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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten

Definitions

• the present invention relates to stainless steels, and, more particularly, to precipitation hardenable stainless steels which afford a level of strength and toughness characteristics of such magnitude that the steels are eminently suitable for structural applications, e.g., pressure vessels, components for aircraft and the like.
• stainless steels have been classified, in the broadest sense, as being austenitic, martensitic or ferritic. Of the three types, the austenitic variety, particularly the AISI 300 series, enjoys the greatest degree of commercial usage. This is readily understandable in view of, inter alia, the outstanding combination of corrosion resistance and ease of fabricability inherently characteristic of these steels. Nonetheless, the yield strengths thereof are relatively low and, as a general proposition, they do not undergo any appreciable hardening upon being subjected to heat treatment whereby a strengthening effect would be induced. High yield strength, e.g., up to 200,000 p.s.i.
• AISI 400 series of martensitic stainless steels e.g., AISI 440
• AISI 440 generally manifest hardening cap-abilities upon heat treatment (quenching followed by tempering) and thus afford strength levels significantly higher than that characteristic of the non-cold rolled austenitic stainless steels.
• the Metals Handbook, 8th ed., (1961) indicates at page 414 that AISI 440 (A) exhibits a yield strength (0.2% offset) as high as 240,000 p.s.i. in the tempered (hardened) condition.
• This particular grade of martensitic stainless is of high carbon content and to achieve the effectiveness of this potent element quenching must be utilized to obtain the hard martensitic structure.
• AISI 440 (A) has a tensile ductibility of but and a reduction of area of only 20% at the 240,000 p.s.i. yield strength level. In accordance with the present invention, a reduction in area value of even 30% is considered brittle.
• the precipitation-hardening stainless steels can, for convenience, be categorized as falling within three major areas (although there are others).
• the first category are those stainless steels which are both martensitic upon cooling from the solution anneal temperature and upon cooling after aging, i.e., they undergo a substantial phase transformation from the austenitic face-centered cubic (f.c.c.) structure to the martensitic body-centered cubic (b.c.c.) structure upon cooling to about room temperature from the annealing treatment.
• the second class includes those stainless steels which are substantially austenitic upon cooling from the annealing temperature (no substantial phase transformation) but which become substantially martensitic after cooling from the aging treatment.
• a conditioning treatment a heat treatment intermediate that of the solution anneal and aging treatments
• a cold treatment e.g., refrigeration or cold working
• Precipitation-hardening stainless steels which are austenitic in both the annealed and aged conditions comprise the third category and thus do not undergo any substantial martensitic transformation.
• Common to all three types is an annealing treatment and an aging treatment, although it is to be understood that sundry complex intermediate treatments have been proposed.
• the present invention is primarily concerned with the first category of precipitation-hardening stainless steels, although the invention also encompasses the second class, as will become clearer herein.
• toughness is rather elusive and defies precise definition. As used herein, toughness connotes and includes more than tensile ductility and reduction of area values. It also includes the ability to manifest high notch tensile strength coupled with the important barometer of a high ratio of notch tensile strength to ultimate tensile strength. Experience has shown that tensile ductibility and reduction of area values arrived at from testing smooth specimens are not always an unqualified indicator as to reliability. This stems from the fact that in commercial usage structural components develop cracks which may be internally or externally induced. Too, it is more than possible that the alloy from which the component was formed contained incipient cracks, notches or other flaws.
• Notch toughness is a reflection of the ability of a material to yield by plastic flow to localized stress.
• a crack, notch or other flaw is an initiating or focal point of self-propagation. It has been established and is generally accepted that stresses tend to concentrate at such points whereby a localized stress concentration is induced. If a material is sufficiently resistant to the propagation of the flaw, i.e., if it is sufficiently self-yielding, it is considered as being notch-ductile; if not, it is deemed notch-sensitive, i.e., it is prone to the development of deleterious brittle fracture or brittle failure characteristics.
• brittle failure problem occurs in many materials notwithstanding that the yield strength, tensile ductility and reduction of area of smooth specimens of the material are otherwise acceptable.
• the propagation of the flaw leading to brittle fracture can be induced by a number of factors, including the heat treatment applied to the material.
• the strength of the material It is known that with an increasing magnitude of yield and tensile strengths the smaller becomes the minimum size of a flaw which can cause or promote subsequent brittle fracture. Thus, even relatively small fiaws must be taken into consideration.
