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
• • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium

Definitions

• the present invention relates to steels and more par ticularly to maraging steels which manifest an exceptional combination of toughness and strength together with good corrosion resistance, including resistance to stress-corrosion cracking in marine atmospheres.
• yield strengths of up to about 300,000 pounds per square inch (p.s.i.) can be attained with the simplest of processing. This is typified by the recently developed maraging steels. But, strength per se does not represent the problem herein concerned. Indeed, there are innumerable commercial applications in which yield strengths on the order of about 150,000 p.s.i. to 200,000 p.s.i. are quite sufficient. But the point of difficulty is in obtaining an exceptional level of toughness at such strength levels Generally speaking and other factors being equal, the greater the strength afforded by a steel of given composition, whether achieved by the treatment, quenching or otherwise, the less tough it becomes.
• a steel must exhibit a yield strength (0.2% offset) on the order of about 150,000 p.s.i. to 200,000 p.s.i. Ultimate tensile strength is of relatively little significance since a designer is governed by the yield strength of a material.
• the ratio between the yield strength and ultimate tensile strength should not fall below about 0.9%; otherwise, there will be a considerable disparity between the respective strengths and this portends other difficulties.
• the Charpy V-notch toughness data should be determined on specimens taken in a direction transverse to the direction of rolling. This stems from the fact that results of tests determined on specimens taken in a direction longitudinal to the direction of rolling are often higher than results obtained on the aforementioned transverse specimens. Accordingly and under the above conditions, at a yield strength of about 150,000 p.s.i. the steels should exhibit a Charpy V-notch value of about 70 ft.-lbs. or above, at 160,000 p.s.i. 60 ft.-lbs. or more, and at 170,000 p.s.i. at least 50 ft.-lbs.
• the steels Insofar as tensile ductility and reduction in area are concerned, at a yield strength of 150,000 p.s.i. the steels should afford a tensile elongation of at least 15% and preferably about 20% or higher together with a reduction in area of at least 60% While the steels, in addition to the aforediscussed mechanical properties, should afford good resistance to a multiple of corrosive environments, in accordance herewith the steels should offer appreciable resistance to stresscorrosion cracking under the commonly employed U-bend testing procedures and using ambient sea atmospheres as a corrosive medium. When so tested, steels which might possibly be considered somewhat similar to the steels of the instant invention have been found to exhibit greater susceptibility to stress-corrosion cracking.
• the low alloy carbon steels might be considered but would be found wanting in view of their lack of corrosion resistance and their strong tendencies to distort and/or warp upon being liquid quenched to develop optimum strength.
• the problems attendant the quench operation are too well documented to discuss herein.
• the stainless steels such as the austenitic type forming the AIS! 300 series
• the yield strengths thereof are exceedingly low, e.g. 35,000 p.s.i. to 40,000 p.s.i., unless they are subjected to cold working.
• T heseparticular steels do not respond to heat treatment whereby hardness and strength might otherwise be increased.
• the martensitic stainless steels as exemplified by the AISI 400 series respond to heat treatment, are quite strong, but, comparatively speaking, are virtually toughless.
• the so-termed precipitation hardenable steels which are in commercial use can be processed or otherwise treated to render a sufficient magnitude of strength but suffer from a lack of toughness.
• steels in accordance with the present invention consist essentially of, in percent by weight, about 8.75% to 11.5% chromium, about 1.4% to about 3.25% molybdenum, about 8% to about 11% nickel, the sum of the chromium, molybdenum and nickel being at least but not exceeding about 23.5% and advantageously not exceeding 23%, at least one element selected from the group consisting of aluminum and titanium in a total amount of at least 0.1% to about 0.65%, the aluminum not exceeding 0.4% and the titanium not exceeding 0.3%, carbon in an amount up to about 0.04%, up to 0.5% manganese, up to 0.5% silicon, and the balance essentially iron.
• balance or balance essentially when used to indicate the amount of iron in the steels does not exclude the presence of other elements commonly present as incidental elements e.g., deoxidizing and clansing elements, and impurities ordinarily associated therewith in small amounts which do not adversely aflfect the basic characteristics of the steel.
