US3364013A - Stainless steel alloy - Google Patents

Description

3,304,013 Patented Jan. 16, 1968 3,364,013 STAINLESS STEEL ALLOY Robert L. Caton, Wyomissing, Pa., assignor to The Carb penter Steel Company, Reading, Pa., a corporation of figfl ifig New Jersey 5 Silicon", No Drawing. Continuation-impart of application Ser. No.

Broad Range Preferred Range .10% max 010% max. .50% max. max. .50% max .10% max.

Phqsphorus 0 476,655, Aug. 2, 1965. This application Feb. 6, 1967, g %??3" 12%) 5 13;? 614029 $850323; itttfiit 6 Clams (CL 75-426) 15% to 3.5% 0 2.67 3.0 0.

Up to .01%. 0.001% 130 0.003%

ABSTRACT OF THE DISCLOSURE Age hardenable martensitic stainless steel and article made thereof containing chromium, cobalt and molyb- 15 denum.

*Except for incidental impurities.

When properly balanced within this broad range, my alloy provides an ultimate tensile strength of about 190,- 000 p.s.i. to 270,000 p.s.i., and a notch tensile strength, stress concentration factor of 10, of about 150,000 p.s.i. to 340,000 p.s.i. This is combined with a .2% yield strength of about 170,000 p.s.i. to 230,000 p.s.i.

Within the broad range, care must be exercised in balancing the composition, particularly with the larger amounts of nickel present to avoid an undesirable amount steel alloy which is age hardenable and, in that condiof retained austenite. Thus, with nickel .in the neighbortion, is characterized by a unique combination of high hood of 3%, I preferably use the lower amounts of the strength and ductility, both at room and at elevated tern alloying elements which tend to form or stabilize ausperatures. tenite to keep the maximum retained austenite below Since the advent of age hardenable martensitic iron about 20%. base alloys, considerable effort has been expended to- The elements cobalt and nickel are austenite formers, toward providing a martensitic stainless steel alloy which although nickel is much stronger in this regard, while the could be readily forged, hot and/or cold Worked, and elements chromium and molybdenum tend to form ferwhich, when suitably heat treated, could be strengthened rite and stabilize austenite. Thus, within the broad ranges so as to provide parts having high strength and good stated, these elements are balanced in my composition stainless properties at temperatures ranging from room so that the alloys attains a stable, primarily martensitic temperatures up to about 1100 F. In such alloys and microstructure. Substantially no retained ferrite or sigma that of the present application, strength is measured by 3 phase is desired in my composition because those phases tensile tests, both at room and elevated temperatures, and cause an excessive loss in ductility and embrittlement. by stress rupture life tests at elevated temperatures. For However, larger amounts of retained austenite can be many purposes, the usefulness of such alloys is detertolerated in my alloy without objectionably affecting its mined to a large extent by the ultimate tensile strength high tensile strength, and when so balanced, is well suited of the part. For example, such parts may include strucfor use where the outstanding stress rupture strength and rural members of steam turbine compressors or supernotch tensile strength of my preferred composition are sonic air vehicles. 011 the other hand, more intricately not required. More consistent results, particularly in the shaped parts of steam turbine compressors or the strucattainment of a notch tensile strength which is at least tural portions of supersonic air vehicles must not be equal to or greater than the ultimate tensile strengh, tonotch sensitive, and in practice, it is usually required that gether with maximum yield strength and transverse duethe notch tensile strength of such parts be at least equal tility, are attained when my alloy is not only free of reto the ultimate tensile strength. tained ferrite and sigma phase but also is essentially free I have discovered that by carefully balancing my comof retained austenite, that is, when retained austentite is position within critically defined ranges, with particular less than about 5%. Within its prefererd range, my alloy attention to the elements chromium, cobalt and molybconsistently provides parts which, in their heat treated denum, anage hardenable stainless steel alloy is provided condition, have a room temperature ultimate tensile which is eminently well suited for meeting the foregoing strength of about 200,000 p.s.i. to 250,000 p.s.i., a .2% requirements. [Not only does my alloy have outstanding yield strength of about 180,000 p.s.i. to 225,000 p.s.i. and strength, but it does not require more than a relatively a notch tensile strength of about 200,000 p.s.i. to 325,000 simple heat treatment to convert it from the condition p.s.i. Furthermore, the NTS/UTS ratio of my preferred in which it is readily forged and hot or cold worked to composition is consistently at least equal to or greater a precipitation hardened martensite which combines than one. highly desirable properties of strength, ductility and cor- Carbon and nitrogen are strong austenite forming elerosion resistance. My alloy readily transforms to a marments and advantageously are kept to a minimum in the tensitic structure and does not require any unusual or alloy. In amounts in excess of about .10%, carbon may hard to control treatments. have a beneficial effect upon the ultimate tensile strength The preferred and broad ranges of my composition are but it excessively impairs the yield strength and results in given in approximate Weight percent in accordance with the presence of relatively hard martensite in the as-solugood metallurgical practice in the following table: tion-treated condition of the composition which detracts This application is a continuation-in-part of my appli' cation Ser. No. 476,655 filed Aug. 2, 1965, now aban cloned. 20