• the steels contemplated must manifest a ratio of notch-tensile strength to ultimate tensile strength of at least unity (the notch acuity factor, K being or greater) to be classed as being notch-ductile.
• the ratio is at least 1.2.
• Another object of the invention is to provide precipitation-hardening stainless steels which exhibit an improved combination of corrosion resistance, fabrication quality, high yield strength and good toughness.
• precipitation-hardening stainless steels contemplated herein consist essentially (by weight) of from 11% to 13% chromium, about 9% to about 11% nickel, about 1.5% to 3% molybdenum with the proviso that the sum of 0.8 times the chromium content plus the nickel and molybdenum contents is at least about 20%, e.g., 20.5% and does not exceed about 23%, e.g., 22.75%, at least one metal selected from the group consisting of titanium and columbium, the titanium being from about 0.1% to not more than 0.5% and the columbium being from 0.05% to 1% and preferably from 0.1% to 0.5%, about 1% to 1.6% aluminum with the proviso that the sum of the aluminum plus any titanium does not exceed about 1.9% and with the further proviso that the ratio of nickel to the sum of the aluminum plus any titanium is at least 5 to 1, carbon in an amount up to about 0.03%, e.g., about 0.005% to
• iron content as constituting the balance or essentially the balance of the stainless steels, it is to be understood, as will be readily appreciated by those skilled in the art, that the presence of other elements is not excluded, such as those commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities ordinarily associated therewith in small amounts which do not adversely affect the basic characteristics of the steels.
• elements such as sulfur, phosphorus, hydrogen, oxygen, nitrogen and the like should be kept at levels as low as is consistent with commercial steelmaking practice.
• Other elements which can be present in the steels include the following: up to 0.5% vanadium, up to 1% tantalum, up to 0.5% copper, up to 0.1% beryllium, up to 0.01% boron and up to 0.05 Zirconium.
• cobalt is not essential and can be kept to impurity levels.
• up to 0.1% of elements, such as calcium, cerium, etc. can be present, e.g., 0.02% or 0.03% calcium or 0.05% or 0.06% cerium.
• the steels be of a composition falling within the following ranges: about 11.5% to 12.75% or 12.5% chromium, from 9% to 10.75% nickel, e.g., 9.75% to 10.5%, about 1.75% to 2.5% molybdenum, with the proviso that the sum of 0.8 times the chromium content plus the nickel and molybdenum contents be from 20.5% to 22%, at least one element selected from the group consisting of about 0.2% to about 0.35% titanium and about 0.2% to about 0.5% columbium, about 1.1% to about 1.5% aluminum with the proviso that the sum of the aluminum plus any titanium does not exceed about 1.8%, up to 0.03% carbon, up to 0.03% carbon, up to
• the chromium content of the steels should not fall below about 11% and preferably should be at least 11.5 otherwise, the basic purpose of maintaining a high degree of corrosion resistance for which the stainless steels are noted would be seriously impaired, notwithstanding the benefits conferred by molybdenum in certain specific corrosive environments.
• chromium contents much above the maximum specified herein the property characteristics of the steels are adversely affected and/or unnecessary heat treatments might be otherwise necessary, e.g., intermediate condi tioning treatments.
• conditioning treatments might well be necessary prior to the final aging treatment and subsequent to the solution annealing treatment in order to efiect a full transformation from the face-centered cubic structure to a body-centered cubic structure. That is to say, after solution annealing at a temperature of about 1600 F. to 1800 IE, it would be necessary to heat treat the steels at some temperature below that of the initial solution anneal temperature, e.g., at about 1100 F. to 1350 F., to precondition the steels such that a transformation to martensite will eventually take place.
• the amount of nickel should be at least 9% in order to obtain a combination of good strength and toughness. Excessive amounts of nickel promote the retention of austenite on aging and also undesirably narrow the range of aging temperature both from the view of retained austenite and overaging. Thus, it is advantageous that the nickel content not exceed 11%. Further, as indicated above herein, the ratio of nickel to the sum of aluminum plus titanium must be at least 5 to 1 to insure good toughness characteristics.
