US3759757A

US3759757A - Aluminum bearing precipitation hardening stainless steel of high retained toughness

Info

Publication number: US3759757A
Application number: US00581584A
Authority: US
Inventors: Cameron Perry D
Original assignee: Armco Inc
Current assignee: Armco Advanced Materials Corp
Priority date: 1966-09-23
Filing date: 1966-09-23
Publication date: 1973-09-18
Anticipated expiration: 1990-09-18

Abstract

A PRECIPITATION-HARDDNABLE SEMI-AUSTENITIC STAINLESS STEEL WHICH BY VIRTURE OF PARTICULAR COMPOSITION BALANCE IS ESSENTIALLY FREE OF DELTA-FERRITE AND READILY LENDS ITSELF TO A VARIETY OF WORKING AND FORMING OPERATIONS, IS READILY HARDENED BY PRECIPITATION HEAT-TREATMENT, AND IN HARDENED CONDITION IS STRONG AND TOUGH, MAINTAINING ITS STRENGTH AND TOUGHNESS OVER LONG PERIODS OF USE AT ELEVATED TEMPERATURE. THE STEEL CONTAINS THE FOUR ESSENTIAL INGREDIENTS CHROMIUM, NICHEL, CARBON AND ELUMINUM IN CRITICAL AMOUNT AND PROPORTION, WITH REMAINDER ESSENTIALLY IRON. THE CHROMIUM CONTENT IS IN THE AMOUNT OF ABOUT 12% TO 19%, NICKEL ABOUT 7% TO 10%, CARBON ABOUT .03% TO .05%, AND ALUMINUM ABOUT .01% TO .48% TO .48%. THE INGREDIENTS MANGANESE, SILICON, PHOSPHORUS, SULPHUR AND NITROGEN COMMONLY PRESENT IN STAINLESS STEEL ARE MAINTAINED IN LOW AND CRITICAL AMOUNT. MOLYBDENUM MAY BE ADDED WHERE DESIRED.

Description

Sept. 18, 1973 D. c. PERRY 3,759,757

ALUMINUM-BEARING PRECIPITATIONHARDENING STAINLESS STEEL OF HIGH RETAINED TOUGHNESS Filed Sept. 23, 1.966 2 Sheets-Sheet 2 D Camera Perry BY L J IS ATTORNEY Fig.2

United States Patent Office US. Cl. 148--38 14 Claims ABSTRACT OF THE DISCLOSURE A precipitation-hardenable semi-austenitic stainless steel which by virtue of particular composition balance is essentially free of delta-ferrite and readily lends itself to a variety of working and forming operations, is readily hardened by precipitation heat-treatment, and in hard ened condition is strong and tough, maintaining its strength and toughness over long periods of use at elevated temperature. The steel contains the four essential ingredients chromium, nickel, carbon and aluminum in critical amount and proportion, with remainder essentially iron. The chromium content is in the amount of about 12% to 19%, nickel about 7% to 10%, carbon about .03% to and aluminum about .01% to .48%. The ingredients manganese, silicon, phosphorus, sulphur and nitrogen commonly present in stainlesssteel are maintained in low and critical amount. Molybdenum may be added where desired.

As a matter of introduction, my invention relates to the chromium-nickel-aluminurn precipitation-hardening stainless steels, in a sense being related to the steel described in my companion co-pending application Ser. No. 334,923 filed Dec. 31, 1963 and entitled Chromium-Nickel Aluminum Steel and Method, and more particularly, concerns a steel of high retained toughness following prolonged exposure to elevated temperatures.

Among the objects of my invention is the provision of a semi-austenitic chromium-nickel-aluminum stainless steel of maximum cleanliness and minimum cost in the ingot; which steel is essentially free of delta-ferrite and readily lends itself to hot-working and cold-working as in the production of plate, sheet, strip, bars, rods, wire, special shapes and like products; which products are readily formed and shaped, as in the fabrication of a wide variety of articles, equipment and the like of ultimate use; and which steel and articles, equipment and the like are hardenable by simple precipitation-hardening heat-treatment at comparatively low heat-treating temperatures, with resultant strength, durability and toughness, properties which are retained even after prolonged exposure to elevated temperatures.

