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

Definitions

• the present invention relates to the production of stainless steel products, and more particularly, to the production of stainless steel products characterized by high strength and improved toughness qualities.
• the art has endeavored to produce special corrosion-resistant metal products, e.g., corrosion-resistant metal sheet, for use in structures which must sustain high stresses while the structures are in contact with corrosive materials or are in the presence of corrosive environments. Pressure vessels for holding corrosive fluids under high pressures are examples of such structures.
• corrosion-resistant metals produced by prior art methods still have serious shortcomings as engineering materials for use where high stresses must be sustained.
• a metal In order for a metal to be entirely satisfactory for use in highly stressed structures, it is not only necessary that the metal be characterized by sufficiently high yield strength or ultimate tensile strength such as are determined by conventional tension tests of smooth (unnotchcd) specimens of the metal, but it is also necessary that the metal be characterized by adequate resistance to brittle failure.
• Past experience has proven that brittle failure can occur in a highly stressed structure made of a ductile metal even though the average stresses in the metal of the structure do not exceed the yield strength of the metal.
• Brittle failures of metal structures, particularly steel structures have occurred since steel was first used as a structural material and have included failures in pressure vessels, gas transmission pipe lines, storage tanks, ships, bridges and power shovel booms. However, it was not until the Second World War that brittle fractures became recognized as a serious problem meriting systematic study.
• stress analysis has established that stresses in a structure can concentrate at the roots of notches in the structure to develop localized stress concentrations that are many times greater than the average stresses in the structure. Stress concentrations can develop at intentionally formed notches such as threads and sharp fillets. Metallic discontinuities such as internal flaws and surface scratches also often act as notches and give rise to stress concentrations.
• Metallurgical investigations of the effects of stress-concentrating notches on the behavior of metals have found that these effects cause some metals to behave in a brittle manner, whereas other metals behave in a ductile manner when such notches are present. These latter metals are referred to as being notch ductile or insensitive to notch effects or as characterized by adequate notch toughness.
• Notch toughness is the capability or capacity of a metal to yield plastical-ly under high localized stress such as might occur atthe root of a notch. By so yielding, a metal characterized by adequate notch toughness or notch ductility can relieve itself of localized stress concentrations. Thus, brittle failure due to stress concentration at notches is avoided if a metal is characterized by notch ductility. Special tests have been developed for the purpose of evaluating notch toughness since it has been found Patented June 28, 1966 that data obtained by conventional testing of smooth specimens, e.g., data such as yield strength, ultimate tensile strength, elongation and reduction of area, do not accurately portray the notch toughness characteristics of metals.
• notch tension test wherein a notched specimen is fractured in tension, has become widely accepted by metallurgists as a means of accurately evaluating notch toughness of metals in sheet form and also in other forms.
• special sharp-notch tensile test specimens having critically dimensioned sharp notches must be employed since it has been shown that the minimum critical size of a flaw that can induce brittle failure in steel decreases with an increase in the strength level of the steel.
• Such a sharp-notch tensile test specimen that is specially adapted for accurate evaluation of notch toughness of high strength steel is described in the paper of G. B. Espey, M. H. Jones and W. F.
• the notch-tensile test is used to determine notch strength and, in conjunction with the conventional tensile test, is also used to determine the notch/ tensile strength ratio.
• This strength ratio is the criterion for determining whether a metal is notch brittle or notch ductile.
• Notch strength notch tensile strength
• the notch/tensile strength ratio is the ratio of notch strength to pusti tensile strength.
• metals characterized by notch/ tensile strength ratios of at least about 1, i.e., 0.95 or greater, are deemed notch ductle, whereas metals characterized by lower notch-tensile strength ratios are deemed notch brittle or notch sensitive.
• the notch toughness characteristics that thereby are determined are referred to as sharp-notch strength, sharpnotch/tensile strength ratio, sharp-notch ductility, etc.
• the sharp-notch tension test provides, in conjunction with the conventional tension test, a direct indication of the ability of a metal to eliminate high stress concentrations by plastic flow.
• A-ustenitic nickel-chromium stainless steels have been found useful and satisfactory for a wide variety of structures wherein corrosion resistance and relatively low, or moderate, levels of strength are required.
• austenitic is used to designate a class of steels on the 3 basis of the behavior of the steels.
• Austenitic steel is defined in the ASM Metals Handbook, 8th edition, page 3, as an alloy steel whose structure is normally austenitic at room temperature.
