US3861909A

US3861909A - High strength steel alloy

Info

Publication number: US3861909A
Application number: US275355A
Authority: US
Inventors: Robert L Caton
Original assignee: Carpenter Technology Corp
Current assignee: Carpenter Technology Corp
Priority date: 1970-05-11
Filing date: 1972-07-26
Publication date: 1975-01-21
Anticipated expiration: 1992-01-21

Abstract

An age-hardenable stainless iron-base alloy and method of making the same containing by weight about 9-13% chromium, about 8-16% cobalt, about 4-8% molybdenum, about 4-8% nickel in which those elements are balanced to provide an austenite retention index (ARI) value of from about 18.0 to 22.8 as calculated from the equation

Description

United States Patent [191 Caton [451 Jan. 21, 1975 Related US. Application Data [63] Continuation-impart of Ser. No. 36,219, May 11,

1970, abandoned.

[52] US. Cl 75/128 B, 75/128 F, 75/128 N,

75/128 W [51] Int. Cl. C22c 39/20 [58] Field of Search 75/128 B, 128 W [56] References Cited UNITED STATES PATENTS 2,306,662 12/1942 Krivobok 75/128W 2,432,618 12/1947 Franks 75/128 B 3,251,683 5/1966 Hammond 75/128 B R26,225 6/1967 Kasak 75/128 B Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or FirmEdgar N. Jay

[57] ABSTRACT An age-hardenable stainless iron-base alloy and method of making the same containing by weight about 9-13% chromium, about 816% cobalt, about 4-8% molybdenum, about 4-8% nickel in which those elements are balanced to provide an austenite retention index (ARI) value of from about 18.0 to 22.8 as calculated from the equation ARl Ni 0.8(% Cr) 0.6(% M0) 0.3(% Co).

8 Claims, 1 Drawing Figure HIGH STRENGTH STEEL ALLOY This application is a continuation-in-part of my copending earlier application filed May 11, 1970, Ser. No. 36,219 and now abandoned.

This invention relates to an age-hardenable stainless steel alloy and method of making the same, and more particularly to a Cr-Co-Ni-Mo stainless steel alloy having high strength, good ductility, and/or good thermal stability.

Age-hardenable martensitic stainless steel alloys have hitherto been provided which combine highly desirable physical and chemical properties. The alloy of my U.S. Pat. No. 3,364,013 granted Jan. 16, 1968 is one such alloy in which about 12-18% Cr, 1.5-3.5% Mo, and 18-26% Co are balanced in an iron matrix so as to provide a primarily martensitic microstructure. No more than about 3% nickel is included in that alloy in order to keep the amount of retained austenite below about 20%. And in order to consistently attain high tensile strength at room temperature with maximum strength at elevated temperatures, the alloy is balanced with no more than about 0.50% nickel present so as to contain less than about 5% retained austenite.

Because nickel is such a powerful austenite former, it has been the usual practice as in said U.S. Pat. No. 3,364,013 as well as others, e.g., U.S. Pat. No. 3,154,412 granted Oct. 27, 1964, to exclude nickel or to restrict it to relatively small amounts even though when present it imparts certain desirable properties to the alloy. For example, in U.S. Pat. No. 3,251,683 granted on May 17, 1966, it is brought out that when cobalt is at about its lower limit of 3%, nickel can be present up to about 7% in order to enhance resistance to stress corrosion if the accompanyng reduction in mechanical properties can be tolerated.

While all three of the aforementioned patents are concerned with martensitic stainless steel, two of them rely upon balancing the socalled nickel equivalent elements and the chromium equivalent elements to establish the properties of the alloy while the remaining U.S. Pat. No. 3,154,412, though also containing the elements, Cr, Co, Ni, Mo and Fe, relies upon substantial amounts of carbon and nitrogen to control the microstructure and thereby the properties of the alloy.