Other objects of my invention in part will be obvious and in part pointed out in the description which follows.

My invention accordingly consists in the combination of elements, composition of ingredients and in the correlation between the same and in the articles, products and the like fashioned thereof, the scope of which invention is defined by the claims at the end of this specification.

BACKGROUND OF THE INVENTION It is though that perhaps a better understanding of my invention may be had by first considering the precipitation-hardening stainless steels presently known in the art. Commonly, these steels essentially require chromium and nickel in such amount that the steel is substantially austenitic. And, in addition, there is required one or more of the ingredients titanium, aluminum, copper and vanadium. For example, the Wyche et al. U.S. Pat. 2,381,416 essentially requires 12% to 20% chromium, 2% to Patented Sept. 18, 1973 nickel, 0.3% to .15% carbon, .25% to 10% manganese, aluminum in the amount of 1% as a maximum, titanium 0.40% to 2%, and remainder iron. Columbium may be present as an optional ingredient. The steels of the Goller U.S. Pats. 2,505,762, 2,205,763, 2,505,764 and 2,506,558 essentially require chromium and nickel in large amount according to the particular relation between the same as set out in diagrams accompanying the patents, together with carbon in the amount of 0.02% to 0.12%, manganese in very incidental amounts up to about 8%, aluminum about 0.50% to about 2.50%, and remainder 11011.

Unfortunately, however, the steels of the prior art leave something to be desired. For example, the titanium-bearing precipitation-hardening stainless steels generally are lacking in desired cleanliness. To some extent the same may be said for the aluminum-bearing precipitationhardening stainless steels, these steels moreover being undesirably inclined to embrittlement developed under long sustained periods of stress at moderately elevated temperatures.

One of the objects of my invention, therefore, is the provision of a precipitation-hardenable chromium-nickel stainless steel employing a minimum of costly alloying ingredients, which steel works well in the furnace, teems well, giving steel of maximum cleanliness which works well in the hot mill as well as the cold mill, which readily lends itself to fabrication as by bending, pressing and drawing, and which is readily hardenable by precipitation heat-tretment methods giving a steel possessing the combination of strength and toughness, with toughness retained after long periods of use under conditions of elevated temperature stress, that is, stress at temperatures up to about 750 F.

SUMMARY OF THE INVENTION Turning now to the practice of my invention, I provide a steel essentially consisting of the ingredients chromium, nickel, carbon and aluminum, all in critical proportions, and remainder substantially all iron. The ingredients manganese, silicon, phosphorus, sulfur and nitrogen commonly present in stainless steels of course are present in my steel, these, however, being maintained in particularly small amount. Where desired, molybdenum may be added to improve the room temperature strength of the metal and the high temperature strength as well. Broadly, the steel of my invention essentially consists of about 12% to about 19% chromium (preferably about 13.5% to about 16% chromium), about 6% to about 10% nickel (preferably about 7% to about 9% nickel), about .03% to about .05% carbon, a manganese content less than about 1%, a silicon content not exceeding about 1%, phosphorus not exceeding about .04%, sulfur not exceeding about .03%, nitrogen not exceeding about .05%, about .05% to about .45% aluminum, or even about .01% to about .48% aluminum, and remainder substantially all iron. Molybdenum, where added, may be employed in amounts up to about 3%. Best results are had where the sum of the molybdenum and chromigm contents is in the amount of about 15% to about 19 0.