This invention relates to an iron base alloy and more particularly to a chromium-cobalt-molybdenum stainless from the formability and machineability of the alloy. Thus carbon is limited to a maximum of .10% in this alloy. However, because carbon is such a strong austenite former, care must be exercised in balancing the alloy by using the smaller amounts of the elements which form or stabilize austenite so as to ensure that the retained austenite in the alloy does not exceed When the most favorable notch tensile strength is not required, carbon may be present in this alloy in an amount of up to .10%, but balancing the alloy to ensure that the amount of retained austenite is not excessive is facilitated by limiting the carbon content to no more than .05 Carbon appears to have its most significant objectionable effect in increasing the amount of retained austenite and in increasing the hardness of the alloy in its as-solutiontreated condition when it is present in this alloy in amounts ranging from about 01% to .05 Thus, carbon is preferably kept below .010% maximum. Nitrogen, in addition to being a strong austenite former, tends to form nitrides. It is, like carbon, therefore, considered to be an undesired impurity and is held to no more than .1%, and preferably to more than 01%.

Manganese is also a strong austenite former and is toerated only in amounts which do not upset the martensitic balance of the alloy. When the alloy is melted in air, manganese may be used as a deoxidizer in controlled amounts so that no more than about 50% is retained in the alloy. Preferably, the amount of retained manganese is kept to no more than .10%.

Silicon may also be advantageously used as a deoxidizer when the alloy is air melted. Although silicon is a ferrite former, it too is not a desirable alloying element and is held to no more than 50% and preferably to no more than .10%.

Aluminum and titanium are commonly used strengthening agents, but in my alloy, when either or both are present in amounts of .5 or more, severe embrittlement results as indicated by a marked reduction of the notch tensile strength.

Nickel causes the formation and/ or retention of undesired austenite in my alloy, and also objectionably depresses the austenite reversion temperature of the alloy With a consequent instability upon long exposure to elevated temperatures. For best results, I preferably limit nickel to no more than .50%, although I may use as much as 3% for its desirable effect upon the alloy notch tensile strength at room temperature when the accompanying loss in stress rupture strength at 1000" F. can be tolerated. With the larger amounts of nickel, the cobalt content should be adjusted downward within the stated range to preserve the martensitic balance of the alloy. The austenite stabilizing elements, e.g., chromium, can also be adjusted downward to avoid the retention of excessive austenite.

Within the limits stated, the elements chromium, cobalt, molybdenum and iron work together, and, unless these elements are carefully maintained within the specified ranges, the unique combination of properties characteristic of my alloy of high strength, together with good stainless properties, good forgeability and hot or cold workability, is not attained.

In my alloy at least about 12% chromium is required to attain the desired stainless properties, together with high ultimate and notch tensile strengths. Chromium is a ferrite forming element and also tends to stabilize austenite formed during the high temperature treatment of the alloy. This effect with the resultant retention of excess ferrite, sigma phase and/or austenite becomes increasing 1y apparent as the chromium content in my alloy is increased above about 18%. For best notch and ultimate tensile strengths, and to eliminate the possibility of retained ferrite, sigma phase and/or excess austenite, my alloy preferably contains from about 14.5% to 16.5% chromium.

