US3262823A - Maraging steel - Google Patents

Description

y 1956 E. P. SADOWSKI ETAL 3,262,823

MARAGING STEEL Filed June 7, 1963 CMN.

N M C M E WWW M TOK N m EAF. V50 4 W F 0 Wm WM [P Y E w F V N M a United States Patent 3,262,823 MARAGING STEEL Edward P. Sadowski, Metuchen, and Raymond F. Decker,

Fanwood, N.J., assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware Filed June 7, 1963, Ser. No. 286,365

12 Claims. (Cl. 148142) The present invention relates to high strength steel and, more particularly, to an exceedingly tough, controlled, high strength maraging steel especially usable as plate and as welded structures made from said plate.

It is well known that many steels can be used in the form of plate having a thickness from below 0.5 inch and up to 4 or 6 inches or greater. It is conventional to form structures such as marine hulls from such steel plate by welding plates to each other and to a structural frame (such structures will be called hereinafter plate structures). The huge sizes of many plate structures very often place severe limitations upon the types of heat treatment which can be applied to the plates either before or after welding. For example, it is practically impossible to normalize, quench and temper any substantial portion of a hull of an ocean-going ship. Accordingly, it has been recognized that certain grades of alloy and/ or carbon steels cannot be used successfully in plate applications since satisfactory welding of these steels requires preheating and/ or the use of post-weld normalizing, quenching and tempering treatments. Among the many problems encountered with such alloy and/or carbon steels are cracking in the heat affected zone and welding-induced distortion.

Heat treatment problems are only some of the many difficulties which must be considered by an engineer designing a structure to be formed from plate. Weight and buoyancy considerations often require the designer to specify a steel of controlled high strength, i.e., a steel having a yield strength of about 140,000 pounds per square inch (p.s.i.) up to about 200,000 -p.s.i. There are, of course, many steels which will exhibit yield strengths of such an order of magnitude. However, as indicated hereinbefore, the steel to be specified as plate must be weldable without heat affected zone (H.A.Z.) cracking under conditions encountered in the practical construction of large structures.

Another of the many considerations which is involved in the selection of a high strength plate steel is toughness of the plate. It is now well recognized that during rolling of plate, factors are brought into play which induce directional anomalies in mechanical characteristics in the final plate. As pointed out in the 8th edition of Metals Handbook, volume I, 1961 on pages 229 to 231, Charpy V-Notch (C.V.N.) specimens taken in a direction parallel to the direction of rolling often exhibit impact values substantially higher than the values exhibited by specimens taken in a direction transverse to the direction of rolling. In other words, longitudinal specimens taken parallel to the rolling direction show greater energy absorption than specimens taken in a transverse direction. Very often the difference between the energy absorption of transverse and longitudinal specimens can be of the order of 100% and higher based upon the transverse impact value. For example, the difference between the C.V.N. impact values in foot-pounds (ft.-lbs.) of transverse and longitudinal specimens of S.A.E. Steel No. 8617 taken at 0 F. is at least about 50 ft.-lbs. This difference is about 200% of the maximum C.V.N. impact value of 25 ft.-lbs., exhibited by transverse specimens of this steel at 0 F. with a hardness of 30 Rockwell C units (R Uniformly, in the 15 graphical representations set forth on pages 230 and 231 of the Metals Handbook, the C.V.N. impact values of longitudinal specimens significantly exceed the C.V.N. impact values of transverse specimens. This directional phenomenon is particularly evident in rolled plate and sheet since the amount of work homogenization which can be induced in plate and sheet by cross rolling is limited by the length of the plate and/or sheet as opposed to the width of the available rolling mill. Thus, in the final stages of reduction of sheet and plate, it is often impossible to insert metal into a roll gap with the length of the sheet or plate parallel to the roll axes because the rolls are not wide enough. In contrast to rolled bar or rod (which are conventionally tested for toughness using longitudinal C.V.N. impact specimens) rolIed plate and sheet almost inherently exhibit directional anomalies which are most evident in ascertaining toughness by the C.V.N. test.

The importance of the foregoing discussion to a designer of plate structures is clearly that the designer must design within toughness limits imposed by the lowest value of toughness criteria (usually impact values) exhibited by the materials available to him. It is also clear that the plate-structure designer must compare truly comparable impact values when deciding upon the relative merits of available material. Thus, for a designer to attempt to evaluate two plate steels solely by comparing C.V.N. impact values taken in one steel from transverse specimens and in the other steel from longitudinal specimens is a very unsound procedure unless the situation is such that the values exhibited by the transverse specimens of the one steel exceed the values exhibited by the longitudinal specimens of the other steel. In the latter situation it is perhaps safe to conclude that the steel from which transverse specimens were taken is at least as tough or tougher than the steel from which longitudinal specimens were taken.

