USRE26225E - Heat-resistant high-strength stainless steel - Google Patents

Description

June 20, 1967 Original Filed Oct. 5,

sTREss (I000 ps1) RUPTURE TIME (Hours) A. KASAK ETAL Re. 26,225 HEAT-RESISTANT HIGH-STRENGTH STAINLESS STEEL 1961 4 Sheets-Sheet 1 TENSILE STRENGTH AT FiOOM TEMPERATURE o 25% FREE FERRITE AT2000F 250- o YIELD STRENGTH AT ROOM TEMPERATURE ,A OGEISILE STRENGTH AT "00F 3 YIELD STRENGTH AT ||O0F X loox g b. 50 v so AT 40 FigJ TEMPERING AT |2oo|= 0 l I I I I l I I I I o 2 4 s e PERCENT MOLYBDEN UM ||00F 75000 psl Fig.2 200- o O Q 5 \LIZOO F 3 OOOPSI G I I I I I I I 0 2 4 s a PERCENT MOLYBDENUM INVENTORS August Kusok BY Edward J.Dulis VIJA! K. Chundhok HEAT-RESISTANT HIGH-STRENGTH STAINLESS STEEL 4 Sheets-Sheet Original Filed Dot. 5, 1961 Fig.5

I I 0.6 0.8 L0 PERCENT VANADIUM m mu w w 2 3 5 E3 MEDEDm STEEL 77 S L E E T S H w T E G E N T M S K T S A H W S m T H H O E l H T C S C W. m E m U m M E S m m m m. 5 MW 5 O 5 2 2 I @380; 15255 wmnEnmiwwmu E6169 Fig.6

United States Patent tion of New Jersey Original No. 3,154,412, dated Oct. 27, 1964, Ser. No. 143,157, Oct. 5, 1961. Application for reissue Oct. 26, 1966, Ser. No. 596,035

9 Claims. (Cl. 75-126) Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

This invention pertains to stainless steel and particularly to stainless steel exhibiting very high strength at room and elevated temperatures. More particularly, the invention pertains to stainless steel having very high strength and exhibiting good oxidation and corrosion resistance under substantially atmospheric conditions at both ambient and elevated temperatures.

Concerning the various steels of the prior art, those known as austenitic stainless steels may be characterized as resistant to corrosion and oxidation and as having good strength at elevated temperatures. A disadvantage of such steels, however, is their relatively low strength at ambient and moderately elevated temperatures. On the other hand, some steels of a heat-treatable nature and having a martensitic structure attain desired high strengths at ambient temperature which are retained at elevated temperatures. Exemplary of such steels are those covered by US. Patent No. 2,986,463, issued May 30, 1961, which was copending herewith. These steels, however are not characterized by good resistance to corrosion and oxidation. Other martensitic steels having chromium contents of about 12% by weight have improved corrosion and oxidation resistance while similarly exhibiting relatively good strength at low temperatures and fairly high strength at elevated temperatures. The latter martensitic steels however, are limited in chromium content to a maximum value of about 12%. Above this value, the microstructure of the steel is characterized by the presence of substantial amounts of free ferrite. Free ferrite in substantial amounts is undesirable, being generally regarded as the probable cause of many hot-working difficulties as well as excessive variations in product properties with regard to the direction of working.

Another group of heat-treatable stainless steels, the socalled semiaustenitic steels, have chromium contents up to about 17%, but their applicability at temperatures above about 900 to 1000 F. is very limited. At such temperatures, particularly after prolonged exposure, these steels have relatively low strength values.

In accordance with a principal object of the present invention, steels are provided which are heat-treatable to very high strength levels and which retain an unusually high percentage of such strength at temperatures up to 1200 F. or higher. These steels offer particularly high strengths, in both showtime and prolonged exposures, at temperatures ranging from about 1000 to 1200 F. and in addition exhibit high resistance to corrosion and oxidation. The inventive steels constitute a substantial improvement over the elevated-temperature structural materials known to the art and are particularly well suited for construction of aircraft and engine parts subjected to wide variations in operating temperatures.