While the essential ingredients chromium and nickel are generally critical in my steel, and so, too, the permisisble amounts of the commonly present ingredients manganese, silicon, phosphorus and nitrogen, it is the ingredients carbon and aluminum which are especially critical. For desired results carbon is present in the amount of about .03% to about .05 And although aluminum, as noted above, may be employed in the amount of about .05% to about .45%, or even to about .48%, the best combination of cleanliness, strength and sustained toughness under long periods of stress at elevated temperatures are had where the aluminum content is in the amount of about .05% to about .25%. For those steels where welding is contemplated, I desirably include titanium in the composition of the metal, this in amounts only up to about .'l0% as a maximum, however, an excess of titanium giving dirty metal and, additionally, undesirably combining with carbon critically necessary to the steel. The small amount of titanium employed is just sufficient to combine with the nitrogen present in the steel in order to assure the production of sound gas-free welds. Because of the high total alloy content of my steel, boron, this in amounts up to about .005 may be included where desired to impart improved hot-working properties.

The chromium of my steel, as indicated above, is in the amount of about 12% to about 19%, at least about 12% being necessary to achieve the desired corrosionresisting characteristics, while chromium in an amount exceeding 19% becomes unduly costly. Moreover, the higher chromium content makes for an undesirably high total alloy content and an undesirably stable metal, requiring an extra effort to elfect transformation prior to precipitation-hardening heat-treatment. An excellent combination of properties is had Where the chromium content is in the range of about 13.5% to about 16%.

An as to the nickel content of my steel, where the nickel content exceeds about there again is too high an alloy content and the steel becomes too stable. With less than about 6%, however, the steel becomes too lean, there being insufiicient nickel to assure the desired semiaustenitic structure. Best results are had with a nickel content of about 7% to about 9%.

Now the manganese content of my steel is in an amount less than 1%. Actually, for best combination of results I prefer a manganese content of about .20% to about .70%. The same is true for the silicon content of the steel, that is about .20% to .70% silicon for best toughness, although silicon may amount to as much as about 1%. Greater amounts of both manganese and silicon are inclined to disturb the austenitic balance of the metal, the one being an austenite former, the other a ferrite former. The phosphorus content in my steel seems to have very little, if any, effect upon the properties of the metal, although phosphorus in amount exceeding about .04% is considered undesirable. The sulfur content ordinarily is in the amount of .02% to .03%, for sulfur in amounts less than .0l% requires extra precautions in melting the steel, this at increased cost. The best toughness is achieved, however, where the sulfur content does not exceed about .015%, and preferably does not exceed .010%. Nitrogen is present in an amount not exceeding about .05%, this being viewed largely as an impurity normally encountered in electric arc furnace practice. Nitrogen is not a purposeful addition and in certain melting operations there is had a steel which is virtually free of nitrogen.

H is in the ingredients carbon and aluminum, however, as suggested above, that my steel is particularly critical. With carbon less than about .03%, I find that there is some difficulty in preventing transformation of the annealed metal, as in cold-weather shipment. With carbon in the amount of about .03% to about .05 however, an ease of transformation is achieved by desired heattreatment. But carbon in an amount exceeding about 0.5% is not desired because the metal becomes undesirably stable and, of even greater significance, during a heating of the metal prior to precipitation-hardening certain precipitates of carbon are inclined to appear in the phase boundaries, resulting in a loss of toughness in the steel, all as more particularly noted in my companion co-pending application for patent.

Aluminum, as noted, is an ingredient essential to the composition of the steel of present interest, this in the amount of about .05 to about .45 or to about .48%,

or even about .0 l% to about .45% or about .48%. For best retained toughness, with good initial strength, an aluminum content of about .01% to about .25% is desired. Aluminum in amounts exceeding .48% I find results in a loss of toughness of the hardened steel following exposure to elevated temperatures for prolonged periods of time. It seems that the higher the aluminum content, the greater is this inclination. Perhaps with elevated temperature exposure there is a continuation of the aluminumnickel precipitation reaction characterizing the precipitation-hardening chromium nickel aluminum stainless steels, this reaction evidently not being completed with the usual aging or precipitation-hardening treatment (some 750 to 1000 F. for about one hour as noted in the Goller patents referred to above). Or perhaps the principal hardening is due not to the formation of the aluminum-nickel constituent had in the known aluminumbearing pre'cipitation-hardenable chromium-nickel stainless steels, but rather to an alpha prime iron-chromium constituent (also referred to as order ferrite) which develops with heating below 885. But irrespective of theoretical considerations, I find that while there is a gain in hardness and strength in service at elevated temperatures these properties are more than offset by the undesired loss in toughness and the increased brittleness which result. While maximum freedom from embrittlement may possibly be had with a complete absence of aluminum, I find that at least .0l% aluminum is necessary in order to achieve the desired preciiptation-hardening effect, at least .05 usually being best for full realization.

