US2614921A - Stainless steel and method - Google Patents

Description

Oct. 21, 1.952 H. TANczYN 2,614,921

STAINLESS STEEL AND METHOD Filed Aug. 25', 195o xo, In v mom man aad INVENTOR HARRY TANczYN /5 ATTORNEY Patented Oct. 21, 1952 STAINLESS STEEL AND METHOD Harry Tanczyn,

Baltimore, Md.,

assigner to Armco Steel Corporation, a corporation of Ohio Application August 25, 1950, Serial No. 181,498

6 Claims.

My invention, relating generally to chromiumnickel'stainless steels, more especially concerns a precipitation-h-ardenable stainless steel of novel composition, as well as a method for hardening this steel and various resulting hardened products and manufactures formed thereo-f.

One object of my invention is to provide chromium-nickel stainless steels suited to hardening, through controlled heat treatment, from an initial condition wherein it is. soft and readily formed and machined.

Another object is to provide -a method, commercially practical in nature and direct and highly effective in execution, for conditioning stainless steels of the general type indicated and wherein, through annealing, the steels yare brought to a condition suited for fabricating operations of which illustratively, cold-rolling, cold-drawing, stamping, punching, upsetting or machining may be listed as typical, and in which method by subsequent do-uble-hardening treatment, the alloys are transformed land precipitation-hardened in thoroughly effective manner.

A further object of my invention is to provide hardened, strong, and wear-resistant chromiumnickel stainless steels and articles and products thereof, which have been precipitation-hardened from a soft, formable and m'achinable condition, and which, thus treated, display high tensile and compressive strengths, coupled with reasonably good ductility, and which further display substantial` freed-om from the directional effects characterizing the usual cold-rolled, chromiumnickel stainless steel sheet and strip.

Other objects of my invention in part will be obvious and in part pointed out hereinafter during the course of the following description.

Accordingly, my invention may be seen to reside in the combination of elements, composition of ingredients, and mixture of materials, and in the various operational steps, as well as in the relation of each of the same to one or more of the others, the scope of the application of all of which is indicated in the claims at the end of this specification.

In the accompanying drawing the single figure graphically displays the proportions of chromium and nickel which may be employed in the composition of my stainless lalloy steel, all las will be more fully pointed out hereinafter.

It will be helpful at this time to give brief consideration to certain aspects of stainless steels, thisto permit a more thorough understanding of certain importa-nt features of my invention. Generally, stainless steels may be considered as low carbon steels comprising about 10% -t-o 35% chromium. There may also be present nickel, with or without supplemental additions, in varying amounts, of say manganese, silicon, cobalt,

molybdenum, tungsten, vanadium, titanium, co-

lumbium, sulphur, and the like, for special pur,- noses. The remainder is substantially all iron. In steels of this type, the carbon contentis usually maintained low, ranging from about 0.03% to around 0.20%, or even more where desired;

Now, amongst the stainless alloys exist certain chromium-nickel stainless steels which remain stably austenitic down to about room temperature upon quenching following an :annealing treatment. It has been found that such steels,

`particularly those classi-able with the more coml are frequently encountered in such fabrication,l troublesome in nature and noticeable especially in a loss in compression strength along the direction of rolling or drawing, and this'characteristic has proved most troublesome and difficult of avoidance.

Following fabrication, the fabricated articles are put into use, either in soft, unstrained condition, or where feasible, in the work-hardened condition. As has been suggested, however, there is not suiicient control over the properties iin-l parted by work-hardening to make this an entirely satisfactory modeof hardening thesteels.

Over a number of years eifort hais been directed in the art towards the provision of stainless steelsA which can be hardened by heating to a temperature which while suiliciently high to bring about the desired hardening, at the same time can be maintained sufhciently low to yavoid or minimize oxidation and undue heat distortion. Thiseffort ha-s 'been directed towards bringingI abolita precipitation, at this relatively low temperature',`

of a critically dispersed phase.

