steel of the invention can readily be welded or brazed and may be cut, drilled, tapped, threaded and machined in other manner in fabrication of articles of ultimate use.
• Austenitic stainless steel having excellent resistance against galling, a high work hardening rate, and excellent resistance against pitting corrosion in chloride-containing environments, consisting essentially of from about 12 percent to about 19 percent chromium, about 4 percent to about 12 percent nickel, about 7 percent to about 13 percent manganese, 3 percent to 5 percent silicon, about 0.01 percent to about 0.12 percent carbon, about 0.03 percent to about 0.3 percent nitrogen, about 0.75 percent maximum molybdenum, about 0.75 percent maximum copper, about 0.09 percent maximum phosphorous about 0.05 percent maximum sulfur, and remainder essentially iron except for incidental impurities, all percentages being by weight, the nickel content being varied directly in proportion to the silicon content.
• the steel of claim 1 consisting essentially of from about 15 percent to about 17 percent chromium, from about 6 percent to about 10 percent nickel, from about 7.5 percent to about 8.5 percent manganese, from about 3.7 percent to about 4.2 silicon, from about 0.05 percent to about 0.10 percent carbon, from about 0.10 percent to about 0.20 percent nitrogen, about 0.5 percent maximum molybdenum, about 0.5 percent maximum copper, about 0.07 percent maximum phosphorus, about 0.03 percent maximum sulfur, and remainder substantially iron except for incidental impurities.
• the steel of claim 3 consisting essentially of about 16 percent chromium, about 7.4 nickel, about 8 manganese, about 4 percent silicon, about 0.09 percent carbon, about 0.14 percent nitrogen, about 0.010 percent phosphorus, about 0.014 percent sulfur, about 0.02 percent molybdenum, about 0.04 percent copper, and

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• 1973
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