US3408178A - Age hardenable stainless steel alloy - Google Patents

Description

% NICKEL OR NICKEL. EQUIVALENT 1968 L. P. MYERS ET Al. 3,498,178

AGE HARDENABLE STAINLESS STEEL ALLOY Filed June 27, 1967 I i l l l l s 9 10 I: l2 l3 United States Patent 3,408,178 AGE HARDENABLE STAINLESS STEEL ALLOY Lewis P. Myers, Mount Penn, and Kermit J. Goda, Jr.,

Leesport, Pa., assignors to The Carpenter Steel Company, Reading, Pa., a corporation of New Jersey Continuation-impart of application Ser. No. 401,831,

Oct. 6, 1964. This application June 27, 1967, Ser.

10 Claims. (Cl. 75-425) ABSTRACT OF THE DISCLOSURE A high strength stainless steel alloy age hardenable to a hardness of at least 45 Rockwell C with no more than about 10% retained austenite containing no more than 0.2% carbon, 8-14% chromium, -12% nickel, up to 7% copper, 0.6 to 2% titanium, no more than 0.5% aluminum, 0.25 to 1% columbium, up to 0.1% boron, and the remainder iron in which the chromium and nickel or nickel equivalent content of the alloy are balanced as indicated by the area ABCDEF in the drawing, in which the nickel equivalent content is calculated by adding onehalf the percent copper to the percent nickel, and in which the copper may replace no more than about 4% nickel.

This application is a continuation-in-part of our application Ser. No. 401,831 filed Oct. 6, 1964, allowed on Mar. 31, 1967, and now abandoned. Except for the larger amounts of carbon that may be present in our alloy, our present application is identical in all essential respects to said application Ser. No. 401,831, now abandoned.

This invention relates to iron-base alloys and more particularly to high strength stainless steel alloys having exceptionally high notch ductility and tensile strength.

Heretofore, stainless steel alloys having good corrosion resistance have left much to be desired when high strength was required. The use of relatively large proportions of such costly alloying elements as nickel and cobalt has met with some success but the highcost of the alloys thus provided has substantially limited their usefulness.

We have discovered that by carefully controlling the relative proportions of the main alloying elements, chromium and nickel or chromium, nickel and copper with respect to each other and together with additive elements as will be more fully described hereinafter, we provide an alloy from which parts may be readily fabricated and age hardened to a substantially fully martensitic condition having an outstanding combination of mechanical and stainless properties. To achieve these properties as well as good ductility and notch tensile strength in our stainless steel alloy, we carefully control the elements present in our alloy to ensure that in its fully heat treated condition there is no more than about and preferably no more than about 1% retained austenite in the alloy.

Important advantages of the present invention reside in the unique combination of mechanical and stainless properties in products manufactured from our alloy. In addition to high strength, ductility and high notch tensile strength, such products are uniformly easily formed and machined while in the solution treated condition prior to aging. When fabricated and heat treated, parts formed from our alloy may be used where good corrosion and oxidation resistance are required. With controlled additions of copper, products made from our alloy are-especially well suited for use where, in addition to high strength, good corrosion resistance to salt spray and atmospheric conditions are required. In addition to being characterized by an outstandingly unique combination of "ice strength and resistance to stress corrosion cracking in certain media, a further advantage resides in the fact that parts formed of our alloy may be readily welded and are not sensitive to weld-cracking. A high proportion of these properties is provided by suitably proportioning the elements chromium, nickel, copper, titanium and columbium, in an iron base alloy in which carbon is kept below a low level and subjecting the alloy to a heat treatment which brings out the desired strength, toughness, ductility and hardness of the alloy while preserving its stainless properties.

In its broader aspects, the present invention provides an alloy consisting essentially of a maximum of about .2% carbon, about 8 to 14% chromium, about 5 to 12% nickel, about .6 to 2% titanium or titanium plus aluminum but not more than about .5 aluminum, about .25 to 1% columbium, about 0 to 7% copper, up to about .l% boron, and the balance being essentially iron except for small amounts of other elements which do not adversely alfect the desired properties of the alloy. Phosphorus and sulfur are not to exceed .05 While manganese up to about 2% and silicon up to about 1% may be present when the alloy is air melted, they are usually present only as an impurity when the alloy is melted or remelted under a controlled atmosphere. Here and throughout this application, percent concentrations refer to percent by weight.