In U.S. Pat. No. 3,508,912, an age-hardenable steel is disclosed containing 2.5-6% Cr, up to 4% Ni, 8-15% cobalt, 10-18% Co Ni and with carbon and nitrogen restricted to no more than 0.08%. The patent points out that larger amounts of chromium result in the formation of objectionable amounts of delta ferrite and also adversely affect the room temperature and elevated temperature strength and ductility of the composition. Another age-hardenable steel is disclosed in U.S. Pat. No. 3,650,845 containing 8-l4% Cr, 3-10% Ni, 4l0% Co, 2-5% Mo, and no more than 0.06% C. This steel contains sufficient chromium to be considered stainless and is balanced within the limits stated in accordance with two equations, one to limit the amount of delta ferrite present and the other to ensure a fully martensitic microstructure at room temperature, that is, an M, 100C.

It is therefore a principal object of this invention to provide an improved age-hardenable martensitic stainless steel alloy and a method for making the same which more consistently can be balanced to satisfy the requirements of modern-day technology. More specifically, it is an object of this invention to provide such an alloy, and method of making the same, having high room temperature strength and toughness, and which also can be provided with elevated temperature strength, toughness and ductility, that are not objectionably affected by prolonged exposure at elevated temperature.

Further objects as well as advantages of the present invention will be apparent from the following detailed description and the accompanying drawing which is a graph showing the interdependence of the alloys room temperature ultimate tensile strength with variations in its composition as measured by its austenite retention index (ARI) in accordance with the present invention.

The present invention also includes a new method for making this essentially martensitic stainless steel as well as a new alloy produced thereby consisting essentially of, in weight percent,

All or part of the molybdenum can be replaced by an equivalent amount of tungsten.

and the balance essentially iron in which the elements nickel, chromium, molybdenum and cobalt are added in such proportions that the austenite retention index ranges from about 18.0 to about 22.8 as calculated from ARI Ni 0.8(% Cr) 0.6(% Mo) 0.3(% Co) and with the elements carbon, nitrogen, manganese and silicon controlled as pointed out hereinafter. The relationship among the elements Ni, Cr, Mo, and Co which is identified herein as the austenite retention index is based upon the relative effect of those elements in depressing the M, temperature of the alloy with the effect of nickel equal to unity. For the stated broad range of this alloy, chromium was determined to be as effective in depressing the temperature (M at which the transformation of austenite to martensite begins, molybdenum was found to be 60% as effective as nickel, and cobalt was found to be 30% as effective as nickel.

By carefully balancing the alloy in this way, an agehardenable stainless steel alloy is provided which, depending upon the amounts of chromium and nickel selected, is characterized by one or more of the following properties, (1) a room temperature ultimate tensile strength of about 225,000 psi to 300,000 psi, (2) a ratio of notch tensile strength to ultimate tensile strength of at least 1, and (3) thermal stability such that mechanical properties undergo a minimum of change on exposure to temperatures of up to about 900F for long periods (e.g., 1,000 hours).

The elements nickel, chromium, molybdenum, cobalt and iron work together but unless maintained within the stated ranges and carefully controlled to provide an austenite retention index within the values specified, the desired properties cannot be consistently attained. The elements nickel and cobalt are austenite formers, nickel being much stronger than cobalt. Both chromium and molybdenum are ferrite formers but stabilize austenite. These factors are taken into account in the manner in which the austenite retention index is computed to provide an essentially martensitic microstructure in the age-hardened condition which is substantially free of retained delta ferrite and which contains no more than a tolerable amount of austenite. Preferably there is about to austenite present to provide maximum toughness with little or no adverse effect on tensile strength. That austenite is normally made up of retained austenite which occurs as a result of solution treatment and reverted austenite which is formed furing the aging treatment.

