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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese

Definitions

• Field of the Invention relates to a modified chromium stainless steel (low'in nickel, copper and cobalt) of stable ferritic-austenitic structure having excellent toughness, ductility, corrosion resistance and welding characteristics.
• the alloy of the invention by reason of its compositional balance, achieves a structure of from percent to 50 percent austenite (preferably percent to 30 percent) in a ferritic matrix which resists transformation into martensite despite cold working, heat treatment, or welding.
• the stainless steel of this invention has particular utility as weldments in straight chromium steels, for fabrication into fasteners which require cold heading, and a variety of other applications requiring relatively high strength and ductility, good weldability, and high resistance to intergranular corrosion in strongly oxidizing media, as well as good resistance to stress corrosion in chloride media.
• Carney consisting of 0.06 percent to 0.15 percent car'- bon, 14 percent to 20 percent manganese, 17 percent to 18.5 percent chromium, 0.05 percent to 1.0 percent nickel, 0.25 percent to 1.0 percent silicon, 0.25 percent to 1.0 percent nitrogen, and remainder iron;
• B '& W CROLOY 299 consisting of 0.20 percent carbon, 15.0 percent manganese, 17.0 percent chromium, 1.5 percent nickel, 0.35 percent nitrogen, and remainder iron; and other fully austenitic steels such as Armco 16-16-1 and Allegheny Ludlum 205.
• Straight chromium stainless steels such as A.l.S.I. Types 430, 442 and 446 have the serious disadvantages of being brittle and subject to corrosion in the heat affected zone of the base metal of a weldment. Further, the unaffected base metal may be low in impact strength at room temperature.
• Typical of austenitic stainless steels which transform with cold working to less ductile martensite is A.l.S.I. Type 304, consisting of 0.08 percent maximum carbon, 2.0 percent maximum manganese, 18 percent to 20 percent chromium, 8 percent to 10.50 percent nickel and balance iron.
• IN 744X An alloy developed for cold heading applications which does not transform to martensite is designated as IN 744X. This steel contains about 26 percent chromium and is about half austenitic and half ferritic. Due to the high alloy content the cost is excessivelyhigh.
• the principal object of this invention is to provide a magnetic austenitic-ferritic stainless steel essentially consisting of'chromium, manganese, carbon and nitrogen which is stable against transformation to martensite regardless of cold working, heat treatment or welding,
• a stainless steel having a two-phase structure comprising between 10 percent and 50 percent austenite in a ferrite matrix consists essentially of from about 4.0 percent to less than 11.0 percent manganese, about 19 percent to about 24 percent chromium, and about 0.12 percent to about 0.26 percent nitrogen.
• Carbon is of course present and is limited to a maximum of about 0.06 percent.
• Phosphorus and sulfur, normally present as impurities are limited to a maximum of about 0.03 percent each.
• Silicon is also normally present, in amounts up to 1.0 percent maximum.
• Nickel may be present, ranging from trace amounts up to about 3.0 percent.
• Copper and cobalt, if present as residual elements, are limited to a maximum of about 0.5 percent each.
• the balance is of course iron, together with incidental impurities.
• Molybdenum may be substituted for chromium on a 1:1 basis in amounts up to about 5 percent for improved resistance to corrosion in pitting media.
• Columbium may be added in amounts up to about 1 percent for improved weld area corrosion resistance.
• the austenite level preferably 20 percent to 30 percent, is achieved through addition of nitrogen (a strong austenite former) within the range of 0.12 percent and 0.26 percent.
• nitrogen a strong austenite former
• Carbon although maintained at a low level, also contributes to some extent to austenite formation.
• the austenite is maintained at a stable level by reason of the chromium, manganese and nitrogen relationship. It is thus apparent that the compositional balance among the essential elements is in every sense critical. Unlike prior art austeniticferritic alloys, the nickel, copper and cobalt contents are maintained at low levels, and hence the steel of the invention is not subject to stress corrosion failure when exposed to hot chloride media.
• the use of manganese to stabilize the austenite balance results in a ductile material which is also resistant to stress cracking in hot chloride media. The low carbon content tends to prevent intergranular corrosion when welded.
• At least about 0.12 percent nitrogen is necessary in order to form sufficient austenite. Nitrogen in excess of about 0.26 percent would exceed the solubility limit of this element and hence would result in porosity and unsoundness in the metal.
• a minimum of about 4 percent manganese is required in order to balance the chromium and thereby stabilize the austenite. Excessive manganese adversely affects the balance with chromium, increasing the austenite level above the desired range, and the maximum manganese content is thus less-than 11.0 percent.
