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 stainless steels and, more particularly, to a low cost, martensitic stainless steel amenable to ease of processing and characterized by a combination of properties, including yield strength and toughness, such that the steels can be used for common constructional as wellv as for other purposes.
• the austenitic, martensitic and ferritic the former, as exemplified by the AISI 300 series, has found much the greater commercial acceptance and usage, a fact attributable to the outstanding corrosion resistance and fabricability characteristics thereof.
• the yield strengths (0.2% offset) of the austenitic type stainless steels are relatively low, being of the order of 35,000 or 40,000 p.s.i. in the non-hardened condition.
• These steels practically speaking, are non-responsive to thermal treatment in the sense of increasing their strength or hardness. To be sure, the strength thereof can be substantially increased via the application of well known plastic deformation processing, such as cold rolling at room or lower temperatures, and yield strengths up to above 200,000 p.s.i.
• the austenitic stainless steels differ from the austenitic (and also the ferritic) in that they undergo strengthening and hardening upon being subjected to heat treatment.
• Many of the martensitic stainless steels can be thermally hardened to yield strengths well above 200,000 p.s.i., e.g., AISI 440A, but the toughness characteristics are very poor, a factor which has well contributed to the commercial development of the ultra high strength, precipitation hardenable stainless steels.
• A181 440A (or 440B or 440C) render such steels more than diificultly weldable. It perhaps should be mentioned that strength levels of 200,000 p.s.i., can be obtained in various ways in stainless steels but the overall product cost would be of a magnitude much too high for applications herein contemplated.
• non-precipitation hardenable martensitic stainless steels which afford yield strengths of 100,000 to 150,000 p.s.i.
• such steels are subjected to a three-stage heat treatment consisting of (depending upon the particular steel) an annealing treatment at a temperature of about 1200 F. to 1600 F. followed by cooling, a hardening and strengthening treatment consisting of austenitizing over a temperature range of about 1700 F. to 1900 F. followed by a cooling operation (often a liquid quench) and finally a tempering treatment at a temperature of about 400 F. to 1000 F. followed by cooling.
• a three-stage heat treatment consisting of (depending upon the particular steel) an annealing treatment at a temperature of about 1200 F. to 1600 F. followed by cooling, a hardening and strengthening treatment consisting of austenitizing over a temperature range of about 1700 F. to 1900 F. followed by a cooling operation (often a liquid quench) and finally a tempering treatment at a temperature
• AISI 431 has a Rockwell hardness of about R 20 to 25.
• the hardness is increased to about 40 to 45 R but toughness is drastically reduced to the order of about 10 to 15 ft.-lbs. (Izod), a prime factor responsible for the application of the tempering treatment.
• Izod ft.-lbs.
• stainless steels such as A181 431 manifest an undesirably strong tendency to form delta ferrite and/or retained austenite, i.e., because of the difliculty in commercial practice in observing the extremely close tolerances necessitated by the chemistry of the steel, the rnicrostructure thereof is not consistently uniform from heat to heat and and too often is characterized by excessive amounts of austenite and/or the delta ferrite phase, the latter being conducive to embrittling.
• These objectionable processing difficulties are perhaps accountable, partially at least, for the reason that the production of A181 431 has not appreciably increased over the last twenty years.
• a particular objective of the invention is to overcome the difficulties attendant stainless steels of the A181 431 type and do so while minimizing processing operations; for example, eliminating the necessity for, inter alia, a tempering treatment.
• the present invention is particularly concerned with stainless steels in the form of plate and attention should be addressed to this aspect. It is more than common, as is well known, to determine toughness characteristics, including tensile elongation, reduction in area and Charpy V-notch energy values, rom tests conducted on steel in the form of bar or rod. Also well known is the fact that such properties are, at times, higher, often substantially so, than those obtained from tests conducted on plate, particularly those performed on transverse sections. Even as to plate, it is documented that the ability to absorb impact energy is usually significantly higher when measured in the longitudinal direction as opposed to the direction transverse to rolling. Thus, since plate is an extremely common mill form, conventionally used, for example, in welding pressure vessels, and since the characteristics thereof are inherently self-imposed, it is with advantage, particularly in designing fabricated structures, to have available experimental data indicative of the expected performance of plate.
