US2505763A - Stainless steel and method - Google Patents

Description

May 2, 1950 G. N. GOLLER STAINLESS STEEL AND'METHOD Filed Sept. 6, 1946 GEORGE N. GOLLER www Patented May 2, 1950 UNITED STATES PATENT FFICEv STAINLESS STEEL AND METHOD George N. Goller, Baltimore, Md., assignor to Armco Steel Corporation, a corporation of Ohio Application September 6, 1946, Serial No. 695,216

(Cl. 'i5-124) 11 Claims.

This invention relates to chromium-nickel stainless steels, more especially to a method for providing the steels in the hardenable and hardened conditions, and to the resulting products and manufactures.

An object of my invention is the provision of chromium-nickel stainless steels which are amenable to hardening by heat treatment from a soft, formable and maehnable condition.

Another object is the provision of a reliable and commercially practical method for hardening stainless steels of the character noted.

A further object of my invention is the provision of a direct and highly effective method for producing chromium-nickel stainless steels to a condition suited for fabricating operations such as those including, cold-rolling, drawing, stamping, punching, upsetting or machining, and to a strong hardened condition by subsequent heat treatment.

A still further object is the provision of hardened, strong and wear-resistant `chromium-nickel stainless steels and articles thereof.

Other objects of my invention will be obvious and in part pointedout hereinafter.

The invention accordingly consists in the combination of elements, composition of materials, and features of production and in the various operational steps and the relation of each of the same to one or more of the others as described herein, the scope of the application of which is indicated in the following claims.

The single ligure of theaccompanying drawing graphically represents proportions of chromium and nickel which are at times employed in the composition of my stainless alloy steel, as will be pointed out more fully hereinafter.

As conducive to a clearer understanding of certain features of my invention, it may be noted at this point that stainless steels by definition are carbon steels which include about to 35% chromium, with or without nickel. and with or without supplemental additions of copper, manganese, silicon, cobalt, molybdenum. aluminum, tungsten, vanadium, titanium. columbium, sulphur, and the like, for special purposes, and a remainder which is substantially all iron. The carbon content usually is low, this being on the order of 0.02% to 0.20%, although it may be higher for particular needs.

In the above group of steels, there are certain chromium-nickel varieties, especially those which are classiable with the more commonly known 18% chromium-8% nickel grades, which remain stably austenitic at room temperatures after quenching from annealing temperature, and in no manner are hardenable by heat treatment. These steels are work-hardenable, .but of course the property of work-hardenability cannot always be relied upon for the satisfactory provision of hardened products. In view of other properties, the steels are in widespread demand for fabrication by such methods as hot-working, cold-forming, machining, punching, drilling, drawing or spinning. After fabrication, the steels are put into use either in soft annealed condition, or in workhardened condition where feasible.

Under certain circumstances, some few of the chromium-nickel stainless steels have been known to respond to hardening heat treatment, this by virtue of the addition of one or more precipitation-hardening agents such as titanium, columbium and copper. 'The use of those hardening materials has depended upon a well studied proportionment of alloy ingredients in the steel, followed by a critical form of heat treatment to eect the hardening. Columbium and titanium, however, are relatively expensive materials. The chromium-nickel-titanium or columbium stainless steels, moreover usually contain stress-laden ferrite as an essential to hardening from the annealed condition. When the steels are produced to such composition as to present a comparatively soft, substantially fully austenitic structure at about room temperature after annealing, they are more amenable to cold-working and forming operations, but the extent of the hardening treatment usually is sacriced.

In the production and treatment of certain stainless steels,.the element aluminum heretofore has been recognized as a deoxidizing material and also one which frequently serves for developing an aluminum-containing oxide film on the steel surface which is resistant to heat and corrosion. The alloying of aluminum with suitable quantities of other included elements of the steels is known to impart high'temperature properties, this at times being sufficient to justify the production from the metal of such articles as heat engine valves, turbine blades, and the like. In many of the stainless steels, aluminum is said by previous investigators to exert an impairing eiect upon the ability to harden. I iind that this by far is not true of all stainless steels, for under certain circumstances the aluminum, which is a relatively cheap material, serves in a highly benecial manner as a precipitation hardening agent.

