US3594158A - Strong,tough,corrosion resistant maraging steel - Google Patents

Abstract

A MARGAGING STAINLESS STEEL CONTAINS CHROMIUM, MOLYBDENUM, NICKEL, ALUMINUM AND/OR TITANIUM, AND CARBON AS ESSENTIAL CONSTITUENTS, THE ALUMINUM AND TITANIUM BEING SPECIALLY CONTROLLED. A SPECIAL RELATIONSHIP IS GIVEN IN RESPECT OF THE ELEMENTS CHROMIUM, MOLYBDENUM, AND NICKEL WHEREBY VARIOUS PROCESSING TREATMENTS ARE RENDERED UNNECESSARY. THE STEELS ARE USEFUL IN THE PRODUCTION OF PRESSURE VESSELS AND AN ILLUSTRATIVE STEEL CONTAINS ABOUT 11% CHROMIUM, 2% MOLYBDENUM, 10% NICKEL, 0.25% ALUMINUM, 0.2% TITANIUM, UP TO 0.02% CARBON, THE BALANCE BEING PRINCIPALLY IRON.

Description

United States Patent Office 3,594,158 Patented July 20, 1971 3,594,158 STRONG, TOUGH, CORROSION RESISTANT MARAGING STEEL Edward Peter Sadowski, Ringwood, N.J., assignor to The International Nickel Company, Inc., New York, N.Y. No Drawing. Continuation-impart of application Ser. No.

530,785, Mar. 1, 1966. This application Apr. 5, 1967,

Ser. No. 628,567

Int. Cl. C22c 39/20 US. Cl. 75-128W 7 Claims ABSTRACT OF THE DISCLOSURE This is a continuation-in-part of application Ser. No. 530,785 filed Mar. 1, 1966 now abandoned.

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.

As is known to those skilled in the metallurgy of steel, during the last score of years considerable emphasis has been directed to the problem of developing new steels capable of exhibiting increasingly higher levels of toughness, particularly notch toughness. As new applications rendered it necessary or as requirements became more stringent, enhanced yield strength characteristics were also desired and good corrosion resistance and tensile ductility as well. Suffice to say, however, such a combination of characteristics involves conflicting modes of metallurigical behavior.

It is well recognized that 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. This difiiculty is compounded when additionally imposed is the requirement that the steel also afford good resistance to corrosive media including resistance to stress-corrosion cracking in marine atmospheres. "Phat which might confer improved corrosion resistance often detracts from some other desired characteristic. With regard to ductility, as a general proposition, if a steel is tough, it will also possess good tensile elongation and reduction in area properties. But by no means does the converse necessarily hold true, as will be shown herein.

Before further considering the problem to which the subject invention is addresed, it is necessary that there be a clear understanding concerning certain fundamental points of consideration. In accordance herewith, 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. However, 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.

With regard to toughness, for a given level of yield strength a steel rnust absorb at least a minimum number of foot-pounds (ft-lbs.) of impact energy as determined by standard Charp V-notch test procedures. Results obtained on smooth bar specimens in contrast to notched specimens are not deemed sufliciently reliable nor sutficiently discriminating. Further, the minimum level of resistance to impact must be exhibited by a steel in the form of plate (say, /2 inch thick or greater) as distinguished from bar, rod, or other mill form. With few exceptions, Charpy V-notch toughness data obtained on bar and rod are significantly higher than the values obtained for the same steel in the form of plate. This is more fully discussed at pages 229 to 231 of the 1961 edition of The Metals Handbook. In addition, where unidirectional rolling is used, 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.

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.

Further, the above-discussed combination of characteristics must obtain with the simplest of processing and the steels should be amenable to air melting practice. Heretofore, it has often been necessary to find recourse in the utilization of cold treatments, whether they take the form of cold working or refrigeration or both, to develop required properties. Another common expedient has been the utilization of intermediate heat treatments (often referred to as preliminary or conditioning heat treatments) whereby a steel is brought to within a certain temperature range and then cooled before subjecting it to the hardening (final) heat treatment. This intermediate heat treatment (which should not be confused with the solution annealing treatment normally applied at higher temperatures) has in common with the cold treatment the undesirable feature of increasing cost. An important feature of the invention is that neither type of treatment is necessary nor a prerequisite herein. And, in any event, conditioning heat treatments are undesired.

In exploring possible approaches to the problem, 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. In respect of the stainless steels such as the austenitic type forming the AIS! 300 series, while these steels are highly corrosion resistant and relatively tough, 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. At the other end of the spectrum, 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 (including those of the stainless steel category) can be processed or otherwise treated to render a sufficient magnitude of strength but suffer from a lack of toughness.

It has now been discovered that by exercising special control over the respective amounts of certain constituents, notably, chromium, molybdenum, nickel, aluminum, titanium, carbon, manganese and silicon, the aforediscussed combination of properties and characteristics can be realized with the simplest of heat treatment to wit, a single aging treatment.

It is an object of the present invention to provide a tough, strong and corrosion resistant steel.

Other objects and advantages will become apparent from the following description.