Illustratively, it has heretofore `been `known that under certainV circumstances, some `few of, the chromium-nickelstainless steels will respond` to hardening heat-treatment due to the addition of one or more precipitation-hardening agents, such as titanium and columbium, the use of which hardening materials has depended upon a well-studied proportioning of the alloy ingredients in the steel, following which a form of critical heat-treatment is employed to achieve the hardening. From a practical standpoint, however, not only are columbium and titanium relatively expensive materials, but as well, the chromium-nickel, titanium or columbium stainless steels usually contain stress-laden ferrite as an essential to hardening from the annealed condition. Steels produced to such composition do not present the soft annealed condition characteristics of the austenitic steels and are not particularly amenable to cold-working and coldforming operations.

In comparatively recent years there has arisen the practice of employing, in a critically-proportioned chromium-nickel stainless steel, an amount of aluminum which is correspondingly critically-proportioned and which will impart to the steel the characteristic of being conditionable, upon annealing, into a fully aluminum-soluble austenitic alloy which thereafter, upon double heat-treatment, can be transformed and brought to a high degree of hardness. Somewhat similar results have been observed, also within recent years, upon the inclusion of copper in critical amounts in lthe critically-proportioned chromium-nickel stainless steel.

Despite the important advantages achieved by the foregoing researches, still further enhancement of these qualities are sought by those skilled in the art, including increase in hardness with retention of moderate ductility and decrease in the amounts of important and costly alloying ingredients such as nickel, and the elimination of others, such as columbium, while retaining the other desired good qualities characterizing such alloys.

An outstanding object of my invention therefore, in the provision of chromium-nickel stainlessv steels, which steels, readily fabricated into diverse products displaying hardening characteristics at temperatures so low as to avoid substantial scaling and distortion due to heat, additionally display, both as alloys and as products fashioned therefrom, other desirable properties in both the pre-hardened and in the hardened conditions, and particularly display such properties which are markedly superior to similar stainless steels in which aluminum or copper, standing alone, is employed as a hardening constituent.

Now, havingreference more particularly to the practice according to my present invention, I provide a chromium-nickel stainless steel of general analysis which will shortly be established, and inwhich, through the close correlation on the one hand of the quantities of chromium and nickel employed with critical quantities of aluminum, copper and carbon on the other hand, ismade highly suitable for ready fabrication into complicated contours and shapes while in the relatively soft, pre-hardened condition, and for subsequent precipitation-hardening by heat treatment to a hardened condition displaying gratifyingly high tensile and yield strengths, to-

gether with a degree of hardness not heretofore F' readily achieved.

At this point, particular notice should be given, with respect to the aluminum additive, and perhaps to a lesser extent the copper additive, of the hardening effect which these elements, in

4 critical proportions as hereinafter recited, have upon the steels. While heretofore it has been recognized that these comparatively inexpensive constituents impart certain advantageous hightemperature properties to the alloy, at the same time it has been thought that they impart an impairing effect upon the ability to harden. Quite on the contrary, my investigations establish conclusively that, in critical proportions, the combination of copper and aluminum additives not only contribute effectively to markedly increased ductility and lowered strength and hardness while in annealed condition, evidenced by austenitic steel wherein both the copper and aluminum remain almost completely soluble down to about room temperatures, nevertheless they display, in the hardened condition, following transforation in a double-heat treatment, markedly enhanced properties of vhardness and strength, and with satisfactory ductility as compared with similar alloys but which contain either aluminum or copper standing alone as a hardening agent.

Accordingly, the stainless steel of my invention contains the ingredients chromium and nickel substantially in accordance with the abscissa and ordinate of any given point of the area ABCD in the accompanying diagram illustratively ranging from about 14.0% to approximately 19.0% chromium and from about 4.0% to approximately 8.0% nickel. Moreover, my steel contains carbon from traces up to about 0.15%, aluminum from about 0.80% to 1.75%, copper from about 2.00% up to about 5.00%, from incidental amounts up to about 2.00% manganese, phosphorus from traces up to about 0.50%, sulphur from traces up to about 0.50% maximum, and silicon up to about 2.00 maximum, with the remainder substantially all iron.

