US2505764A - Stainless steel and method - Google Patents

Description

May 2, 1950 G. N. GOLLER STAINLESS STEEL AND METHOD Filed Sept. 6, 1946 mom mao aad rGEQRGE N. QQLLER ...j Z/KGM www Patenad Mq 2, 195o STAINLESS STEEL AND METHOD George N. Goller, Baltimore, Md., assilnor to Armco Steel Corporation, a corporation o! Ohio Application September 6, 1946, Serial No. 695,217

(Cl. 'l5-124) Claims.

The present invention relates to the production and treatment of chromium-nickel stainless steels of low-carbon content, more particularly to a method for conditioning the steels to the hardenable or hardened state, and to the resulting products and manufactures.

An object of my invention is the provision cf chromium-nickel stainless steels which are hardenable, with substantially no scaling or warping due to heat effects during the hardening steps employed.

A further object of my invention is the provision oi' a simple, practical and reliable method for conditioning chromium-nickel stainless steels to a soft, hardenable and workable state suited for such operations as cold working, forming, stamping, punching, spinning, drawing, bending. heading, and machining, and to the hardened state by heat exchange treatment at temperatures suillciently low to avoid substantial warping and the formation of heat scale.

Another object is the provision of chromiumnickel stainless steels so proportioned in composition as to be amenable to annealing, and working and fabricating in the annealed condition, and thereafter to hardening.

A still further object of my invention is the provision of hardened chromium-nickel stainless steels, as for example cold worked and hardened articles and products, which are characterized by excellent physical properties including high yield strength and high ultimate strength.

Other objects in part will be obvious and in part pointed out hereinafter.

The invention accordingly consists in the cornbination of elements, composition of ingredients, and in the articles, products and manufactures thereof, also in the several method 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 the accompanying 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 the better known chromium-nickel stainless steels, as for example the 18% chromium-8% nickel steels, are stably austenitic at room temperatures and do not appreciably respond to hardening by heat treatment. They do attain hardness by cold working but, in actual practice, there are sharp limitations upon the possible use of this property to achieve desired hardness. The products so hardened are under strain, and the customary annealing treatment to relieve the strain tends to reduce the hardness. There are many valuable properties of the steels, for instance cold workability as by rolling, swaging, cold heading and drawing, machinability, and, as well, many additional features such as excellent resistance to corrosion, which place the steels in very considerable demand.

In accordance with the general definition of stainless steels, the chromium-nickel varieties frequently include additions of such elements as copper, manganese, silicon, cobalt, molybdenum, aluminum, tungsten, vanadium, titanium, columbium, and sulphur, for special purposes. Some few of the chromium-nickel stainless steels have been known to respond to hardening heat treatment, this by virue of the addition of ti- 'taniuni or columbium in well studied proportionoperations, but hardenabilityv by heat treatment is usually sacriiiced.

The production and treatment of certain grades of stainless steels heretofore has involved the introduction of aluminum into the steel melt for serving as a deoxidizing material, and also as an alloying element to develop an aluminumcontaining oxide lm on the metal which is resistant to heat and corrosion. The alloying of aluminum with suitable quantities of other included elements of the steel is known to impart 'high temperature properties which, depending upon degree, have justified the production from the metal of such articles as heat engine valves, turbine blades, and the like. Aluminum, in many of the stainless steels, is said by previous investigators to exert an impairing eiect upon the ability of these steels to harden. I find that this by far is not true of all stainless steels; instead, aluminum offers great benefits as a relatively cheap hardening agent when properly correlated with other elements in the steel.