Within the ranges stated, it is essential, in order to achieve among other things the high strength and ductility of our alloy, that the amounts of chromium, nickel and copper present in our alloy be carefully balanced. Unless the combined total of chromium and nickel or chromium and nickel plus one-half the copper content of our alloy is at least equal to 18%, the high notch tensile strength and ductility characteristic of our alloy is not attainable. In our alloy, copper, which may be present in amounts ranging up to about 7% and may be used to replace up to about 4% of the nickel, is only about one-half as effective as nickel in its effect on causing the formation of austenite in the alloy. Thus, 1% of copper is equivalent to .5 of nickel in this respect and in computing the nickel equivalent content of our alloy, one-half of the copper content is added to the nickel content. As the chromium conent of our alloy is raised from about 10% to about 14%, the maximum amount of nickel or nickel equivalent tolerable in our alloy decreases from 12% with a chromium content of 10%, to about 9% with a chromium content of about 14%. We have found that unless the nickel or nickel equivalent content is kept below 12% for the smaller amounts of chromium, that is, between about 8 to 10% chromium, and unless it is carefully reduced to 9% as the chromium content is increased from 10 to 14%, an excessive amount of stable austenite,

that is more than about 10%, is retained in the composition, and the unique properties of our alloy are not attainable.

The critical interrelationship of the chromium and nickel or the chromium plus nickel and copper content of our alloy may be best understood with reference to the drawing in which the percent chromium is plotted along the abscissa and the percent nickel equivalent is plotted along the ordinate. The area ABCDEF clearly brings out the manner in which the chromium, nickel and copper content in our alloy is balanced at each level of chromium and nickel within the ranges of 8 to 14% chromium and 5 to 12% nickel. In practicing the broader aspects of our invention, chromium and nickel or chromium and nickel equivalent contents are balanced so that when their contents expressed as percent are plotted respectively along the abscissa and ordinate of the accompanying drawing, the point defined thereby will lie on or within the area 3 ABCDEF. Thus, for a given chromium content of 12%, the minimum nickel or nickel equivalent content must be about 6% and the maximum must not exceed about 10.5%. Similarly, if the nickel or nickel equivalent content of the alloy is desired to be about 9.5%, then the chromium content may range from about 8 /2 %1 to 13.3

Our age hardenable stainless martensitic alloy is especially suitable for making parts having intricate shapes because of the ease with which it may be fabricated in its annealed condition from which, by a relatively simple heat treatment, it is converted to an age hardened martensite having a unique combination of strength, toughness, durability and hardness. Better tensile strength, notch tensile strength and ductility in our alloy are attained in accordance with our invention by controlling the chromium and nickel or the chromium and nickel equivalent content of our alloy so that it falls within the more restricted area AHDEF as shown in the accompanying drawing. Here, while the chromium content may range from 8 to 14%, the nickel content or the nickel equivalent content of our alloy ranges from 10 to 12% for 8% chromium, to to 9% when the chromium content is as high as 14%. In the area AHDEF, an ultimate tensile strength of about 225,000 p.s.i. or more is attainable. However, we prefer to utilize a minimum of 7% nickel or nickel equivalent for consistent attainment of high notch strength and ductility.

To enhance the stainless properties of our alloy, we utilize a minimum of 10% chromium and at least 1% copper in our alloy so that it falls within the area BCDEK. The better strength and ductility together with enhanced stainless properties are thus attained in our alloy when it is controlled so as to fall within the area GHDEK.

