US2506558A - Stainless steel and method - Google Patents

Description

May 2, 1950 G. N.v GOLLER STAINLESS STEEL AND METHOD Filed Dec. 14, 1949 TBXOIN N30 83d @0265 N. Gouf/z.

mw f4 f '-11.3 flllmnm! Patented May 2, 1950 UNITED STATES PATENT OFFICE STAINLESS STEEL AND METHOD George N. Goller, Baltimore, Md., assignor 4to Armco Steel Corporation, a corporation of Ohio Application December 14,1949, Serial No. 132,832 7 Claims.A (Cl. 14S-21.54)

My application for patent is a continuationin-part of my copending application, Serial No. 695,216 filed September 6, 1946, and entitled Stainless steel and method and also it is a continuation-in-part of my copending application, Serial No. 695,217 of the same date and title, and the invention relates to 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 themselves.

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 machinable condition.

Another object is the provision of chromiumnickel stainless steels so proportioned in composition as to be consistently and reliably produced to analysis specification and of such composition as to be amenable to annealing, and working and fabricating in the annealed condition, and thereafter to hardening treatment Without, however, being unduly sensitive to cold weather in the prefabricated condition.

A further object of my invention is the provision of a simple, practical and reliable method for conditioning chromium-nickel stainless steels to a 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 sufficiently low to avoid substantial warping and the formation of heat scale.

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 combination of elements, composition of ingredients, and in the several method steps and the relation of each of the same to one or more of the others as described herein, and in the resulting articles and products had thereby, the scope of the application of which is indicated in the following claims.

The single figure of the accompanying drawing graphically represents proportions of chromium and nickel which are 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 chromiumnickel 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, tungsten, vanadium, titanium, columbium, and sulphur, for special purposes. Some few of the chromium-nickel stainless steels have been known to respond to hardening by heat treatment, this by virtueof the addition of titanium or colum- 'bium in well studied proportionment with other elements present coupled with a critical form of heat treatment to effect precipitation hardening. Columbium and titanium, however, are relatively expensive materials. The chromium-nickeltitanium or chromium-nickel-columbium stainless steels, moreover, usually contain stress laden ferrite as an essential to hardenability from Athe annealed condition. When the steels are produced to such composition as to present a comparatively soft, substantially fully Iaustenitic structure at about room temperatures after annealing, they are more amenable to cold working and forming operations, but hardenability by heat treatment is usually sacrificed.

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 film 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,

aooaose tion 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. 'I'he 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. 1

I '11nd, as briefly noted above, that with the quenching, however, certain internal stresses apparently are set up in the steel which permit the transformation from austenite to martensite upon subsequent severe cooling'. Thus, in shipping the steel during unusually cold winter weather, I note that a hardening occurs which in many cases is most objectionable. Under unusual conditions, the hardness had may amount to as much as Rockwell C-30. This hardening notably increases the difficulties of the forming, machining or like operations.

In accordance with my invention, I subject the steel, following the annealing and quenching steps, to a stabilizing treatment, this by heating the steel to a temperature of some 600 F. to 1100" F. for a short period of time. Ordinarily, heating at a temperature of 900 F. for about one hour is found to give good results. Where the lower temperatures are employed, however, a longer period of time is used and conversely where the higher temperature is used less time of treatment is required. This treatment leaves the steel with substantially no change in hardness or other physical properties.

My stainless steel at any time subsequent t from the standpoints of yield, in tension and compression coupled with toughness and corrosion resistance. I provide 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 articles of ultimate use, 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 of the metal. Although I prefer to conduct the working and fabricating operations prior to any -hardeningv treatment, I may delay part or all of the forming and fabricating operations until after the achievement of a preliminary hardening 'pre-treatment now to be described.

To preliminarily harden the annealed and stabilized chromium nickelaluminum stainless steel, as after the forming and fabricating steps illustratively conducted before undertaking the hardening treatment, I subject the steel toa heat exchange treatment consisting either of heating the steel to substantial tem-peratures or of cooling it to very low temperatures. For the one, I reheateit, 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 approximately 1A 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-llke constituent. The transformation occurs at quite low temperature such as between about 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.

Where I employ a low temperature heat exchange treatment, I cold-treat the metal, as in fabricated condition, at temperatures of about 30 F. to 100 F. or lower, this preferably being in a suitable cooling compartment, or the like. A satisfactory temperature is 60 F. It will be understood that because of the extremely low temperature required, my steel does not preliminarily harden to any substantial extent during shipment even under the most severe cold weather conditions.