An outstanding object of my invention accordingly is the provision of chromium-nickel-aluminum stainless steels which are readily fabricated into 9, host of diiierent products hardenable at temperatures sufficiently low to avoid substantial scaling and distortion due to heat effect, and which steels and products fashioned thereof are characterized by other desirable properties in both the prehardened and hardened conditions. t

Referring now more particularly to the practice of my invention, I find that by closely correlating the chromium and nickel contents of the steel, and in addition including the ingredient aluminum in critical amounts, a stainless steel is had possessing the surprising feature of hardenability of low temperature exchange treatment from a very soft, workable condition. The hardening treatment comprises successive cold temperature and heating steps, the cold treatment being conducted at temperatures considerably below the freezing point of water, and the heating operation likewise at temperatures sutilciently low to prevent substantial surface scaling and warping of the metal due to heat effects.

.The stainless steel which I provide in accordance with my invention has chromium and nickel contents which are in substantial accord with the abscissa and ordinate of any given point of area ABCD in the accompanying diagram, the diagram itself being based upon contents of actual chromium and nickel, and further upon contents of about 0.06% to 0.08% carbon, 0.60% to 1.0% aluminum, incidental amounts of manganese and silicon up to about 1.0% each, sulphur and phosphorus up to 0.040% each, and the remainder substantially all iron. My steel, however, may contain 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% man.. ganese, 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. Where the carbon, aluminum, silicon or manganese content is somewhat different from the amount upon which the accompanying diagram is based, 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 austente forming relation to those amounts of chromium and nickel and oi 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 1 ratio with respect to chromium, and for such purpose as maintaining a desired relation between the austente 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 ilnd is particularly advantageous where either the aluminumor 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 accompanying diagram and still, in view of the eect 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 proportionality 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 even at times instead increase the manganese content. For each part of nickel, I add about 2 parts manganese or on the order of about 1/an to 1/so 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 sumcient to enhance free-machining properties. The advantages of such elements are noted especially in machining the steel in the annealed state.

A preferred form of the steel which I produce contains chromium' and nickel in amounts substantially in accord with the abscissa and ordinate respectively of any given point substantially falling within area abcd in the accompanying diagram, the amounts advantageously being about 16.75% to 17.3% actual chromium and approximately 7.0% to 7.6% 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% to 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%.

To condition the steel, either as wrought or cast, for subsequent forming and fabrication and for hardening of the formed and fabricated products, I heat the metal in a temperature range preferably not lower than about 1800 F. and extending up to about 2000" F. for such time as to achieve an austenitic aluminum-soluble condition which is retainable down to substantially below ordinary room temperature. This heating is in the nature of an annealing treatment. To achieve the annealing, I usually adjust the metal to temperature and maintain or hold the same at temperature as in a. conventional heat treating furnace for a suitable period of time. The holding time as applied to the steel is not too critical. About one-half hour usually is quite satisfactory from the standpoint of economy and of ensuring adequate solubility of the aluminum'. I then quench the annealed metal as in air, oil or water, preferably to around room temperature. As a result of the annealing and quenching, the stainless steel displays a soft, substantially fully austenitic structure, is ductile, has good directional properties, hardnesses usually below about Rockwell B-92, and readily lends itself to forming, machining, and the like for fabrication at this point into any of a wide variety of prehardened products.

From the quenched or prehardened chromiumnickel-aluminum austenitic stainless steels, which are now precipitation hardenable by subsequent treatment to be described, I provide products in such forms as bars, wire, rounds, sheet, strip or plate. Also, I frequently provide shapes of a more intricate character, illustratively trim and structural members such as parts for aircraft including those components requiring great strength from the standpoints of yield, in tension and compression coupled with toughness and corrosion resistance; cold-headed bolts and screws, as for example those eventually to include hardened shanks; surgical instruments and dental tools; valve and valve seats; die blocks; and the like. In all of these I take advantage of the excellent workability and formability of the soft annealed metal through such operations as cold-forming, upsetting, drawing, spinning, stamping, punching. machining, and other steps consistent with the properties o! the metal. At times, I delay part or all of the forming and fabricating operations until after the achievement of an initial hardening treatment now to be described.