• Austenitic nickel-chromium stainless steels ordinarily contain about 16% to about 26% chromium, about 6% to about 22% nickel, about 0.5% to about 2% manganese, about 0.5% to about 1% silicon and small amounts of carbon, e.g., 0.1% carbon.
• austenitic stainless steel is so proportioned that the steel is austenitically structured, i.e., at least a major proportion of the steel in austenite, when the steel is at room temperature and in the annealed condition.
• Some compositions of austenite are described as metastable to denote the fact that such austenite is not a phase in a physico-chemical sense because it does not maintain its identity and characteristics under all conditions of heat or mechanical treatment but instead can become transformed into martensitic ferrite (martensite).
• the metastability of austenite is relative in the sense that the chemical composition of austenite effects its transformability by thermal and/ or mechanical methods.
• Stainless steels containing metastable austenite of such a chemical composition that this austenite is stable at room temperature and can be transformed by some practical method, such as cold working at a temperature not lower than the boiling point of liquid hydrogen, are described as transformable austenite stainless steels.
• ENI equivalent nickel index
• ENI Percent Ni+0.68 (percent Cr) +0.55 (percent Mn)+0.45 (percent Si)-[-27 (percent C-l-percent N) wherein the numerical factors are the empirical values of nickel equivalents determined by G. H. Eichelman and F. C. Hull and reported in Transactions of American Society for Metals, vol. 45, page 95, and wherein percent Ni, percent Cr, percent Mn, percent Si, percent C and percent N are the weight percentages of these elements in the steel.
• transformable austenitic stainless steels have an equivalent nickel index of at least about 17 and not greater than about 30.
• Transformable austenitic stainless steels are well known in the metallurgical art and this art includes a number of teachings whereby one skilled in the art can readily determine Whether a stainless steel is transformable.
• austenitic nickel-chromium stainless steel products characterized by an improved combination of high strength and sharp-notch ductility can be produced by a new and improved process whereby a special austenitic nickel-chromium stainless steel workpiece is deformed by cold working at or below room temperature including temperatures below zero degrees Fahrenheit.
• the invention also contemplates providing a new and improved austenitic nickel-chromium stainless steel product characterized by an improved combination of high strength and sharp-notch ductility.
• the present invention contemplates the production of a sharp-notch ductile, nickel-chromium stainless steel product by a new process comprising the steps of providing a workpiece of a special transformable austenitic, low-silicon, nickel chromium stainless steel, the structure of said workpiece comprising a major proportion of austenite, cold working the workpiece at or below room temperature such as by subzero working the workpiece to thereby produce a martensitic structure therein and thereafter heat treating the cold worked workpiece for about 1 hour to about 48 hours at about 700 F. to about 850 F.
• the special low-silicon stainless steel of the workpiece contains, in weight percent, not more than about 0.15% silicon, about 4% to about 12% nickel and about 15% to about 22% chromium and is a transformable austenitic steel that is normally austenitically structured when at room temperature or when at subzero working temperatures for short periods of time, e.g., up to about 1 hour, and can be transformed to be martensitically structured by cold Working at room or subzero temperatures.
• Metalworking operations which can be performed to cold work (plastically deform) the special workpiece to produce new and improved stainless steel products in accordance with the invention include rolling, forging, stretching, drawing, spinning, bending, swa'ging, hydroforming, explosive forming and roll forming.
• Stainless steel products produced in accordance with the inven tion have improved strength and hardness and improved notch toughness characteristics, including sharp-notch ductility and notch impact strength and are also characterized by the corrosion resistance of austenitic stainless steel in normal conditions.
• the present invention incorporates the discovery made by us that in processes of subzero working or cold working at about room temperature and subsequently heat treating transformable austenitic nickel-chromium stainless steel workpieces to produce stainless steel products of improved strength and hardness, it is advantageous for the purpose of obtaining sharpnotch ductility, especially in high strength stainless steel products, that the stainless steel workpieces be characterized by containing not more than 0.15 silicon, i.e., that the workpieces be of low-silicon stainless steel.
• a transformable austenitic stainless steel workpiece which contains at least about 90% austenite, which has an equivalent nickel index of not less than about 19 and not more than about 25 and which contains not more than about 0.15% silicon, about 6% to about 10% nickel, about 16% to about 21% chromium, about 0.01% to about 1% manganese, about 0.01% to about 0.1% carbon, about 0.005% to about 0.1% nitrogen, with the balance essentially iron.
• the new low-silicon stainless steel cold worked at room temperature e.g., 40 F. to 100 F and/ or slightly below room temperature, i.e., temperatures as low as zero degrees Fahrenheit F.