Cobalt, like nickel, causes the formation and/or retention of austenite but has much less of an effect in this regard than nickel. Thus, as much as 26% cobalt can be used in my alloy by adjusting the austenite stabilizing elements downward Within their broad ranges without its causing the retention of an undesirable amount of austenite. Below about 18% cobalt, the unique strength characteristic of my alloy is not attained. Over this range, the cobalt works with the broad ranges of chromium and molybdenum to provide my high strength stainless steel alloy, but to consistently attain high notch tensile strength so that the NTS/UTS ratio is at least equal to or greater than one and minimize retained austenite, I preferably use about 19% to 21% cobalt in my alloy. Also, the austenite stabilizing effect of chromium and cobalt is preferably offset by balancing the larger amounts of chromium with the lower amounts of cobalt, and vice versa, within the ranges already stated. Thus, with about 14.5% chromium 1 prefer to use about 21% cobalt, and with about 16.5% chromium I preferably balance about 19% cobalt.

Molybdenum and tungsten may be used interchangeably in my 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. It is therefore to be understood that throughout this application, when molybdenum is referred to, it is intended to include molybdenum and tungsten either together or individually with the tungsten replacing all or parts of the molybdenum in the proportion stated.

Molybdenum critically afiects the strength of my alloy and from about 1.5% to 3.5% molybdenum (or the equiv alent amount of tungsten) is essential for this purpose. The notch tensile strength of this alloy is especially sensitive to the molybdenum content. Below about 2% molybdenum, the high notch tensile strength of my preferred composition cannot be consistently attained. This is the case also when molybdenum is present in an amount greater than about 3%. Thus, to ensure consistent attain" ment of a NTS/UTS ratio of at least one, I preferably use from about 2% to 3% molybdenum. However, when somewhat lower notch tensile strength can be tolerated, I use the larger amounts of molybdenum to attain higher ultimate tensile and yield strengths.

When my alloy is intended for a use which requires a NTS/UTS ratio equal to or greater than one, it is balanced within the preferred range with boron ranging from about; 0.001% to about 0.003%. Unless boron in this critically defined range is included when balancing the composition Within the preferred range, a NTS/UTS ratio of at least equal to or greater than one cannot be consistently attained. On the other hand, when the alloy is balanced Within the preferred range for a use which requires a mini mum notch tensile strength of no more than about 150,- 000 p.s.i., then a minimum of about 0.0005% and no more than about 0.007% boron is included in the composition.

When this alloy is intended for use in the fabrication of parts or articles in which a notch tensile strength at room temperature of less than 150,000 p.s.i. can be tolerated, then boron need not be present in this alloy, or if desired, boron can be included in an amount up to about 0.01% because of its beneficial effect upon the ultimate tensile strength, the .2% yield strength and the stress rupture strength of the alloy. Furthermore, When a composition within the broader range of this alloy, but outside the preferred range, is intended for a use which requires a minimum notch tensile strength of about 150,000 p.s.i., boron may or may not be included, depending upon its effect on the notch tensile strength of the composition.

My alloy is readily prepared and formed into parts. It may 'be melted in air in the usual way but better results are more readily attained when the alloy is vacuuminduction melted. If desired, a double melting process may be used in which an ingot is air or vacuum-induction melted and cast, and then remelted under vacuum as a consumable electrode. Whether the alloy is air or vacuum melted, the heat treatment required to bring out the unique properties of the alloy is relatively easy to carry out. The alloy can be solution treated from about 1400 F. to 1800 F., preferably from about 1500 F. to 1700 F.,

Examples 1-14 had a major gauge diameter of .357 in., a notch diameter of .252 in. and a notch root radius of .001 in., a stress concentration factor (K of about 10. The results of the tests are shown in the following table and are the average of several tests, except in the case for a suflicient time to ensure complete austenitizing. 5 of the notch tensile strength tests Where the results ob- Usually about one hour for each inch of thickness is tained from the individual specimens of each example are sufficient. After cooling to room temperature, it is regiven.