In addition to the aforediscussed factors of weldability, strength and toughness of the plate steel, a designer of plate structures must take into consideration the toughness of the weld-affected area in the assembled plate structure. It is known that many instances of brittle fracture of plate structures have originated in weld-affected areas of the structure. For example, on page 235 of the 1961 edition of the Metals Handbook, it is stated that the brittle fracture of a T2-type tanker occurring while the vessel was moored dockside, originated in a welding arc strike. On page 236 of the Metals Handbook, an example of the failure of a pressure vessel designed for petroleum refining is set forth. In this example, it is stated that during hydrostatic pressure testing, failure occurred in a knuckle plate at the base of a fillet weld. In view of the fact that in many instances, weld-affected areas of plate structures and cracks induced by welding are the originating points of brittle failures, it is necessary that a designer seriously consider the effect of welding upon the strength and toughness of the metal adjacent wel-d deposits. In this regard, it is not only necessary that commercially practical post-weld heat treatments give high strength efficiency in the weld-affected areas, but also such post-weld heat treatments must not detrimentally affect the toughness of the metal in the weld-affected areas to an extent that such toughness would fall below tolerable design limitations.

Keeping in mind all the foregoing difficulties and other problems, plate-structure designing and, in particular, specifying alloys for plate structures, is a very complex activity. As far as is known, the metallurgical art has never provided the designer with a wholly satisfactory commercial steel of controlled high strength suitable for use in plate structures adapted to be subjected in use to high pressures and violent impacts at temperatures from above room temperature down to about 100 F. and lower. Although many attempts were made to provide such a satisfactory, tough, controlled high strength plate steel, none, as far as we are aware, was entirely than the aforelisted maximum amounts should be used and, in any event, no greater than 2% total of beryllium, vanadium, columbium, tantalum and tungsten should be present in alloys in accordance with the present invention.

successful when carried into practice commercially on 5 More advantageous embodiments contemplated inacan industrial scale. cordance with the present invention include, with respect It has now been discovered that an alloy steel conto alloy composition, alloys containing about 11% to taining specially controlled and interrelated amounts of about 12% nickel and/or about 3% to about 5% chromialloying elements is especially suited for use in the manuum and/ or about 2.5% to about 3.5% molybdenum, and/ facture of plate structures having a high strength to or about 0.05% to about 0.15% aluminum and/or about weight ratio. 0.20% to about 0.30% aluminum. It is generally advan- It is an object of the present invention to provide a tageous to limit the amount of chromium in the alloys novel steel. of the invention to a maximum of about 5.5% in order Another object of the invention is to provide a novel to obtain the best combination of strength and toughness. maraged steel. 15 The strength and toughness of the alloys of the present The invention also contemplates providing a novel invention are also greatly dependent upon the aluminum process of heat treating said novel steel. content of the alloys. FIGURE 1 of the drawing shows It is a further object of the invention to provide novel that as the aluminum content of alloys in accordance with welded plate structures made from said novel steel. the invention (containing nominally 12% nickel, 5% The invention further contemplates providing a novel chromium and 3% molybdenum) is increased from 0.1% process of producing said novel plate structures. to about 0.3%, the yield strength of the alloys increases Other objects and advantages will become apparent sharply while the Charpy V-Notch impact values at 70 F. from the following description taken in conjunction with decrease only slightly. Above about 0.3% aluminum the accompanying drawing in which: the impact values tend to decrease at an increasingly FIGURE 1 is a graphical representation of mechanical rapid rate to totally unacceptable levels. Consequently, characteristics of a typical steel of the present invention it is important to specially maintain the amount of alumias related to the aluminum content of the alloy; and mum in the alloys of the invention below an ultimate FIG. 2 is a graphical representation of the interrelation maximum of 0.4% and, more advantageously, below existing between yield strength and C.V.N. impact values about 0.35%. Each of the aforementioned more advanas exhibited by alloys both within and without the present tageous ranges of nickel, chromium, molybdenum and invention. aluminum can be employed either singly or in combi- Broadly stated, the present invention contemplates novel nation with the ranges of alloy composition set forth maraging steel alloys consisting essentially, in percent by hereinbefore. For reasons set forth in detail hereinweight, of about 9.5% to about 13.5% nickel, about 2.5% after, it is advantageous to interrelate the nickel and to about 8% chromium, about 1.9% to about 4.2% mochromium contents within the aforementioned broader lybdenum, about 0.05% to about 0.4% aluminum, up to range and the more advantageous ranges so that the sum about 0.30% titanium, and advantageously, about 0.1% of the percentages of nickel and chromium in a given to about 0.2% titanium, about 0.001% to about 0.03% specific alloy is about 14% to about 19%. As high as carbon, up to not more than 0.25% manganese, up to about 0.50% silicon can be tolerated in alloys of the about 0.30% silicon, up to about 0.01% boron, up to present invention provided that the aluminum copresent about 0.1% zirconium, with the balance being essentially with amounts of silicon in excess of about 0.30% be no iron in a amount of at least about 74%. The alloys greater than about 0.30%. can also contain impurities and incidental elements in The alloy ranges set forth in Table I are illustrative small amounts which do not deleteriously affect the of the broadly stated and more advantageous alloy ranges basic and novel characteristics of the alloy. Impurities of nickel, chromium, molybdenum, titanium and alumisuch as sulfur, phosphorous, hydrogen, oxygen, nitrogen num as set forth hereinbefore.