• Nickel if present, is limited to a maximum of about 3.0 percent. It has been found that the stress corrosion resistance of the metal will be adversely affected if the nickel content exceeds 3.0 percent. Within the prescribed range, nickel will of course increase the austenite level and thus cooperates with the nitrogen in this function, without adversely affecting toughness.
• the steel of the invention consists essentially of carbon up to about 0.06 percent, manganese about 4.0 percent to less than I 1.0 percent, chromium about l9 percent to about 24 percent, nitrogen about 0.12 percent to about 0.26 percent, nickel up to about 3.0 percent, phosphorus and sulfur up to about 0.03 percent each, silicon up to about 1.0 percent, copper and cobalt up to about 0.5 percent each, and remainder substantially iron
• a preferred composition comprises about 0.02 percent carbon, about 6.0 percent manganese, phosphorus and sulfur low, about 0.40 percent silicon, about 2l.O percent chromium, about 0.20 percent nickel, about 0.20 percent nitrogen, copper and cobalt low, and balance substantially iron.
• Heats designated as B, E, H, I, J, K, L, P and Q are steels of the invention TABLE 1 Heat 0. Mn P S Si Cr Ni N A 0. 75 Low 0. 015 0. 50 26. 00 0. 25 0. 20 B 5. 85 0. 017 0. 023 0. 41 21. 12 0. 14 0. 23 C. 6. 20 0. 019 O. 014 0. 52 27. 26 0120 0. 23 D 6. 09 0. 009' 0. 008 0. 30 24. 80 0. 13 0. 24' E. 5. 96 0.004 0. 008 0. 38 21. 07 2. 58 0. 23 F 5. 67 0. 005 0.
• Table II summarizes the effect of the austenite percentage level on the hardness and toughness of the heats of Table I both in the annealed and austenitized condition.
• the heats are listed in the order of increasing austenite content. Heats L, I, B, J and K fall within the preferred austenite levels of 20 percent to percent, and heats I and B have optimum properties. i
• the austenite percentage was measured on a calibrated permanent magnet gauge known as a MAGNE- GAGE.
• Tables III through VIII below list certain selected heats and compare the effect on hardness and toughness of variation of chromium, manganese, nitrogen, carbon, nickel, and chromium plus nickel, respectively, all other elements in each Table being substantially constant.
• the data on hardness are included to show transformation to martensite. High hardness indicates that transformation to martensite has occurred.
• the magnetism values measure both ferrite and martensite since both are magnetic, but if the hardness does not increase after annealing or austenitizing, substantially all the magnetic phase remains as ferrite.
• a Charpy V notch impact strength of 2 kgm/cm in the annealed condition is considered the minimum acceptable toughness.
• Table IV indicates that with all other elements substantially constant manganese contents of less than about 4 percent result in a transformation of austenite to ferrite.
• Table V shows the effect of nitrogen in control of the austenite-ferrite balance. Nitrogen contents of less than about 0.12 percent, with all other elements substantially constant, result in too low an austenite content to provide good toughness.
• the stainless steel of claim 1 containing about 0.02 percent carbon, about 6.0 percent manganese, about 21.0 percent chromium, about 0.20 percent nitrogen, about 0.20 percent nickel, and about 0.40 per cent silicon.
• the stainless steel of claim 1 including columbium in amounts up to about 1 percent.
• a stainless steel having a two-phase structure comprising between 20 percent and 30 percent austenite in a ferrite matrix, consisting essentially of about 0.02 percent carbon, about 6.0 percent manganese, about 21.0 percent chromium, about 0.20 percent nitrogen, about 0.20 percent nickel, phosphorus and sulfur low, about 0.40 percent silicon, copper and cobalt low, and balance substantially iron.
• Welded articles having a. two-phase structure comprising between percent and 50 percent austen ite in a ferrite matrix, consisting essentially of up to about 0.06 percent carbon, about 4.0 percent to less than 11.0 percent manganese, about 19 percent to about 24 percent chromium, about 0.12 percent to about 0.26 percent nitrogen, nickel up to about 3.0 percent, phosphorus and sulfur up to about 0.03 percent each, silicon up to about 1.0 percent, copper and cobalt up to about 0.5 percent each, and remainder substantially iron.

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• Chemical & Material Sciences (AREA)
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• Mechanical Engineering (AREA)
• Metallurgy (AREA)
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• Heat Treatment Of Steel (AREA)
• Arc Welding In General (AREA)

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Citations (3)

• 1970
  • 1970-12-23 US US00101096A patent/US3736131A/en not_active Expired - Lifetime
• 1971
  • 1971-11-24 CA CA128,473A patent/CA961310A/en not_active Expired
  • 1971-12-13 GB GB5778571A patent/GB1363087A/en not_active Expired
  • 1971-12-13 FI FI3542/71A patent/FI56201C/fi active
  • 1971-12-20 IT IT54883/71A patent/IT945560B/it active
  • 1971-12-22 BR BR8516/71A patent/BR7108516D0/pt unknown
  • 1971-12-22 FR FR7146137A patent/FR2119612A5/fr not_active Expired
  • 1971-12-22 SE SE16483/71A patent/SE363350B/xx unknown
  • 1971-12-23 JP JP10513171A patent/JPS5651222B1/ja active Pending

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