Actually, a best combination of respons to precipitationhardening heat-treatment and sustained toughness in the precipitation-hardened condition is achieved with an aluminum content of about .05 to about .25 And with this lower aluminum content, freedom from undesired transformation, as in cold-weather shipment of the steel, is had with carbon in the amount of about .03%. If the carbon content is as much as .05% the toughness of the steel is inclined to suffer.

The steel of my invention conveniently is melted in the electric arc furnace, although where desired it may be melted in the induction furnace or otherwise. But for the production of steel of minimum cost, I prefer the electric arc furnace. The steel handles well in the furnace and in teeming. Moreover, it works Well in the hot-mill, being virtually free of delta-ferrite. The steel conveniently is produced in the form of slabs, blooms and billets and desirably cold-rolled into sheet, strip, bars, wire and the like. The metal usually is marketed in the annealed or solution-treated condition. Where desired, it may be supplied in the form of forging billets, or even for remelt casting, excellent castings being had by reason of freedom from the usual aluminum oxide scum encountered in many of the aluminum-bearing precipitationhardening steels.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings illustrating certain features of my invention:

FIG. 1 depicts the gain in tensile strength had with long-time exposure at high temperatures by a series of precipitation-hardened chromium-nickel stainless steels of differing aluminum contents; and

2 illustrates the loss in toughness suffered by these steels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As particularly illustrative of the steel of my inven tion this as compared to steels of similar composition but departing from that steel in the amount of aluminum employed, I give below the analyses of some 27 heats of differing aluminum contents. Additionally, I give below the mechanical properties of these steels -in the precipitation-hardened condition, both before and after exposure to high temperature for a prolonged period of time, exposure at 750 F. for 1,000 hours being adopted as accelerated test procedure (exposure at 750 F. for even -20 hours being considered equivalent to exposure at some 550-600 F. for 50,000 hours). Further, as a matter of convenience, I give the changes in mechanical properties as a result of the long-time exposure at high temperature, this for the steels of differing aluminum contents. The chemical analyses of the 27 steels, being steels of the present invention and 10 being steels which either are martensitic and not semi-austenitic or which differ therefrom in the matter of aluminum content (and resulting properties), are presented in Table 1(a) below:

TABLE 1(a) [Chemical composition or chiol'lllsiilm nickel molybdenum stainless S ec C Mn Si Cr Ni Mo Al B N Steels of present invention.

Nora-Phosphorus, .O0l% to 004%; sullur, .0031% to .0079%.

Certain physical characteristics of the steels of Table 1(a), particularly the hardness in the annealed condition (from which certain of the teels are seen to be martensitic and not semi-austenitic), the grain size, and the deltaferrite content are set out below in Table (I)b:

TABLE 1(b) [Hardness, grain size and delta-ferrite content of the steels of TableI (a)] Annealed hardness, Percent Percent Rockwell delta- ASTM,

A1 B/O ferrite G/ 28C 20 12 76 32 14 12 1. 16 86B 14 11 77 83 3 12 48 76. 5 1 12 1. 08 88. 5 9 12 71 88 2 11/12 48 97 1 12 049 84 0 9/ 10 15 99 0 9/10 38 82 0 9/ 10 11 83 0 8/ 10 010 83. 5 0 10/11 14 33C 0 10/ 11 060 33 0 10/11 42 B 0 10/11 12 75 0 9/ 10 22 82 1 10/11 094 82 1 10/11 52 27C 1 10/1 1 25 74B 1 9/12 10 8O 1 8/0 52 76 1 9/10 26 74 1 9/10 *Steels of present interest.

*Marteusitic in the annealed condition.