Fabricators of parts having intricate shapes, which in use are subjected to stress, require that the alloy utilized be sufliciently strong and ductile so that the part is not likely to fail because of high stress concentrations that occur at sharp angles or fine cracks. In accordance with our invention, when the chromium and nickel or the chromium and nickel equivalent content of our alloy are maintained so as to fall within the more restricted area AH'NLF', not only is the ultimate tensile strength of our alloy equal to at least 225,000 p.s.i. or more, but with a stress concentration factor (K of 10 in the notch, the notch tensile strength of any given analysis in that area is greater than the ultimate tensile strength. In other words, the ratio of the notch tensile strength and the ultimate tensile strength is greater than one and the part fails in the smooth part of the test specimen before failing at the notch.

As shown in the drawing, in the area AHNLF', chromium ranges from about 8% to 13.5% while nickel or the nickel equivalent range from about 7.8% to 12%, to provide high strength in our alloy combined with exceptional notch ductility. To enhance corrosion resistance we use at least about 10% chromium and at least about 1% copper as was pointed out hereinabove. Thus, our alloys falling in the area GH'NLM are characterized by a unique combination of stainless and mechanical properties. Furthermore, to provide maximum corrosion resistance we prefer to limit carbon to 0.03% and for best results to .01% or less.

In addition to the careful balance of the elements chro mium, nickel and copper in our alloy, it is also necessary to carefuHy control the amounts of titanium, aluminum, columbium and boron. In our alloy, at least .6% titanium is required to impart hardness and strength, while above about 2%, titanium tends to cause embrittlement of the alloy. For best results, we preferably utilize titanium in an amount ranging from .75 to 1.6%. However, when the larger amounts of chromium, that is in the neighborhood of 14%, are present in our alloy, best results are attained with from about .75 to 1.2% titanium.

Columbium works to enhance notch ductility and toughness and for this purpose we utilize columbium in an amount ranging from about .25 to about 1%. Below about .25%, the desired effect is not attained. Above about 1%, columbium does not appear to have any beneficial effect and because it is a strong austenite stabilizer, columbium may upset the martensitic balance of the alloy when present in amounts in excess of about 1%. Preferably, columbium in an amount ranging from about .35% to .65% is used.

Aluminum is a desirable addition in our alloy in amounts ranging up to .5% and preferably when used is not present in an amount in excess of 35%. When present, aluminum works together with the titanium to ensure maximum strength with high notch ductility in our alloy. When both aluminum and titanium are present in our alloy, their combined content should not exceed about 1.5%. Because titanium is a powerful austenite stabilizer, aluminum in the stated amount of up to about .5 is advantageously utilized as a substitute for a like amount of the titanium to provide ultimate tensile strength in the neighborhood of 290,000 p.s.i. However, when maximum transverse ductility is required rather than maximum ultimate strength, titanium without aluminum is preferably utilized. Boron is also useful in enhancing the transverse ductility of parts formed from our alloy and has a beneficial effect upon the hot workability of our alloy in amounts ranging up to about .1%. Preferably, our alloy includes from about .002% to .005 boron.

Carbon is not an essential constituent of our alloy and preferably is limited to no more than .03%. Carbon is more or less tolerable in our alloy, depending upon its intended use. Most notably affected by the presence of larger amounts of carbon are the transverse properties (e.g., transverse ductility and toughness) of parts such as bars having a substantial cross section. Larger amounts of carbon may also tend to detract from the corrosion resistance of our alloy. Thus, carbon is preferably limited to .03 maximum and best results are achieved when carbon is limited to no more than .0l%. On the other hand, when the better transverse properties of the alloy are not required, as for example, when the alloy is provided in the form of sheet material, larger amounts of carbon can be present in our alloy so long as it is not present in suificient quantity to upset the martensitic balance of the alloy or adversely affect the desired strength. Because carbon tends to tie up titanium and columbium which are believed to be involved in the hardening, strengthening and/or toughening mechanism of our alloy, no more than about .2% carbon should be present in our alloy unless the elements titanium and columbium are present in the amounts designated by the upper portions of their respective ranges.

While the best properties of our alloy are brought out when such techniques as melting under vacuum are utilized, good ingots well suited for many less demanding uses are obtained when the alloy is made using conventional air melting techniques such as air induction or are melting. Best results are attained when the alloy is made by first casting an ingot using air induction or are melting practices or vacuum induction melting, forming the ingot thus obtained into an electrode and remeltin-g it under vacuum following conventional vacuum consumable electrode arc melting techniques.