It is to be noted, however, that should there be any appreciable variance cf the carbon, aluminum, copper, silicon or manganese contents or any combination thereof, from the amounts upon which the accompanying diagram is based, carbon 0.05% to 0.08%, manganese up to 1.00% maximum, silicon up to 1.00% maximum, about 3.00% to 4.00% copper, aluminum about 0.85% to 1.10%, phosphorous up to about 0.030% maximum, sulphur up to about 0.030% maximum, and the balance substantially all iron, then it may prove advantageous to modify either the chromium or nickel content of the steel as the case may be, or perhaps both, this to achieve chromium-like and nickel-like components in the steel which are substantially equivalent in ferriteforming `and austenite-forming relationship to the quantities of chromium and nickel representcd in the diagram, as related to the other elements represented therein. Illustratively, I sometimes replace a part of the chromium called for in the diagram with va quantity of aluminum, copper, silicon or molybdenum, and in approximately 1-1 ratio with respect to the chromium. And f replace part of the nickel with carbon or manganese in the ratio of 1/20 to 1/30 part carbon to one of nickel and two parts manganese to one of nickel. In this way I maintain the desired relationship between the austenite-forming and the ferrite-forming components of the steel in the same manner as would be achieved by rigid adherence to the analysis upon which the diagram is based.

In the preferred practice of my invention I produce a stainless steel in which the chromium and nickel contents respond to the requirements of any given point falling within the area abcd (a vabout 16.5% to 17.5% chromium, and `approxihardening, is `to place the'metal in an austenitic,V

aluminum-solubleand copper-soluble condition, whichcondition'can'beiretained down to atleast about room temperature: This I accomplish bya iirst',preliminary or annealing `treatmenttwherein I 'bring "thewalloy to iatemperature rpreferably ranging betweenabout 1700 as a lower 4limit toapproximately'20`00 Fsas an upper limit, preferably 1900my accomplish'this Ain -a suitable heat-treating furnacetin which `I'hold Atherrietal for a .time vvwhich in itself-is vnot toocritical,` and at the desired/temperature, until'the austenitic condition is achievedv Usually about-cnehalf hour proves'tobe` quite satisfactory, andthis both from the standpoint of 4eccnomyandof assured copper-solubility and aluminumsolub`ility5-"Once I have broughttthe steel into-this `desired tausteni-tic -condition',-in which `both thefialuminum and copper contents' areheld `in "solution, I `at once discontinue the annealing operation `and follow this by-quenehing in a` desiredi medium, conveniently `toroom temperature. This quenching is achieved either in air or water.

It is noteworthy that following the `quenching operation and after the annealing.A treatment, both the aluminum `and copper constituents remain fully in solution. As may be expected, the metal is soft, substantially fully austen'itic and is characterized by-its extreme ductilityiandli-ts `low degree of hardness. Surprisingly enough,.Ifha=ve found such steels to have a hardness, in annealed condition, appreciably less than that of a similar steel Witha copper-hardening additive,f.andalso less than that :of one with an aluminumfhardeni ing agent.` Moreover, my newsteel displays an ultimate tensile strength incannealed condition substantially lower than that of either the aluminum-containing steel or ofthe copper-containing steek Thus, as maybe expected, mynew alloy is-'more readilyformabler and machinable in the `annealed condition than-eitherfof thesesteels.

surgical instruments,'including those lused in dentistry, .die blocks 'and the like.`

Itis to be noted that in all these illustrative I examples, I take advantage of the softness `and ductility `ofthe metaltin lits austenitic` coppersolubleand aluminum-soluble condition, with its excellentl workability-and formability before hardening;` ltov subjectf theA sametoany `cnefof a widely `varied zfabricating steps; illustratively including `cold-forming, y upsetting, stamping, punching, drawing-machining, or Vother manipulativeftechniques to which the kmetal may :readily be subjected.` Additionally, Ait is `feasible at times to Adelay atleastfpart `of the formi-ng and fabricating operations Auntil the .metal has been subjected-to the initial one ofk the -ltwo-stageharden-` ing treatment-howto beidescribed;` l t l Following the annealing treatment'and subsequent to `any fabricating steps to which the annealed metal may thereafter be subjected, I subject `my -annealed chromium-nickel-aluminum copper'stainless steel to a preliminary hardening heat. This may vbe termed a re-heat if `considered in the light of the preliminary or annealingtreatment. In this `re-heating stage, for pr'eliminarily hardening the steel, illustrati-vely Iv employ the same heat-treating furnace used in 'the annealing stage. i In this instance it is my `objective `to achieve a precipitation oi carbides,` accompanied possibly by some `precipitationfofaluminum and/or copper compounds, `to gether with increase or :raising of the transformation point' of the austenitic matrix. i