TABLE I Alloy Percent Percent Percent Percent Percent Percent Percent Type 0 Ni 01- Mo T1 Fe 1 0.03max 10-10.5 4.5-5 2-4 0. 05-0. 15 0.1-0.2 Bel. E. 0.03 I1121X 11.5-12.5 3-3.5 2.75-3.25 0. 05-0. 15 0.1-0.2 13:11. 0.03 max l0-11 4-5 3-4 0.2-0.3 0.10.2 Bal. E. 003 max..." 11.5-12.5 3-5 2. 5-3.5 0.2-0.3 0.1-0.2 Bal. E. 0025 max 11.5-12.5 4. 75-5. 25 2.75-3.25 0. 2-03 0.1-02 1321. E.

elements.

and the like, should be kept at the lowest possible levels consistent with economical commercial production. Incidental elements such as cobalt and copper are not specifically beneficial in the alloys of the present invention and should be limited to those small amounts which are unavoidably introduced into the composition during commercial manufacture. Small amounts of elements such as beryllium, vanadium, columbium, tantalum and tungsten can be employed in the alloys of the present invention. These elements, when used singly, should be limited to the following maximum amounts: 0.2% beryllium, 1% vanadium, 0.4% columbium, 0.8% tantalum Alloys in accordance with the present invention have, at normal temperatures of usage, essentially a martensitic structure, that is, a structure resulting from a low temperature (below about 700 F.), substantially diffusionless transformation from austenite. During aging, as described in detail hereinafter, the alloys increase in hardness, for example, from about 25 R to between about 33 R and about 43 R This hardening response during aging distinguishes the alloys of the present invention from substantially all known prior art carbon steels (i.e., steels containing more than about 0.1% carbon) in that or 2% tungsten. When employed in combination, less during tempering at temperatures akin to the aging temperatures for the present alloys, prior art carbon steels exhibit a softening response.

Generally speaking, the alloys of the present invention are heat treated by a process which includes a first transformation to martensite after hot working, a solution anneal at a temperature in excess of about 1400 F., a second transformation to martensite following said solution anneal, and a final aging treatment in the vicinity of about 900 F. for a time in excess of about 1 hour and up to about hours to harden and strengthen the alloy. As will be discussed hereinafter, the transformation to martensite can be accomplished by merely cooling the alloys to a temperature below about 150 F. The rate of cooling is not critical and drastic quenching operations are definitely not needed in order to obtain the advantageous mechanical characteristics of the alloys of the present invention.

Alloys according to the present invention are made by conventional high strength steel melting practice. For example, alloys of the present invention can be made by melting together relatively pure iron, nickel, chromium and molybdenum; decarburizing the resultant molten metal to produce a carbon level below about 0.03% by weight; deoxidizing the decarburized molten metal with silicon and manganese so as to limit residual silicon and especially residual manganese in the metal to within the aforementioned ranges for these elements; completing deoxidation by adding aluminum and titanium to provide residual amounts within the aforementioned ranges of aluminum and titanium; adding any desired boron and/ or zirconium and casting the metal into ingots. In' carrying out the manufacturing process, care must be taken to avoid introducing into the metal excessive amounts of sulfur, phosphorous, nitrogen, hydrogen and the like. Additions of small amounts of calcium which leave a residual of no more than about 0.02% in the metal can be beneficial in reducing sulfur to acceptable levels, e.g., about 0.01% maximum. Addition of malleableizing and deoxidizing amounts of rare earth elements (from the addition of misch metal), lithium, magnesium, uranium, etc., can be beneficial. Best results are obtained when premium grades of ingredients such as Armco iron, electrolytic nickel, electrolytic chromium and the like are employed in making alloys in accordance with the present invention. The alloys can be melted in high frequency induction furnaces advantageously under a blanket of inert gas such as argon. Alternatively, vacuum melting can be employed. Selected scrap can be used in the manufacture of alloys of the present invention provided proper basic electric furnace practice is employed and oxygen lancing is used to reduce carbon to levels below about 0.03%.

After the metal has been cast, the ingots are subjected to hot working operations at starting temperatures of about 2300 F., to mechanically homogenize the as-cast therein. In the event sheet is desired, the cooled alloys can be worked further by hot-cold working and/or cold working to sheet thicknesses. After forming has been completed, the alloy is usually annealed at temperatures of about 1450 F. to about 1900 F. for about one to about four hours. The result of the annealing (or solution) treatment is to retransform any martensite to austenite, to place substantially all the alloying elements in solution in the matrix of the alloy and to remove the residual elfects of hot and cold work.