• Elements such as sulfur, phosphorus, hydrogen, oxygen and nitrogen and the like should be kept at low levels consistent with good commercial steelmaking practice.
• Boron and zirconium should not exceed 0.01% and 0.1%, respectively, but beneficially do not exceed 0.0015 and 0.01%, respectively, since these elements detract from toughness.
• Auxiliary elements such as beryllium, vanadium, tantalum and tungsten can be utilized and when present should not exceed the following amounts: 0.2% beryllium, 1% vanadium, 0.8% tantalum, and 1% tungsten. When two or more such auxiliary elements are used, the total should not exceed 2%. Constituents such as cobalt and copper confer no particular attribute but can be present in small amounts.
• the chemistry of the steels must be critically balanced.
• the amount of chromium should not fall below 8.75%, e.g., 9%, and preferably should be at least 9.75% in providing enhanced resistance to corrosive media.
• chromium appreciably in excess of about 11.5% considerable danger is invited that an undesired amount of austenite will be retained upon cooling from solution treatment or will be formed during age hardening, unless the sum of the chromium, nickel, and molybdenum is maintained such that it does not exceed about 23.5 or 23%; otherwise, there would be a degrading effect on yield strength.
• the sum of the chromium, nickel, and molybdenum is maintained such that it does not exceed about 23.5 or 23%; otherwise, there would be a degrading effect on yield strength.
• the chromium content can be as high as 13.5% or even up to 14.5%, but should the total chromium, nickel, and molybdenum much exceed 23.5%, e.g., 24% to about 25% or 25.5%, a cold treatment as by either refrigeration at a low temperature (say, down to minus 300 F.) or cold working or both would be necessary to eifect the transformation from austenite to martensite to the fullest extent possible. It is noteworthy to mention that heretofore it has been expressed with'regard to similar prior art steels of significantly lower chromium contents, that to raise the chromium level would appreciably lower toughness.
• the nickel content should not fall below 8% and advantageously should be at least 9.5% to achieve a high level of strength.
• unduly high amounts of nickel markedly contribute to retained austenite or reversion to austenite.
• the nickel content not exceed 10.5% and in no event should it exceed 11%. It perhaps should be noted that nickel within the prescribed ranges imparts excellent toughness characteristics at cryogenic temperatures, e.g., minus 300 F. and below.
• Molybdenum is quite beneficial apart from its function of promoting resistance to corrosion. Since one of the attributes of the subject steels is their amenability to air melting practice, it has been found that molybdenum is beneficial in combination with titanium and aluminum in tolerating the presence of constituents such as sulfur and nitrogen in amounts which might otherwise dictate the use of vacuum processing. Molybdenum contents appreciably below 1.5% result in an undesirable loss in strength and toughness, whereas amounts above 3.25% render it difficult to achieve a substantially complete martensitic structure when nickel and chromium are at the higher end of H their respective ranges. The total chromium, nickel, and
• molybdenum should not, as mentioned above herein, exceeed about 23.5% if, for example, refrigeration treatment is to be avoided; otherwise, yield strength can be markedly impaired through the formation of deleterious amounts of austenite.
• Aluminum and titanium must be specially controlled. As an illustration of this point and as is shown hereinafter, it has been found that aluminum in an amount of but 0.9% virtually completely destroyed the impact resistant characteristics (transverse direction) of a unidirectionally rolled plate of what would have been an otherwise satisfactory steel. Even a level of 0.5% to 0.6% aluminum is detrimental. Accordingly, while aluminum and/or titanium must be present, inter alia, in order to confer adequate strength and to assist in minimizing the detrimental effects otherwise induced by sulfur, nitrogen, etc., the aluminum should not exceed 0.4% nor should the titanium be in excess of 0.3% and the sum total of these constituents must not venture beyond 0.65%. An aluminum range of 0.1% to 0.35% is most advantageous, although a range of 0.05% to 0.375% is satisfactory.
• steels characterized by an optimum combination of properties contain both aluminum and titanium, the minimum sum thereof being 0.25 and the maximum being about 0.5%.