To achieve this I bring the metal in .the heattreating furnace upto an approximate temperaturerange of 1200 F'. to 1600 F.,-preferably about 14:00o F. `At.` this desired temperatureI hold- `the metal for `approximately one-quarter hour-or more,` during which the carbide precipitation .takes` place. l Subsequently, and at. the completion'of the re-heating stage, I quench the steel in suitable manner as in air, oil, or water.`

V as by cold-rolling` or cold-drawing, while at the i Sametime theyare still machinable and `capable of fabrication through cutting, punching and drilling operations and the like. Accordingly, and since the metal can be further hardened without `transformation or" .phase and without the ydimensional changes usually attending such phase-transformation, I .frequently elect to.` delay certain parts of forming and fabricatingoperations, particularly those final intoperation, until after the preliminary-hardeningtreatment.` .Followingtthis technique, I then produce a-widefvariety! of` fabricatedarticles finished toA iinal dimensionsr l s N In theforegoing I have described., the kpreliminary :hardening asA brought about by heatvtreatment'wata temperature; of `1200 F. .tow1600 F., that lis well below that at which extensivescaling or warping of the metal takes place, and by virtue ofuwhichthe carbides are precipitated andthe transformation point raised of the austenitic matrix, so. that upon subsequent. `quenching .a softy martensite-like. constituent is produced.Y I

haveobserved that it-is also possible in certain instances, where-operational conditionsare conducive to such results, to achievethis `transformation and `preliminary hardening with partial change in phase, by cold-working the metal `to a considerable and controlled extent from the annealed condition. Thus it is entirely within the contemplation of my invention to subject the soft metal, following preliminary anneal, to coldforming and cold-fabricating to desired specifications Which are found to give rise to a desired phase-transformation and a desired preliminaryhardening. This alternative technique is Vparticularly feasible in those instances wherequantity production is sought, responding to identical dimensional specifications, and the cold-working of which is found through empirical investigation to result in the required transformation.

Once transformation of the metal has been achieved, together with preliminary hardening, through either the reheating and quenching process or the cold-Working alternative technique as hitherto described, and once the forming and fabricating operations have been completed either before or following the preliminary hardening treatment, should such operations be desired'and required, then the steel and the articles formed therefrom is ready for the nal hardening step. This final hardening I accomplish by subjecting the metal to further heat-treatment at temperatures sufliciently low to avoid scaling and warping of the metal due to excess heat. This further heating constitutes the iinal stage of my entire treatment.

According to the practice of my invention and to achieve the final hardening, I re-heat-thealloy, illustratively as fabricated products, at comparatively low temperatures, illustratively 800 F. to approximately 1100 F. In a preferred embodiment I bring these products to about 900 F. and hold them at such temperature for a desired period, say one hour, and then quench in air or The finally hardened steel not only displays a substantial freedom from directionality and high:

resistance to corrosion, occasioned perhaps by the recapture of the chromium from the rst heat-treatment stage into the metal and replacement of the copper and aluminum precipitated into the metal, but as well, the hardened metal displays nall strength values substantially greater than do the corresponding metal in which copper or aluminum alone, as the case may be, are ernployed as the hardening constituents.V Moreover, the low temperatures at which both the preliminary hardening treatment and particularly the nal hardening treatment are conducted, gives a steel which emerges substantially Without heatscaling, and without heat-warping.