The alloy is solution treated (that is, austenitized) by heating to 1400-1700 F. for from one quarter of an hour to two hours followed by air cooling. In some instances it may be desirable to use a two step solution treatment in which event the alloy is first heated to from 1700-1900 F. followed by air cooling and reheating to 1400 to 1600 F. and again air cooled. In its solution treated condition, our composition is soft and may readily be worked as desired to form a wide variety of intricate shapes. Cooling below room temperature is not required. The parts which have been formed from the alloy in its soft condition are strengthened, hardened and toughened by aging for a suitable length of time at a temperature of about 800 to 1100 F. followed by cooling in air. The

optimum duration of the aging treatment may be readily determined for each analysis. In practice, aging for from two to sixteen hours has proven satisfactory although shorter aging treatments, fifteen minutes or even less, may prove advantageous.

Age hardenable martensitic stainless steel alloys illustrative of our invention are set forth in Table I. Unless otherwise indicated, each of these alloys as well as those set forth following Table I, was hammer forged to bars from ingots cast under vacuum. The bars were solution treated or annealed at about 1500" F. for about one-half hour, formed into test specimens which were then aged for eight hours at about 950 F.

TABLE 1 Ex. C Cr N1 Ti Al Cb Cu B In the alloys of the foregoing examples and in those set forth hereinafter unless otherwise noted, manganese and silicon each were present in amounts less than .1% and phosphorus and sulfur were below .01%. For boron, the amount shown was the amount added.

TABLE IL-MECHANICAL PROPERTIES Percent Percent Ultimate Notch Ex. Hardness, Elonga- Reduction Tensile Tensile No Rockwell C tion of Area Strength, Strength,

p.s.1. p.s.1.

Solution treated at 1500 F. and aged at 900 F.

The hardness of the alloys of Examples 1-9 was also measured on specimens in the annealed or solution treated condition and all were found to be between Rockwell C 28 and Rockwell C 32. Smooth ultimate tensile strength specimens were readily formed having a gauge length of one inch, a .252 inch gauge diameter and one-half inch diameter threads at each end. The notch tensile strength specimens having a stress concentration factor (K of 10 were formed 4 /2 inches long having /2-inch diameter threads at each end, a diameter of .357 inch notched to .252 inch diameter with a root radius of .001 inch and a 60 notch angle.

TABLE I11 Ex. C Cr Ni Ti Al Cb Cu B N o.

As in the case of the examples in Table 1, Examples 10-13 were vacuum melted and contained less than .1% manganese, less than .1% silicon, less than .01% phosphorus and less than .0l% sulfur, the balance of Examples 1-l3 was essentially iron in each instance.

TAB LE IV.-MEGHANICAL PROPE RTIES Percent Percent Ultimate Notch Ex. Hardness, Elonga- Reduction Tensile Tensile No. Rockwell 0 tion of Area Strength, Strength,

p.s.i. p.s.1.

In marked contrast to the alloys of the present inven tion set forth in Table 1, Examples 10, 11 and 13 exhibit extremely poor notch tensile strength, while Example 12 is characterized by extremely low ultimate tensile strength. Furthermore, Examples 10, 11 and 13 exhibit the effects of poor ductility and Example 12 although exceedingly ductile illustrates the adverse effect of excessive retained austenite upon the strength of the alloy.

As was described in connection with Examples l-9, the alloys of Examples 14 and 15 were prepared having analyses as set forth in Table V. Test specimens Were made from the alloys of Examples 14 and 15 also as was described in connection with Examples l-9 except that the parts were aged for four hours instead of eight hours at about 950 F. The mechanical properties thereof set forth in Table VI were obtained from tests carried out on the specimens as was described in connection with the examples of Table I.

TABLE V Ex. No.

C Cr Ti Al Cb Cu B Al was less than 03%.