Generally speaking, 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. As will be understood by those skilled in the art, the term 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.

As indicated above herein, 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. On the other hand, with 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. However,

where maximum corrosion resistance is necessary, 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. I so indicated in a paper presented in 1963 and published (together with additional information) in the February 1965 issue of Metals Engineering Quarterly by the American Society for Metals, pages 56 to 64. For this reason, among others, it was recommended in US. Pats. Nos. 3,262,777 and 3,262,823 that for the best combination of strength and toughness, the chromium content of the steels described therein should not exceed about 5.5%. However, provided that the steel compositions herein are properly balanced, no significant loss of toughness, if any, is experienced.

The nickel content should not fall below 8% and advantageously should be at least 9.5% to achieve a high level of strength. On the other hand and as is the case with chromium, unduly high amounts of nickel markedly contribute to retained austenite or reversion to austenite. Thus, it is advantageous that 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. As to titanium, a range of 0.05% to 0.3% is satisfactory, but a range of 0.1% to 0.25% is more advantageous. 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%.

Carbon, manganese and silcon markedly impair toughness and even amounts on the order of 0.04% carbon and 0.5% each of manganese and silicon prevent achieving best results. Thus, 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%.

To achieve an optimum combination of mechanical characteristics, 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.

In carrying the invention into practice, it is to be noted that an optimum combination of properties is achieved using standard vacuum processing techniques. However, a satisfactory combination of mechanical characteristics is obtainable with air melting practice. This is a specific benefit from the view of economics, air melting procedures anneal to achieve the desired mechanical characteristics is the application of a simple aging treatment at a temperature of about 800 F. to 1000 F. for from about one to 24 hours, the longer period being used with the lower temperature. Higher temperatures are to be avoided since undesirable austenite reversion can result. A temperature range of 850 F. to 950 F., e.g., 900 F., is quite suitable. With chromium contents above about 12%, a maximum aging temperature of about 900 F. is recommended.

For the purpose of giving those skilled in the art a beter understanding of the invention and/or a better appreciation of the invention, the following illustrative data and description are given.

Several 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.

TABLE I Percent. 01 Mo Ni Al Ti C Mn Si S P 10. 2 2. 06 10. 2 0. 36 0. 08 0. 006 0. 053 0. 024 0. 0018 0. 003 10. 4 2. 2 10.4 0. 17 0. 24 0. 004 0. 073 0. 1O 0. 0060 0. 001 10. 3 2. 06 10. 2 0. 35 0. 17 0. 004 0. 052 0. 024 0. 0034 0. 001 10.3 2. 06 10. 3 0. 09 0. 09 0. 002 0. 049 0. 029 0. 0024 0. 002 8. 9 3. 8. 0. 25 0. 2O 0. 007 0. 080 0. 12 0. 0023 0. 001 8. 9 3. 25 8. 45 0. 15 0. 23 0. 028 0. 072 0. 04 0. 0016 0. 001 9. 4 3. 2 10. 7 0. 27 0. 22 0. 011 0. 076 0. 05 0. 0049 0. 001 11. 3 2. 05 10.3 0. 24 0. 23 0. 007 0. 069 0. 11 0. 0054 O. 003 12. 0 2. 05 9. 9 0. 37 0. 19 0. 047 9. 1 3. 2 12. 1 0. 2 0. 24 0. 005 0. 066 O. 11 0. 0055 0. 001 9. 0 3. 25 8. 5 0. 90 0. 23 0. 008 0. 080 0. 12 0. 0023 0. 001 11. 5 3. 0 10. 2 0. 41 0. 12 0. 011 0. 024 0. 048 0. 0029 0. 001

NoTE.Balance iron and impurities.

being decidedly less costly. Use of relatively high purity alloying constituents is beneficial although scrap material can be used. In the latter event, care must be exercised with regard to insuring the proper chemical balance among the alloying constituents.

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.

Subsequent to hot working, 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. Following the solution anneal, 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%. All that is otherwise required subsequent to the aforediscussed Each of the steels was subjected to test as reported in Table II, the yield strength (Y.S., 0.2% offset) and ultimate tensile strength (U.T.S.) being given in thousands of pounds per square inch (K.S.I.), the tensile elongation (1 inch gage length) and reduction in area (R.A.) being given in percent and the Charpy V-notch energy absorption values (C.V.N.) being given in foot-pounds (ft-lbs.) at room temperature. The tensile properties were obtained in the longitudinal direction and the Charpy V-notch impact properties were obtained in the transverse direction.

TABLE II Reduction U.T.S. in area, O.V.N., K, s 1 percent percent t.-lbs.