As a specific illustration, a chromium-nickelalum-inum-copper stainless steel having a composition falling within the general range hereto-Y fore recited, and more specifically, containing approximately 0.057% carbon, 16.85% chromium, 5.85% nickel, 0.92% aluminum, 3.20% copper, 0.50% silicon, 0.01% manganese, 0.011% phosphorus, and 0.009% sulphur, and the remainder iron was found by treatment in accordance with my invention to have the approximate physical values presented in the table given below. In this connection, all values tabulated for the annealed condition resulted from one-half hour treating of the steel at 1900 E'. followed by water-quenching; those for the preliminarily-hardened condition resulted from reheating the metal at 1400D F. for three hours followed by waterquenching; and the values for the final hardening condition followed upon heating the metal one hour at 900 F. followed by air-cooling.

TABLE Mechanical properties of chromium-nickelaZamznam-copper stainless steel water. `The holding period, however, is not highly critical and may vary from one-half hour to some two hours or more, and this with entirely satisfactory results.

The final yheat treatment serves to achieve a a substantially complete precipitation of a copper-rich and aluminum-rich phase through the metal grains, which precipitate, While not visible under an ordinary light microscope, can be photographed with the aid of an'electron microscope. Although I cannot authoritatively explain the exact phenomenon which takes place during the precipitation-hardening treatment itis clear that both copper and aluminum are precipitated in some form. It is established with reasonable certainty by dilatometer tests that no phase transformation occurs; nor does substantial change in volume of the metal take place during the hardening reaction. While I suggest that the reaction involves a precipitated nickel-aluminumcopper compound within the matrix which thereafter exerts an interference hardening effect, I have not fully confirmed this theory, and accordingly, do not desire to be bound thereby.

t is noted from the foregoing that in the hardened condition, not only is the hardness substantially greater than is true of generally similar steels, but which contain aluminum or copper alone as the precipitating element, but thatas well, the ultimate tensile strength is substantially greater as is also the yield strength. In this connection, it may be noted that a seemingly slight increase in the inal hardness as for example from a C39 to a C/ifirange for the copper-containing stainless steels, and a C43 to C46- 9 comparatively cheap, with the other ingredients in the steel, I achieved in my steel a commercially valuable precipitation-hardening effect.

This effect becomes active with proper treatment i of the metal from the soft, workable condition, or from a preliminarily work-hardened condition, as desired. And the-eiTect is fully realized with the final heat-treating step. Moreover, the copper and aluminum additives permit the use of low temperatures to achieve heat-treatment and thereby preclude heat scale formation, a point of particular importance in the respective hardening steps.

i From the foregoing it will be seen that I provide a chromium-nickel stainless steel, as well as a method offorming and hardening the same, in which the various objects hereinbefore noted are successfully achieved, along with many practical advantages. Moreover, my new chromiumnickel-copper aluminum stainless steel makes possible the provision of wrought or cast articles which are preliminarily formed, machined, or fabricated in any one of a variety of operations and are effectively and reliably hardened by heat-treatment from a soft, ductile condition, this by a method which is readily practiced and which enables the production of chromium-nickel stainless steels of hardened quality, with a minimum of ancillary treatments such as pickling, and which is otherwise quite suitable for commercial use.

Since many embodiments may be made of my invention, and since many changes may be made in the illustrative embodiment herein set forth, I desire the foregoing description to be considered simply as illustrative, and not by way of limitation.

I claim as my invention:

1. A chromium-nickel stainless steel capable of precipitation-hardening by double heat-treatment from a soft, workable and substantially fully austenitic condition, said steel containing chromium and nickel in amounts substantially in accordance with area ABCD in the accompanying diagram, carbon from traces up to about 0.15% maximum, aluminum about 0.80% to 1.75%, copper between 2.00% to 5.00%, manganese from incidental amounts. up to about 2.00% maximum, silicon from incidental amounts upto about 2.00% maximum, and the remainder substantially all iron.

2. A chromium-nickel stainless steel capable of being precipitation-hardened by double heattreatment following annealing and quenching and containing approximately 16.5% to 17.5% chromium, about 4.50% to 6.20% nickel, about 0.85 to 1.10% aluminum, about 3.00% to 4.00% copper, about 0.05% to 0.08% carbon andthe remainder substantially ,all iron.