• the carbon content should be held to about 003% maximum and advantageously below about 0.02% with the respective amounts of manganese and silicon not exceeding 0.25% and preferably not above 0.1%.
• the steels should contain about to 11% chromium, about 1.5% to 2.25% molybdenum, about 9.5% to 10.5% nickel, the sum of the chromium, molybdenum and nickel not exceeding 23%, about 0.15% to 0.35% aluminum, 0.1% to 0.25% titanium, the sum of the aluminum plus titanium not exceeding 0.5%, up to 0.02% carbon, up to 0.1% manganese, up to 0.1% silicon, balance being essentially iron.
• a suitable steel contains about 11% chromium, 2% molybdenum, 10% nickel, 0.25% aluminum, 0.2% titanium and up to 0.02% carbon.
• alloy steels having compositions within the invention (Alloys Nos. 1 to 8, Table I) or outside the scope of the invention (Alloys A to D, Table I) were prepared by vacuum induction melting and the ingots obtained therefrom were hot Worked to inch thick plate, the steels being unidirectionally rolled. Thereafter the steels were solution annealed at 1500 F. for about one hour, air cooled and then aged for about three hours at 900 F. Neither a cold nor conditioning heat treatment was used.
• chrominum Upon melting a basic charge, molybdenum, nickel, iron and, after completion of a carbon boil, chrominum are added. Calcium or such other constituent can be used to effect desulfurization (although the use of calcium is not necessary in vacuum processing), with silicon or siliconmanganese being used for deoxidation. Thereafter, the aluminum and/or titanium addition is then made.
• the cast ingots should be first homogenized by soaking at temperatures in the range of about 2100 F. to 2300 F., followed by hot working and, if desired, cold working to desired shape (this cold working should be distinguished from that heretofore necessary to achieve certain properties, particularly strength). Suitable hot Working temperatures include 1800 F. to 2000 F., a recommended finishing temperature being about 1500 F. to 1700 F.
• the steels are then preferably solution annealed over a temperature range snfficient to obtain recrystallization of the hot worked microstructure.
• a temperature of about 1400 F. to 1700 F. is suitable, a holding period of up to about four hours being satisfactory.
• Temperature as high as 1900 F. or higher can be used but are not recommended since grain coarsening can occur and this would impair stress-corrosion resistance. While a solution anneal is not indispensable, it is recommended for consistently obtaining uniform results.
• the steels are cooled to room temperature to effect transformation to martensite.
• Transformation is substantially complete at this point and no cold treatment or any preliminary or preconditioning heat treatment is necessary, although, as indicated herein, a cold treatment may be necessary, and is considerably beneficial, when the chromium plus nickel plus molybdenum is much above about 23.5%.
• Alloy B which contained 12.1% nickel in comparison with otherwise similar alloys, e.g., Alloys Nos. and 7, was lower by a factor of 30,000 p.s.i. to 45,000 p.s.i. This significant deficiency was not compensated for by any increase in resistance to impact.
• Alloy C is illustrative of the destructive influence of excess aluminum, this alloy being virtually toughless with an impact energy of but 2 foot-pounds.
• Alloy D the sum of the chroumium, nickel and molybdenum, being 24.7%, was too high in the absence of a cold treatment.
• the yield strength of the steel increased to 200,000 p.s.i., the ultimate tensile strength to 206,000 p.s.i., the tensile elongation was with the reduction in area being 42% and the average of three separate Charpy V-notch tests was 39 foot-pounds. As to physical appearance, the tested sepcimen was deemed exceptionally good. On the basis of this test, the steel performed quite satisfactorily.
• hydrocracker vessels in use currently have a yield strength (before use) of about 100,000 p.s.i. with the ability of absorbing only about foot-pounds of impact energy. Further, such vessels are normally lined with stainless steel for purposes of corrosion resistance. Accordingly, the Wall thicknesses of presently used hydrocracker vessels are comparatively thick and are thus characterized by undue heat loss. Such disadvantages would be eliminated in accordance herewith.