An outstanding object of my invention accordingly is the provision of chromium-nickel-aluminum stainless steels which are readily fabricated into a host of different products having hardening characteristics at temperatures sufficiently low to avoid substantial scaling and distortion due to heat, and which steels and products fashioned thereof are characterized by other desirable properties in both the prehardened and hardened conditions.

Referring now more particularly to the practice of my invention, I provide an especial quality atoaves of chromium-nickel stainless steel which, through the close correlation of amounts of chromium and nickel, with critical amounts of the ingredients aluminum and carbon, in the composition thereof, is highly suitable for fabrication in the relatively soft condition and for precipitation hardening by heat treatment to a corresponding condition of increased tensile strength and yield strength. In the practice of my invention, I provide stainless steel in which the chromium and nickel contents are in substantial accord with the abscissa and ordinate of any given point of area ABCD in the accompanying diagram, and which further contains anywhere from about 0.02%,to 0.12% carbon, aluminum from about 0.50% to 2.50%, from incidental amounts up to about 8.0% manganese, from incidental amounts up to approximately 2.0% silicon, with or without molybdenum ranging up to about 3.0% illustratively 'to enhance corrosion resistance of the steel, and the remainder substantially all iron. In this, however, should the carbon, aluminum, silicon, or manganese content be somewhat different from the amount upon which the accompanying diagram is based (the basis of the diagram being amounts of actual chromium and nickel as called for, about 0.06% to 0.08% carbon, 0.60% to 1.0% aluminum, incidental amounts of manganese and silicon up to 1.0% each, sulphur and phosphorus up to 0.040% each, and the remainder substantially all iron) or should molybdenum be used, I find it preferable to modify the chromium or nickel content of the steel, or both of the contents as the case may be, so as to achieve chromium-like and nickel-like components in the steel which are substantially equivalent respectively in ferrite and austenite-forming relation to those amounts of chromium and nickel and of the other elements represented in'the diagram. For example, I often replace a part of the chromium called for in the diagram with a quantity of aluminum, silicon or molybdenum, the replacement being approximately on a 1 to l ratio with respect to chromium, and for such purpose as maintaining a desired relation between the austenite and ferrite forming components of the steel substantially as would be achieved by rigid adherence to the diagram. Similarly, I occasionally add several or all of the replacement elements in partial substitution for the chromium. The practice of substitution I find is particularly advantageous where either the aluminum or silicon is to exceed about 1.0%, or where molybdenum is a constituent of the steel. Following substitution, the actual chromium content of the steel may at times be somewhat below those amounts prescribed by area ABCD in the diagram and still, in view of the eiIect of the substituted element or elements, be in substantial accord with the area.

Where the permissible quantity of carbon exceeds about 0.08%, or where more than small amounts of manganese are present (say for example amounts in excess of about 1.0%) I usually employ a proportionally decreased quantity of nickel in the steel as compared with the accompanying diagram. The actual nickel content of the steel, however, importantly is not less than about 3.5% after substitution. Should the permissible quantity of carbon be on the low side, i. e. below about 0.06%, I usually increase the nickel content as compared with the diagram, or evenat times instead increase the manganese content. For each part of nickel I add'about 2 parts manganese or on the order of about 1/zo to 1/am part carbon as the substantial equivalent.

The actual nickel content of the steel, after partial substitution, consequently may on occasions fall even considerably outside those amounts prescribed by area ABCD in the diagram or remain inside and still, in View of the substituted element or elements and the contributed effect thereof, be in substantial accord with the area.

Where desired the steel contains such addition elements as sulphur and/ or selenium in amounts sufficient to enhance free-machining properties. The value of such elements is realized especially in machining the steel in the annealed state.

In accordance with the preferred practice of my invention, I produce a stainless steel containing chromium and nickel in amount substantially in accord with the abscissa and ordinate respectively ol any given point substantially falling within area abcd in the accompanying diagram, the amounts advantageously being about 16.65% to 17.5% actual chromium and approximately 6.75% to 7.5% actual nickel as represented by the area abcd. The steel, also preferably contains, on the basis of the diagram, aluminum in amounts between about 0.60% and 1,0%, carbon within the approximate range of 0.06% to 0.08%, and the remainder substantially all iron. There are preferably small amounts of such elements as manganese, silicon, sulphur and phosphorus in the steel, as for example manganese and silicon each not exceeding about 1.0% and sulphur and phosphorus each up to about 0.040%.