• Another object of the invention is to provide a new and improved martensitic stainless steel characterized by a yield strength of about 100,000 p.s.i. and above together with good toughness.
• An additional object of the invention is to provide a martensitic stainless steel plate which manifests all of the aforedescribed characteristics.
• the present invention contemplates martensitic stainless steels consisting essentially of (by weight) from 12% to 16.5% chromium, from 3% to 6.5% nickel, the sum of the chrominum plus nickel not exceeding about 21.5 and advantageously not exceeding 21.25%, carbon up to 0.12%, up to 0.1% nitrogen, with the sum of the carbon and nitrogen not exceeding 0.13% and preferably not exceeding 0.12%, up to 1% and preferably not more than 0.75% manganese, up to 1% silicon, up to 0.15% aluminum, the balance being essentially iron.
• iron content as constituting the balance or essentially the balance, it is to be understood, as will be 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 and oxygen should be kept at levels as low as is consistent with good commercial steelmaking practice.
• up to 1.5% chromium can be replaced by molybdenum, the chromium not falling below 12%.
• the stainless steels herein advantageously contain about 13% to chromium, about 4% to 6% and preferably 4.5% to 6% nickel, about 0.001 to 0.05% carbon, up to 0.05 nitrogen, the sum of the carbon plus nitrogen being not more than 0.07%, up to 0.1% aluminum (e.g., 0.01% to 0.1% aluminum) to provide good deoxidation characteristics during melting, up to 0.5% and preferably not more than 0.25% silicon, up to about 0.5% manganese and the balance essentially iron and normal impurities.
• the temperature range of about 1450 F When normalized in the temperature range of about 1450 F.
• chromium confers corrosion resistance and advantageously at least 13% chromium should be present since even at a level of 12%, resistance to various corrosive media is marginal. Amounts below 12% are simply unsatisfactory. On the other hand, excessive amounts of chromium are causative of or promote the formation of delta ferrite, a weak and embrittling phase which also gives rise to hot working difficulties. Thus, while the percentage of chromium can be as high as 16.5 it preferably should not exceed about 16% and to consistently achieve an optimum combination of processing characteristics and mechanical properties, it should not exceed about 15%.
• Nickel counteracts the tendency for delta ferrite formation; however, excess nickel can lead to undesirable quantities of retained austenite on cooling from the austenitic condition. Stable or retained austenite is detrimental since various properties, notably yield strength, are adversely affected and to avoid the same, the nickel content should not exceed 6.5 and preferably is not greater than 6%. It should be mentioned, as will be appreciated by those skilled in the art, that at elevated temperatures, a completely austenitic structure is desired for good processing characteristics, particularly forgeability. However, the steels should transform at least substantially to martensite upon cooling to about room temperature and to that end the sum of the percentages of nickel and chromium should not exceed 21.5% and preferably should be less than 21.25%. Thus, a high martensitic transformation (M temperature is most desirable, a temperature on the order of above 200 F. and advantageously above 250 F.
• Carbon and nitrogen also exert a strong influence in resisting the formation of the delta ferrite phase but, as with nickel, excessive amounts thereof, though relatively small, can impair strength characteristics, generally as a result of retained austenite, as will be shown herein.
• the total sum of carbon and nitrogen should not exceed 0.07%. in this connection, the carbon content can be below 0.01% and the desired combination of properties are readily attainable, a reflection of the fact that steels within the invention do not depend upon the presence of carbon to achieve a required level of properties.
• Silicon and aluminum being ferrite formers and thus capable of promoting delta ferrite, should not exceed 1% and 0.15%, respectively. It is preferred that the silicon content be less than 0.5 and most advantageously is not greater than 0.25%.
• Aluminum (also silicon) can adversely affect toughness and preferably does not exceed 0.1% although it is beneficial as an addition during melting in an amount sufiicient to provide good deoxidation. The use of aluminum for precipitation hardening purposes, e.g., 0.5%, is quite inconsistent with the invention.