:From the high hardnes of the steels of Codes 575, 576, 615, 616 and 723 in the annealed condition (respectively Rockwell C28, C32, C33, C33 and C27) it will be seen that they are martensitic rather than semiaustenitic. They may be immediately dismissed from further consideration.

It is to be noted here that of the remaining steels the Codes 577, 578, 580, 581 and 726 are of substantial deltaferrite content, ranging from 14% for the first steel down to 1% for the latter steel. These steels are not considered to be a part of my invention as appears more fully hereinafter.

The mechanical properties of the 20 semi-austenitic stainless steels included in Tables 1(a) and 1(b) in the precipitation-hardened condition are given below in Table II(a), these being the yield strength and ultimate tensile strength, both in kilopounds per square inch, percent elongation in 2", the Rockwell hardness and the precracked sheet Charpy value W/A. All properties were taken on cold-rolled strip of .050" thickness. The mechanical propertie of these steels following exposure at 750 F. for 1,000 hours are given in Table II(b). And the diiferences, that is, the changes in the mechanical properties resulting from exposure at 750 F. for 1,000 hours, are given in Table II(c).

TABLE 11(5).)

[Mechanical properties of the 20 precipitation-hardened serni-austeniti stainless steels of Table 1(a)] Y.S UIS, Percent k.s.1 k.s.i. E, 2" Re W/A 238. 1 250. 5 6 50. 5 804 230. 0 241. 2 6 49 1, 127 219. O 226. 3 6 49 1, 285 227. 0 241. 7 6 49. 5 862 222. 0 234. O 6 48. 5 1, 425 213. 0 222. 0 6 47. 5 745 166. 0 178. 8 8 40 2, 814 170. 5 182. 5 8 40 2, 385 197. 8 208. 7 7 45 881 180. 0 192. O 9 43 2, 349 176. 3 186. 9 10 43 2, 387 195. 8 207. 5 5 45 1, 354 180. 9 191. 2 8 41. 5 2, 676 111. 6 146. 8 18 40. 5 2,130 167. 8 182. 0 14 44 2, 237 189. 5 200.8 6 44. 5 1, 883 179. 0 189. 2 8 44. 5 2, 434 196. 1 211. 0 8 46. 5 792 183.8 194.1 7 44. 5 1,930 171. 8 182. 5 10 43 2, 203

' Steels of present invention.

NorE.-All steels solution-treated at 1,700 F. for 1 hr.; refrigerated at minus 100 F. for 8 hours; and aged at 900 F. for 8 hours.

The mechanical properties of the semi-austenitic steels of Table 1(a) change substantially with prolonged exposure at high temperature. The yield strengths and tensile strengths are found to increase along with hardness while the ductility as gauged by the elongation figures and the toughness as gauged by the pre-cracked sheet Charpy values W/A sufier considerably. The several mechanical properties for the precipitation-hardened semi-austenitic steels following exposure at 750 F. for 1,000 hours are given below in Table 11(b):

TABLE II(b') [Mechanical properties of the 20 precipitation hardened semi-austenitic steels of Table 1(a) following exposure at 750 F. for 1,000 hours] Y.S. UTS, Percent k.s.i. k.s.i. E, 2 Re W/A *Steels of present invention.

As a matter of convenience the individual changes in yield strength, ultimate tensile strength and toughness, with indication of the percent of toughness remaining following the exposure at high temperature, are given below in Table II(c) this with indication of the aluminum contents of the various steels:

TABLE II(c) [Change in mechanical properties of the 20 precipitation hardened semiaustegitic steels of Table 1(a) resulting from exposure at 750 F. for 1,000 ours Percent Change, Change, Change, W/A

Y.S. UTS W/A remaining Steels of present invention.

The gain in the ultimate tensile strength of the various precipitation-hardened semi-austenitic steels as a result of the 1,000-hour treatment at 750 F. as reported in Table II(c) above is graphically illustrated in FIG. 1 of the drawings. It will be immediately noted that the gain in strength is substantially the same for all steels, that is, irrespective of the aluminum content of the steel. Perhaps it may be said that a somewhat greater gain in strength is had for the steels of the lower aluminum contents, although this difference in gain appears to be rather slight, say, a gain of about 32,000 p.s.i. for the steels of the lower aluminum contents as against a gain of 27,000 p.s.i. for the steels of the higher aluminum contents.