• room temperature e.g. 40 F. to 100 F and/ or slightly below room temperature, i.e., temperatures as low as zero degrees Fahrenheit F.
• the new low-silicon stainless steel cold worked at room temperature e.g. 40 F. to 100 F and/ or slightly below room temperature, i.e., temperatures as low as zero degrees Fahrenheit F.
• These steels contain not more than 0.15% silicon, about 4% to about 10% nickel, e.g., 5% to nickel, and about to about 19% chromium with the total percentage of nickel plus chromium being not greater than 25.2% and advantageously being within the range of from 22% to 25%, about 0.01% to about 1% manganese, 0.01% to about 0.1% carbon, about 0.005% to about 0.1% nitrogen with the balance iron.
• the stainless steels rolled at room temperature contain 5% to 7% or 8% nickel, about 16%; to 19% chromium and have an equivalent nickel index of 21.1 to about 22, more advantageously 21.1 to 21.6, and the nickel plus chromium content is about 23 to 25.
• the carbon content be 0.04% to 0.10% and it is essential that the steel contain at least 0.01% carbon.
• balance of the stainless steel compositions that are satisfactory for the workpiece of the invention are characterized herein as being essentially iron, it is to be under-stood 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 1% columbium, up to about 0.5% titanium, 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.
• Colurnbium or titanium can serve the purpose of carbide stabilization and, for this purpose, columbium should be present in amounts equal to about ten times the carbon content or titanium should be present in amounts equal to about five times the carbon content of the steel.
• Titanium, aluminum, calcium, magnesium, zirconium and boron can serve purposes of deoxidation, malleabilization and/or purification.
• the balance of the stainless steels used in practicing the invention may also contain very small amounts of impurities such as sulfur, phosphorus, bismuth, antimony, tin, lead, arsenic, 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 aforedescribed workpiece in advantageously subzero Worked at a deformation temperature, i.e., the temperature of the workpiece at the time when deformation is commenced, not higher than about minus 40 F. and is thereafter heat treated as described hereinbefore.
• a deformation temperature i.e., the temperature of the workpiece at the time when deformation is commenced, not higher than about minus 40 F. and is thereafter heat treated as described hereinbefore.
• Sufiicient deformation is accomplished to transform austenite of the workpiece to martensite and to provide that the product comprise at least about 60% but not more than about 99% martensite, advantageously to about 99% martensite.
• Amounts of deformation that are equivalent to reductions in thickness of about 20% to about 50% of the original thickness are sufiicient for producing high strength products.
• the optimum amount of deformation required for developing high yield strength is a function of the nature of the deforming operation, the temperature of the metal at the start of deformation and the equivalent nickel index of the composition of the workpiece. In processes of the invention wherein deformation temperatures are in the range of about minus 40 F. to about minus 320 F.
• deformation is accomplished by rolling a workpiece of the aforedescribed composition characterized by an equivalent nickel index in the range of about 20 to about 25, sufficient amounts of rolling are those which reduce the thickness by at least about 20%.
• the deformation temperature is about minus 106 F.
• a satisfactory amount of deformation for producing high strength stainless steel products of the invention is accomplished by rolling to reduce the thickness of the workpiece by about 40%.
• the workpiece When cold working is performed at near room temperature or a little lower, the workpiece, which is in an austenitic condition, is deformed an amount equivalent to a reduction in thickness of about 20% to about 50% and the deformation temperature is controlled, if necessary, by cooling the sheets between passes to obtain a deformation temperature in the range from 0 F. to about 100 F.
• the product After cold working is completed, the product comprises at least about 60% but not more than about 99% martensite.
• the summation of the nickel plus chromium contents advantageously does not exceed a level of 25% 'In carrying the invention into 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 necessary for attaining the objects of the invention.
• the stain-less steel products of the invention are produced commercially, the workpieces will usually be hot worked and sometimes cold worked before being worked at subzero temperatures.
• workpieces which have been hot and/or cold worked are annealed before being cold worked at room temperature or subzero worked.
• 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. to about 2050 F. and thereafter air cooling to room temperature.
• Annealing serves the purpose of austeni tizing and softening the workpiece.
• the workpieces are cooled prior to subzero working by any method that lowers the temperature of the workpieces to the deformation temperature of the process without transforming more than a small amount of austenite.
• workpieces that are not more than 1 inch thick can be satisfactorily cooled to about minus 106 F. by immersion in a bath of Dry Ice and isopentane for about one-half hour, or such workpieces can be satisfactorily cooled to about minus 320 F.