In preliminarily hardening the steeleby cold treatment, a holding time of about 1/2 hour 0r more at cooling temperatures within the cooling range noted is preferred to effect phase transformation and the preliminary hardening. The cooling medium illustratively is dry ice in acetone. Hardnesses of the chromium-nickel 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 whether by heat treatment or by cold treatment, retains a reasonable amount of workability and/formability, 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 possi'- bility of further hardening the metal without phase transformation and changes in dimensions of the metal accompanying transformation that 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 articles and products, as to substantially iinished dimensions, of the partially hardened steel.

When transformation has been achieved by the heat exchange treatment 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, representing a further axiomatic4 part army conditioning treatinent, I neat the steel within a temperature range of about 750 F. to approximately 1000 F., preferably at /about 900 F.,.and there hold vthe same at temperature for about one hour. The time of holding under treatment, however, may vary from approximaten ly one-half hour for the high temperature of 1000 F. to some ve hours or more for the low temperature of 750 F., this with satisfactory re- 1 sults. About one hour treatment at 900 F. is preferred, however. part a material gain in hardness to the metal. This hardness, I ilnd, `upon quenching the metal as in air, oil or water from the hardening temperatures, comes within the approximate range of C40 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 Ivery strong both in tension and compression, have The treatment serves to imi high yield strength, good directional qualities,

and are quite resistant to corrosion. The steel emerges from the preliminary hardening treatment and precipitation hardening heat treatment substantially free of heat scale and unwarped by heat at the low temperatures employed.

As illustrative of the stainless steel and method l 2. By combined cold treatment and heat treatment: i f

F. Samples ol E iient No. `E'nnsil-Jtlrllnctsodof-33' T110 Precipitation F 20mm hardened 900 F.

' l hr. air cooled C35 C43 C34 C42 C34 C41 TABLE IV Hardness of unstabilized steels as compared to stabilized steels G. Samples of A (un- H. Samples of B Heut No stabilized) subjected (Stabilized) subjected 32 F. 20 mins. 32 F. 20 mins.

C35 B90 C33 B79 C B75 of my invention, three samples of stainless steel i of composition noted in Table I were subjected to various conditions of heat-treatment with the I results as reported below in Tables 1I and III.

TABLE I Composition of stainless steels testedv HentNo. c Mn 1 s Isi cr Ni A1 .oso .6i 62o .oi5 .4a 16.54 1.01 1.o:

051 .11 .624 .oio 55 16.11 1.06 .as

TABLEH stabilized following annealing i Hardness of annealed samples and of samples It win be seen that with the stabilizing treatment there is substantially no change in hardness.

TABLE III B given heat exchange treatment and then precipttation hardened l. By combined double heat-treatment:

C- Steps?. tit;- D-Trmpla 0 Y re a en precipi ion Heat No 3 hrs. water hardened 900 F.

quenched l hr. air cooled c2c C43 C C42 C C43 Hardness of annealed and stabilized samples of The superior working and forming characteristics of the stabilized steels are evident.

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 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 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 an improved precipitation hardening stainless steel and method oi treating the same in which the various objects hereinbei'ore. noted together with many thoroughly practical advantages are successfully achieved. It will be ,Y

seen that the method makes possible the provision of wrought or cast chromium-nickelaluminum stainless steel subjected to any of a number of forming, machining or fabricating operations and effectively and reliably hardened by heat exchange either at substantial temperatures or at low temperatures.` Likewise, it will be seen that my method is readily practiced, and enables the production of chromium-nickel grade stainless steel oi?v 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 condition and in the annealed and stabilized condition, and are,y 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. In a method of hardening chromiumnickel 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 approximately 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 aluminumsoluble structure transformable below usual room temperature; heating the steel to a temperature of about 600 F. to 1100 F. to stabilize the same; subjecting said steel to heat exchange pre-treatment at such temperatures and for long enough time to achieve transformation; and heating the transformed alloy at a sufllciently high temperature and for a sufiicient period of time to precipitate an aluminum compound and obtain 'a substantial increase in the hardness of the steel.