The steels of the invention comprise iron-base alloys of the following broad composition ranges:

Percent Carbon 0.01 to 0.25 Nitrogen 0.0l to 0.25 Sum of carbon and nitrogen 0.06 to 0.35 Manganese Up to 1.0 Silicon Up to 1.0 Chromium 11 to 16 Vanadium Up to 1.0 Molybdenum Up to 10 Tungsten Up to 10 Sum of molybdenum and tungsten 3.5to 20 Cobalt 10 to 20 Aluminum Up to 0.25 Boron Up to 0.025

Preferred composition ranges of the inventive steels are as follows:

Carbon 0.05 to 0.17% 0.00 to 017 Nitrogen 0.0lto0.07f 13 Sum of Carbon and Nitrogen 0.10 to 0.20%

Manganese. 0.10 to 0.40%.

Silicone-.- 0 10 to 0.40"

In addition to the elements listed in the above broad and preferred composition ranges, the steels may contain impurities commonly encountered in the steelmaking practice (such as sulfur and phosphorus) and tramp elements. i

The steels of this invention are typified by Steels 77 and 107, the chemical compositions of which, together with those of a sizeable number of steels of similar compositions are given in Tables LA and I-B. Table I-A lists these steels according to significant compositional variable, whereas Table I-B lists the same steels according to identification number. Microstructural characteristics of the steels are determined by microstructural examination and mechanical properties of the steels as determined by hardness, tension, and creep-rupture tests are set forth in Tables Il through VI.

TABL

Results of Tension Tests at 1100 R 5 Austenitiz 0.2%Oi1set Tensile Elong. in Reduction Steel ing Tem- Yield Strength 1 inch of Area.

perature Strength (1,000 psi.) (Percent) (Percent) quenched, refrigerated at, 100 F. 2+2 hr.

TABLE V Results of Creep-Rupture Tests at 1100 F.

Rupture Aust. Stress Steel Temp. (p.s.i.)

( F1) Life Elong. Red. of

thr.) (percent) Area (percent) 2, 000 75, 000 112 8 11 l' 1, 900 75, 000 239 8 10 t 2, 000 2 15, 000 365 5 9 2, 000 75, 000 224 17 51 2, 100 15, 000 195 13 35 1, 900 75, 000 221 13 31 1 1,900 00, 000 137 21 47 77 2, 000 15, 000 366i 8 14 2,000 00, 000 104 13 15 2, 100 I5, 000 406 4 7 2, 000 75, 000 243 5 8 2, 100 75, 000 259 3 8 J 2, 000 75, one use 9 9 i 2, 000 so, 000 31 I8 11 11 1, 900 75, 000 296 13 16 2, 000 75, 000 309 7 12 2, 100 75, 000 275 5 11 2, 000 000 214 12 23 2, 000 75, 000 269 11 21 2, 000 75, 000 224 9 22 2, 000 75, 000 259 8 15 2, non 75,000 242 7 13 f 1, 900 75. D00 1119 12 30 l 2, 000 I5. 000 254 11 20 2, 000 15, 000 159 11 27 2, 000 T5, 000 274 13 26 2, 000 75, 000 7 19 55 2, 000 75, 000 0, 1 21 01 2,000 75,000 115 21 49 2, D00 75. 000 212 13 30 2, 000 75, 000 271i 5 8 1, 900 75, 000 185 20 55 2, 000 75. 000 694 B 14 2, 000 75, 000 257 11 PR 2, 000 75, 000 338 11 17 2, 000 75, 000 782 5 7 1 Heat treatment: Austenitired at the indicated temperature, oil quenehed,reirigerated at 100 F. 16 hr,, and tempered at 1,100 F. for 2 2 hr.

I; Heat treatment: Austenitized at the indicated temperature, oil quenched, refrigerated at 100 F. 15 hr., and tempered at 1,100 F. for

6 TABLE VI Results of Creep-Rupture Tests at 1200 F. Under 21 Stress or 35,000 p.s.i.

Rupture Steel Aust.