To preliminar-ily harden the soft, annealed chromium-nickel-aluminum stainless steel, as after the forming and fabricating steps illustratively conducted before undertaking the hardening treatment, I subject the steel to a low temperature non-scaling heat exchange treatment. More particularly, I cold-treat the metal, as in fabricated condition, at temperatures substantially below usual room temperatures to achieve transformation and a consequent increase in the metal hardness; this preferably being in a suitable cooling compartment, or the like, while employing a holding temperature of about 30 F. to 100 F. or lower. It will be readily understood that my steel'does not preliminarily harden to any substantial extent during shipment under ordinary cold weather conditions, this being advantageous for frequently desired customer fabrication of the soft, annealed metal directly after shipment. A holding time of about 1/2 hour or more at cooling temperature within the cooling range noted is preferred to effect phase transformation and preliminary hardening. The cooling medium illustratively is Dry Ice in acetone. Hardnesses of the chromiumnickel stainless steel after this transformation vary in or near the range of Rockwell C-32 to C-36 depending upon the particular composition and conditions of treatment within the limits mentioned.

The chromium-nickel-aluminum stainless steel in the preliminarily hardened condition retains a reasonable amount of workability and forma- 4bility, illustratively cold rolling or drawing properties, and the steel is machinable and may be fabricated as by cutting, punching, drilling, and the like. It is for this reason and also the possibility of further hardening the metal without phase transformation and changes in dimensions of the metal accompanying transformation that l frequently put ofi' all or part of certain forming and fabricating operations to be aecomplished on the same until after the preliminary hardening treatment, and then produce any of a wide variety of articles and products, as to substantially finished dimensions, of the partially hardened steel.

When transformation has been achieved by the low temperature conditioning as described, and appropriate working and forming operations effected where desired, the alloy steel then is ready for positive precipitation hardening treatment. In performing this operation,` represent-V ing a further part of my conditioning treatment, I heat the steel within a temperature range of about 750 F. to approximatly 1000 F., preferably at about 900 F., and there hold the same at temperature for about one hour. The time of holding under treatment, however, may vary from approximately one-half hour to some two hours or more with satisfactory results. About one hour is preferred. The treatment serves to impart a material gain in hardness to the metal. This hardness, I find, upon quenching the metal as in air, oil or water from the hardening temperatures, cornes within the approximate range of C2-40 to C-46 Rockwell. A precipitated aluminum-rich phase is instrumental to the hardness. This I believe to be an aluminum-nickel compound.

In the precipitation hardened condition which.

results from the heating at 750 F. to 1000 F., the so-conditioned steel or steel products are very strong both in tension and compression, have high yield strength, good directional qualities, and are quite resistant to corrosion. The steel emerges from the preliminary hardening cold treatment and precipitation hardening heat treatment substantially free of heat scale and unwarped by heat at the low temperatures employed.

Certain properties of several illustrative steels which were treated in accordance with my invention are set forth in the table below. lSteel A noted was made to contain ln approximate amounts 0.064% carbon, 16.82% chromium, 7.58% nickel. 1.00% aluminum, and the remainder substantially all iron, while steel B contained about .071% carbon, 16.72% chromium, 7.00% nickel, .95% aluminum, and the remainder substantially all iron. Both of the steels had therein only small amounts of manganese, silicon, sulphur and phosphorus. The steels were heated at 1800 F. for about 1/ hour and thereafter' quenched in water for obtaining the annealed condition. This was followed by cold treatment at 35 F. for about 1A? hour, and quenching the same in air, was employed for achieving the finally hardened condition. Those properties given in the table are representative of the steels after each of the steps of annealing, preliminarily hardening and finally hardening in the manner just described.