TABLE II Ex. UTS* N TS* Percent Percent Percent Percent No. Y.S.* Elong. R.A. Austenite 19s 15 52 Nil 259 275 235 214 i 259 283 295 i 17 64 N11 295 515 237 279 205 301 15 5s 4 235 315 17 51 Nil i 212 239 i 265 211 195 15 53 N11 249 220 15 55 4 244 275 230 284 17 55 7 235 ggg 309 211 15 59 2 255 340 223 15 57 9 254 225 17 55 5 257 130 298 313 215 15 4s 10 230 257 308 314 52 3 *1,000 p.s.i. heated to about 900 F. to 1100 F., preferably 1000 F. In addition to the foregoing tests, elevated temperature to 1050 F., for about two to four hours followed by tensile and stress rupture tests were carried out in the case cooling in air. In the case of Work pieces of substantial of selected examples. Recorded in Table III are the results thickness, rapid cooling from the austenitizing temperaof tensile tests carried out at 1000 F. on test specimens ture, as by quenching in oil or water, is used to ensure of Examples 4 and 7-14 forged from bars prepared as a fully transformed, martensitic microstr'ucture. was described hereina-bove in connection with the room The following specific examples of my alloy, the 40 temperature test specimens but which had a gauge diamanalyses of which, in weight percent, are set forth in eter of .357 in. and a gauge length four times the Table I, as well as the examples given hereinafter for diameter. purposes of comparison, were forged into bars which TABLE III were then formed into the required test specimens. The compositions were vacuum melted, and, unless otherwise 2% Percent Percent noted, were heat treated for one-half hour at 1600 F. and Eloug. 11.11. oil quenched followed 'by heatingfor four hours at 1000 F. and then cooled in air. 157 171 15 54 1 10 170 15 52 TABLE I 170 100 15 51 157 150 15 157 155 14 52 Ex. No. Cr Co M0 180 205 12 54 173 197 14 52 172 104 13 54 15. 0 20. 0 1. 55 155 123 13 59 15.0 19.5 2.10 15.0 19.3 2. 45 15. 0 20.1 2.86 55 *1,000 p.s.i. 15.0 20.0 3.02 15 0, 32; Stress rupture speclmens of Examples 7, 8 and 10-13 were formed from forged bars prepared as previously 15.1 19.8 3.00 15.8 19.8 3.01 descnbed and were comblnatlon notch/ smooth test speclgf-g 3:33 mens having a smooth bar gauge diameter of .178 in. and 1521 22.7 1 3.05 60 gauge length of .712 in. The notch portion of each speci- 32 3:82 men had a notch diameter of .178 in. and major diameter 1 of .252 in. and a notch root radius of .005 in., giving a stress concentration factor (K of about 3.8. In each of Examples 1-14, carbon, nitrogen, phosphorus TABLE? Iv and sulfur were each less than .01%, manganese and silicon were each less than .1%, nickel was less than .15 E N st R t P t El boron was between 0.0005% and 0.0015 and the re- 132 men Imam mainder was iron except for incidental impurities.

Smooth bar specimens of Examples l-14 used in carry- 57 20 G4 ing out measurements of ultimate tensile strength (UTS), 52g 55 .2% yield strength (.2% Y.S.), percent elongation (per- 505 2 if cent elong.) and percent reduction in area (percent R.A.), 28g 50 all of which were carried out at room temperature, had 0 a gauge diameter of .252 in. and a gauge length of 1 in. Notch tensile strength (NTS) specimens formed of Examples 1-6 demonstrate the effect in my composition of variations in the molybdenum content. With as little as 1.55% molybdenum and with about chromium, cobalt and the balance essentially iron, my composition has good ultimate strength but does not provide the NTS/UTS ratio of at least one, characteristic of my preferred composition demonstrated by Examples 3, 4 and 5. It is to be noted that Example 4 is considered to be within the range of my preferred composition because the slightly higher cobalt content of 20.1% is offset by the chromium content of 15.0% being close to the lower end of the preferred chromium range. Example 6 demonstrates that high ultimate strength is attainable in my composition with molybdenum present in an amount greater than 3%, but the reliable notch tensile strength of my preferred composition as represented by a NTS/UTS ratio consistently at least equal to one is not provided.