• the workpieces should consist entirely of austenite at the beginning of su bzero deformation, although the presence of small amounts, e.g., about 5%, of delta fernte in the workpieces at this stage of the process are not highly detrimental to the characteristics of the finished product and satisfactory results can be produced even with workpieces comprising as little as 75% austenite.
• Maximum strength and hardness in the Subzero rolled and heat treated product of the invention is achieved by producing a product comprising a very high proportion of martensite, e.g., 75 or 95% martensite.
• nickel-chromium stainless steel products characterized by yield strengths of at least 250,000 p.s.i. and sharp-notch tensile strength ratios of at least 0.95 are achieved in accordance with the invention by employing workpieces of chemical compositions characterized by an equivalent nickel index of about 21 to about 23 and containing about 0.01% to about 0.15% silicon, about 7% to about 8.5% nickel, about 18% to about 19% chromium, about 0.1% to about 1% manganese, about 0.04% to about 0.08% carbon, about 0.01% to about 0.04% nitrogen with the balance essentially iron.
• a satisfactory subzero working operation for producing such a product is to roll the workpiece at a deformation temperature in the range of about minus 100 F. to about minus 150 F., e.g., about minus 106 F., to reduce the thickness thereof by about 30% to about 50%, e.g., about 40%.
• the product is heat treated for about 4 hours to about 48 hours, e.g., 24 hours, at a temperature in the range of about 750 F. to about 850 F., e.g., 800 F.
• Alloys 1 through 8 are stainless steels of the special compositions contemplated by the invention.
• the compositions of Alloys A through D are not in accordance with the invention; instead, these compositions have silicon contents which are representative of the typical silicon contents of stainless steels usually sold in commerce.
• the description of the compositions, processing and results of testing of Alloys A through D are included herein to illustrate the inferior sharp-notch toughness qualities of stainless steel products produced by using stainless steel workpieces that are outside the invention.
• Ingots of the aforementioned Alloys 1 through 8 and A through D were hot forged and hot rolled to one-quarter inch thick plates and then cold rolled to sheets about 0.110 inch thick. These sheets were annealed by heat treating for one hour at 1950 F. and air cooling to room temperature in order to insure that the sheets consisted predominantly of austenite and to eliminate hardening resulting from prior cold work. The annealed sheets Were finish machined to 0.100 inch thickness. This machining operation was simply for the purpose of providing that all of the sheets be equal in thickness at the start of the subzero Working operation and did not afiect the metallurgical characteristics of the finished products.
• the machined sheets were vsubzero worked by rolling the sheets at a deformation temperature of about minus 106 F.
• the temperature of the rolls was about room temperature (65 F. to 75 F.).
• the sheets were cooled to a temperature of about minus 106 F. in a bath of Dry Ice and isopentane.
• Each sheet was removed from the bath, immediately thereafter passed once through a pair of rolls, and then returned to the bath and cooled again to minus 106 F.
• the sheets were rolled a sufiicient number of passes to reduce the thickness of the sheets to 0.060 inch (40% reduction in thickness).
• the reduction in thickness per pass was generally 0.005 to 0.008 inch except for that of the final pass, which was usually a lesser reduction in order to produce the desired final thickness of 0.060 inch.
• both smooth (unnotched) and sharp-notched sheet tensile specimens were machined from the sheets. Longitudinal axes of the specimens were in alignment with the direction in which the sheets were subzero rolled. The dimensions of these sheet tensile specimens were the dimensions shown in FIGURE 1 of the aforeidentified paper of G. B. Espey, M. H. Jones and W. F. Brown.
• 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 radii not greater than 0.001 inch.
• Ratio, SN TS/TS Ratio of sharp-notch tensile strength to ultimate tensile strength.
• Table IV Chemical compositions of low-silicon stainless steel products produced by cold working at room temperature and heat treating in accordance with the invention are set forth in Table IV, wherein the compositions of Alloys Nos. 9 through 13 are in accordance with the invention. Also, for purposes of further illustrating advantageous and novel features of the invention, Table IV shows chemical compositions of two alloys, Alloys E and F, which are unsatisfactory for cold working at room temperatures. Thus, Alloy E has a nickel plus chromium content of about 26.1, which is too high, and Alloy F has an equivalent nickel index of only 19.4, which is too low.
• Ratio, SNTSITS Ratio of sharp-notch tensile trength to ultimate tensile trength.
• the process of the invention provides sharp-notch ductile stainless steel products characterized by the corrosion resistance of austenitic nickel-chromium stainless steel. Further, such products of the invention can be produced as products characterized by high yield strengths of at least about 230,000 p.s.i. and even as high as at least about 250,000 p.s.i.