It is in the matter of toughness, however, that a sharp and critical difference appears between the critically low aluminum-bearing steels of present interest and the stainless steels of high aluminum content. This is seen from the data presented above, particularly the original toughness remaining following long-time exposure at high temperature. This difference is graphically presented in FIG. 2 of the accompanying drawings, based on the data given above. It will be seen that for the steels with aluminum contents of about .05% to about .45 or even to about .48%, most of the original toughness is retained even after long-time exposure at high temperatures, this amounting to about 75% of the original toughness figures for the precipitation-hardened steels. But for the steels of the higher aluminum contents, the toughness sharply falls, only about 60% remaining for a steel of about .6% aluminum, about 50% remaining for a steel of about .8% aluminum, about 30% remaining for a steel of about 1.0% aluminum and on the order of 10% remaining for a steel of about 1.2% aluminum. In short, the steels having the higher aluminum values, namely from about .50% to 1.2%, are inclined to loss of toughness, and increasing brittleness, with an increase in aluminum content. Moreover, it will 8 be noted that of the 15 steels illustrative of the present invention, those steels having an aluminum content of about .05% to about .25% (Codes 608, 609, 612, 618, 718, 719, 724, 725, 727 and 728), and especially those having the even lower and more limited aluminum content of .05% to .15% (Codes 608, 609, 612, 618, 719, 725 and 728) enjoy best initial toughness (see Table II(a)) and best toughness after exposure to high temperatures (see Table II(a)). It is in the steels of about .05 to about -.25% aluminum, especially .05% to .15 aluminum, that a best combination of strength and retained toughness is enjoyed.

The critically low aluminum, low carbon, chromiumnickel stainless steel of my invention enjoys a maximum of cleanliness, along with toughness and strength under conditions of long service at high temperatures. The steel, as suggested above, is made available to the trade in the form of cold-rolled sheet, strip, bars, rods and wire in the annealed or solution-treated condition. In this condition it is semi-austenitic. It also is made available in the form of forging billets. It readily lends itself to fabrication as by spinning, press-forming, bending, roll-forming, drawing, punching, blanking, and the like. And it readily lends itself to Welding as in the fabrication of a variety of articles and equipment of ultimate use. For example, it is admirably suited to the production of cutter knives for garbage disposal units, for the production of various articles, such as springs which are subjected to high temperature stresses for long periods of time, and for the production of structural members for motor trucks, railroad cars and the like whee a combination of strength, toughness and corrosion-resistance, all with reduced Weight, is desired.

The properties of hardness, strength and toughness are developed by the customer-fabricator by proper heattreatment. In this regard I find that the best combination of results is had by conditioning the semi-austenitic steel, and the articles, equipment and the like fashioned thereof, at a temperature of some l300 to 1700 F. for about an hour and cooling in air or otherwise as desired. Following this, the steel and articles are maintained at room temperature or substantially below room temperature to effect transformation to martensite. In this respect where the steel is conditioned at a temperature of 1400 F. a mere cooling at 60 F. effects transformation. Where, however, the metal is conditioned at the higher temperature of, say, 1700 F., refrigeration at a temperature of about minus F. is required. Actually, I find that the higher conditioning temperatures, that is, about 1700 F., with subsequent treatment at minus 100 F., gives a steel of maximum toughness in the precipitation-hardened con dition. Urecipitation-hardening is achieved by reheating the transformed metal at a temperature of some 700 to 950 F., preferably 800 to 900 F., for a period of some 1 to 50 hours, the length of time depending upon the particular temperature employed and the particular hardness desired.