I condition the steel, either as wrought or cast, for subsequent forming and fabrication and for hardening of the formed and fabricated products, by heating the same in a temperature range preferably not lower than about 1800 F. and extending up to about 2000 F. for such period of time as to place the metal in an austenitic aluminumsoluble condition which is retainable down to at least about room temperature. This heating is in the nature of an annealing treatment. To achieve the annealing, I usually adjust the met-al to temperature and maintain or hold the same at temperature as in a suitable heat treating furnace. The holding time as applied to the steel is not too critical. About one-half hour is quite satisfactory from the standpoint of economy and of ensuring solubility of the aluminum.

With the steel in aluminum-soluble austenitic condition, I discontinue the annealing operation and quench the heated metal as in air, oil or water, conveniently to around room temperature. After the quenching, the aluminum constituent remains in solution. The metal is of soft,- substantially full," austenitic quality characterized by ductility, hardnesses usually below about Rockwell B-92, is formable, and machinable, one or more of such properties make it possible at this point to effect fabrication into any of a host of prehardened chromium-nickel stainless steel products.

From my annealed and quenched steel, which then is in the prehardened condition and capable of being hardened by subsequent treatment hereinafter described, I provide products in shapes which are less basic or more intricate, illustratively structural members for aeroplanes, as those parts requiring great strength from the standpoints of yield in tension and compression coupled with corrosion resistance and toughness: coldheaded bolts and screws, as for example those eventually to include hardened Shanks; surgical instruments including those in the eld of dentistry; valves and valve seats; die blocks; ilxtures and trim. In these, I take advantage of the exnickel-aluminum stainless steel to a preliminary hardening heat, this being a, reheat considered in the light of the already completed annealing treatment. Therefore, to preliminarily harden the steel, I reheat it, illustratively in the same heat treating furnace employed for the annealing, this time up to within the approximate temperature range of 1200" F. to 1600 F., preferably up to about 1400" F., and there hold the metal to desired temperature or temperatures for ap-` proximately A hour or more. During the holding operation, as for example with the steel at around 1400 F., there results a precipitation of carbides, possibly some precipitation of aluminum compounds, and a raising of the transformation point of the austenitic matrix. The steel, upon cooling, consequently transforms to include a "soft martensite-like constituent. The transformation occurs at quite low temperature such as between about 175 F. and 225 F. Thus, at the completion of the reheating stay, I quench the steel as in air, oil or water with the result that transformation does occur above room temperature to achieve a` partial hardening of the metal, or of the articles made thereof as described hereinbefore. Hardnesses after this transformation vary in or near the range of Rockwell C-24 to C-28 depending upon the particular composition and the conditions of treatment within the limits noted.

My chromium-nickel-aluminum stainless steels retain sufficient softness and ductility in the preliminarily hardened, transformed condition to permit working illustratively by cold rolling, or drawing and still are machinable and capable of being fabricated through cutting, punching, drilling and the like. For this reason. and because the metal can be further hardened without phase transformation and changes in dimensions usually accompanying transformation, I frequently put off all or part of certain forming and fabricating operations to be accomplished on the same until after the preliminary hardening treatment, and then produce any of a wide variety of chromium-nickel-aluminum stainless steel articles, as to achieve substantially finished dimensions.