Temp. F. Elong. Red. 01

Lite (hlx) (percent) Area.

(percent) 2, 000 9|] 13 13 2, 000 148 16 26 2, 000 103 18 17 2, 000 76 16 24 2, 000 114 11 11 1, 900 63 32 50 2, 000 135 21 28 2, 100 109 25 33 2, 000 as 27 27 2, 000 63 17 22 2, 000 66 51 47 2, 000 16 19 2, 000 69 9 9 l, 900 21 29 54 2, 000 73 32 43 2, 000 84 7 12 2, 000 56 21 39 2, 000 27 16 21 2, 000 6 19 31 2, 000 36 8 19 2, 000 72 43 38 2, 000 103 11 15 1, 900 54 42 53 2, 000 98 13 28 2, 000 146 16 1 7 2, 000 70 22 24 2, 000 82 27 38 2, 000 101 25 33 2, 000 175 11 13 lleat treatment: Austenitized at the indicated temperature, oil quenched, refrigerated at F. )6 hr., and tempered nt U F. for 2+2 hr.

Graphical illustrations of the foregoing, as well as graphical comparisons of the steel of the invention with prior-art steels, are depicted by the drawings, wherein;

FIG. 1 shows the effect of molybdenum on tensile properties and hardness;

FIG. 2 shows the efi'ect of molybdenum on creep-rupture strength;

FIG. 3 shows the eiTect of cobalt on tensile properties, hardness and microstructure;

FIG. 4 shows the eifect of cobalt on creep-rupture strength;

FIG. 5 shows the effect of vanadium on creep-rupture strength;

FIG. 6 shows the creep-rupture strength of various prior-art steels together with that of a steel typical of the invention;

FIG. 7 shows the tensile strength of various prior-art steels together with that of a steel typical of the invention;

FIG. 8 shows the oxidation resistance of various priorart steels together with that of a steel typical of the invention.

As indicated by the test data set out in the aforementioned tables and drawings, steels within the invention are capable of exhibiting hardness of Rockwell C 48 or greater and tensile strength of 250,000 p.s.i. or greater at room temperature, tensile strength of 170,000 p.s.i. or greater at a temperature of 1100 F., creep-rupture life of 100 hours or more for an applied stress of 75,000 p.s.i. at a tempcrturc of 1100 F., and creep-rupture life of 50 hours or more for an applied stress of 35,000 psi. at a temperature of 1200" F. In addition to exhibiting superior strength properties the steels of the invention exhibit good ductility at both room and elevated temperatures and have excellent weldability.

In accordance with the basic concepts of the present invention, the foregoing superior mechanical properties are obtained only upon the achievement of a proper compositional balance and the application of a proper heat treatment. Such heat treatment may involve the steps of austenitizing at an elevated temperature, preferably in the range of 1900 to 2100 F., cooling to room temperature, and heating at an elevated temperature below about 1500 F. Refrigeration at a subzero temperature, e.g. l F., is preferably included in the heat-treating schedule immediately following the austenitizing step for the purpose of facilitating the transformation of austenite. While the recited sequence of heattreating steps has been found admirably suited for the contemplated purpose, it is well to note that other sequences of heattreating steps, including conditioning treatments, may be employed, the principal criteria being the complete or substantially complete elimination of free ferrite at elevated temperatures and the complete or substantially complete elimination of austenite thereafter as well as the obtention of high strength.

It is also understood that a combination of thermal and mechanical treatments may result in further improvement of some of the important properties of the inventive steels. For example, when a sheet sample of Steel 77 was cold reduced 50% in thickness and subsequently heated at 1100" F. for two plus two hours, a tensile strength of 341,000 psi. and an elongation of 6% were obtained at room temperature. In contrast, it will be noted from Table III that thermal treatment alone of Steel 77 resulted in a maximum tensile strength of only 294,000 p.s.i.