Where large sections are involved the annealing at the mill, or subsequently by customer-fabricator, preferably is had at a temperature of about 1800 to 1850" F., for I find that the austenite is less stable both at higher and at lower temperatures. And the steel is maintained at annealing temperature for a sufiicient period of time to assure a thorough soaking. For heavy sections this may be a matter of something under an hour but for light sections, with continuous annealing, a minute or less is sufficient. And in cooling from annealing temperatures a rapid air-cool is sufficient for the light sections but quenching in oil, water or brine is desirable Where heavy sections are to be treated, it being necessary to get the metal below a temperature of 1000 F. as quickly as possible.

Thus, in conclusion, it will be seen that I provide in my invention a steel in which the various objects hereinbefore set forth are successfully achieved. The steel is strong, ductile and tough, the toughness being retained under long-time conditions of service at high temperature. Moreover, the steel handles well in the furnace, it readily lends itself to conversion into plate, sheet, strip, bars, rods, wire and the like in both the hot-mill and the cold-mill. And the metal readily lends itself to fabrication as by cutting, bending, pressing, spinning and the like, as well as by welding, as in the production of a host of articles and equipment of ultimate use.

Inasmuch as many embodiments of my invention may occur to those skilled in the art to which the invention relates, and inasmuch as many variations of the embodiments set out above also may occur to them, it will be understood that all matter described herein, or shown in the accompanying drawings, is to be interpreted as illustrative and not by way of limitation.

CLAIMS 1. A precipitation-hardenable semi-austenitic stainless steel substantially free of delta-ferrite and essentially consisting of about 12% to about 19% chromium, about, 7% to about 10% nickel, about .03% to about carbon, about .0l% to about .45% aluminum, manganese less than about 1%, silicon not exceeding about 1%, phosphorus not exceeding about .04%, sulfur not exceeding about .03%, nitrogen not exceeding about .05%, molybdenum up to about 3%, and remainder iron.

2. A precipitation-hardenable semi-austenitic stainless steel substantially free of delta-ferrite and essentially consisting of about 12% to about 19% chromium, about 7% to about 10% nickel, about .03% to about .05% carbon, about .01% to about .45% aluminum, about .20% to about .70% manganese, silicon not exceeding about 1%, phosphorus not exceeding about .04%, sulfur not exceeding about .015 nitrogen not exceeding about .05%, molybdenum up to about 3%, and remainder iron.

3. A precipitation-hardenable semi-austenitic stainless steel essentially consisting of about 12% to about 19% chromium, about 7% to about 10% nickel, about .03% to about .05% carbon, about .05% to about .45% aluminum, about, 20% to about .70% manganese, about .20%- to about .70% silicon, phosphorus not exceeding about .04%, sulfur not exceeding about .03%, nitrogen not exceeding about .05%, molybdenum up to about 3%, with titanium present but in amounts not exceeding about .10%, and remainder iron, in which steel any deltaferrite present is in amount less than 1% by volume.

4. A precipitation-hardenable semi-austenitic stainless steel essentially consisting of about 12% to about 19% chromium, about 7% to about 10% nickel, about .03% to about .05%- carbon, about .01% to about .25% aluminum, manganese less than about 1%, silicon not exceeding about 1%, phosphorus not exceeding about .04%,

sulfur not exceeding about .03%, nitrogen not exceeding about .05%, molybdenum up to about 3%, with the chromium content plus the molybdenum content about 15% to about 19%, and remainder iron.

5. A precipitation-hardenable semi-austenitic stainless steel essentially consisting of about 12% to about 19% chromium, about 7% to about 10% nickel, about .03%- to about .05 carbon, about .50% to about .25 aluminum, manganese less than about 1%, silicon not exceeding about 1%, phosphorus not exceeding about .04%, sulfur not exceeding about .015 nitrogen not exceeding about .05%, molybdenum up to about 3%, and remainder iron.

6. A precipitation-hardenable semi-austenitic stainless steel essentially consisting of about 12% to about 19% chromium, about 7% to about 9% nickel, about .03% to about .05% carbon, about .05% to about .45% aluminum, manganese less than about 1%, silicon not exceeding about 1%, phosphorus not exceeding about .04%, sulfur not exceeding about .03%, nitrogen not exceeding about .05%, molybdenum up to about 3%, and remainder iron.