Following annealing (or in special instances the finish of hot Working) the alloy is cooled to room temperature or to a temperature at least below about 150 F. The rate of cooling after annealing is not critical but it is preferred to cool at a rate at least equal to the rate of air cooling. During cooling, the alloy structure changes from austenite (face-centered cubic) to martensite (bodycentered cubic) by means of an essentially diffusionless process. The annealed martensitic alloy has a hardness of about 25 R Cold working, as mentioned hereinbefore, can be performed on the annealed martensitic alloy to the extent equivalent to about reduction by rolling. Likewise, the annealed martensitic alloy of the present invention can be readily machined by sawing, drilling, turning, planing, milling and the like to final dimension. During subsequent aging, there is extremely little dimensional change.

After the annealed martensitic alloy has been formed or machined into the desired configuration, it is aged for about one to about ten hours at temperatures of about 800 F. to about 1000 F. A practical and recommended aging treatment is heating for about 3 hours at 900 F. During aging, samples of the annealed martensitic alloy increased in hardness from a level in the range of about 25 R, to about 32 R in the annealed condition up to a level in the range of about 33 to 43 R in the aged condition. Primarily depending upon composition, the yield strength of the aged alloys is of the order of about 140,000 p.s.i. to about 190,000 p.s.i. or even to about 200,000 p.s.i. The yield strength (Y.S.) level of aged samples of the alloy types disclosed in Table I are set forth in Table II:

TABLE II Alloy type: Y.S., in p.s.i. I 145,000155,000 I I 145,000-155,000 II 165,000-175,000 IV 165,000175,000 V 170,000-188,000

For the purpose of giving those skilled in the art a better understanding of the invention, examples of alloy compositions in accordance with the present invention are set forth in Table III:

TABLE III Alloy Percent Percent Percent Percent Percent Percent Percent Percent No. Ni Cr Mo A Ti 0 Mn Fe 1 12.03 4. 77 2. 00 0.07 0.20 0.015 0. 01 Bal. 12. 04 2. 3. 00 0. 12 0.18 0. 011 0.03 Bal. 10. 30 4. 95 3. 02 0. 11 0. 18 0. 008 0. 02 Bal. 12. 09 3. 25 2. 07 0.23 0.20 0.008 0. 04 139.1. 12. 16 5.00 3. 00 0.31 0.21 0. 008 0. 02 Bal. 10. 2G 4. 87 4. 03 0.31 0.009 0. 02 Bal. 10. 37 5. 10 2. 10 0- 30 0. 007 0. 02 Bal. 2

9. 67 4. 90 1. 95 0.21 0.10 0.025 0.08 Bal. 10.80 5. 20 3. 16 0. 06 0.12 0. 030 0.07 Bal. 10. 05 4. 73 4. 18 0. 07 0. 12 0.027 0. 07 Bal.

1 Bal. means balance iron, inclusive of small amounts of silicon, boron, zirconium and/0r calcium together with amounts of phosphorus below about 0.007% and sulfur below about 0.004%.

2 Alloy No. 7 contains 0.48% silicon.

structure and to form hot rolled plate or other hot worked shapes. Finish hot working temperatures can be as low as about 1500 F. After 'hot working, the hot worked alloys are cooled to room temperature or at least below tensitic and age hardened condition the best combination of room temperature strength (as measured by yield strength) and room temperature toughness (as measured about F. to induce a martensitic transformation 75 by C.V.N. values obtained with transverse specimens from The alloys set forth in Table III, exhibit in the marrolled plate). Additional examples of alloys in accordance with the present invention are set forth in Table IV:

TABLE IV Alloy Percent Percent Percent Percent Percent Percent Percent Percent No. N 1 Cr Mo A T1 Mn Fe 1 10.40 5.15 2.15 0. 4o 0.008 0. 01 Bal. 12. 35 4.85 3. 00 0. i2 0. 19 0. 015 0. 1351. 11.17 3.85 2. 55 0.19 0.10 0. 010 0. 02 B31. 10. 35 4. s0 4. 00 0. 00 0.14 0. 007 0. 02 B31. 12. 05 2. 04 2. 93 0. 05 0. 13 0. 032 0. 00 Bal. 11.25 3. 35 2.00 0. 00 0. 13 0. 022 0. 05 Bal. 10.20 4. 72 2. 05 0. 05 0. 12 0. 031 0. 06 Bal. 10.05 4. 05 1.00 0. 05 0. 12 0. 019 0. 00 B21. 10. 2s 2. 35 3.00 0. 32 0. 21 0. 000 0. 02 Bal.

1 Del. means balance iron, inclusive of small amounts of silicon, boron, zirconium and/or calcium together with amounts of phosphorus below about 0.007% and sulfur below about 0.004%.

2 Alloy N0. 12 contains 0.31% silicon.

TABLE v Hardness in Re Units Alloy No.