As in the case of the examples in Table 1, Examples 14 and 15 were vacuum melted. While the silicon content of each was .04%, Examples 14 and 15 contained less than .0l% manganese, less than .0l% phosphorus, less than .0l% sulfur and the balance was essentially iron in each instance.

dicated a hardness in their aged condition of Rockwell C 44 but that alloy is capable of being aged to a higher hardness level by using a somewhat lower aging temperature without substantially affecting the other properties of the alloy. When a specimen of Example 15 Was prepared and heat treated as was previously described except that aging was carried out at 900 F., it was found to have a hardness of 46 Rockwell C.

It is apparent from Examples 14 and 15 that it is the critical interrelationship between the chromium and nickel or nickel plus one-half the copper content in our alloy which determines its outstanding properties of ultimate and notch tensile strength and ductility, and as was noted hereinabove, these properties are not unduly affected by the larger amounts of carbon so long as the martensitic balance of the alloy is not upset. Specimens of Examples 14 and 15 in their heat treated condition showed no more retained austenite, less than about 3%, than would be consistent with their position in the graph of the drawing, slightly above the line AGH, determined by their chromium content and their nickel plus one-half their copper content.

The outstanding properties of our alloy are readily obtained when it is prepared in commercial quantities so as to contain a maximum of .01% carbon, about 10.75%

7 to 11.5% chromium, about 8.5% to 9% nickel, about 1% to 1.25% titanium, about .35% to .45% columbium, about 1.2% to 1.8% copper, about 002% to 004% boron, a maximum of .1% manganese,'a maximum of .1% silicon, a maximum of .01% each of phosphorus, sulfur and nitrogen with the remainder consisting essentially of iron. For example, a 5,000 pound ingot was prepared by first melting a heat in a vacuum induction furnace and casting it into a 14-inch round ingot which was then remelted in a vacuum consumable electrode furnace to a 20-inch round ingot having the following analysis in percent by weight:

1 Balance except for incidental impurities.

The thus formed ingot was readily hot worked to a 9 inch square billet. A section was cut from the billet and solution treated at 1500" F. for one-half hour and then water quenched. As solution treated, the alloy had a hardness of Rockwell C 30. Test specimens were prepared from the section as was previously described in connection with Examples 1-9 and were then age hardened at 950 F. for eight hours followed by air cooling to a hardness of Rockwell C 48. The specimens thus prepared had a room temperature ultimate tensile strength of 235,000 p.s.i. and a notch tensile strength of 290,000 p.s.i. The .2% yield strength was 223,000 p.s.i. with an elongation of 11% and with 49% reduction in area.

The 9 inch square billet was further hot worked to 4 inches square and, after surface preparation, was hot rolled to bars about .813 inch round. Following solution treatment at about 1500 F. for one-half hour and water quenching, test specimens were prepared from these rounds as was previously described in connection with Examples 1-9. The test specimens were aged by heating at about 900 F. for about 4 hours followed by air cooling to a hardness of Rockwell C 51. The specimens thus prepared had a room temperature ultimate tensile strength of 257,000 p.s.i. and a notch tensile strength of 301,000 p.s.i. The .2% yield strength was 245,000 p.s.i. with an elongation of 12% and with 47% reduction in area.

The following test is illustrative of the corrosion resistance of our alloy. A specimen having the analysis of Example 1 was prepared in the shape of a tensile specimen and was immersed in a boiling aqueous solution of 6% sodium chloride and 1.5% sodium dichromate (Na Cr O with the specimen loaded at about 75% of its .2% yield strength, that is, at about 190,000 p.s.i. The thus tested specimen failed after 97 hours. Two such specimens of essentially identical analysis as Example 1 were subjected to the same test at a load of about 184,000 p.s.i. with one failing at 265 hours and the other at 318 hours. Specimens of essentially the identical analysis as Example 1 were also subjected to salt spray at 95 F. for 14 days without undergoing any noticeable discoloration.

The high strength and ductility of our alloy coupled with its good stainless properties and the readiness with which it may be aged to a hardness of at least Rockwell C 45 make it eminently well suited for a wide variety of uses.