The data tabulated in Table II rather clearly illustrate the markedly superior combination of strength and toughness characteristic of steels within the invention in contrast to those without the scope thereof. In this connec tion, it will be noted that Alloy A, containing 12% chromium (and also 0.047% carbon) with the sum of the chromium, nickel and molybdenum being 23.9%, manifested poor yield strength. When viewed against Alloy No. 1, for example, .Alloy A suffers greatly by way of comparison, its yield strength being about 40,000 p.s.i. lower and its impact strength being lower by approximately 30 footpounds despite the lower level of yield strength. Such data are indicative that more is involved than merely using a level of chromium which is relatively high in comparison with steels such as those described in US. Pat. No. 3,262,823. Had the carbon content of Alloy A not ex ceeded about 0.04% and if either the chemistry thereof was properly balanced such that the sum of the chromium, nickel and molybdenum did not exceed about 23.5% (the respective amount of each constituent, of course, being within the ranges given herein), or if the alloy was subjected to a cold treatment, it would be Within the invention.

The yield strength of 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. As to Alloy D, the sum of the chroumium, nickel and molybdenum, being 24.7%, was too high in the absence of a cold treatment.

An intended application of the subject steels is in the fabrication of hydrocracker vessels. In connection therewith, Alloy No. 8 in plate form (4 /2 inches x 2% inches .x /2 inch) was suspended in an actual running hydrocracker vessel, the plate being exposed to the hydrogen atmosphere therein at a temperature of about 750 F. under a pressure of about 1000 p.s.i.g. for 500 hours. The specimen Was then removed and tested at room temperature to determine the mechanical properties thereof, particularly the degree of embrittling as determined by the standard Charpy V-notch testing procedure. 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. Another steel containing about 9.9% chromium, 10.5% nickel, 2.1% molybdenum, 0.23% aluminum, 0.22% titanium, 0.002% carbon, 0.07% silicon, 0.12% manganese manifested an average impact strength of 39 footpounds together with a yield strength of 207,000 p.s.i. after exposure at 750 F. for 1000 hours in the absence of hydrogen. It should be perhaps mentioned that 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.

'In respect of resistance to stress-corrosion cracking, U-bends (2 each) of Alloys Nos. 2,6,7, and 8 have been under test in sea water and the marine atmosphere at the well known testing stations at Harbor Island and Kure beach, respectively, for over 250 days and no failures have been observed. Laboratory tests of 3-point loaded specimens of Alloys Nos. 2, 7, and 8 tested in 3.5% NaCl and stressed at 90% of yield strength were discontinued without failure after 100 days. Alloys Nos. 2, 7, and 8 have also been exposed (U-bend test) to the industrial atmosphere at Newark, NJ. for over 215 days without failure or surface corrosion.

Since it is contemplated that 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. This was done in accordance with the basic purpose of obtaining a critical evaluation of segregation characteristics. The ingot was thereafter hot worked to 5/8 inch plate by cross rolling and the plate was annealed at 1500 -F. for one hour and aged at 900 F. for three hours. Microstructural examination of both the as cast ingot and the /8 inch plate did not reveal any of the usual forms of segregation such as freckling or banding. It may be noteworthy of mention that the Charpy V-notch impact strength of this alloy (transverse direction) at room temperature was about 32 foot-pounds. While this is on the low side due to the processing performed to simulate a structure to be obtained on a large size ingot, nonetheless it is deemed that the absence of titanium contributed notably to this comparatively low value.

A second air melted alloy (30 lbs.) was prepared but no intentional sulfur addition was made. This alloy contained about 10.2% chromium, 2.15% molybdenum, 10.4% nickel, 0.08% aluminum, 0.14% titanium, 0.03% carbon 0.0034% sulfur and the nitrogen content was estimated to be about 0=.0035% to 0.005%. After being unidirectionally rolled to inch plate, 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. These data indicate that alloys within the invention can be air melted with satisfactory results assuming, of course, that good commercial steelmaking practice is used.

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.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. 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.

2. 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%.

3. 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%.

4. 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.

to about 11% 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.

References Cited UNITED STATES PATENTS 2,220,932 11/1940 Krivobok 75128.8 3,262,777 7/1966 Sadowski 75128.8 3,288,611 11/1966 Lula 75-1289 3,347,663 10/ 19 67 Bieber 75124 HYLAND B-IZOT, Primary Examiner US. Cl. X.R.

1 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3.594458 Dated July 2g 197;

Inventg -(g) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 54, after "levels" insert a period line 56, for "the treatment" read --heat treatment--; and line 70, for "addresed" read --addressed--.

Column 2, line 12, for "Charp" read --Charpy--; lines 22 and 23,

for "The Metals Handbook" read --THE METALS HANDBOOK-.

Column 3, line 7, after "e.g." insert a comma line 2 after "treatment" insert a comma line +6 before "e.g., insert a comma and line 47, for 'clansing" read --cleansing-.

Column l, line +8, for "ceeed" read --ceed--.

Column 5, line 43 for "chrominum" read --chromium--; and line 62, for Temperature" read --Temperatures--.

Column 6, line 12, for "beter" read --better--.

Column 7, line l7 for "chroumium" read --chromium--; and

line 35, for 'sepcimen" read --specimen--.

Column 8, line 2 after "carbon" insert a comma )5 and line 52 {Claim 1, line 4) after "chromium" insert a comma Signed and sealed this 8th day of February 1972.

(SEAL) Attest: W

EDWARD MELETCHERJR. ROBERT GOTTSCHALK eating Officer Commissioner of Patents