Before Aging 1 After Aging 2 The alloys as set forth in Tables III and IV were 25 25 0 3 made by vacuum melting and/or by melting under an 3 1 5.5 37.5 argon blanket as described hereinbefore. The carbon 2 level of the molten metal was reduced by a carbon boil g g fig and initial deoxidation was accomplished by the use 3 3- 3 of silico-manganese. After all alloying ingredients were 3% 35.2 added, the metal was cast and hot worked as described %.2 28.8 hereinbefore to provide plate. The plates were then ggig 301g permitted to cool to room temperature to effect a first 23.5 33.0 martensitic transformation therein. The alloys were then 3L0 5 subjected to a solution treatment (annealing) at 1500 F. 31:2 25:3 for 1 hour followed by cooling to room temperature to effect a second transformation to martensite. The alloys Annealed for 1 11011! at 1500 R followed y r were then aged for 3 hours at 900 F. Hardnesses before and after aging are set forth in Table V:

cooling.

2 Aged for 3 hours at 900 F. followed by air cooling.

The data set forth in Table V demonstrates that during aging at temperatures of about 800 F. to about 1000 F., the martensitic alloys of the present invention increase in hardness by up to about 15 Rockwell C units. After aging, the martensitic alloys of the present invention exhibit mechanical characteristics as set forth in Table VI.

TABLE VI O.V.N. Impact Strength Alloy 0.2% Y.S., U.T.S EL, R.A., (ft.-lbs.) 4

No. k.s.l. k.s.l. percent percent 70 F. -100 F. 320 F.

1 U .;I.S., is ultimate tensile strength measured in thousands of pounds per square inch .s. 2 E1. is elongation expressed in percent based upon a gage length of 4 times the test bar diameter.

In addition to the mechanical characteristics set forth in Table VI, which are representative of the mechanical characteristics of aged martensitic alloys in accordance with the present invention, alloys of the present invention also exhibit excellent notch tensile strengths of the order of 1.5 (or greater) times the ultimate tensile strength. It is also characteristic of alloys of the present invention that the ratios of yield strength to ultimate tensile strength are in excess of 0.9 and usually in excess of 0.95.

In the as-annealed condition, i.e., after solution treatment at 1500 F. for 1 hour, followed by air cooling, alloys in accordance with the present invention also exhibit an excellent combination of characteristics. The data set forth in Table VIA illustrates this with respect to alloys containing, in addition to the indicated aluminum percentages, nominally about 12% nickel, chromium, 3% molybdenum, about 0.01% carbon, with the balance being essentially iron.

1 At room temperature.

As indicated hereinbefore, weld-affected areas of aged martensitic alloys of the present invention can be restored in properties by a simple postweld heat treatment at about 900 F. Such a heat treatment can be applied to a large Welded plate structure by means, for example, of strip heaters, electrical resistance heating, induction heating, torch heating, etc. Macro and micro examinations of the weld-affected areas of welded plate samples disclosed only sound metal with no discernible cracks. During welding, heat affected zones of the metal of the welded samples softened somewhat. Subsequent trnaraging resulted in complete restoration to the hardness of the plate body. In cruciform Welding tests embodying a high degree of restraint, no cracking was discernible in heat affected zones of 1 inch plate samples in accordance with the present invention.

To achieve the objects of the present invention, it is vitally important that the chemical composition of the alloys of the present invention be maintained within the Both chromium and molybdenum within the ranges of about 2.5% to 5.5% and about 2% to about 4%, respectively, are advantageous because with amounts of these elements lower than those specified, alloys of much lower strength are attained. Increasing the amounts of molybdenum above those specified as a maximum results in drastically lowering the toughness of the alloys and lowering strength. An excessive amount of chromium in the alloy tends to lower strength. Amounts of chromium in excess of about 5.5% and up to about 8% can be advantageous from a corrosion standpoint. Amounts of manganese above about 0.25%, which are normally considered as beneficial in good alloy steel manufacturing practice, are detrimental to the toughness of the alloys of the present invention and, accordingly, the maximum manganese permitted in the alloys of the present invention is about 0.25%. It is essential that in the presently disclosed alloys the carbon be kept at a maximum of about 0.03%, i.e., less than 0.033%. Amounts of carbon even slightly in excess of 0.033% have a deleterious effect on toughness of the alloys. Raising carbon in the alloys of the present invention to very (comparatively speaking) high levels of 0.1% or more, results in the production of alloys akin to known carbon steels, said alloys having the defects and deficiencies of carbon steels as discussed in detail herein before. The maximum specified carbon content is also highly critical in that the deleterious effects of amounts in excess of about 0.03% in the alloys of the invention cannot be remedied by the use of carbide formers such as titanium. Like carbon, metal carbides such as titanium carbide, drastically reduce the toughness of alloys which oth erwise would be within the ambit of the present invention. Amounts of aluminum up to about 0.3% or 0.4% assist in strengthening the alloys of the present invention as shown in FIGURE 1 of the drawing. Titanium in amounts up to about 0.3%, e.g., about 0.1% to about 0.2%, in combination with less than about 0.03% carbon is advantageous in that it tends to minimize the deleterious effects of small amounts of sulfur which may inadvertently be present in the alloys.