• Aluminum is particularly responsible for the precipitation-hardening effect. If excessive amounts of aluminum are present, e.g., 2% to 3%, notch toughness is adversely afiected and other toughness characteristics can be detrimentally impaired. On the other hand, with aluminum contents below about 1% the highest yield strengths are not obtained. Of course, yield strength levels of 150,000 p.s.i. to 200,000 p.s.i. are quite acceptable for sundry application. Where such yield strengths would be satisfactory, aluminum contents of 0.5% to 1%, e.g., 0.6% to 0.9%, could be utilized.
• Carbon should be maintained at the lowest possible levels. High amounts thereof, apart from promoting intergranular corrosion, drastically reduce the M M transformation range and impair toughness characteristics.
• the formation ofchromium carbides might effect an increase in the M temperature but it is the undesirable precipitation of chromium carbides during heat treatment which can impair toughness.
• opposed reactions would be involved. That is to say, with relatively high carbon contents the carbon would react in both a tempering and hardening manner. Unstabilized carbon would, per se, confer a hardening reaction upon cooling from the solution anneal treatment and this hardening reaction would be tempered (lower strength and softer material) upon aging during the precipitation hardening treatment.
• the carbon content should be kept at a low order of magnitude and preferably not above 0.02%. It should be mentioned that carbon essentially has no substantive relation with regard to such elements as nickel and manganese of the steels contemplated herein. Carbon cannot (nor, for that matter, can manganese) be used to replace nickel; carbon simply must be maintained at a low level.
• silicon and manganese should also be kept to a minimum; otherwise, toughness can be adversely affected.
• silicon and manganese levels of, say, 0.5 or above detrimentally affect the notch ductility of the steels. It is most advantageous that the total amount of these elements does not exceed 0.25%. It is most preferred to keep these elements at a level of not more than about 0.1%, respectively; however, this is difficult to consistently achieve commercially because of pickup of these elements from raw materials, slags, refractories, etc.
• Molybdenum confers additional corrosion resistance in certain specific environments, particularly chloridecontaining environments, e.g., sea water, hydrochloric acid, etc. Molybdenum also affords improved resistance to such acids as sulfuric and phosphoric. With amounts of molybdenum below about 1.5% the resistance to such corrosive environments is lessened. On the other hand, amounts of molybdenum much above 3% promote the retention of austenite. In this connection, the amount of molybdenum must be specially controlled in correlation with the other constituents of the alloys to thereby obviate the necessity of employing conditioning treatments prior to the final aging treatment. Accordingly, it is advantageous that the molybdenum content not exceed about 2.5%.
• columbium is beneficial, partciularly where optimum corrosion resistance and strength combined with optimum toughness characteristics are necessary. While the columbium can be present up to 1%, it is preferred that not more than 0.5 columbium be present. This constituent will preferentially combine with carbon and preclude the precipitation of detrimental chromium carbides in the grain boundaries during aging, thereby enhancing toughness and corrosion characteristics.
• the amount of titanium should not exceed 0.5 and advantageously does not exceed 0.35%; otherwise, segregation and other problems arise in processing the steels.
• air melting may be utilized, preferably followed by vacuum, consumable electrode melting for optimum effects.
• Use of materials of good purity is advantageous.
• the initially formed cast ingots should be thoroughly homogenized as by soaking at temperatures of about 2200 F. to about 2250 F. for about 1 hour per inch of cross section followed by hot working (forging, pressing, rolling, etc.) and, if desired, cold working to desired shape.
• a plurality of heating and hot working operations are desirable for purposes of assuring thorough homogenization of the cast structure through diffusion. Satisfactory hot working temperatures include 1800 F. to 2000 F. with suitable finishing temperatures being about 1600" F. to 1500 F.
• the steels are solution annealed at temperatures of about 160 F. to about 1800 F. for about 4 hour to several hours, depending upon section size. In the production of sheet or strip, short annealing periods, e.g., 10 minutes, can be employed.
• the steels are cooled, e.g., air cooled. No liquid quench is necessary and thus the attendant difiiculties associated therewith are obviated.
• the steels transform to the martensitic or substantially martensitic condition or can be readily transformed thereto by subjecting the steels to a cold treatment as by refrigerating the steels at a low temperature, e.g., F. or below, or by cold working the steels. Both refrigerating and cold working can be utilized if desired.
• an attribute of the steels of the invention having a combined chromium plus nickel plus molybdenum content of not more than about 23.5%, and particularly not more than about 23%, is that refrigeration and/or cold working is unnecessary. This aspect minimizes cost and processing difliculties.