Table Per Cent Net. Tens. 0.27 Yield Per Cent Rockwell steel Condition Str., p. s. l. Str.,o p. s. i. E1ong2 feda Hardness A Annealed 147, 500 42,100 19. 0 58. 8 B89 V Preliminarily Hardened 179, 200 113. 000 10. 5 58. 4 036/37 inally Hardened--- 205, 300 184, 800 10. 5 57. 5 C44/46 B Annealed 141, 600 45, 900 21. 0 60. 5 B88 Preliminarily Hardened 172, 114, 10') 10.0 52. 2 C36 Finally Hardened 212, 100 190, 100 10. 5 42. 6 C47 The close correlation of aluminum, a comparatively cheap material, with other elements in the steel accordingly enables a commercially valuable hardening effect, which becomes active with 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 asduring the low temperature precipitation hardening heat in the vicinity of 750 F. to 1000 F. and in the resulting precipitation hardened corrosion resistant products.

Thus, it will be seen that in this invention there is provided a method and a low temperature hardening stainless steel in which the various objects hereinbefore noted together with many 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 steel subjected to any of a number of forming, machining or fabricating operations and eectively and reliably hardened by heat treatment at low temperatures. 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 any such treatment as pickling, and otherwise is quite suitable for commercial use. It will further be noted that the steels provided have good directional properties, are quite soft and ductile in the annealed conditionfand are hardenable to a condition of reasonable ductility and high yield and ultimate strengths.

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 heat exchange treatment involving low temperature cold treatment, said steel containing chromium and nickel in amounts substantially in accordance with area ABCD in the accompanying diagram, carbon between about 0.02% to 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, and the remainder substantially all iron.

2. In a method of hardening chromium-nickel alloy stainless steel, the art which includes providing a steel containing chromium and nickel in substantial accord with area ABCD of the, accompanying diagram, aluminum in amounts between about 0.50% to 2.50%, and from incidental amounts up to approximately 2.0% silicon, manganese from incidental amounts up to about 8.0% and carbon between about 0.02% and 0.12%, and the remainder substantially all iron; then annealing and quenching the steel to provide an austenitic aluminum-soluble structure transformable below usual room temperature; subjecting said steel to heat exchange cold treatment at sufllciently low temperature and for long enough time to achieve transformation; and heatlng the transformed alloy at a sullciently high temperature and for a sufficient period of time to precipitate an aluminum compound and obtain a substantial increase in the hardness of the steel.

3. In a method of hardening chromium-nickel 'alloy stainless steel, the art which includes pro- 8 vidinga steel containing about 16.75% to 17.3%

chromium, with about 7.0% to 7.6% nickel, aluminum in amounts between about 0.60% to 1.0%, and about 0.06% to 0.08% carbon, and the remainder substantially all iron; then heating the steel at temperatures between approximately 1800* F. to 2000 F. and quenching the same to provide an austenitic aluminum-soluble structure transformable below usual room temperature; subjecting said steel to heat exchange cold treatment at suiilciently low temperature and for long enough time to achieve transformation; and heating the transformed alloy at temperatures within the approximate range of 750 F. to 1000 F. for a suilicient period of time to precipitate an aluminum compound and obtain a substantial increase in hardness of the steel.

4. In a method of hardening chromium-nickel alloy stainless steel, the art which includes providing a steel containing about 16.75% to 17.3% chromium, with about 7.0% to 7.6% nickel, aluminum in amounts between about 0.60% to 1.0%, and about 0.06% to 0.08% carbon, and the remainder substantially all iron; then annealing and quenching the steel to provide an austenitic aluminum-soluble structure transformable below usual room temperature; subjecting said steel to heat exchange cold treatment below room temperature; subjecting said steel to heat exchange cold treatment below about 30 F. for sufllcient time to achieve transformation; and heating the transformed alloy at a sufliciently high temperature for a sufiicient period of time to precipitate an aluminum compound and obtain a substantial increase in hardness of the steel.