When transformation and preliminary hardness have been achieved through reheating and quenching, as hitherto described, and following the performance of forming and fabricating operations before or after the preliminary hardening treatment, should the operations be desired, the steel then is ready for additional hardening,

sumciently low to avoid scaling and warping of the metal due to heat and constitutes a further part of my entire hardening treatment. In effecting the additional hardening, I heat the alloy metal, as in the form of fabricated products, within a low temperature range of about 750 F. to approximately 1000 F., preferably at about 900 F., and hold the same at temperature for about one hour. The time of holding under treatment, howevenmay vary from approximately one-half hour to some two hours or more with satisfactory results. About one hour is preferred. The treatment serves to precipitate aluminum as a compound and imparts a material gain in hardness to the alloy metal. This hardness, I find, upon quenching the steel as in air, oil or water from the hardening temperature, comes within the approximate range of C-39 to C-46 Rockwell. The exact phenomena which takes place during this precipitation hardening treatment are not so clear although aluminum in some form is precipitated. From indications of dilatometer tests no phase transformation or' substantial changes in volume of the metal are to be noted during the hardening reaction. It is believed that the reaction may involve some rearrangement or ordering of the lattice structure of a precipitated nickel-aluminum compound within the lattice `structure of the matrix which thereafter exerts an interference hardening effect. This belief, however, is not fully conrmed and, therefore, I do not wish to be bound by the same.

In the hardened condition achieved through quenching from the hereinbefore mentioned reheat at 750 F. to 1000 F., the so-conditioned steel or steel products are very strong having a high yield strength in tension and compression, are substantially free from directionality, and are quite resistant to corrosion. By virtue of the low temperatures at which the preliminary hardening treatment and especially the additional hardening treatment are conducted, the steel emerges substantially without heat scale and unwarped by heat. The hardened steel usually has a straw surface color.

In way of illustration, a chromium-nickelaluminum stainless steel provided having a composition falling within the `heretofore noted general range, and more specifically containing approximately 0.07% carbon, 16.7% chromium, 7.3% nickel, 1.0% aluminum, 0.6% silicon, 0.4% manganese, about 0.015% sulphur and phosphorus together, and remainder iron, was found by treatment in accordance with my invention to have the approximate physical property values Ipresented below in the table. All values tabulated for the annealed condition resulted from one-half hour heating of the steel at 1850 F. followed by quenching in water, those for the preliminarily hardened condition from heating the metal at 1400 F. for three hours and Waterquenching, and values for the additionally hardened condition resulted from heating the metal this being by heat treatment at temperatures o5 one hour at 900 F. and cooling in air.`

Table Ult. 0.29:7 P Tens. Yield Swen Femm Rockwell Condition Str" r., Elozngfa. Hardness p.s.i. p. s. i.

annealed 143,500 43,100 23.5 58.5 B454 Preliminarnynardenemm. 143,100 103,450 10.0 53.0 C-a sdditionauynardened 214,400 202,500 0.0 21.0 c-m 'I'he close correlation of aluminum, a comparatively cheap material, with other elements in the steel accordingly enables a commercially valuable precipitation hardening effect which becomes active with 'proper treatment of the metal from a soft workable, or worked condition. It will further be appreciated that the aluminum contributes to heat resistance and to the prevention of heat scale formation, as during the respective hardening heats and in the resulting -hardened corrosion-resistant products.

Thus it will be seen that in this invention there is provided a method and a chromium-nickel hardened stainless steel in which the various objects hereinbefore noted together with many other thoroughly practical advantages are successfully achieved. It will be seen that the method makes possible the provision of wrought or cast chromium-nickel-aluminum stainless steelrsubjected to any of a number of forming, machining or fabricating operations and eiectively and reliably hardened from a soft, ductile condition by heat treatment. Likewise, it will be seen that my method is readily practiced, and enables the production of chromium-nickel grade stainless steel of hardened quality with a minimum of such treatments as pickling, and otherwise quite suitable for commercial use. It will further be noted that the hardened products havehigh yield and ultimate strengths, good directional properties, and a reasonable amount of ductility.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiment hereinbefore set forth, it is to be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

I claim:

1. A chromium-nickel stainless steel susceptible to precipitation hardening by double heat treatment from a soft workable substantially fully austenitic condition, said steel containing chromium' and nickel in amounts substantially in accordance with area ABCD in the accompanying` diagram, carbon between about 0.02% and 0.12%, about 0.50% to 2.50% aluminum, from incidental amounts up to about 8.0% manganese, from incidental amounts up to about 2.0% silicon, and the remainder substantially all iron.