• the steels are quite ductile and are characterized by a Rockwell C hardness of above about R 25 and up to about R 35. Thus, the steels can be fabricated to shape before aging.
• the steels are then aged in the martensitic condition to effect the high levels of strength.
• the aging treatment comprises heating the steels to a temperature of about 800 F. to 1000 F. for about A to 4 hours, the longer aging periods being used in conjunction with the lower aging temperatures. Aging at 900 F. to 950 F. for about 1 to 4 hours has been found very satisfactory. Temperatures on the order of above about 1050 F. or 1100 F. or higher should not be utilized since reversion to austen- 7 ite and/ or overaging can occur with adverse consequential effects, e.g., loss of strength. Further, in accordance with the invention the Rockwell C hardness levels of the steels preferably should not be more than about R 51; otherwise, the toughness characteristics thereof are impaired.
• the determined Rockwell C hardness values are given in Table II, as ascertained from cooling after the solution treatment, the refrigerated condition where used and after cooling from the aging treatment.
• the cast ingots 4 by 4 inches, were soaked at about 2250 F. to eifect good homogenization, forged to billets 2 by 2 inches square and thereafter reheated to 1800 F. and hot rolled to rods about inch in diameter and about 12 to 15 feet long.
• the alloys were then machined to tensile specimens having a reduced section of about 0.25 inch diameter.
• the notch specimens had a V-notch of about 60.
• the radius of the notch was 0.0005 inch or less and the notch acuity factor, K was 10 or greater. Notch acuity factors of less than 10 are not considered adequate to provide sufiiciently severe test conditions.
• the yield strength (Y.S., 0.2% offset) is given in thousands of pounds per square inch (K.S.I.) as are the ultimate tensile (U.T.S.) and notch tensile strengths (N.T.S.).
• Tensile elongation (EL) is given in percent using a standard gage length of 4 times the diameter of the specimen and the reduction in area (R.A.) is also given in percent
• the designation N.T.S./U.T.S. is the ratio of the notch tensile to ultimate tensile strength. None of the steels was cold worked prior to or after aging. Cold working would enhance the property characteristics of the steels, but would interfere with the attaining of a good analysis of results as a reflection of composition and heat treatment.
• Alloy A was, as a practical matter, completely austenitic upon cooling from the solution treatment as evident from the very low Rockwell hardness level of R 6 set for in Table II. In addition, this alloy had a high amount of retained austenite after cooling from the aging treatment. Concerning Alloys D and E, the yield strength of these alloys was good but the ratio of the notch tensile strength to ultimate tensile strength for these alloys was below unity; thus these steels lacked sulficient toughness. Alloys F and G contained too much nickel and, as mentioned above, the yield strengths did not satisfy the high yield strength minimum of 200,000 p.s.i.
• Alloys 1 to 4 formulated in accordance herewith all manifested a satisfactory level of properties.
• Alloys 3 and 4 which nominally contained an amount of chromium plus nickel plus molybdenum of 23% illustrate that a high level of properties can be obtained without any cold treatment, e.g., refrigeration and/or cold working, whatever.
• This aspect is quite beneficial where it is necessary to form large vessels. To form such vessels, a welding treatment would be necessary and this in turn would require a subsequent solution and aging treatment to restore the properties of the parent metal contiguous to the weld zone. Obviously, it would be quite desirable, if possible, to avoid subjecting such vessels to a refrigeration treatment prior to aging in view of the impracticality of so doing.
• condition treatments are not only unnecessary but utilization of refrigeration techniques can be avoided where desired.
• a comparison of Alloy 1 with Alloys 2, 3 and 4 further shows the benefit of maintaining the sum of 0.8 times the chromium content plus the nickel and molybdenum contents at an amount not greater than about 22%.
• aluminum can be used in the steels as referred to above herein.
• Such steels manifest a unique capability of absorbing extremely high levels of impact energy. This is illustrated by the alloys in Tables IV (composition) and V (test results), the alloys having been prepared and tested, except for heat treatment in the same manner as the alloys of Table I.
• the heat treatment consisted of solution treating at 1800 F. for 1 hour followed by air cooling and then aging at either 900 F. for 4 hours (Heat Treatment D) or at 1000 F. for 2 hours (Heat Treatment E).
• the steels were neither refrigerated nor cold Worked.