Examples 7-10 demonstrate the effect of varying amounts of chromium with about 20% cobalt, 3% molybdenum and the balance iron. With chromium within its preferred range as in Examples 8 and 10, the exceptional notch tensile strength of my preferred composition, together with good room temperature and elevated temperature tensile properties and good stress rupture life at elevated temperatures are obtained. With a chromium content of 12%, the good tensile strength characteristic of my composition is obtained, but the stress rupture strength at elevated temperatures and the TABLE VII Ex. No. 2% Y.S.* UIS Percent Percent Elong. .A.

*1 ,000 p.s.i.

Examples 19-24 were prepared to demonstrate the effect of appreciable amounts of such elements as carbon, aluminum and titanium in my composition.

*Boron less than .0005%.

Specimens for carrying out room temperature tensile tests were prepared from each of Examples 19-24 in the same manner as was described in connection with the room temperature tensile tests described above. The results of such tests carried out on specimens of Exnotch tensile strength at room temperature is adversely amples 19-24 are recorded in Table IX. affected. Example 9 demonstrates the effect of chromium I in amounts appreciably greater than 16.5% in causing TABLE X g theretention of 1n excess of 5% austemt the maxlmum EX. Na UTS, Percent Pemn Percent desired 1n my preferred composition. Elong. R.A. Austenite Examples 11-14 demonstrate that my composition, within its broad range, provides highly desirable room 19 250 2 g 29g 301 62 11 temperature and elevated temperature tensile properties. However, as the cobalt content is increased above 21%, the outstanding notch tensile strength of my preferred composition is no longer consistently attainable and a 24 228 g1 g 83 10 37 tendency toward notch sensitivity may be noted.

The following examples demonstrate the equivalence of molybdenum and tungsten in my composition in the approximate proportion of 1 to 1.2-1.6 in weight percent. These analyses and the test specimens used in car-rying out the tests recorded in Tables V-VII were prepared as previously described. In the examples of Table V, as well as all those described hereinafter, the balance of each alloy was essentially iron and other elements were held to the amounts pointed out in connection with Table I or to inconsequential amounts, unless otherwise indi- *1,000 p.s.i.

Examples 19, 20 and 20A demonstrate the effect of the larger amounts of carbon upon the retention of austenite and the notch tensile strength of my alloy. In Example 20A the effect of carbon on the retention of austenite is counter-acted by the reduction in the chromium and cobalt contents as compared to Examples 19 and 20. Examples 21-24 demonstrate that appreciable amounts of aluminum and titanium severely reduce the ductility and notch tensile strength of my alloy.

cated' Examples 25, 26 and 27 were prepared to demonstrate TABLE V the effect of nickel in my composition and the analyses and test data are set forth in Table X. Ex. No. Cr 00 M0 W TABLE X 15.2 19.5 1.93 15. 2 19. 5 3. 80 Elements and Tests Ex. 25 Ex. 26 Ex. 27 15.2 19.7 1.99 15.2 19.7 1.99

2. 86 2. 86 2. 36 TABLE VI N1 1. 00 3.03 2. 93

RODOHt1 Temp, Tensile a a: Ex. N6.- UTS* NTS" Percent Percent Percent UTS* 244 222 139 Elong. R. Austemte NTs* 327, 334,336 331,326,331 149,150,166 Percent Elong 17 17 38 Percent R.A 61 63 72 15". 197 270 190 137 15 61 Percent Austenit 10 13 239 292 293 325 17 63 5 1,000 F., Tensile Data: 224 293 230 16 62 Nil a 163 242 257 195 232 17 60 4 14 58 1,000 p.s.i. In Table VII are recorded the results of tensile tests car-ried out at 1000 F. on test specimens of Examples 54 16, 17 and 18 prepared as was described in connection with Table III. 75 *1,000 p.s.l.