7. A precipitation-hardenable semi-austenitic stainless steel essentially consisting of about 13.5% to about 16% chromium, about 7% to about 10% nickel, about 0.3% to about .05% carbon, about .05% to about .45% aluminum, manganese less than about l%, silicon not exceeding about 1%, phosphorus not exceding about 0.4%, sulfur not exceeding about .03%, nitrogen not exceeding about .05%, molybdenum up to about 3%, and remainder 1ron.

8. A precipitation-hardenable semi-austenitic stainless steel essentially consisting of about 13.5% to about 16% chromium, about 7% to about 9% nickel, about .03% to about .05% carbon, about .01% to about .45 aluminum, manganese less than about 1%, silicon not exceeding about 1%, phosphorus not exceeding about .04%, sulfur not exceeding about .03%, nitrogen not exceeding about .05%, molybdenum up to about 3%, and remainder iron.

9. A precipitation-hardenable semi-austenitic stainless steel essentially consisting of about 13.5% to about 16% chromium, about 7% to about 9% nickel, about .03% to about .05% carbon, about .0l% to about .45% aluminum, about .20% to about .70% manganese, about .20% to about .70% silicon, phosphorus not exceeding about .04%, sulfur not exceeding about .015%, nitrogen not exceeding about .05%, molybdenum up to about 3%, and remainder iron.

10. Precipitation-hardened stainless steel articles, equipment and the like essentially consisting of about 12% to about 19% chromium, about 7% to about 10% nickel, about .03% to about .05% carbon, about .05% to about .45 aluminum, manganese less than about 1%, silicon not exceeding about 1%, phosphorus not exceeding about .04%, sulfur not exceeding about .03%, nitrogen not exceeding about .05%, molybdenum. up to about 3%, with the chromium content plus the molybdenum content about 15% to about 19%, titanium but in amounts not exceeding about .10%, and remainder iron.

11. Precipitation-hardened stainless steel articles, equipment and the like essentially consisting of about 12% to about 19% chromium, about 7% to about 10% nickel, about .03% to about .05% carbon, about .05% to about .25 aluminum, manganese about .20% to about .70%, silicon about .20% to about .70%, phosphorus not exceeding about .04%, sulfur not exceeding about .015 nitrogen not exceeding about .05%, molybdenum up to about 3%, with the chromium content plus the molybdenum content about 15% to about 19%, titanium but in amounts not exceeding about .10%, and remainder iron.

12. A precipitation hardenable stainless steel characterized by a good combination of strength and toughness and consisting essentially of about 12% to 15.5% chromium, about 9% to about 10% nickel, about 0.1% titanium, about 0.2% to 0.45% aluminum, carbon in an amount 0.03%, up to 0.2% manganese, up to 0.2% silicon, up to 0.005% boron and the balance essentially iron.

13. A precipitation hardenable stainless steel characterized by a good combination of strength and toughness, and consisting essentially of about 13.5% to about 15% chromium, about 9% to 9.75% nickel, about 0.1% titanium, about 0.2% to about 0.4% aluminum, carbon in an amount of about 0.03%, up to 0.15% manganese, up to 0.15 silicon and the balance essentially iron.

14. A precipitation hardenable stainless steel characterized by a good combination of strength and toughness and consisting essentially of about 12% to about 13% chromium, about 10% nickel, about 0.1% titanium, about 0.2% to about 0.4% aluminum, carbon in an amount of about 0.03%, up to 0.15% manganese, up to 0.15 silicon, and the balance essentially iron.

(References on following page) References Cited UNITED OTHER REFERENCES STATES PATENTS Wyche et a1. 148-142 X Perry X 5 CHARLES N. LOVELL, Primary Examiner Perry et a1. 75124 X Bieber 75 124 Us. 01 X.R.

Tuffnell et a1. 75-124 X 75 124, 728 N, 128 T, 128 w Transactions of AIMME, vol. 167, 19 46, relied on pp.