2. A chromium-nickel stainless steel susceptible to precipitation hardening by'double heat treatment from a soft workable substantially fully austenitic condition, said steel containing chromium and nickel in amounts substantially in'accordance with area ABCD in the accompanying diagram, carbon between about 0.02% and 0.12%, about 0.50% to 2.50% aluminum, from incidental amounts up to about 8.0% manganese, from incidental amounts up to about 2.0% silicon, at least one of the group consisting' 16.65% to 17.5% chromium, about 6.75% to 7.5% nickel, aluminum between about 0.60% to 1.0%, about 0.06% to 0.08% carbon, and the remainder substantially all iron, the steel being precipitation hardenable by double heat treatment after the annealing.

4. In a method of hardening chromium-nickel stainless steel, providing a steel containing chromium and nickel in amounts substantially in accordance with area ABCD in the accompanying diagram, aluminum in amounts between about 0.50% to 2.50%, about 0.02% to 0.12% carbon, from incidental amounts up to approximately 8.0% manganese, from incidental amounts up to about 2.0% silicon, and the remainder substantially all iron; then heating the steel at such temperature as to provide an austenitic aluminum soluble condition stable down to at least about room temperature; transforming the alloy by heat treating and quenching; and heating the same at a temperature suiciently high and for such Iperiod of time to precipitate an aluminum compound and obtain a substantial increase in the metal hardness.

5. In a method of hardening chromium-nickel stainless steel, correlating about 16.65% to 17.5% chromium, with about 6.75% to 7.5% nickel, aluminum in amounts between about 0.60% and 1.0%, about 0.06% to 0.08% carbon, and the remainder substantially all iron; treating said steel within a temperature range of about 1800 F. to 2000 F. to provide an aluminum soluble austenitic condition stable down to at least about room temperature; transforming the alloy by heat treatment; and heating the transformed alloy within a temperature range of approximately 750 F. to 1000 F. for such period of time to precipitate an aluminum compound and obtain a substantial increase in the metal hardness.

6. In the production of hardened chromiumnickel stainless steel articles and products. providing stainless steel containing about 16.65% to 17.5% chromium, with about 6.75% to 7.5% nickel, aluminum in amounts between about 0.60% to 1.0%, about 0.06% to 0.08% carbon, and the remainder substantially all iron; annealing said steel within a temperature range sufllciently high to provide an aluminum soluble austenitic condition stable down to at least about room temperature; fabricating the articles and products of said steel; reheating the fabricated metal within the approximate temperature range of 1200 F, to 1600 F. to eiiect carbide precipitation and quenching the same to achieve transformation; and heating the transformed metal within a temperature range of approximately 750 F. to 1000 F. for such period of time as to precipitate an aluminum compound and obtain a substantial increase in hardness of the products and articles.

7. In the production of hardened chromiumnickel stainless steel articles and products, providing stainless steel containing about 16.65% to 17.5% chromium, with about 6.75% to 7.5% nickel, aluminum in amounts between about 0.60% and 1.0%, about 0.06% to 0.08% carbon, and the remainder substantially all iron; annealing said steel within a temperature range sutilciently high to provide an aluminum soluble austenitic condition stable down to at least about room temperature; reheating the steel within the approximate temperature range of 1200 F. to 1600 F. to effect carbide precipitation, and quenching the same to achieve transformation; fabricating the articles and products of said steel, and heating said fabricated metal within a temperature range of approximately 750 F. to 1000" F. for such period of time as to precipitate an aluminum compound and obtain a substantial in- ,crease in the metal hardness.

8. A chromium-nickel stainless steel precipita- 0.06% to 0.08% carbon, and the remainder sub- 5 stantally all iron.

9. A chromium-nickel stainless steel aluminumprecipitation hardened by double heat treatment from the annealed condition containing chromium and nickel in amounts substantially in acl cordance with area ABCD in the accompanying diagram, carbon between about 0.02% and 0.12%, about 0.50% to 2.50% aluminum, from incidental amounts up to approximately 8.0% manganese,

from incidental amounts up to about 2.0% silicon, l

and the remainder substantially all iron.