• 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. Products of the invention are useful for making highly stressed structures and articles for use in corrosive environments. Such structures and articles include pipes, couplings, pressure vessels, beer barrels, wheel spokes, hydrofoils, bolts, rivets and screws.
• Processes and products of the invention are also useful for making hard, corrosion resisting articles including knives, surgical instruments, dental tools, saws and chisels. Since the stainless steel products of the invention are sharp-notch ductile at subzero temperatures as low as minus 320 F. or lower, the process of the invention is particularly applicable to the production Table IV Composition in Weight Percent Alloy N0.
• the products of the invention are particularly useful for making rocket motor cases that are to be subjected -to subzero temperatures and are also useful for making articles, structures and apparatus for producing, handling, storing and transporting liquefied gases, e.g., liquid propane, liquid oxygen or liquid nitrogen, including metal flanks, storage tanks, tanks for ships, railroad cars, trucks, airplanes, rockets and spacecraft, pumps, condensers, piping and tubing.
• liquefied gases e.g., liquid propane, liquid oxygen or liquid nitrogen, including metal flanks, storage tanks, tanks for ships, railroad cars, trucks, airplanes, rockets and spacecraft, pumps, condensers, piping and tubing.
• the process of the invention is applicable in the production of stainless steel articles by room temperature working down through subzero working operations including rolling, forging, drawing, extruding, spinning, stretching, hydroforming, roll forming, upsetting, swaging, peening and explosive loading.
• the invention is also applicable to the production of welded structures and articles whereby a welded stainless steel workpiece including weld metal of the special low-silicon transformable austenitic nickel-chromium stainless steel of the invention is deformed at or below room temperature including subzero temperatures.
• an article such as a tank or barrel can be produced by expanding a welded workpiece of low-silicon stainless steel in accordance with the invention in a closed die while the workpiece is at a subzero temperature by internal hydraulic pressure using a very cold liquid such as liquid nitrogen.
• the present invention provides a new process for making stainless steel products characterized by sharp-notch ductility and high strength.
• the invention provides a new stainless steel product characterized by sharp-notch ductility and high strength at room temperature and at subzero temperatures.
• the product of the invention is also charact-erized by a very high ratio of strength to density (the Shapiro index).
• a process for producing a stainless steel product characterized by improved notch toughness characteristics including sharp-notch ductility comprising the steps of providing a low-silicon transformable austenitic nickelchromium stainless steel workpiece consisting essentially of not more than about 0.15% silicon, about 4% to about 12% nickel, about 15% to about 22% chromium, about 0.1% to about 1% manganese, about 0.01% to about 0.1% carbon, about 0.005 to about 0.1% nitrogen, up to about 0.2% aluminum, up to about 0.1% each of calcium, magnesium and zirconium, up to about 0.5% titanium, up to about 1% columbium, up to about 0.01% boron, with the balance essentially iron and also characterized by an equivalent nickel index (ENI) of at least about 17 but not greater than about 30 as computed by the formula ENI: percent Ni 0.68 (percent Cr) 0.55 (percent Mn) +0.45 (percent Si) +27 (percent C+percent N) the structure of said workpiece comprising at least 75% auste
• ENI equivalent nickel index
• a wrought, sharp-notch ductile stainless steel product having a microstructure consisting essentially of martensite and austenite with about 85% to about 99% of the structure being martensite, made of an alloy consisting of about 0.01% to about 0.15 silicon, about 7% to about 8.5% nickel, about r18% to about 19% chromium, about 0.01% to about 1% manganese, about 0.04% to about 0.08% carbon, about 0.01% to about 0.04% nitrogen with the balance essentially iron, characterized by an equivalent nickel index (ENI) of at least about 21 but not greater than about 23 as computed by the formula ENI: percent Ni+0.68 (percent Cr) +0.55 (per-cent Mn) +0.45 (percent Si) +27 (percent C+percent N) and also characterized by a sharp-notch/tensile strength ratio of at least 0.95 and a yield strength of at least about 25 0,000 pounds per square inch.
• ENI equivalent nickel index

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• Heat Treatment Of Steel (AREA)

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Cited By (15)

• 1964
  • 1964-07-27 US US385459A patent/US3258370A/en not_active Expired - Lifetime
• 1965
  • 1965-07-14 GB GB29879/65A patent/GB1097527A/en not_active Expired
  • 1965-07-21 AT AT669265A patent/AT259607B/de active

Non-Patent Citations (1)

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