Of the main alloying elements, nickel is the strongest austenite former and, as was noted, is assigned a value of unity in my austenite retention index. Because it has such a strong effect in depressing the M, temperature, nickel is limited to no more than about 8% while a minimum of 4% nickel is required to ensure the desired essentially martensitic microstructure by offsetting the effect of the ferriteforming elements such as chromium and molybdenum. Within its broad range, the nickel content is adjusted depending upon the intended use for the parts to be fabricated from the alloy and the amount of the other alloying elements, particularly chromium and molybdenum, that is present. Preferably nickel is present in an amount ranging from about 5% to 7%. For best room temperature corrosion and/or tensile properties in the case of parts that will not be subjected to prolonged exposure at elevated temperatures in service, this alloy is balanced with the smaller amounts of nickel and the larger amount of chromium within the stated ranges while maintaining the austenite retention index within specified limits. On the other hand, when maximum tensile properties are required in parts intended for prolonged exposure to elevated temperatures of up to about 900F, then the larger amounts of nickel are balanced with the smaller amounts of chronium within the stated ranges.

Chromium imparts stainless properties to the alloy. While as little as 9% chromium can be used, preferably about 9.5-11.5% is used, while no more than about 13% chromium is present, to ensure good stainless properties and enhance response to age hardening.

Cobalt, like nickel, is an austenite-forming element, but in this alloy has less than about one-third the effect in depressing the M temperature as an equal weight percent of nickel. Thus, larger amounts of cobalt can be used without overbalancing the alloy to work with the other elements to provide high strength and notch tensile strength. Cobalt may range from about 8 to 16% in balancing my alloy over its broad range to provide the desired austenite retention index. Preferably about 9.5l3.5% cobalt is present.

Molybdenum in an amount ranging from about 4% to 8% is essential to the strength of my alloy. Apparently, the larger amounts of molybdenum work with the larger amounts of nickel to provide strength properties which are not adversely affected by prolonged exposure to elevated temperatures of up to about 900F. For best results in providing the best combination of strength, toughness and stability at elevated temperature, about 5% to 6% molybdenum is used.

It is to be noted that tungsten can be substituted for molybdenum in this alloy. At any level of molybdenum, the amount of tungsten required to replace a given amount of molybdenum with an equivalent effect is in the proportion of about 1.2% to 1.6% tungsten to 1% molybdenum. Therefore, throughout this application it is to be understood that when molybdenum is referred to, it is intended to include molybdenum and tungsten either together or individually with the tungsten replacing all or part of the molybdenum in the proportion stated.

Carbon and nitrogen are not considered to be desired additions in this alloy, and each is preferably held to no more than about 0.01% because each is a very strong austenite former when present in solid solution about 30 times more effective in that regard than an equal amount of nickel. When carbon or nitrogen is present in solid solution in an amount of about 0.01% or less, the effect of either or both is negligible and can be ignored. In the age-hardened condition of the alloy, a substantial part of the carbon and nitrogen is not present in solution but is present in the form of precipitated carbides and nitrides and so do not affect the presence of austenite. In amounts in excess of about 0.1%, carbon may have a beneficial effect upon the ultimate tensile strength, but it excessively impairs the yield strength and results in the presence of relatively hard martensite in the as-solution-treated condition of the aloy which detracts from its formability and machinability. When the most favorable notch tensile strength is not required, carbon may be present in an amount of up to about 0.1%, but then the amount of carbon in solution multiplied by 30 must be added in calculating the ARI value when balancing the alloy to avoid the presence of excessive austenite in the as-age-hardened con dition. Like carbon, nitrogen may also be present in amounts up to about 0.1%; but when present in amounts greater than 0.01%, the amount in solid solution multiplied by 30 should be added in computing the austenite retention index.

Manganese may be used as a deoxidizer if the alloy is melted in air, but is not considered a desirable alloy ing addition in this alloy because it is a relatively strong austenite former, about one-half as effective as an equal weight percent of nickel. Insofar as the calculation of the austenite retention index of this alloy is concerned, manganese in amounts of about 0.5% or less can be ignored although manganese is preferably kept below about 0.1%. Up to about 2% manganese can be tolerated in the alloy, but above 0.5% the manganese present should be taken into account when calculating the austenite retention index by adding one half of all the percent manganese.