The terms and expressions which havexbeen employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recog-' nized that various modifications are possible within"the scope of the invention claimed.

We claim:

1. A stainless steel alloy capable of being age hardened to a hardness of at least 45 Rockwell C with an essentially martensitic structure having no more than about 10% retained austenite, said alloy having high tensile strength and high notch tensile strength and consisting essentially of chromium and a nickel equivalent selected from the group consisting of nickel and nickel plus up to about 7%- copper in amounts substantially in accordance with the area ABCDEF in the accompanying drawing with cop per replacing no more than about 4% nickel and in which the nickel equivalent content is calculated by adding one-half the percent copper to the percent nickel, up to about .2% carbon, about .6% to 2% titanium, up to about .5% aluminum,about .25% to 1% columbium, up to about .1% boron, up to about 2% manganese, up to about 1% silicon, and the remainder consisting essentially of iron except for incidental impurities.

2. The stainless steel alloy as set forth in claim 1 in which said nickel equivalent is at least about 7%.

3. A stainless steel alloy capable of being age hardened to a hardness of at least 45 Rockwell C with an essentially martensitic structure having no more than about 10% retained austenite, said alloy having high tensile strength and high notch tensile strength and consisting essentially of chromium and a nickel equivalent selected from the group consisting of nickel and nickel plus from, about 1 to 7% copper in amounts substantially in accordance with the area ABCDEF in the accompanying drawing with copper replacing no more than about 4% nickel and in which the nickel equivalent content is calculated by adding one-half the percent copper to the percent nickel, up to about .2% carbon, about .6% to 2% titanium, up to about .5% aluminum, about .25% to 1% columbium, up to about .1% boron, up to about 2% manganese, up to about 1% silicon, and the remainder consisting essentially of iron except for incidental impurities.

4. A stainless steel alloy capable of being age hardened to a hardness of at least 45 Rockwell C with an essentially martensitic structure having no more than about 10% retained austenite, said alloy having high tensile strength and high notch tensile strength and consisting essentially of chromium and a nickel equivalent selected from the group consisting of nickel and nickel plus from about 1 to about 7% copper in amounts substantially in accordance with the area ABCDEF in the accompanying drawing with copper replacing no more than about 4% nickel and in which the nickel equivalent content is calculated by adding one-half the percent copper to the percent nickel, up to about .03% carbon, about .75 to 1.6% titanium with no more than about 1.2% titanium when chromium is present in an amount of about 14%, up to about 35% aluminum, about 35% to .65 columbium, about 002% to .005 boron, up to about 2% manganese, up to about 1% silicon, and the remainder consisting essentially of iron except for incidental impurities.-

5. A stainless steel alloy capable of being age hardened to a hardness of at least 45 Rockwell C with an essentially martensitic structure having no more than about 1% retained austenite, said alloy having high tensile strength and high notch tensile strength and consisting essentially of chromium and nickel equivalent selected from the group consisting of nickel and nickel plus from about 1 to about 7% copper in amounts substantially in accordance with the area AHDEF in the accompanying drawing with copper replacing no more than about 4% nickel and in which the nickel equivalent content is calculated by adding one-half the precent copper to the percent nickel, up to about .2% carbon, about .6% to 2% titanium, up to about .5% aluminum, about .25% to 1% columbium, up to about .1% boron, up to about .2%

manganese, up to about l% silicon, and the remainder consisting essentially of iron except for incidental impurities.

6. A stainless steel alloy capable of being age hardened to a hardness of at least 45 Rockwell C with an essentially martensitic structure having no more than about 1% retained austenite, said alloy having high tensile strength and high notch tensile strength and consisting essentially of chromium and a nickel equivalent selected from the group consisting of nickel and nickel plus from about 1 to about 7% copper in amounts substantially in accordance with the area AHDEF in the accompanying drawing with copper replacing no more than about 4% nickel and in which the nickel equivalent content is calculated by adding one-half the percent copper to the percent nickel, up to about .03% carbon, about .75 to 1.6% titanium with no more than about 1.2% titanium when chromium is present in an amount of about 14%, up to about .35% aluminum, about 35% to .65% columbium, about .002% to .005% boron, up to about 2% manganese, up to about 1% silicon, and the remainder consisting essentially of iron except for incidental impurities.