The critical nature of composition limits discussed hereinbefore is exemplified by reference to Tables VII and VIII. Compositions of alloys (designated by letters) outside the present invention are set forth in Table IX together with compositions of alloys (designated by munbers) within the present invention to provide a comlimits of the ranges set forth hereinbefore. The nickel parison.

TABLE VII Alloy Percent Percent Percent Percent Percent Percent Percent Percent Ni Cr M0 A Ti 0 Mn Fe 12, 10 3. 01 0. 06 0. 14 0. 021 0. 06 Bal. 10. 80 5. 20 3. l6 0. 06 O. 12 0. 030 0. 07 13:11. 11. 95 3. 43 1. 01 0. 24 0. 10 0. 021 0. 00 Bal. 10. 25 4. 5. 20 0. 07 0. 11 0. 028 0. 07 Bal. 12. 09 3. 25 2. 07 0. 23 0. 20 0. 008 0. 04 Bal. 9.80 2. 75 2. 88 0. 06 0.12 0.04 Bald 11. 90 3. l8 3. 30 0. 06 0. 11 0.05 Bal. 11. 50 4. 75 2. 00 0. 26 0. 10 0. 06 0. 07 1361. 10. 00 4. 80 2. 00 0. 25 0. 20 0.06 0. 07 13211. 12. 6 5. 4 2. 9 0. 22 0. 14 0. 023 0. 29 Ball 12. 5 5. 5 3. 0 0. 11 0. l3 0. 024 0. 29 Bal. 12. 4 5. 5 3. 0 0. 14 0. 14 0. 024 0. 42 Bal. 12. 08 4. 87 2. 97 0. 25 0. l5 0. 009 0. 10 13211. 12. 26 4. 98 2. 97 0. 23 0. l4 0. 015 0. 21 13211. 12. 16 5. 00 3. 00 0. 31 0. 21 0. 008 0. 02 Bal. 10. 4 6. 4 0. 76 0. 022 0. 18 0. 122 0. 44 Bal. 10.3 6. 5 0. 77 0. 024 0. 16 0. 113 0. 38 Bal.

1 Bal. means balance iron, inclusive of small amounts of silicon, boron, zirconium and/or calcium, together with amounts of phosphorus below about 0.007% and sulfur below about 0.004%.

1 Also contain about 0.45% copper.

content must be maintained Within the range of about 9.5% to about 13.5% because lower amounts of nickel, especially in combination with amounts of chromium below about 3%, lead to decreased strength and to decreased low temperature toughness characteristics. Amounts of nickel higher than about 13.5% are uneconomical and,

All of the alloys in Table VII were made, worked and heat treated in accordance with the procedure disclosed hereinbefore as to alloys of Tables III and IV. Duplicate samples of alloys K and L after melting, casting and working to plate in the usual manner were first normalized in association with high chromium, lead to lower strength. at 1625 F., second normalized at 1450 F. and tempered at 1050 F. Mechanical characteristics of the alloys of Table VII are set forth in Table VIII.

12 loys D, E, F, G, K and L of Table VII. As set forth in Table VIII, none of these alloys exhibit a room tempera- IABLE VIII C.V.N. Impact Strength 0.2% Y.S., U.T.S., El., R.A., (it-lbs.) 1 Alloy k.s.i. k.s.l. percent percent 70 F. 100 F. --320 F.

the direction of rolling.

2 Heat treated by double normalizing followed by tempering at 1,050 F.

The data set forth in Table VIII is to be considered in conjunction with FIG. 2 of the drawing. FIG. 2 of the drawing shows the interrelation between 0.2% yield strength and room temperature (70 F.) Charpy V-Notch impact strengths of alloys set forth in the present specification. It is known in general for steels that there is an inverse relationship between strength and toughness. Thus, for any given steel as the strength is increased, the toughness decreases. For alloys in accordance with the present invention containing alloying elements within the ranges set forth in Table IX, this inverse relationship is generally representable by approximately the curve DE as shown in FIG. 2.

Further, with respect to alloys of the present invention having compositions falling (except for manganese and silicon) within the ranges set forth in Table IX, the curves AB and BC effectively define the minimum strength and toughness limits obtainable. Such alloys can be represented by a point lying above and to the right of curves AB and BC when heat treated in accordance with the presently disclosed process to 0.2% yield strengths of about 140 k.s.i. to about 200 k.s.i. As a general approximation, such alloys are characterized by 0.2% yield strengths and room temperature C.V.N. impact strengths such that both of the following quasi-mathematical criteria are satisfied:

ture C.V.N. impact strength as high as 40 foot-pounds. The yield strengths and impact strengths of alloys D, E, F, G, K and L have been plotted on FIG. 2 of the drawing (as circles) and can be compared to the yield strengths and impact strengths of the alloys of the present invention plotted (as Xs) on the same figure. Tables VII and VIII also show that when chromium is deleted from the alloys of the present invention the strength is low; that when molybdenum is below about 1.9% the strength and toughness is low; that when molybdenum exceeds about 4.2% the toughness is low; and that when manganese exceeds about 0.25% (alloys H, I and J) the toughness of the alloys is low.