It is well known to the art that in order for a steel to exhibit satisfactory corrosion resistance at ambient temperature and satisfactory oxidation resistance at temperatures up to about 1200 F., the chromium content of such steel should be about 12% or more. In general, the higher the chromium content of the steel, the higher its corrosion and oxidation resistance. In view of these considerations, chromium has been included as an essential element in the steels of the invention, the minimum content thereof being set at 11%. As it is desirable to maintain the chromium content at the highest level compatible with desired microstructural characteristics, i.e., essential freedom from free ferrite and retained austenite, the maximum chromium content of the inventive steels has been set at about 16%. The latter figure finds support in the accompanying data wherein it is observed from Tables I-A and II that when the compositional variable is chromium, values of 14.36% (Steel 77) and 15.54% (Steel 97) result in a microstructure having no free ferrite after an austenitizing treatment at 1900 F., whereas, a value of 16.20% (Steel 98) results in a microstructure having 10% free ferrite after the same austenitizing treatment.

Molybdenum has been included in the steels of the invention as an effective strengthener. Supporting data therefor were obtained by testing steels of the basic inventive composition wherein the molybdenum content was varied (see Table IA, Steels 102, 103, 77 and 104). As shown in FIGS. 1 and 2, all the important strength properties, i.e., yield and tensile strengths at room temperature and at 1100 F., hardness after tempering at 1100 F. and 1200 F., and rupture times in creep-rupture tests at 1100 F. and 1200 F., are steadily enhanced as the molybdenum content is increased from about 0.1 to about 5%. Tables III through VI indicate that for this same molybdenum range, ductility is maintained at a high level. Molybdenum contents in excess of about 5% have some additional strengthening effects in short-time tests; however, the tendency for formation of free ferrite at high temperatures is increased. Therefore, although molybdenum contents up to about are included within the broad scope of the invention, a molybdenum content of about 4 to 6% is preferred for an optimum combination of properties. In addition to its strengthening role, molybdenum functions to increase significantly the corrosion resistance of the inventive steels in such media as chloride solutions.

Tungsten is also an effective elevated-temperature strengthener in the inventive steels. For example, a steel containing 5.11% tungsten and only 0.04% molybdenum 8 (Steel 122) exhibited a higher creep-rupture life (782 hours) than any other stecl when tested at 1100" F. under a stress of 75,000 psi. (see Table V). Similarly, when tested at 1200 F. under a stress of 35,000 p.s.i., this same steel again exhibited a higher creeprupture life (175 hours) than any other steel tested (see Table VI).

In addition to the benefits derived from the individual uses of tungsten and molybdenum, test data show that a combination of these elements is an effective strengthening agent in the inventive steels. In support thereof, it will be noted that Steel 75, containing 3.01% molybdenum and 2.35% tungsten, exhibited a creep-rupture life of 365 hours when tested at 1100 F. under a stress of 75,000 psi. (see Table V).

Cobalt has been incorporated as an essential ingredient in the inventive steels for the purpose of obtaining desired microstructural characteristics, a critical cobalt range of 10 to 20% having been determined therefor. In this regard, Table II strikingly reveals that cobalt values below about 10% (Steels 111, 112 and 113) result in the formation of excessive amounts of free ferrite, and that cobalt values above about 10% (Steels 76, 106, 77, 78 and 67) permit the complete or substantial elimination of free ferrite by proper compositional balance and/or heat-treatment selection. On the other hand, cobalt contents in excess of about 15% permit the retention of increasing amounts of austenite, the effect of which is to lower the room-temperature strength properties of the steels to some extent. This is graphically depicted in FIGS. 3 and 4, wherein it is observed that optimum strength values at both room temperature and 1100 F. are obtained in a cobalt range of about 13 to 15% and that said values fall oflf with increasing cobalt content until an extrapolated maximum of about 20% cobalt is reached, beyond which value strength properties become unsatisfactory.