5. In the production of hardened chromiumnickel alloy stainless steel articles and products, the art which includes providing a steel containlng about 16.75% to 17.3% chromium, with about 7.0% to 7.6% nickel, aluminum in amounts between about 0.60% and 1.0%, and about 0.06% to 0.08% carbon, and the remainder substantially all iron; annealing said steel at approximately 1800o F. to 2000 F. and quenching the same to provide an austenitic aluminum-soluble structure transformable below usual room temperature; fabricating articles and products of the austenitic steel; subjecting said fabricated articles and products to heat exchange cold treatment below about 30 F. for sufficient time to achieve transformation; and heating the transformed articles or products within the approximate temperature range of 750 F. to 1000 F. for a sufficient period of time to precipitate an aluminum compound and obtain a substantial increase in hardness of the articles and products.

6. A chromium-nickel stainless steel machined article aluminum-precipitation hardened by heat exchange treatment involving low temperature cold treatment from the annealed condition, said steel containing chromium and nickel in amounts substantlallyin accordance with area ABCD in the accompanying diagram, carbon between about 0.02% to 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, at least one element of the group consisting of sulphur and selenium sumcient for free-machining properties in the annealed condition, and the remainder substantially all iron, said aluminum being precipitated as aluminum compound within the matrix of the steel to give substantial hardness.

7. A chromium-nickel stainless steel aluminumprecipitation hardened by heat exchange treatment involving low temperature cold treatment from the annealed 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 fractional percentages up to about 3.0% molybdenum, manganese from incidental amounts up to approximately 8.0%, from incidental amounts up to about 2.0% silicon, and the remainder substantially all iron, said aluminum being precipitated as aluminum compound within the matrix of the steel.

8. A chromium-nickel stainless steel susceptible to precipitation hardening by heat exchange treatment involving low temperature cold treatment, said steel containing: the chromium-like components aluminum in the amount 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, said components in total meeting the terms for chromium of area ABCD of 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 actuJl amount not less than about 3.5%, said nickel-like components in total meeting the terms for nickel of areaABCD of the accompanying diagram, and said carbon and manganese serving as a substitute for nickel on the ratios of about 1/zo to 1/ao part carbon to l, and approximately 2 parts manganese to l; and the remainder substantially all iron.

9. A chromium-nickel stainless steel aluminumprecipitation hardened by heat exchange treatment involving low temperature cold treatment from the annealed condition, said steel containing: the chromium-like ,components aluminum in the amount `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, said components in total meeting the terms for chromium of area ABCD of the accompanying diagram, said chromium and silicon serving as a substitute for chromium on about a l 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 les than about 3.5%, said nickel-like components in total meeting the terms for nickel oi area ABCD of the accompanying diagram, and said carbon and manganese serving as a substitute for nickel on the ratios of about 1/20 to l/ao part carbon to l, and approximately 2 parts manganese to l; and the remainder substantially all iron, said aluminum being precipitated to give substantial hardness.

l0. A chromium-nickel stainless steel aluminum-precipitation hardened by heat exchange treatment involving low temperature cold treatment from the annealed condition, said steel 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 oi.' the steel, and the remainder substantially all chromium, said components in total meeting the terms for chromium oi' area ABCD of the accompanying diagram, said aluminum, molybdenum and silicon serving as a substitute for chromium on about a 1 to l ratio; the nickel-like components carbon 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 suicient 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 oi' about 1/20 to 1/su part carbon to 1, and approximately I2 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 oi' record. in the ille of this patent:

UNITED STATES PATENTS OTHERREFERENCES American Society for Steel Treating, vol. 19. page 501. Published in 1932 by the American Society for Steel Treating, Cleveland. Ohio.

Claims (1)

1. A CHROMIUM-NICKEL STAINLESS STEEL SUSCEPTIBLE TO PRECIPITATION HARDENING BY HEAT EXCHANGE TREATMENT INVOLVING LOW TEMPERATURE COLD TREATMENT, SAID STEEL CONTAINING CHROMIUM AND NICKEL IN AMOUNTS SUBSTANTIALLY IN ACCORDANCE WITH AREA ABCD IN THE ACCOMPANYING DIAGRAM, CARBON BE-

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