When the chemistry of the alloys of the present invention is controlled within the more advantageous limits set forth he'hreinbefore and welds are made with suitable and compatible fillers, highly advantageous weld joints can be produced. Tests have shown that in tungsteninert-gas weld joints of /2 inch plate samples of alloys in accordance with the present invention, combinations of characteristics as set forth in Table X can be obtained.

TABLE X Room temperature C.V.N. values, 0.2% yield strength, k.s.i.: ft.-lbs.

The ranges of alloying elements in accordance with the present invention as set forth hereinbefore include certain interrelationships among alloying elements. One of the essential concepts of the present invention is that in alloys in accordance with the invention, the total nickel plus chromium content must be between about 13.5% and about 19%. When nickel and. chromium are copresent in the alloys of the present invention in the amounts set forth hereinbefore, these elements appear to interact during maraging (i.e., aging in the martensitic condition after martensitic transformation), to harden and strengthen the alloy. This interaction during maraging is most pronounced when the nickel content is about 11% to about 12%, the chromium content is about 3% t0 about and the total of the nickel plus chromium is about 14% to about 16%. It is essential that at least 0.05% aluminum be present in the alloys of the present invention. As shown in FIGURE 1 of the drawing, aluminum in amounts between about 0.1% and 0.3% contributes greatly to the strength of the alloys and permits this strength to be attained without great sacrifice of toughness.

The present invention is particularly applicable to the provision of plate for high strength, welded structures. Examples of such structures are ships hulls, pressure vessels for chemical reactors and high pressure equipment, rotors for electrical generators, etc. The alloys of the present invention can, of course, be provided in forms other than plate, for example, sheet, bar, rod, wire, strip, tube and the like. These forms can be produced by hot working, e.g., forging, rolling, extrusion and the like, cold working, e.g., trolling, drawing, pressing, etc., hotcold working, machining including turning, drilling, milling and the like and, in general, by any conventional shaping or cutting operation used in ferrous metal technology. While not necessary in the usual instance, the alloys of the present invention can be formed by any of the relatively new high-energy processes, including high-energy impact processes and explosive forming. The alloys of the present invention can also be provided with a hard surface by conventional low temperature nitriding.

It is to be observed that the present invention provides a novel maraging steel having in the age hardened condition highly useful levels of strength in excess of about 140 k.s.i. yield strength coupled with high levels of toughness indicated by C.V.N. impact values in transverse plate samples at room temperatures in excess of about 70 ft.- lbs. at yield strengths up to about 150 k.s.i. and in excess of about 50 ft.-lbs. at yield strength-s up to about 170 k.s.i. or higher. This advantageous combination of strength and toughness is highly useful since it can be readily attained in welded plate structures. The alloys also exhibit advantageous combinations of mechanical characteristics at cryogenic temperatures. For example, in general the alloys of the present invention exhibit C.V.N. impact values of the order of 30 to 60 ft.-lbs. at 100 F. and C.V.N. impact values of the order of 20 to 40 -ft.-lbs. at -320 F. Other metals such as alloy carbon steels cannot, under commercial conditions, attain the required strength and toughness combination in welded plate structures without resorting to uneconomical and/ or technically impractical preand/or postweld heat treatments.

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.

We claim:

1. A maraging steel for use at 0.2% yield strengths in excess of about 140,000 pounds per square inch in thick sections in plate structures consisting essentially, in weight percent, of about 9.5% to about 13.5% nickel, about 2.5% to about 8% chromium, said nickel and said chromium being interrelated so that the sum thereof is about 13.5% to 19%, about 1.9% to about 4.2% molybdenum, about 0.05% to about 0.40% aluminum, up to about 0.3% titanium, about 0.001% to about 0.03% carbon, up to about 0.25% manganese, up to about 0.50% silicon with the silicon content not exceeding about 0.3% when the aluminum exceeds about 0.3%, up to about 0.01% boron, up to about 0.1% zirconium, up to about 2% total of metal from the group consisting of beryllium, vanadium, columbium, tantalum and tungsten, said elements in said group being present individually in 14 amounts of 0% to 0.2% beryllium, 0% to 1% vanadium, 0% to 0.4% columbium, 0% to 0.8% tantalum and 0% to 2% tungsten, the balance of the steel being essentially iron in an amount of at least about 74%.

2. A plate structure made of the maraging steel of claim 1.

3. The steel composition as set forth in claim 1 in which the silicon content does not exceed about 0.3%.

4. A steel as in claim 1 heat treated by subjecting said steel to a first martensitic transformation, annealing said steel to convert the thus-formed martensite to austenite, cooling the annealed steel to effect a second transformation to martensite and thereafter aging the thus-treated martensitic steel at about 800 F. to about 1000 F. to increase the 0.2% yield strength thereof to at least about 140,000 pounds per square inch.