10. A chromium-nickel stainless steel aluminum-precipitation hardened by double heat treatment from the annealed condition, containing the chromium-like components aluminum in the amounts of about 0.50% to 2.50%, silicon from incidental amounts up toY about 2.0% based on total content of the steel, and the remainder substantially all chromium in amounts sufficient with chromium substitute for meeting'the terms for 2.

chromium of area ABCD in the accompanying diagram, said aluminum and silicon serving as a substitute for chromium on about a 1 to 1 ratio; the nickel-like components carbon in the amount of about 0.02% to 0.12%, manganese from incidental amounts up to about 8.0%, and the remainder substantially all nickel in actual amount not less than about 3.5% and sufficient with nickel substitute for meeting the terms for nickel of area ABCD of the accompanying diagram, said carbon and manganese serving as a substitute for nickel on the ratios of about 1/20 to l/an part carbon to 1, and approximately 2 parts manganese to 1; and the remainder substantially all iron, said aluminum being precipitated as aluminum compound within the matrix of the steel.

11. A chromium-nickel stainless steel aluminumprecipitation hardened by double heat treatment from the annealed condition containing: the chromium-like components aluminum in the amount of about 0.50% to 2.50%, molybdenum from fractional percentages up to about 3.0%, silicon from incidental amounts up to approximately 2.0% all based on total content of the steel, and the remainder substantially all chromium in amount suflicient with chromium substitute for meeting the terms for chromium of area ABCD in the accompanying diagram, said aluminum, molybdenum and silicon serving as a substitute for chromium on about a 1 to 1 ratio; the nickel-like components carbon in the amount of about 0.02% to 0.12%, manganese from incidental amounts up to about 8.0%, and the remainder substantially al1 nickel in actual amount not less than about 3.5% and suiiicient with nickel substitute for substantially meeting the terms for nickel of area ABCD in the accompanying diagram, said carbon and manganese serving as a substitute for nickel on the ratios of about 1/zo to 1/:0 part carbon to 1, and approximately 2 parts manganese to 1; and the remainder substantially all iron,` said aluminum being precipitated to give substantial hardness.

GEORGE N. GOLLER.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,943,595 Foley Jan. 16, 1934 2,048,163 Pilling July 21, 1938 2,384,567 Schaufus Sept 4, 1945 2,390,023 Wyche Nov. 27, 1945

Claims (1)

10. A CHROMIUM-NICKEL STAINLESS STEEL ALUMINUM-PRECIPITATION HARDENED BY DOUBLE HEAT TREATMENT FROM THE ANNEALED CONDITION, CONTAINING THE CHROMIUM-LIKE COMPONENTS ALUMINUM IN THE AMOUNTS OF ABOUT 0.50% TO 2.50%, SILICON FROM INCIDENTAL AMOUNTS UP TO ABOUT 2.0% BASED ON TOTAL CONTENT OF THE STEEL, AND THE REMAINDER SUBSTANTIALLY ALL CHROMIUM IN AMOUNT SUFFICIENT WITH CHROMIUM SUBSTITUTE FOR MEETING THE TERMS FOR CHROMIUM AREA ABCD IN THE ACCOMPANYING DIAGRAM, SAID ALUMINUM AND SILICON SERVING AS A SUBSTITUTE FOR CHROMIUM ON ABOUT A 1 TO 1 RATIO; THE NICKEL-LIKE COMPONENTS CARBON IN THE AMOUNT OF ABOUT 0.2% TO 0.12, MANGANESE FROM INCIDENTAL AMOUNTS UP TO ABOUT 8.0%, AND THE REMAINDER SUBSTANTIALLY ALL NICKEL IN ACTUAL AMOUNT NOT LESS THAN ABOUT 3.5% AND SUFFICIENT WITH NICKEL SUBSTITUTE FOR MEETING THE TERMS FOR NICKEL OF AREA ABCD OF THE ACCOMPANYING DIAGRAM SAID CARBON AND MANGANESE SERVING AS A SUBSTITUTE FOR NICKEL ON THE RATIOS OF ABOUT 1/20 TO 1/30 PART CARBON TO 1, AND APPROXIMATELY 2 PARTS MANGANESE TO 1; AND THE REMAINDER SUBSTANTIALLY ALL IRON, SAID ALUMINUM BEING PRECIPITATED AS ALUMINUM COMPOUND WITHIN THE MATRIX OF THE STEEL.

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