Silicon can also be used as a deoxidizer when the alloy is melted in air. Silicon is a strong ferrite former, being 1.5 times as effective as an equal weight percent of chromium. Though when present silicon affords some oxidation resistance, it is not considered a desirable alloying addition but can be tolerated up to about 1%. Above about 0.5% silicon must be taken into account when calculating the austenite retention index and is adjusted for by adding an amount equal to 1.5 times the amount of silicon present. In amounts less than about 0.5%, silicon can be ignored when calculating the austenite retention index and preferably silicon is kept below about 0.1%.

A relatively small amount of boron is beneficial in this alloy when best toughness properties are desired. In that event, the alloy preferably contains about 0.001% to 0.003% boron. When optimum toughness is result in an effective amount of carbon in solution cither the aged or the aged-and-exposed condition inso far as the austenite retention index is concerned (by exposed is meant heated at about 900F for 1.000

not q then P 10 8 ut 0.02% boron can be inhours), and no adjustment in the ARI value was found cludedm the alloy because of its beneficial effect upon to be necessary because f h carbon Content the ult1mate tensile strength, the 0.2% y1eld strength and the Stress rupture Strength of the alloy. Three notch and three smooth tenslle specimens The alloy is readily prepared and formed into parts. were machmed d. tested at room iempcmture for It can be melted in air in the usual way, but better re- 10 g of q (1) 3 se i sults are attained when the alloy is vacuum-induction age an E gg gifi ggg melted. Preferably, a double-melting process is used in temperature 0 out or f tut which an ingot is air or vacuum-induction melted and Speclmen followlpg Solunor, treatment wah rough cast, and then remelted under vacuum or a controlled machmed 0010 'f overslze then after havmg atmosphere as a consumable electrode. Whether the aged or after havmg been aged and exposedto 900 F alloy is air or vacuum melted, only relatively simple F 1000 W as the Case may h was i" heat treatment is required to bring out the unique propmachmed' The Smooth bar specmens f p erties of the alloy. The alloy can be solution treated Carrymg 9" room temperature meflsuremems o u nfrom about 1,4000 to ZOOOOF! preferably from about mate tensile-strength (UTS), 0.2% yleld strength (0.2% 1,500 to 1,800F, for a sufficient time to ensure compefcent elonganon Song) percent plete austenitizing. Usually about 1 hour for each inch Fiucnon m area RA) had a gauge dlameier of 0252 of thickness is sufficient. After cooling to room temperand a gauge length of Notch, tensfle Strength atul'e, it is reheated 16 about 900 to 1,100F, prefera- ,(NTS) specmlens had a gauge 0357 bly 6 to 1 OOOOF for about 2 to 4 hours followed by in., a notch diameter of 0.252 in. and a notch root racooling in Cooling below room temperature during dius of 0.001 in., a stress concentration factor (K,) of any stage of the heat treatment is not required. In the about case of work pieces of substantial thickness, rapid cool- Th results f il tests i d out at room g from the auslenitiling e p as y q nchperature on specimens of Examples l-6 in the solution ing in oil or water, is used to ensure maximum response treated and aged condition are set out in Table II. The to the age-hardening treatment. results under 0.2% yield strength are in each instance The following illustrative examples of this alloy, the an average of two tests, and the remaining values are analyses of which in weight percent are set forth in the average of three. In Table III are set out the results Table I as well as the further examples given hereinafof tests carried out at room temperature on specimens ter for purposes of comparison, (unless otherwise indiof Examples 1-6 which, following solution treatment cated) were prepared as l7-pound experimental heats and aging, were then exposed to a temperature of using vacuum-induction melting, were cast into ingots 900F for 1,000 hours. Again, the 0.2% YS data are the 2 3/8 in. sq. and then forged using a furnace temperaaverages of two tests each while the remaining results ture of about 2,100F into 5/8 in. sq. bars which were were obtained by averaging the measurements of three then formed into the required test specimens. Heat tests in each case. Hardness data and the percent austreatment was carried out at about 1,700F for 1 hour tenite present as determined by means of standard followed b uenchin in water, then heatin at about X-ra diffraction techni ues for each of three condi q 8 g y 1 1,000F for 4 hours followed by an cooling. tions ((1) as solution treated, (2) as solution treated TABLE 1 Example No. 1 2 3 4 5 0 ARI 19.3 21.3 21.0 21.2 22.5 22.0