• steels within the invention be amenable to air melting practice and useful in heavy section applications
• additional alloys were prepared using air melting techniques.
• One alloy a 100 pound air melt, was prepared with parts per million of sulfur being intentionally added and titanium being deliberately omitted.
• the alloy otherwise contained about 9.8% chromium, 1.95% molybdenum, 9.6% nickel, 0.33% aluminum, less than 0.03% carbon, 0.006% sulfur, 0.16% vanadium and the nitrogen content was estimated to be about 0.01%.
• the ingot formed (6 inches x 6 inches x 6 inches) was slowly cooled, the rate of cooling being controlled to simulate the solidification rate expected were the ingot of heavy section of about 40 inches in diameter.
• a second air melted alloy (30 lbs.) was prepared but no intentional sulfur addition was made.
• the steel was given the usual heat treatment consisting of an anneal at 1500 F. for one hour followed by an age at 900 F. for three hours.
• the yield strength of this alloy was 159,000 p.s.i. and it manifested a Charpy V-notch impact strength (transverse direction) of foot-pounds.
• the present invention is broadly applicable in providing strong, tough, corrosion-resistant maraging steels in the form of strip, bar, rod, sheet, etc., and is particularly applicable in providing structures fabricated from plate, e.g., pressure vessels.
• a strong, tough, corrosion resistant maraging steel consisting of about 8.75% to 11.5% chromium, about 1.4% to about 3.25% molybdenum, about 8% to about 11% nickel, the sum of the chromium molybdenum and nickel being at least 20% but not exceeding about 23.5%, at least one element selected from the group consisting of aluminum and titanium in a total amount of at least 0.1 to about 0.65%, the aluminum not exceeding 0.4% and the titanium not exceeding about 0.3%, carbon in an amount up to about 0.04%, up to 0.5% manganese, up to 0.5% silicon, up to 0.1% zirconium, up to 0.01% boron, up to 0.2% beryllium, up to 1% vanadium, up to 0.8% tantalum, up to 1% tungsten, the sum of the beryllium, vanadium, tantalum and tungsten not exceeding 2%, and the balance essentially iron.
• a steel in accordance with claim 1 which possesses both a yield strength of at least 150,000 p.s.i. and a notch toughness of at least 50 ft.-lbs. and in which the chromium is at least 9% and the nickel does not exceed 10.5%.
• a steel in accordance with claim 1 which possesses both a yield strength of at least 150,000 p.s.i. and a notch toughness of at least 5 0 ft.-lbs. and in which the chromium is at least 9.75% and the nickel is at least 9.5%.
• a steel in accordance with claim 1 in which aluminum is present in an amount of from 0.1% to 0.35% and the balance is essentially iron.
• chromium about 1.5% to 2.25% molyb- 10 denum, about 9.5% to about 10.5% nickel, the sum of the chromium plus molybdenum plus nickel not exceeding about 23%, about 0.15% to about 0.35% aluminum, about 0.1% to about 0.25% titanium, the sum of the 10 aluminum plus titanium not exceeding 0.5% up to 0.02% carbon, up to 0.1% manganese and up to 0.1% silicon.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Steel (AREA)

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Cited By (6)

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• 1967
  • 1967-02-17 GB GB7780/67A patent/GB1128284A/en not_active Expired
  • 1967-02-23 NL NL6702778A patent/NL6702778A/xx unknown
  • 1967-02-28 FR FR96893A patent/FR1512858A/fr not_active Expired
  • 1967-02-28 DE DE19671558508 patent/DE1558508B2/de active Pending
  • 1967-03-01 BE BE694836D patent/BE694836A/xx unknown
  • 1967-03-01 SE SE2791/67A patent/SE310267B/xx unknown
  • 1967-04-05 US US628567A patent/US3594158A/en not_active Expired - Lifetime
• 1968
  • 1968-03-21 GB GB03737/68A patent/GB1153968A/en not_active Expired
  • 1968-04-04 FR FR147077A patent/FR94951E/fr not_active Expired
  • 1968-04-05 BE BE713336D patent/BE713336A/xx unknown

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