5. A maraging steel for use at 0.2% yield strengths in excess of about 140,000 pounds per square inch in thick sections in plate structures consisting essentially, in weight percent, of about 9.5% to about 13.5% nickel, about 2.5% to 5.5% chromium, said nickel and said chromium being interrelated so that the sum thereof is about 13.5% to 17%, about 1.9% to about 4.2% molybdenum, about 0.05% to about 0.40% aluminum, up to about 0.3% titanium, about 0.001% to about 0.03% carbon, up to about 0.25 manganese, up to about 0.50% silicon with the silicon content not exceeding about 0.3% when the aluminum exceeds about 0.3%, up to about 0.01% boron, up to about 0.1% zirconium, up to about 2% total of metal from the group consisting of beryllium, vanadium, columbium, tantalum and tungsten, said elements in said group being present individually in amounts of 0% to 0.2% beryllium, 0% to 1% vanadium, 0% to 0.4% columbium, 0% to 0.8% tantalum and 0% to 2% tungsten, the balance of the steel being essentially iron in an amount of at least about 74%.

6. A maraging steel as in claim '5 containing about 11% to about 12% nickel and about 2.5% to about 3.5% molybdenum.

7. A maraging steel for use at 0.2% yield strengths in excess of about 140,000 pounds per square inch in thick sections in plate structures consisting essentially, in weight percent, of about 9.5% to about 13.5% nickel, about 2.5% to about 5.5% chromium, said nickel and said chromium being interrelated so that the sum thereof is about 14% to about 16%, about 1.9% to about 4.2% molybdenum, about 0.05% to about 0.40% aluminum, up to about 0.3% titanium, up to about 0.03% carbon, up to about 0.15% manganese, up to about 0.3% silicon with the balance being essentially iron.

8. A process of heat treating maraging steel containing about 9.5% to about 13.5% nickel, about 2.5% to 5.5% chromium, about 1.9% to about 4.2% molybdenum, about 0.05% to about 0.4% aluminum and less than about 0.03% carbon, comprising subjecting said steel to a first martensitic transformation, annealing said steel to convert the thus-formed martensite to austenite, cooling the annealed steel to effect a second transformation to martensite and thereafter aging the thus-treated martensitic steel at about 800 F. to about 1000 F. for about 1 to about 10 hours.

9. A maraging steel for use at 0.2% yield strengths in excess of about 140,000 pounds per square inch in thick sections in welded plate structures consisting essentially, in weight percent, of about 11% to about 12% nickel, about 2.5% to about 5% chromium, said nickel and said chromium being interrelated so that the sum thereof is about 14% to about 17%, about 1.9% to about 4.2% molybdenum, about 0.05% to about 0.2% aluminum, up to about 0.3% titanium, about 0.001% to 0.033% carbon, up to about 0.2% manganese, up to about 0.25% silicon, up to about 0.01% boron, up to about 0.1% zirconium, with the balance being essentially iron.

1 5 1 6 10. A maraging steel consisting essentially, in weight References Cited by the Examiner percent, of about 10% to about 12.5% nickel, about 3% to about 5.5% chromium, the total of said nickel and said UNITED STATES PATENTS chromium being at least about 14%, about 2% to about 2999039 9/1961 Lula et 14837 4% molybdenum, about 0.05% to about 0.3% aluminum, 5 3,093,519 6/1963 Decker et 148-442 X about 0.1% to about 0.2% titanium, up to about 0.03% 3,123,506 3/1964 Tanclyn 14831 carbon, up to about 0.25% manganese, up to about 0.3% 3,151,978 10/1964 Perry et a1 148 37 silicon, with the balance being essentially iron. 3,164,497 1/1965 Matsuda 148-142 11. A plate structure made of the maraging steel of HYLAND BIZOT, Primary Examiner claim 10.

10 12. A maraging steel as in claim 10 containing about DAVID RECK Exammer' 2.5% to about 3.5% molybdenum. C. N. LOVELL, Assistant Examiner.

Claims (1)

1. 8. A PROCESS OF HEAT TREATING MARAGING STEEL CONTAINING ABOUT 9.5% TO ABOUT 13.5% NICKEL, ABOUT 2.5% TO 5.5% CHROMIUM, ABOUT 1.9% TO ABOUT 4.2% MOLYBDENUM,, ABOUT 0.05% TO ABOUT 0.4% ALIMINUM AND LESS THAN ABOUT 0.03% RCARBON, COMPRISING SUBJECTING SAID STEEL TO A FIRST MARTENSITIC TRANSFORMATIION, ANNEALING SAID STEEL TO CONVERT THE THUS-FORMED MARTENSITE TO AUSTENITE, COOLING THE ANNEALED STEEL TOEFFECT A SECOND TRANSFORMATION TO MARTENSITE AND THEREAFTER AGING THE THUS-TREATED MARTENSITIC STEEL AT ABOUT 800*F. TO ABOUT 1000*F. FOR ABOUT 1 TO ABOUT 10 HOURS.

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