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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys

Definitions

• the present invention relates to production of stainless steel products and more particularly to production of stainless steelv products characterized by improved high strengthand toughness qualities.
• Sharp-notch toughness is the capability or capacity of a metal to yield plastically under high localized stress such as might occur at the root of a sharp notch and steels having adequate sharpnotch toughness are referred to as being sharp-notch ductile.
• the sharp-notch (tensile strength) tensile strength ratio is used as a criterion of toughness and, as referred to herein for sheet specimens, metals characterized by sharpnotch/tensile strength ratios of at least about 1, i.e., 0.95 or greater, are deemed sharp-notch ductile, whereas metals characterized by lower sharp-notch/ tensile strength ratios are deemed notch brittle or notch sensitive.
• a further aspect of the problem of producing high strength stainless steel products is that although it is possible to greatly strengthen some known stainless steels by cold rolling the steels to very high reductions in cross-sectional area, it has been found that stainless steels which are in the condition obtained by cold rolling to cross-sectional reductions of 50% or greater are often adversely characterized by poor characteristics in the direction transverse to the rolling direction. Accordingly, there are needs for special stainless steel alloys and production processes therefor which enable production of high strength, sharp-notch ductile stainless steel products without necessity of cold working the steel to amounts of deformation equivalent to cross-sectional reductions in area of 50% or greater.
• Another object of the invention is to provide improved stainless steel products, including stainless steel sheet, strip, bar, rod, wire, tubing and the like, produced by a special process.
• the invention also contemplates providing a new stainless steel alloy having a controlled chemical composition that is of sufiicient scope to enable commercial production of stainless steel products which are consistently characterized by a combination of high yield strength and sharp-notch ductility.
• the present invention contemplates production of a high strength, sharp-notch ductile stainless steel product in accordance with a new strengthening process comprising the steps of providing an austenitically structured workpiece of a special nickel-chromium-cobalt stainless steel alloy which is initially of a structure including at least about austenite, plastically deforming the initially austenitically structured workpiece at a temperature within the range of about 5 F. to about F.
• the balance of the nickel-chromium-cobalt stainless steel alloy composition of the invention is characterized herein as being essentially iron, it is to be understood that the term balance essentially iron does not exclude small amounts of other elements which can serve some useful purpose ancillary to the objects of the invention, e.g., up to about 0.2% aluminum, up to about 0.1% each of calcium, magnesium and/or zirconium, and up to about 0.01% boron.
• Aluminum, calcium, magnesium, zirconium, and/or boron can serve purposes of deoxidation, malleabilization and/or purification.
• the maximum silicon content of the alloy of the invention is restricted to about 0.15% in order that the alloy be characterized by sharp-notch ductility.
• the balance of the alloy of the invention may contain small amounts of relatively harmless impurities such as up to 2% copper and may also contain very small amounts of other more detrimental impurities such as sulfur, phosphorus, bismuth, antimony, tin, lead, arsenic, beryllium, etc. However, the total amount of these impurities must be less than 0.03% of the steel, e.g., about 0.02% or less.
• the alloy of the invention is characterized by an improved combination of high strength, hardness and toughness, including a yield strength which is consistently at least 240,000 pounds per square inch (p.s.i.) together with sharp-notch ductility, i.e., a sharp-notch/tensile strength ratio of at'least about 1 or greater, when in a martensitic condition obtained by plastically deforming and heat treating the alloy in accordance with the process of the invention.
• the present invention provides a new alloy characterized by ranges of chemical composition and equivalent nickel index which are specially controlled to provide for consistently obtaining a high yield strength of at least 240,000 p.s.i in combination with sharp-notch ductility and which are also of sufficient scope to permit consistent commercial production of specific embodiments of this new high strength alloy on a practical basis and with the normal tolerances for variation in composition (within the ranges set forth herein) demanded for successful commercial practice.
• the ingredients for the stainless steel are melted in an induction furnace or any of the other furnaces employed for production of similar alloys. Vacuum melting or inert atmosphere melting can be employed if desired but such practices are not necessarily required for successfully practicing the invention.
• the workpieces will usually be hot Worked and sometimes cold worked before being processed in accordance with the process of the invention.
• workpieces which have been hot worked and especially workpieces which have been cold worked are annealed prior to being subjected to a deformation step of the process of the invention.
• a satisfactory annealing treatment is accomplished by heating the workpiece for about 1 hour to about 24 hours at a temperature of about 1800 F.
• annealing temperatures within the range 1800 F. to 2 l F. have little effect but annealing below 1800 F. is to be avoided because of possible embrittlement by carbides or sigma phase.
• the plastic deformation of the workpiece which is accomplished in the process of the invention can be performed by rolling the workpiece at near room temperature and deformation temperatures in the range 5 F. to 100 F. are satisfactory for obtaining good results. Rolling is normally accomplished by multiple passes and it is to be recognized that it may be necessary to allow the workpiece to cool between passes in order that the workpiece temperature be within the range of 5 F. to 100 F. at the start of each pass.