• a precipitation hardenable stainless steel characterized by a yield strength (0.2% offset) of at least about 215,000 p.s.i., a tensile ductility of at least about 10%, a reduction in area of at least about 45% and a ratio of notch tensile strength to ultimate tensile strength of at least unity, said steel consisting essentially of a composition falling within the following ranges: about 11.5% to about 12.75% chromium, about 9.75% to 10.75% nickel, about 1.75% to 2.5% molybdenum with the proviso that the sum of 0.8 times the chromium content plus the nickel and molybdenum contents is at least 20.5% and not greater than 22%, at least one metal selected from the group consisting of titanium and columbium, the titanium being from 0.2% to 0.35% and the columbium being from about 0.1% to about 0.5%, about 1.1% to about 1.5% aluminum with the proviso that the sum of the aluminum and any co-present titanium does not exceed 1.8%, carbon in an amount
• a precipitation hardenable stainless steel characterized by a good combination of strength and toughness and consisting essentially of about 11% to 13% chromium, about 9% to about 11% nickel, about 1.5% to 3% molybdenum with the proviso that the sum of 0.8 times the chromium content plus the nickel and molybdenum content is at least about 20% and not more than about 23%, at least one metal selected from the group consisting of titanium and columbium, the titanium being from about 0.1% to not more than 0.5% and the columbium being from 0.1% to 1%, about 1% to 1.6% aluminum with the proviso that the sum of the aluminum and any copresent titanium does not exceed 1.9% and with the further proviso that the ratio of nickel to the sum of aluminum and any co-present titanium is at least 5 to 1, carbon in an amount up to 0.03%, up to 0.2% manganese, up to 0.2% silicon, up to 0.5% vanadium, up to 1% tantalum, up to 0.5 copper, up to 0.1% beryllium, up
• a precipitation hardenable stainless steel characterized by a good combination of strength and the ability to absorb high levels of impact energy, said steel consisting essentially of 11% to 13% chromium, from 9% to 11% nickel, about 1.5% to 3% molybdenum with the proviso that the sum of 0.8 times the chromium content plus the nickel and molybdenum contents is at least 20% and not more than 23%, at least one element selected from the group consisting of titanium and columbium, with the titanium content being about 0.1% to 0.5% and the columbium content being about 0.1% to about 1%, about 0.5 to 1% aluminum with the proviso that the sum of the aluminum plus any co-present titanium being not greater than 1.4%, carbon in an amount up to about 0.03%, up to 0.15% manganese, up to 0.15% silicon, and the balance essentially iron.
• a precipitation hardenable stainless steel characterized by a good combination of strength and toughness and consisting essentially of about 11% to 13% chromium, about 9% to about 11% nickel, about 1.5% to 3% molybdenum with the proviso that the sum of 0.8 times the chromium content plus the nickel and molybdenum contents is at least about 20% and not more than about 23%, at least one metal selected from the group consisting of titanium and columbium, the titanium being from about 0.1% to not more than 0.5% and the columbium being from 0.1% to 1%, about 0.5% to 1.6% aluminum with the proviso that the sum of the aluminum and any co-present titanium does not exceed 1.9% and with the further proviso that the ratio of nickel to the sum of aluminum and any co-present titanium is at least 5 to 1, carbon in an amount up to 0.03% up to 0.2% manganese, up to 0.2% silicon, up to 0.5% vanadium, up to 1% tantalum, up to 0.5% copper, up to 0.1% beryllium, up
• An alloy in accordance with claim 9 containing about 0.2% to about 0.5% columbium.
• An alloy in accordance with claim 6 containing at least 0.2% titanium.
• An alloy in accordance with claim 11 containing about 0.2% to about 0.5% columbium and with any titanium not exceeding 0.35%.

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• 1964
  • 1964-09-23 US US398771A patent/US3342590A/en not_active Expired - Lifetime
• 1965
  • 1965-09-15 GB GB39429/65A patent/GB1046300A/en not_active Expired
  • 1965-09-22 DE DEJ29037A patent/DE1232759B/de active Pending
  • 1965-09-22 AT AT862765A patent/AT265346B/de active
  • 1965-09-22 ES ES0317692A patent/ES317692A1/es not_active Expired
  • 1965-09-23 BE BE670021D patent/BE670021A/xx unknown
  • 1965-09-23 NL NL6512391A patent/NL6512391A/xx unknown

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