2. In a method of hardening chromium-nickel alloy stainless steel, the art which includes providing a steel containing about 16.5% to 17.5% chromium, with about 6.75% to 7.5% nickel, aluminum in amounts between about 0.60% to 1.1%, 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; heating the steel to a temperature of 600 F. to 1100 F. to stabilize the same; subjecting said steel to heat exchange pretreatment consisting of cold treatment below 4about 301 F. for sufficient time to achieve 3. In the production of hardened cliromiumu.

nickel alloy stainless steel articles and products, the art which includes providing a steel containing about 16.5% to 17.5% chromium, with about 6.75% to 7.5% nickel, aluminum in amounts between about 0.60% and 1.1%, and about 0.06% to 0.08% carbon, and the remainder substantially all iron; annealing saidv steel at approximately 1800 F. to 2000 F.` and quenching thev same to provide an austenitic aluminum-soluble structure transformable below usual room-temperature; stabilizing the steel by treatment at 600 F. to 1100 F.; fabricating articles and products of the austenitic stabilized steel; subjecting said fabricated articles and products to heat exchange pre-treatment at 1200 F. to 1600 F. for suicient time to achieve transformation; andy heating the transformed articles or products within the approximate temperature range of 750 F. to 1000 F. for a sufllcient period of time to precipitate an aluminum compound and obtain a substantial increase in hardness of the articles and products.

4. An annealed and stabilized chromiumnickel stainless steel susceptible to aluminumprecipitation-hardening by heat exchange treatment from the annealed and stabilized condition involving a preliminary heat-exchange treatment followed by linal precipitation hardening heat 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 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 8.0%, and the remainder substantially all nickel in actual amount not less than about 3.5%, said nickel-like components in total meeting the terms for nickel of area ABCD of the accompanying diagram. and said carbon and manganese serving as a substitute for nickel on the ratios of about 1/iui to 1/zm part carbon to 1, and approximately 2 parts manganese to 1; and the remainder substantially all iron.

5. An annealed and stabilized chromiumnickel stainless steel of substantially fully'austenitic structure at usual room temperatures, said steel containing about 16.5% to 17.5% chromium, about 6.75% to 7.5% nickel, aluminum between about 0.60% and 1.1%, about 0.06% to 0.08% carbon, and the remainder substantially all iron, and the steel being susceptible to hardening by heat exchange treatment involving pretreatment at 1200 F. to 1600 F., after the annealing and stabilizing followed lby linal precipitation hardening.

6. A chromium-nickel stainless steel aluminum-precipitation-hardened by heat exchange treatment involving preliminary treatment below about 30 F. from the annealed and stabilized condition followed by precipitation-hardening final heat 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, said aluminum being precipitated as aluminum compound within the matrix of the steel to sive substantial hardness.

7. In the production of a stabilized precipitation-hardenable chromium-nickel alloy stainless steel, the art which includes providing a steel containing the chromium-like components aluminum in the amount of about 0.05% 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 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 8.0%, and the remainder substantially all nickel in actual amount not less than about 3.5%, said nickel-like components in total meeting the terms fornickel of area ABCD of the No references cited.

Claims (2)

1. IN A METHOD OF HARDENING CHROMIUMNICKEL ALLOY STAINLESS STEEL, THE ART WHICH INCLUDES PROVIDING A STEEL CONTAINING CHROMIUM AND ANICKEL 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 APPROXIMATELY 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 ALUMINUMSOLUBLE STRUCTURE TRANSFORMABLE BELOW USUAL ROOM TEMPERATURE; HEATING THE STEEL TO A TEMPERATURE OF ABOUT 600*F. TO 1100*F. TO STABILIZE THE SAME; SUBJECTING SAID STEEL TO HEAT EXCHANGE PRE-TREATMENT AT SUCH TEMPERATURES AND FOR LONG ENOUGH TIME TO ACHIEVE TRANSFORMATION; AND HEATING THE TRANSFORMED ALLOY AT A SUFFICIENTLY 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.

2. IN A METHOD OF HARDENING CHROMIUM-NICKEL ALLOY STAINLESS STEEL, THE ART WHICH INCLUDES PROVIDING A STEEL CONTAINING ABOUT 16.5% TO 17.5% CHROMIUM, WITH ABOUT 6.75% TO 7.5% NICKEL, ALUMINUM IN AMOUNTSA BETWEEN ABOUT 0.60% TO 1.1%, 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; HEATING THE STEEL TO A TEMPERATURE OF 600*F. TO 1100*F. TO STABILIZE THE SAME; SUBJECTING SAID STEEL TO HEAT EXCHANGE PRETREATMENT CONSISTING OF COLD TREATMENT BELOW ABOUT - 30*F. FOR SUFFICIENT TIME TO ACHIEVE TRANSFORMATION; AND FINALLY HEATING THE TRANSFORMED ALLOY AT A SUFFICIENTLY HIGH TEMPERATURE FOR A SUFFICIENT PERIOD OF TIME TO PRECIPITATE AN ALIMINUM COMPOUND AND OBTAIN A SUBSTANTIAL INCREASE IN HARDNESS OF THE STEEL.

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