In examples 1-6, the balance was iron except for inciand aged, and (3) as solution treated, aged and exdental impurities. In preparing each of the composiposed) are set out in Table IV.

tions, the amounts of each of the elements Cr, Ni, Mo, and Co added were adjusted so as to provide an austenite retention index for each less than about 22.8 but not less than 18.0 according to the relationship.

ARI Ni 0.8(% Cr) 0.6(% M0) 0.3(% Co) TABLE II Ex. .2% Y8 UTS NTS No. X1000 X1000 X 1000 Elong. RA

psi psi psi TABLE lI-Continued Ex. .2% Y UTS NTS No. X1000 X1000 X1000 Elong. RA

psi psi psi Result of one test.

' Average of two tests.

TABLE III Ex. .2% Y5 UTS NTS No. X1000 X1000 X1000 Elong. RA

psi psi psi TABLE IV Hardness R Austenite Ex. Solution Solution No. Treated Aged Exposed Treated Aged Exposed 1 22,5 46.5 48,5 N.D." N.D. N.D. 2 27.5 48 49.5 4 12 19 3 28.5 48 51 N.D. 4 5 4 31.5 51.5 53 N.D. 4 12 5 29.5 48.5 49 ll 21 27 6 31.5 55.5 55 5 12 Average of two tests except the exposed hardnesses of Exs. 3, 4 and 6. None detected Example 1 illustrates the good tensile properties that are readily obtained with the alloy of this invention. The hardness of about Rockwell C 23 in the assolution-treated condition provides good formability while the simple aging treatment brings out a hardness of about Rockwell C 46.5.

The less than 10% increase in ultimate tensile strength and about 10% reduction in notch tensile strength between the results shown in Tables 11 and Ill reflect good thermal stability in spite of the relatively low nickel content of 5.26%. The ultimate tensile strength of 240,000 psi is approximately the average value to be expected from the calculated austenite retention index of 19.3 as can be observed from the drawing. Example 2 differs from Example 1 primarily in the increased nickel content although the increase in molybdenum content to 5.48% would be expected to have a noticeable effect in increasing the stength of the alloy as compared to Example 1. However, it will be observed that the room temperature ultimate tensile strength of Example 2 in the as-aged condition is essentially the same as that of Example 1, and this is believed to be attributable to the fact that the increase in molybdenum which tends to increase the strength is offset by the increase in nickel content which, by increasing the austenite content, reduces the ultimate tensile strength. A comparison of the tensile data in Tables 11 and 111 shows that Example 2 is more stable after the prolonged expposure at elevated temperature than Example l. The relatively small change in tensile properties is indicative of the outstanding thermal stability of this composition. On the other hand, the effect of increasing the chromium content of Example 1 by about 2% as in Example 3, without increasing the nickel content, makes the alloy thermally unstable as is clearly demonstrated. The alloy of Example 3 is well suited for use at room temperature where high yield strength, ultimate tensile strength, notch tensile strength and good corrosion resistance are required, but is not intended for use where prolonged exposure to elevated temperature would be encountered. Example 4 with an ARI value of 21.2, very close to the value of 21.3 for Example 2, demonstrates an outstanding degree of tensile strength and thermal stability. The increased strength of Example 4, both the 0.2% yield strength and ultimate tensile strength, is believed to be attributable to the molybdenum content of about 7% as compared to the approximate 5.5% molybdenum content of Example 2. Example 5 illustrates a composition prepared in accordance with the present invention so as to have an austenite retention index of 22.5 at a somewhat higher carbon level than preferred which has good tensile strength and good thermal stability. Example 6 with an austenite retention index of 22.0 is essentially the same composition as Example 5 except that instead of about 7% nickel and 5% molybdenum, Example 6 contains about 5% nickel and 7% molybdenum. The outstanding yield and ultimate strength of Example 6 is believed to be primarily the result of the increased molybdenum content, the cobalt level of about 15% being used to establish the desired ARI value. Thus, at a given value of the austenite retention index, one reason for the vertical dispersion (in ultimate tensile strength values) resides in the relative proportions of nickel and molybdenum present in the composition. The composition of Example 6 is well suited for use where its exceptional ultimate tensile strength and 0.2% yield strength are required. However, other compositions, Example 1 or Example 2, would be preferable where higher notch tensile strength is desired and in particular where a NTS/UTS ratio of at least equal to one is desired.