In addition to the employment of a critical cobalt range, the invention contemplates as a preferred embodiment a proper balancing of the cobalt content within said range with the nitrogen content. This proper balancing, as evidenced by Steels 76, 106, 77, 78 and 67, involves the restriction of the cobalt content to values in excess of 12% where the steel is essentially nitrogen-free or low in nitrogen, i.e., less than about 0.1%. Where the nitrogen present amounts to about 0.10% or greater, the cobalt content may be lowered to the range 10 to 12%. It will be noted from Table II after an austenitizing treatment at 1900 F. that Steel 76, embraced within the broad concept of the invention, exhibits 5% free ferrite and that Steels 106, 77, 78 and 67, comprising preferred embodiments of the invention, exhibit not more than 1% free ferrite after the same austenitizing treatment.

As is known to the art, the alloying elements cobalt and nickel are interchangeable to some extent for the formation of an austenitic structure and for the elimination of free ferrite in steel. As increasing amounts of nickel up to 10.14% (Steel 118, and 114) were used in the steels of this invention as a replacement for cobalt, it became evident that While nickel effectively caused the amount of free ferrite to decrease, it also stabilized the austenitic structure of the steel so that when the free ferrite was fully eliminated (as in Steel 114 containing 10.14% nickel), the steel was not hardenable and thus incapable of exhibiting very high strengths at ambient and moderately elevated temperatures. Therefore, it is recognized that although to some extent nickel may be substituted for cobalt in these steels, no complete or substantial replacement of cobalt with nickel is possible.

The elements carbon and nitrogen both constitute strong austenite formers and are essential in the steels of this invention for the elimination or minimization of free ferrite in the microstructure. Quantitatively, it appears that about 0.06 to 0.35% carbon plus nitrogen (combined) is required to avoid substantially the formation of free ferrite upon heat treatment at high temperatures. However, it is important to select quantities Within the foregoing range so as to maintain a proper compositional balance vis-a-vis ferrite formers, lest the ferritizing effect of the latter become dominant. For example, Where the sum of carbon plus nitrogen is less than about 0.1% the presence of vanadium (a strong ferrite former) cannot be tolerated above about 0.1%. In this regard it will be noted that Steel 121 (containing 0.068% carbon plus nitrogen and no intentionally added vanadium) was heattreatable to zero precent free ferrite, while Steels 11S and 116 (containing 0.076%

of molybdenum because of the substantial difference in the atomic weights of these two elements. In the steels of this invention we consider tungsten as at least a partial replacement for molybdenum.

For the purpose of comparing relative strength and corrosion properties, an investigation was undertaken along these lines with respect to the steels of the invention and prior-art steels of the semiaustenitic, the 12% chromium high-strength, and the chromium hot-work tool-steel types. The nominal compositions of representative examples of carbon plus nitrogen and 0.039% carbon plus nitrogen, respectively as well as about the latter steel types are set forth in Table VII.

Table VII Nominal Compositions of Comparison Steels Designation t C l Mn I S1 Cr N1 V Mo W Al N F0 Semiaustenitle Steels:

AM 350 0.10 1.00 0. 40 1e 5 PH-7M0 0. 09 1. 0 1. 0 14. 0-15. 0

HIQX. IIIELX 1118K.

12% Cr High-Strength Steels:

Crucible 422 1 0 22 0. 55 0. 35 12.0

Crucible 422M 0. 2s 0. s4 0. 25 12. 0

Lapelloy 4 0. 1. 0 0. 25 12. 0 Hot-Work Steels:

H-ll 3 0. 38-0 43 0. 20-040 0. 50-1. 00 4. 75-5. 25 0. -0. 50 1.20-1.40 m1.

Crucible 21s 0. 35 0.35 1.00 5.00 0. 45 1. Bel.

1 Grade of Allegheny-Ludluni Steel Co.

2 Grade of Armoo Steel Co.

3 AISI Type.

4 Grade of General Electric Co.

6 Grade of Crucible Steel Co. of America.

one-half percent each of vanadium) were heat-treatable to no less than 20% and 30% free ferrite, respectively. On the other hand, it has been found that high carbon plus nitrogen contents have an adverse effect on ductility at high strength levels and promote the retention of austenite in the microstructure. Therefore, to ensure proper compositional balance, it is preferred in the steels of this invention to keep the carbon plus nitrogen content between about 0.10% and 0.20% when the content of cobalt (as indicated heretofore, also a strong austenite former) is about 12% or higher, and between about 0.20% to 0.30% when the content of cobalt is below about 12%.