With only 1% nickel, the adverse effect upon the stress rupture life of my composition at elevated temperatures is apparent from Example 25, but it will be noted that Example 25 and Example 26 demonstrate highly desirable room temperature properties. Example 26 further demonstrates the severe effect of 3% nickel upon the elevated temperature stress rupture life of my composition and that it is not compensated for even when the cobalt content is reduced below the minimum of 18% to as low as 15.1%. When the consequent impairment of elevated temperature stress rupture life can be tolerated, I may use as much as 3% nickel, but in that event, the other elements which form or stabilize austenite are held to the lower amounts hereinbefore indicated in order to maintain the essentially martensitic balance of the composition with preferably no more than about 20% retained austenite. Example 27 demonstrates that unless the balance of the alloy is adjusted by reducing the cobalt content to avoid the retention of excessive amounts of austenite, the room temperature tensile properties are severely affected by an addition of 2.93% nickel.

In my composition, the elements chromium, cobalt and molybdenum must be carefully maintained within the ranges stated. When any one of those elements is present either in an amount appreciably below or above the broad range stated for that element, then the tensile properties or strength characteristic of my alloy is not attained. Referring to Table XI, Examples 28 and 29 demonstrate the severe effect of excessive amounts of molybdenum and/or tungsten upon the notch tensile strength of the alloy. With 4.03% and 3.90% molybdenum in those respective examples, there is a marked loss in notch tensile strength, although an ultimate tensile strength of about 250,000 p.s.i. is attained.

TABLE XI Elements and Tests Ex. 28 Ex. 29 Ex. 30 Ex. 31 Ex. 32Ex. 33

Cr 15.0 15.2 13.0 18. 2 15.0 14. 9 Co"... 10. 20.1 19. 8 10.8 16.0 25. 0 M0 4. 03 3. 90 3.03 3.00 2. 95 3.10

Room Temp, Tensile Data:

UTS 1 253 255 200 199 198 207 NIS 1 100 139 195 135 155 211 Percent Elong:

Percent R.A 66 64 42 65 62 1,000 R, 120,000 p.s.i.:

Stress Rupture Life, hrs Z 24 81 Percent Elong 30 19 Percent R.A 53 60 1 1,000 p.s.i. 2 100,000 p.s.i.

Example 30 demonstrates the effect of less than my preferred amounts of chromium upon notch tensile strength, but this analysis is suitable for use where an NTS/UTS ratio greater than 1 is not required. Example 31 is essentially the same as Example 30 but for the chromium content of 18.2%. The test data clearly demonstrates the loss in strength and the excessive amount of retained austenite. Example 32 demonstrates the effect of an insutficient amount of cobalt while Example 33 demonstrates the need for care in balancing the alloy when the larger amounts of cobalt within the broad range are used. Better over-all properties are attained when, with as much as 25.6% cobalt, the chromium content is reduced somewhat. Also the molybdenum content may be reduced to the neighborhood of about 2% with such a larger amount of cobalt present.

The outstanding properties of my alloy are readily obtained when it is prepared in commercial quantities so as to contain 14.8% to 15.5% chromium, 19.75% to 20.25% cobalt, 2.75% to 3.00% molybdenum, and the the remaining elements as stated in connection with the preferred range set forth hereinabove. For example a 2500 pound ingot was vacuum induction melted and then remelted as a consumable electrode under vacuum. The final ingot thus formed had the following analysis, in weight percent:

Percent Carbon 0.008 Manganese 0.08 Silicon 0.06 Phosphorus 0.007 Sulfur 0.005 Nickel 0.22 Chromium 15.00 Cobalt 20.16 Molybdenum 2.97 Boron 0.001 Iron Balance except for incidential impurities The ingot was heated to its hot working range, 1900-2100 F. and was readily hot worked to form various products including billets, bars, strip and wire. Examination of the microstructure of specimens of the various products showed that they contained less than 5% retained austenite with no detectable retained ferrite or sigma phase. Test specimens were formed as was described in connection with Examples 114, the heat treatment of which consisted of heating to 1700 F. for onehalf hour followed by oil quenching and then aging at 1025 F. for four hours followed by cooling in air.