• Annealing and/or austenitizing and/or tempering treatments hitherto employed in connection with prior art martensitic stainless steels, while not excluded from 15 the scope of the invention, are not mandatory in accord- 6 Heat Treatment BHeat Treatment A followed by heating to 800 F. for one hour and then air cooling to room temperature. Heat Treatment CHeated to 1800 F., held about one hour, then quenched.
• Heat Treatment D-Heat Treatment C followed by heat-- ing to 800 F. for one hour, then air cooling to room temperature.
• cent; cent; cent cent cent e cent ment Gent 001111 1 15,88 .019 0,023 0.042 0.41 0.0 0 .04 A 17, 153.000 66 56 V C 116, 700 147, 900 15 66 88 B 127, 900 158, 400 17 66 52 D 131, 500 155. 500 19 66 48 2 1 ,30 5, 0 0,034 0,023 0.057 0.45 0.49 0.033 A 1 17 17 65 7 0 128,100 166, 800 15 07 76 B 133, 300 165, 700 19 08 77 D 135, 100 166, 00 18 68 88 3 15.02 5 7() 09 0,014 (1104 0.49 0.49 0036 A 8 0 9 55 62 C 96.
• alloying constituents were of good purity and final deoxidation just prior to pouring was accomplished with an aluminum addition of 0.1%.
• the ingots were soaked for about two hours at about 2200 F., forged at 2150 F. first on one diagonal and then on the opposite one, and,
• Table I The data in Table I (plate specimens) illustrate the advantages in maintaining the carbon content below about 0.12%, the total carbon plus nitrogen below about 0.13% and the sum of the chromium plus nickel below about 21.5%.
• Alloy No. 5 having a carbon content of 0.13% and a carbon plus nitrogen content of 0.143% was of low yield strength, the magnitude of which remained unchanged notwithstanding the utilization of a second thermal treatment as with Heat Treatments B and D.
• the low yield strength was attributable to an excessive amount of retained austenite, examination revealing the presence of about 74% austenite.
• Each of the Alloys Nos. 1 through 7 was normalized at temperatures other than 1800" F., to wit, 1500 F., 1600 F. and 1700 F. (Heat Treatments E, F and G, Table II). The data resulting therefrom are given in Table II, the data of Heat Treatment A being included for convenience.
• Alloys Nos. 13 and 14 pact energy with increasing contents of carbon, a point further illustrated by Alloys Nos. 13 and 14 at a different level of nickel.
• Alloy No. 11 exhibited a 15 ft.-lb. impact transition temperature of minus 225 F. in the transverse direction (minus 200 F.
• nickel, carbon and nitrogen are all austenite formers, it is beneficial when using a nickel percentage at the high end of the range to use an amount of carbon plus nitrogen which is relatively at the lower end of" its range. Refrigeration and/or cold working would result in an increase in yield strength of Alloy No. 23 since a greater amount of austenite would be transformed to' martensite. Reducing'the nickel and carbon plus nitrogen slightly, Alloy No. 22, would provide a higher minimum yield strength in the absence of refrigeration or other processing.
• the martensitic stainless steels of the invention are suitable for diverse use, particularly applications requiring steels having a good combination of strength and toughness, e.g., pressure vessels, suction press rolls, transportation equipment, such as truck frames, etc.
• the steels can also be used in applications requiring a reasonably high hardness, e.g., a Rockwell hardness of about R 35 to 45.
• carbon contents above 0.05% are beneficial, e.g., 0.07% to 0.12% carbon. Items of cutlery, particularly knives, would be an example where such hardness values would be desirable.
• the steels can be provided in common mill forms, including plate, bar, rod, etc., or in the form of castings. Corrosion resistance of the steels compares more than favorably with the prior art martensitic stainless steel AISI 431. Tests conducted for a period of four months in ammonium nitrate, for example, have given excellent results.