The relationship between the austenite retention index as calculated from AR1= Ni 0.8(% Cr) 0.6(% Mo) 0.3(% Co) and room temperature ultimate tensile strength of the alloy in its as-aged condition will be better appreciated by reference to the drawing where the calculated values of the austenite retention index have been plotted along the abscissa and stress in thousands of pounds per square inch is plotted along the ordinate. Curves A and B are respectively the envelopes of the maximum and minimum values of the results of room temperature ultimate tensile strength tests carried out on specimens of about 50 different compositions. The various heats and the specimens thereof were prepared and the tests were carried out as described in connection with Examples l-6. Curve C in dashed line is a curve of the average values at each level of the austenite retention index. Curves A, B and C clearly reflect the close interdependence of room temperature ultimate tensile strength with the austenite retention index as calculated in accordance with the present invention.

The steepness and relatively close spacing of the curves A, B and C for austenite retention index values between 22.5 and 23.2 demonstrates the care with which the alloying elements must be balanced. This may be better appreciated from the three following heats, identified as Alloys 7, 8 and 9, when compared with Examples 16. Alloys 7-9 and test specimens thereof were prepared as described in connection with Examples l-6. Alloys 7, 8 and 9 had the following composition in weight percent:

TABLE VI Alloy .2% Y UTS NTS No. X 1000 X1000 X1000 Elong. RA

psi psi psi The adverse effect upon the 0.2% yield strength and ultimate tensile strength of a composition balanced so that the austenite retention index is above 22.8 is clearly demonstrated by Alloys 7, 8 and 9. Alloy 8 is essentially the same composition as Example 5 except for the increase in the molybdenum content of about 1 percent and a resultant increase in the austenite retention index from 22.5 to 23.0. The loss in strength of Alloy 9 as compared to Example 5 is all the more significant when it is borne in mind that molybdenum would have had a noticeable strengthening effect but for the fact that the increase served to overbalance the alloy. The effect of the individual elements in causing a nonhomogeneous microstructure, or what may be termed a patchy effect (because of austenite retention and/or reversion), is evident from a comparison of Alloys 8 and 9 which have a significant difference in 0.2% yield strength and ultimate tensile strength.

In Table VII are set forth the results of room temper ature tensile tests carried out on specimens of each of the Alloys 7, 8 and 9 which had been solution treated, aged and exposed, all as was described in connection with Examples 1-6 (see Table III).

In Table VI there is set forth hardness data and the percent austenite present as determined by means of standard X-ray diffraction techniques from specimens of Alloys 7, 8 and 9 for each of the three conditions as had been previously set out in Table IV in connection with Examples l-6.

TABLE VIII Hardness Austenite Allo solution Ex- Solution Ex- No. Treated Aged posed Treated Aged posed 7 97.5 R, 35.5 R 28 R, 13 67 84 8 91 R, 95.5 R,, 97.5 R 16 71 60 9 24.5 R, 42 R 39.5 R, 24 42 45 When comparing the data in Tables V-VIII with that set forth in Tables I-IV, it is important to keep in mind that each of the Alloys 7, 8 and 9 falls within the composition range of the alloy of the present invention. It is only by carefully following and using the austenite retention index in balancing the composition within the specified ranges for the various elements that the outstanding results characteristic of the present invention can be attained.