3. In the production `of hardened chromiumnickel stainless steel articles and products, the steps comprising providing a stainless steel containing about 16.5% to 17.5% chromium, about 4.50% to 6.20% nickel, about 0.85% to 1.10%

. aluminum, about 3.00% to 4.00% copper, about 0.05% to 0.08% carbon, and the remainder substantially all iron; annealing said steel within a temperature range which is sulciently high, and for a time sufciently long, to provide an aluminumand copper-soluble austenitic condition which is stable down to at least about room temperature; reheating the steel within the approximate temperature range of 1200 F. to l600 F. for a time sufficient to eiIect carbide precipitation and thereupon quenching the steel to achieve transformation; fabricating the articles and products of said steel in the transformed condition; and then heating said fabricated metal within a temperature range of approximately 800 F. to approximately 1100 F. for a period of time sufficient to precipitate aluminum and copper compounds into the metal and to obtain a substantial increase in the hardness of the metal.

4. In the production of hardened chromiumnickel stainless steel articles and products, the steps comprising providing a stainless steel containing about 16.5% to 17.5% chromium, about 4.50% to 6.20% nickel, about 0.85% to 1.10% aluminum, about 3.00% to 4.00% copper, about 0.05% to 0.08 carbon, and the remainder substantially all iron; annealing said steel within a temperature range which is sufficiently high, and for a time sufficiently long, to provide an aluminumand copper-soluble austenitic condition which is stable down to at least about room temperature; subjecting the annealed steel to cold-working and fabricating to a determined extent which is suicient to work-harden the steel and to achieve a transformation of the steel; and then heating said fabricated and preliminarilyhardened and transformed metal within a temperature range of approximately 800' F. to 1100 F. for a period of time suilicient to precipitate aluminum and copper compounds into the metal of the fabricated metals, and to obtain a substantial increase in the amount of hardness of the metal thereof.

5. A chromium-nickel stainless steel, precipitation-hardened from the annealed condition, and containing chromium and nickel in amounts substantially in accordance with area ABCD in the accompanying diagram, carbon from traces up to 0.15% maximum, about 0.80% to 1.75%

aluminum, about 2.00% to 5.00% copper, from incidental amounts up to approximately 2.00% manganese, from incidental amounts up to about 2.00% silicon, and the remainder substantially all iron, with significant amounts of the aluminum and copper being dispersed throughout the steel in ne form.

6. A chromium-nickel stainless steel, precipitation-hardened by double heat-treatment from the annealed condition, containing about 16.5% to 17.5% chromium, about 4.50% to 6.20% nickel, about 0.85% to 1.10% aluminum, about 3.00% to 4.00% copper, about 0.05% to .08% carbon, and the remainder substantially all iron, with significant amounts of the aluminum and copper being dispersed throughout the steel in ne form.

HARRY TAN CZYN.

REFERENCES CITED The following references are of record in the file of this Ipatent:

UNITED STATES PATENTS Goller May 2, 1950

Claims (1)

1. A CHROMIUM-NICKEL STAINLES STEEL CAPABLE OF PRECIPITATION-HARDENING BY DOUBLE HEAT-TREATMENT FROM A SOFT, WORKABLE AND SUBSTANTIALLY FULLY AUSTENITIC CONDITION, SAID STEEL CONTAINING CHROMIUM AND NICKEL IN AMOUNTS SUBSTANTIALLY IN ACCORDANCE WITH AREA ABCD IN THE ACCOMPANYING DIAGRAM, CABRON FROM TRACES UP TO ABOUT 0.15% MAXIMUM, ALUMINUM ABOUT 0.80% TO 1.75%, COPPER BETWEEN 2.00% TO 5.00%, MANGANESE FROM INCIDENTAL AMOUNTS UP TO ABOUT 2.00% MAXIMUM, SILICON FROM INCIDENTAL AMOUNTS UP TO ABOUT 2.00% MAXIMUM, AND THE REMAINDER SUBSTANTIALLY ALL IRON.

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