Vanadium in amounts up to about 1.0% may be added to the steels of this invention to improve certain properties. As shown in FIG. 5, additions of vanadium to the steels of the invention increased rupture life at 1100 F., as determined by creep-rupture tests, the highest rupture life being obtained with a steel containing about 0.5% vanadium.

It is generally agreed that tungsten has alloying effects Comparative data relating to creep-rupture strength and elevated-temperature tensile strength for the steels in Table VII, representative of the types recited therein, and a sample of Steel 77, representative of the steels of the invention, are given in Tables VIII and IX, respectively.

Semiaustenitie Steels:

PHlb-H'lo 12% Cr High-Strength Steels:

Crucible 422 Crucible 42201..."

Lapelloy Ho t-Work Steel: ru-

. bl 21s 155 100 in steels quite slmllar to those of molybdenum except s i s S Invumion; 40 that the amounts (in weight percent) necessary to produce S1081 77 169 35 the effects with tungsten are about twice as great as those TABLE IX Comparison of Elevated-Temperature Tensile Strengths Steel Condition R.T. 200 300 400 500 500 700 800 000 1,000 1,100 1,200

F. F. F. F. F. F. F. F. F. F. F.

Semiaustenitic Steels:

11111-355 SCT l 222 208 20s 20s 197 180 200 185 101 170 179 155 105 08 45 5-7Mo 238 227 211 203 182 101 12% Chromium High-Strength Steels:

Crucible 422 57 Crucible 422M T4 Hot-Work Steel: H-ll 100 Steal of the Invention: Steel 77 121 l Subzero-cooled and tempered. 2 Roll-hardened at room temperature and aged 950 F. B quenched and tempered.

Graphical depiction of the foregoing data is shown in FIGS. 6 and 7, wherein the superiority of the steels of the invention over the comparison steels of the prior art is clearly evident.

To assess the corrosion resistance of the steels of this invention, a heat-treated sample of Steel 77 was subjected to the water-vapor-column corrosion test and to the CASS test along with heat-treated samples of a number of the aforementioned prior-are comparison steels. These tests have been used for many years as laboratory tests to indicate the relative corrosion resistance of stainless steels under essentially atmospheric conditions.

In the water-vapor-column test, the samples are exposed to water vapor condensation for a period of 8 hours and then allowed to dry for a period of 4 hours. This procedure is repeated several times. Results of this test after six wet-and-dry cycles showed the H-ll specimen to be covered with heavy rust, and the Crucible 422 steel to contain local rust spots, whereas it showed Steel 77 of this invention to be only slightly stained, as were the samples of AM 350 and PH157M0.

In the so-called CASS test, the specimens are sprayed with an acidified aqueous solution of NaCl and CuCl for 16 hours at 120 F. The samples are then cleaned with a detergent and the surface appearance is rated by visual examination. In this test the H-ll sample was severely attacked and Crucible 422 was seriously pitted; Steel 77 along with other stainless steels showed only slight pitting and all to about the same degree.

To determine the oxidation resistance, samples of Steel 77 were exposed to a static air atmosphere at 1200 F. and measurements of weight gain were made periodically; in this test, a higher rate of weight gain indicates a higher rate of deterioration (scaling) of the steel in air at the test temperature. The results of this test, given in FIG. 8, show clearly that Steel 77 of this invention exhibited a superior resistance to oxidation attack.

For the purpose of evaluating Welding characteristics of steels of the invention, welding tests were conducted on sheet samples of Steel 77 using the semiautomatic inert-gas-shielded-tungsten-arc process. The tests involved tension testing of one-eighth inch thick specimens, both welded and unwelded. The average results of these tests are given in Table X.