The alloy of this invention is suitable for a wide variety of demanding uses. For example, it is especially well suited for making such parts as compressor discs in jet engines because of its outstanding thermal stability combined with ultra high strength and ductility both at room temperature and at elevated temperatures. For such uses, the alloy, in addition to being balanced by using the austenite retention index so as to obtain optimum strength, also has the lower amounts of chromium combined with the larger amounts of nickel. This alloy is also outstanding for use in forming fasteners and structural members, such as are used in aerospace vehicles, which are not subjected in use to temperatures higher than about 500F. For such uses, where thermal stability is not required, to provide the best combination of strength and corrosion resistance, the higher amounts of chromium are balanced with the lower amounts of nickel.

The terms and expressions which have been 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 recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. An age-hardenable stainless iron-base alloy which in its age-hardened condition is characterized by an ultimate tensile strength of about 225,000 psi or more with good ductility and which consists essentially in weight percent of about Carbon 00.l Nitrogen 0-0. 1 Manganese 0-2 Silicon 0-1 Phosphorus 00.05 Sulfur 00.05 Boron 0-0.02 Chromium 9-13 (obalt 8-16 Molybdenum 4-8 Nickel 4-8 up to about 12.8% tungsten as a replacement for all or part of the molybdenum content in the ratio of about 1.2% to 1.6% tungsten to 1% molybdenum, the balance being essentially iron and incidental impurities, the elements nickel, chromium, molybdenum and cobalt being balanced to provide an austenite retention index (ARI) value of from about 18.0 to 22.8 as calculated from the equation ARI Ni 0.8(% Cr) 0.6(% Mo) 0.3(% Co) by 30 is added.

2. The alloy is set forth in claim 1 which contains less than about 0.01% carbon.

3. The alloy as set forth in claim 2 which contains less than about 0.01% nitrogen.

4. The alloy as set forth in claim 1 which contains less than about 0.01% carbon, less than about 0.01% nitrogen, less than about 0.1% manganese and less than about 0.1% silicon.

5. The alloy as set forth in claim 1 which contains about 9.511.5% chromium, about 9.513.5% cobalt, about 56% molybdenum, and about 57% nickel.

6. The alloy as set forth in claim 5 which contains less than about 0.01% carbon, less than about 0.01% nitrogen, less than about 0.1% manganese, and less than about 0.1% silicon.

7. The alloy as set forth in claim 6 which contains about 0.0010.003% boron.

8. The alloy as set forth in claim 7 which contains less than about 0.01% sulfur and less than about 0.01% phosphorus.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,861,909

DATED I January 21, 1975 INVENTOR(S) 1 Robert L. Caton it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, line 35, for "accompanyng" read accompanying Col. 3, line 16, for "furing" read during line 25, for "ferriteforming" read ferriteforming line 43, for "chronium" read chromium Col. 4, line 30, for "aloy" read alloy Col. 6, line 1, after "solution", insert in line 17, for "finished" read finish Col. 7, line 64, for "expposure" read exposure Col. 11, line 19, after "carbon", for "is" read in Col. 12, line 1, for "is" read as Signed and sealed this 6th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks

Claims

2. The alloy is set forth in claim 1 which contains less than about 0.01% carbon.
3. The alloy as set forth in claim 2 which contains less than about 0.01% nitrogen.
4. The alloy as set forth in claim 1 which contaiNs less than about 0.01% carbon, less than about 0.01% nitrogen, less than about 0.1% manganese and less than about 0.1% silicon.
5. The alloy as set forth in claim 1 which contains about 9.5-11.5% chromium, about 9.5-13.5% cobalt, about 5-6% molybdenum, and about 5-7% nickel.
6. The alloy as set forth in claim 5 which contains less than about 0.01% carbon, less than about 0.01% nitrogen, less than about 0.1% manganese, and less than about 0.1% silicon.
7. The alloy as set forth in claim 6 which contains about 0.001-0.003% boron.
8. The alloy as set forth in claim 7 which contains less than about 0.01% sulfur and less than about 0.01% phosphorus.