7. A stainless steel alloy capable of being age hardened to a hardness of at least 45 Rockwell C with an essentially martensitic structure with no more than about 1% retained austenite, said alloy having high tensile strength and high notch tensile strength and consisting essentially of chromium and a nickel equivalent selected from the group consisting of nickel and nickel plus from about 1 to about 7% copper in amounts substantially in accordance with the area AH'NLF' in the accompanying drawing with copper replacing no more than about 4% nickel and in which the nickel equivalent content is calculated by adding onehalf the percent copper to the percent nickel, up to about .2% carbon, about .6% to 2% titanium, up to about .5% aluminum, about to 1% columbium, up to about .1% boron, up to about 2% manganese, up to about 1% silicon, and the remainder consisting essentially of iron except for incidental impurities.

8. A stainless steel alloy capable of being age hardened to a hardness of at least 45 Rockwell C with an essentially martensitic structure having no more than about 1% retained austenite, said alloy having high tensile strength and high notch tensile strength and consisting essentially of chromium and a nickel equivalent selected from the group consisting of nickel and nickel plus from about 1 to about 7% copper in amounts substantially in accordance with the area AHNLF in the accompanying drawing wtih copper replacing no more than about 4% nickel and in which the nickel equivalent content is calculated by adding one-half the percent copper to the percent nickel, up to about .03% carbon, about .75 to 1.6% titanium with no more than about 1.2% titanium when chromium is present in an amount of about 14%, up to about 35% aluminum, about .35 to .65% columbium, about .002% to .005% boron, up to about 2% manganese, up to about 1% silicon, and the remainder consisting essentially of iron except for incidental impurities.

9. A stainless steel alloy capable of being age hardened to a hardness of at least 45 Rockwell C with an essentially martensitic structure having no more than about 1% retained austenite, said alloy having high tensile strength and high notch tensile strength and consisting essentially of chromium and a nickel equivalent selected from the group consisting of nickel and nickel plus from about 1 to about 7% copper in amounts substantially in accordance with the area GHNLM in the accompanying drawing with copper replacing no more than about 4% nickel and in which the nickel equivalent content is calculated by adding one-half the percent copper to the percent nickel, up to about .03% carbon, about .6% t0 2% titanium, up to about .5 aluminum, about .25% to 1% columbium, up to about .1% boron, up to about 2% manganese, up to about 1% silicon, and the remainder consisting essentially of iron except for incidental impurities.

10. A stainless steel alloy capable of being age hardened to a hardness of at least 45 Rockwell C with an essentially martensitic structure having no more than about 1% retained austenite, said alloy having high tensile strength and high notch tensile strength and consisting essentially of no more than about .01% carbon, about 10.75% to 11.5% chromium, about 8.5% to 9% nickel, about 1% to 1.25% titanium, about .35 to .45 columbium, about 1.2% to 1.8% copper, about .002% to 004% boron, no more than about .1% manganese, no more than about .1% silicon, and the remainder consisting essentially of iron except for incidental impurities.

References Cited UNITED STATES PATENTS 2,381,416 8/1945 Wyche et al 148-38 XR 2,447,897 8/ 1948 Clark -125 2,482,096 9/1949 Clark 75-125 2,528,497 11/1950 Clark 75-125 2,747,989 5/1956 Kirkby 75-128 2,850,380 9/1958 Clark 75-125 2,999,039 9/1961 Lula et a1. 148-37 3,210,224 10/1965 Argo 148-142 3,258,370 6/1966 Floreen et al 75-128 XR 3,288,611 11/1966 Lula et a1. 75-128 3,347,663 10/1967 Bieber 75-128 XR 3,357,868 12/1967 Tanczyn 148-37 XR L. DEWAYNE RUTLEDGE, Primary Examiner.

P. WEINSTEIN, Assistant Examiner.

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