At room temperature the .2% yield strength was from 220,000 p.s.i.-22,000 p.s.i. The ultimate tensile strength was from 240,000 to 245,000 p.s.i., with about 18% elongation and 60% reduction in area. The notch tensile strength was from 280,000 to 330,000 p.s.i. The alloy as solution treated was found to have a hardness of Rockwell C 30, and as aged, a hardness of Rockwell C 49.

At 1000 F. the .2% yield strength was 175,000 p.s.i. and the ultimate tensile strength was 190,000 p.s.i. with 16% elongation and 60% reduction in area. The stress rupture life at 1000 F. under a load of 125,000 p.s.i. was measured on specimens formed both from billets and from bar stock. One test specimen formed from a billet withstood the load of 125,000 p.s.i. for a 133 hour period and showed an elongation of 26% and a reduction in area of 64%. A second specimen withstood the load for hours and showed an 18% elongation and a 56% reduction in area. A third specimen withstood the load of 125,000 p.s.i. for 121 hours and showed a 25% elongation and 65% reduction in area. One test specimen formed from bar stock withstood the load of 125,000 p.s.i. for hours with 21% elongation and 59% reduction in area.

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 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 above about 190,000 p.s.i. with good ductility and which consists essentially of by weight a maximum of 0.10% carbon, a maximum of 0.50% manganese, a maximum of 0.50% silicon, a maximum of 0.030% phosphorus, a maximum of 0.030% sulfur, no more than about 3% nickel, from about 12% to 18% chromium, from about 18% to 26% cobalt, from about 1.5% to 3.5% molybdenum or an equivalent amount of tungsten with tungsten replacing molybdenum in the ratio of about 1.2% to 1.6% tungsten to 1% molybdenum, up

to about 0.01% boron, the remainder being substantially iron, and said alloy being balanced so as to have a primarily martensitic microstructure substantially free of retained ferrite and sigma phase following heat treatment and aging at elevated temperatures.

2. An age hardenable stainless iron base alloy which in its age hardened condition is characterized by an ultimate tensile strength of at least about 200,000 p.s.i. with good ductility and with a ratio of notch tensile strength to ultimate tensile strength at least equal to or greater than one at a stress concentration factor of 10 and which consists essentially of by weight a maximum of 0.010% carbon, a maximum of 0.10% manganese, a maximum of 0.10% silicon, a maximum of 0.010% phosphorus, a maximum of 0.010% sulfur, no more than about 0.50% nickel, from about 14.5% to 16.5% chromium, from about 19.0% to 21.0% cobalt, from about 2.0% to 3.0% molybdenum or an equivalent amount of tungsten with tungsten replacing molybdenum in the ratio of about 1.2% to 1.6% tungsten to 1% molybdenum, from about 0.001 to 0.003% boron, the remainder being substantially iron, and said alloy following heat treatment and aging at elevated temperatures having an essentially martensitic microstructure substantially free of retained ferrite and sigma phase and containing no more than about 5% retained austenite.

3. A stainless steel aiticle formed from an alloy which consists essentially of by weight a maximum of 0.10% carbon, a maximum of 0.50% manganese, a maximum of 0.50% silicon, a maximum of 0.030% phosphorus, a maximum of 0.030% sulfur, no more than about 3% nickel, from about 12% to 18% chromium, from about 18% to 26% cobalt, from about 1.5% to 3.5% molybdenum or an equivalent amount of tungsten with tungsten replacing molybdenum in the ratio of about 1.2% to 1.6% tungsten to 1% molybdenum, up to about 0.01% boron, the remainder being substantially iron, the composition of said article being balanced and the article being heat treated and age hardened at elevated temperatures to have a primarily martensitic microstructure substantially free of retained ferrite and sigma phase with an ultimate tensile strength of above about 190,000 p.s.i., good ductility, a notch tensile strength of above about 150,000 p.s.i. at a stress concentration factor of and with a .2% yield strength of above about 170,000 p.s.i.