• the amount of reduction in cross-sectional area is advantageously at least 20% and less than 50%, e.g., 45%. Reductions of less than 20% do not provide for developing full strength in the products and larger reductions, e.g., 60%, can cause mechanical anisotropy and poor transverse toughness in the product.
• Deformation in accordance with the invention can be accomplished not only by rolling but also by drawing, forging, spinning, or other forming techniques. Of course,
• the deformation step can com
• the strengthening heat treatment which is accomplished in the process of the invention is performed by heating the workpiece, after deformation is complete, for a pe-.
• the alloy of the invention contain about 4.5% to about 5.5% nickel, about 14.5% to about 16% chromium, about 7.5% to about 10% cobalt, about 0.1% to about 0.5% manganese, about 0.04% to about 0.09% carbon, about 0.01% to about 0.05% nitrogen, not more than about 0.1% silicon, with the balance essentially iron and have an equivalent nickel index of about 20 to about 21.
• Stainless steel products of this advantageous composition when in a martensitically strucured condition obtained by deforming a workpiece of this advantageous alloy by an amount equivalent to at least about 30% reduction in area and by heat treating at about 750 F. to about 850 F.
• Alloys 2 and 3 referred to hereinafter are particularly advantageous alloys of the invention.
• Alloys W and X contain only about 3.5% and about 3.1% cobalt, respectively, which amounts are less than the amount of cobalt required in the alloys of the invention. These low cobalt contents of about 3.1% and about 3.5% are insufficient to provide for obtaining the high yield strength of at least about 240,000 p.s.i. which is a characteristic of the alloy of the invention. In order to provide for consistently obtaining high strength and hardness, the alloy of the invention contains at least about 7% cobalt. Alloys Y and Z are characterized by equivalent nickel indexes of only about 18.7 and 18.5, respectively, whereas for the purpose of obtaining high strength and hardness, the alloy of the invention is characterized by an equivalent nickel index of about 19.5 to about 22.
• Ingots of Alloys 1 through 5 and W through Z were hot forged and hot rolled to about one-quarter inch thick plates and then cold rolled to about 0.1 inch thick sheet. These sheets were annealed by heat treating for one hour at 1950 F. and air coiling to room temperature to insure that the sheets consisted predominantly of austenite and to eliminate hardening effects resulting from the prior hot and cold working steps.
• sheets of Alloys 1 through 5 which were characterized by a structure comprising at least about 90% austeuite, up to about 10% martensite and possibly also delta ferrite in small amounts not greater than about 1%, were provided as workpieces for carrying out the process of the invention.
• the annealed sheets of Alloys 1 through 5 and Alloys W through Z were cold rolled at room temperature (about 70 F.) to a reduction of about 40% in thickness, which reduction is approximately equal to a cross-sectional reduction in area of about 40%. Rolling was accomplished in several passes and the sheets were cooled to room temperature between passes. As a result of rolling, austenite in the sheets was transformed to martensite. After rolling was completed, the sheets, which were then about 0.06 inch thick, were heat treated for 24 hours at 800 F. for the purpose of increasing the yield strength and insuring the toughness thereof.
• finished stainless steel products of Alloys 1 through 5 were produced in accordance with the invention along with finished products of Alloys W through Z which were made by the same rolling and heat treating steps which were performed on Alloys 1 through 5.
• the sharp-notched specimens used to test the stainless steels of the foregoing examples had stress-concentration factors, K of at least 18 and sharp notches with root was reached and thereafter at an increased strain rate of 0.05 inch per inch per minute until the specimens fractured.
• yield strength as employed herein, is the yield strength of 0.2% offset.
• the present invention is particularly applicable to the production of sharp-notch ductile stainless steel products characterized by high corrosion resistance, hardness, yield strength and tensile strength.
• Stainless steel products that can be produced in accordance with the invention include sheets, plates, strips, rods, bars, tubing, forgings, wire, extrusions, stampings and pressings.
• the alloy and process of the invention are useful for making highly stressed structures and articles for use in corrosive environments and also in cryogenic temperature environments. Such structures and articles include pipes, couplings, pressure vessels, barrels, wheel spokes, bolts, rivets and screws.
• the alloy and process of the invention are also useful for making hard, corrosion resisting articles including knives, surgical instruments, dental tools, saws and chisels.
• the alloy of the invention is a transformable stainless steel that is particularly adapted for cold working at near room temperature in the austenitic condition to transform a ustenite of the alloy into martensite and increase the strength of the alloy and thereby produce a martensitically structured alloy which can be heat treated to still further increase the strength thereof. Accordingly, it is to be understood that the alloy of the invention is especially useful where it is desired to make stainless steel products and articles characterized by high strength in combination with sharpnotch ductility by methods involving plastically deforming a stainless steel alloy at or near room temperature.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Steel (AREA)
• Heat Treatment Of Articles (AREA)
• Heat Treatment Of Sheet Steel (AREA)

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Cited By (18)

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Citations (4)

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• 1964
  • 1964-03-31 US US356037A patent/US3340048A/en not_active Expired - Lifetime
• 1965
  • 1965-03-17 GB GB11358/65A patent/GB1026257A/en not_active Expired
  • 1965-03-25 AT AT272165A patent/AT261649B/de active
  • 1965-03-26 DE DEJ27788A patent/DE1231439B/de active Pending
  • 1965-03-30 ES ES0311194A patent/ES311194A1/es not_active Expired
  • 1965-03-31 BE BE661896A patent/BE661896A/xx unknown
  • 1965-03-31 NL NL6504056A patent/NL6504056A/xx unknown

Patent Citations (4)

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