These results indicate a weld efliciency (i.e., ratio of tensile strengths of the welded specimens to those of the unwelded specimens) of 98%. Also, it is important to note that no high-temperature heat treatment is needed after the welding operation. This feature is extremely important in fabrication, since it virtually precludes any ditliculties resulting from scaling and warping of a formed and welded article.

While the invention has been described and disclosed in connection with specific embodiments thereof, it will be understood that the invention is not limited thereto but may be otherwise embodied or practiced within the scope of the appended claims.

We claim:

1. A stainless steel havnig enhanced strength and useful ductility over a temperature range from subzero to about 1200 F., consisting essentially, by weight percent, of:

Element: Percent Carbon+nitrogen 0.06 to 0.35. Chromium From 11 to 16. Molybdenum-l- /z tungsten From 3 to less than 7. Nickel Up to 3. Vandium Up to 1. Nickel+cobalt 10 to 20. Manganese Up to 1. Silicon Up to 1. Aluminum Up to 0.25. Boron Up to 0.025. Iron Balance.

wherein vanadium is present on the low side of its range when the sum of carbon+nitrogen is on the [high] low side of the combined range of those elements.

2. A stainless steel having enhanced strength and usewhere the sum of carbons-nitrogen is in the range of 0.10 to 0.20 when the sum of nickel-l-cobalt is at Least 12% and is in the range of 0.20 to 0.30 when the sum of nickel-l-cobalt is less than 12%.

3. A stainless steel having enhanced strength and useful ductility over a temperature range from subzero to about 1200 F., consisting essentially, by weight percent, of:

Element: Percent Carbon 0.01 to 0.25. Nitrogen Less than 0.15. Carbon+nitrogen 0.06 to 0.35. Chromium 12 to 15.5. Molybdenum-P/z tungsten 4to 6. Cobalt 12 to 14. Vanadium Up to 0.5. Manganese Up to 0.4. Silicon Up to 0.4. Aluminum Up to 0.25. Boron Up to 0.025. Iron Balance.

4. A heat-treated stainless steel consisting essentially of the composition of claim 1, said steel being characterized by the following minimum mechanical properties:

Hardness Ro-ckwell C 48 Tensile strength (room temperature) p.s.i 250,000 Tensile strength (1100 F.) p.s.i 170,000

5. A heat-treated stainless steel consisting essentially of the composition of claim 2, said steel being characterized by the following minimum mechanical properties:

Hardness Rockwell C 48 Tensile strength (room temperature) p.s.i 250,000 Tensile strength (1100 F.) p.s.i 170,000

6. A heat-treated stainless steel consisting essentially of the composition of claim 3, said steel being characterized by the following minimum mechanical properties:

Hardness Rockwcll C 48 Tensile strength (room temperature) p.s.i 250,000 Tensile strength (1100 F.) p.s.i 170,000

Hrs. Creep-rupture life (75,000 p.s.i. at 1100 F.) 100 Creep-rupture life (35,000 p.s.i. at 1200 F.) 50

8. A heat-treated stainless steel as in claim 5, said steel being additionally characterized by the following minimum mechanical properties:

Hrs. Creep-rupture life (75,000 psi. at 1100 F.) 100 Creep-rupture life (35,000 p.s.i. at 1200 F.) 50

9. A heat-treated stainless steel as in claim 6, said steel being additionally characterized by the following minimum mechanical properties:

Hrs. Creep-rupture life (75,000 p.s.i. at 1100 F.) 100 Creep-rupture life (35,000 p.s.i. at 1200 F.)

References Cited The following references. cited by the Examiner, are of record in the patented file of this patent or the original patent.

UNITED STATES PATENTS 1,357,549 11/1920 Fahrenwald 128 2,432,614 12/1947 Franks 75126 2,462,665 2/1949 Olcott 75126 2,848,323 8/1958 Harris et al. 75-126 2,880,085 3/1959 Kirkby et al. 75-126 2,990,275 6/1961 Binder et al 75-126 DAVID L. RECK, Primary Examiner. P. WEINSTEIN, Assistant Examiner.

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