4. A stainless steel article formed from an alloy which consists essentially of by weight a maximum of 0.010% carbon, a maximum of 0.10% manganese, a maximum of 0.10% silicon, a maximum of 0.010% phosphorus, a maximum of 0.010% sulfur, no more than about 0.50% nickel, from about 14.5% to 16.5% chromium, from about 19.0% to 21.0% cobalt, from about 2.0% to 3.0% molybdenum or an equivalent amount of tungsten with tungsten replacing molybdenum in the ratio of about 1.2% to 1.6% tungsten to 1% molybdenum, from about 0.001% to 0.003% boron, the remainder being substantially iron, said article being heat treated and age hardened at elevated temperatures and having an ultimate tensile strength of above about 200,000 p.s.i. with good ductility and with a ratio of notch tensile strength to ultimate tensile strength at least equal to or greater than one at a stress concentration factor of 10 and a .2% yield strength'of above about 180,000 p.s.i. and said article having an essentially martensitic microstructure substantially free of retained ferrite and sigma phase and containing no more than about 5% retained austenite.

5. An age hardenable stainless iron base alloy which in its age hardened condition is characterized by an ultimate tensile strength of at least about 200,000 p.s.i. with good ductility and with a ratio of notch tensile strength to ultimate tensile strength at least equal to or greater than one at a stress concentration factor of 10 and which consists essentially of by weight a maximum of 0.010% carbon, a maxi-mum of 0.10% manganese, a maximum of 0.10% silicon, a maximum of 0.010% phosphorus, a maximum of 0.010% sulfur, no more than about 0.50% nickel, from about 14.8% to 15.5% chromium, from about 19.75% to 20.25% cobalt, from about 2.75% to 3.0% molybdenum or an equivalent amount of tungsten with tungsten replacing molybdenum in the ratio of about 1.2% to 1.6% tungsten to 1% molybdenum, from about 0.001% to 0.003% boron, the remainder being substantially iron, and said alloy following heat treatment and aging at elevated temperatures having an essentially martensitic microstructure substantially free of retained ferrite and sigma phase and containing no more than about 5% retained austenite.

6. A stainless steel article formed from an alloy which consists essentially of by weight a maximum of 0.010% carbon, a maximum of 0.10% manganese, a maximum of 0.10% silicon, a maximum of 0.010% phosphorus, a maximum of 0.010% sulfur, no more than about 0.50% nickel, from about 14.8% to 15.5% chromium, from about 19.75% to 20.25% cobalt, from about 2.75% to 3.0% molybdenum or an equivalent amount of tungsten with tungsten replacing molybdenum in the ratio of about 1.2% to 1.6% tungsten to 1% molybdenum, from about 0.001% to 0.003% boron, the remainder being substantially iron, said article being heat treated and age hardened at elevated temperatures and having an ultimate tensile strength of above about 200,000 p.s.i. with good ductility and with a ratio of notch tensile strength to ultimate tensile strength at least equal to or greater than one at a stress concentration factor of 10 and a .2% yield strength of above about 180,000 p.s.i., and said article having an essentially martensitic microstructure substantially free of retained ferrite and sigma phase and containing no more than about 5% retained austenite.

References Cited UNITED STATES PATENTS 2,848,323 8/1958 Harris l26 X 2,880,085 3/1959 Kirkby 75l26 2,985,529 5/1961 Harris 75l26 2,990,275 6/1961 Binder 75l26 3,154,412 10/1964 Basak 75l26 3,251,683 5/1966 Hammond 75l28 DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,364,013 January 16, 1968 Robert L. Caton It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the heading to the printed specification, line 3, for

"Wyomissing" read Wyomissing Hills column 2, line 33, for "alloys" read alloy line 54, for "325,000" read 300,000 column 3, line 22, for "to more" read to no more column 4, line 28, for "parts" read part column 5, line 72, for "elong." read Elong. column 10, line 36, for "22,000" read 225,000 column 12, line 55, for "Basak" read Kasak Signed and sealed this 1st day of April 1969.

(SEAL) Attest:

Edward M. F letcher, J r. EDWARD J. BRENNER Attesting Officer Commissioner of Patents