• martensite includes the low temperature transformation and decomposition products of austenite. Less than 10% and preferably less than 5% or 3% of delta ferrite should be present. The optimum is a steel devoid of delta ferrite. In addition, austenite, retained or otherwise, should be kept to a minimum, to Wit, not more than 5% and preferably less than about 3%.
• a low cost, easily processed, stainless steel in the normalized condition having a martensitic microstructure with less than 10% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch, a tensile elongation of at least about 14%, a reduction in area of at least 55% and a Charpy V-notch impact value of about at least 40 foot-pounds when in the form of plate up to a thickness of one-half inch, the plate having been normalized at a temperature of about 1500" F., said steel consisting essentially of about 13% to 16% chromium, about 4% to 6% nickel, the sum of the chromium plus nickel not exceeding 21.25%, carbon in an amount up to 0.05%, nitrogen in an amount up to 0.05%, the sum of the carbon plus nitrogen not exceeding 0.07%, up to 0.5% manganese, up to 0.5% silicon, up to 0.1% aluminum and the balance essentially iron.
• a low cost, easily processed, stainless steel in the normalized condition having a martensitic microstructure With less than 10% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch, a tensile elongation of at least about 14%, a reduction in area of at least 55% and a Charpy V-notch impact value of about at least 40 foot-pounds when in the form of plate up to a thickness of one-half inchfthe plate having been normalized at a temperature of about 1500 F., said steel consisting essentially of about 13% to 15% chromium, about 4.5% to 6% nickel, about 0.001% to 0.05% carbon, nitrogen in an amount up to 0.05 the sum of the carbon plus nitrogen not exceeding 0.07%, up to 0.5 manganese, up to about 0.25% silicon, about 0.01% to 0.1% aluminum and the balance essentially iron.
• a low cost, easily processed, stainless steel in the normalized condition having a martensitic microstructure with less than 10% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch in combination with good toughness, said steel consisting essentially of about 13% to 15% chromium, about 4% to 6% nickel, about 0.001% to 0.03% carbon, up to 0.05% nitrogen, the sum of the carbon plus nitrogen not exceeding 0.07%, up to 0.5% manganese, up to 0.25% silicon, about 0.01% to about 0.1% aluminum and the balance essentially iron.
• a stainless steel in the normalized condition having a martensitic microstructure with less than 10% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch, said steel consisting essentially of about 12% to 16.5% chromium, about 3% to 6% nickel, the sum of the chromium plus nickel not exceeding about 21.25 carbon up to about 0.12%, nitrogen in an amount up to about 0.1%, the sum of the carbon plus nitrogen not exceeding about 0.13%, up to about 1.5% molybdenum, up to about 1% manganese, up to about 1% silicon, up to about 0.15% aluminum, and the balance essentially iron.
• a stainless steel in the normalized condition having a martensitic microstructure with less than 5% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch, said steel consisting essentially of about about 13%- to 16% chromium, about 4% to 6% nickel, the sum of the chromium plus nickel not exceeding about 21.25%, carbon up to about 0.12%, nitrogen in an amount up to about 0.1%, the sum of the carbon plus nitrogen not exceeding about 0.13%, up to about 1% manganese, up to about 0.5 silicon, up to about 0.15% aluminum, and the balance essentially iron.
• An alloy as set forth in claim 5 containing 14% to 16% chromium, up to 0.25% silicon and a martensitic microstructure containing less than about 3% austenite.
• a stainless steel having a martensitic microstruct-ure with less than 5% delta ferrite and less than about 5% austenite and characterized by a yield strength of at least 100,000 pounds per square inch, said steel consisting essentially of 13% to 15% chromium, 4% to 6% nickel, carbon up to 0.12%, nitrogen in an amount up to about 0.1%, the sum of the carbon plus nitrogen not exceeding about 0.13%, up to about 1.5% molybdenum, up to about 1% manganese, up to about 1% silicon, up to about 0.15 aluminum, and the balance essentially iron.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
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• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Steel (AREA)
• Treatment Of Steel In Its Molten State (AREA)
• Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)

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• 1965
  • 1965-06-25 US US467104A patent/US3355280A/en not_active Expired - Lifetime
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