US2694626A - Stainless steels - Google Patents

Description

Nov. 16, 1954 H. TANczYN STAINLESS STEELS Filed Aug. 25, 1950 HARRY TANCZYN. BY

ATTORNEY United States Patent STAINLESS STEELS Harry Tanczyn, Baltimore, Md., assgnor to Armco Steel Corporation, a corporation of Ohio Application August 25, 1950, Serial No. 181,499

7 Claims. (Cl. 75-124) Relating generally to stainless steels, my invention concerns more particularly chromium-nickel stainless steels which display highly desirable and advantageous properties, both in the hardenable and in the hardened conditions.

Accordingly, among the objects of my invention is the provision of chromium-nickel stainless steels, which although hardenable, nevertheless are comparatively soft in the pre-hardened condition and are suited to a wide variety of forming and fabricating operations, among which may be included cold rolling, upsetting, colddrawing, machining, stamping or punching; and which steels may be transformed following such working into the strong, hardened condition by subsequent heat-treatment, all in the substantial absence of warping or heatscaling at the hardening temperatures employed, and with substantial retention of the close dimensional tolerances to which the steel was originally fabricated in its pre-hardened condition.

All these as well as many other highly advantageous objects attend upon the practice of my invention, which objects in part will be obvious and in part more fully pointed out hereinafter during the course of the following description.

Accordingly, my invention may be defined as residing in the combination of elements, composition of materials, and conditions of treatment, as well as in the various operationalv steps, and in the relation of each of the same to one or more of the others as described herein, the scope of the application of all of which is indicated in the claims which follow.

In the single ligure of the accompanying drawing, I disclose the proportions of chromium and nickel which I employ in the composition of my stainless steel; as well, I disclose the preferred range of these and other materials included in my steel.

It will be helpful to a clear understanding of certain important features of my invention to note at this point that certain characteristics of stainless steels are unique to particular grades of such steel; and for a number of years it was conceived that these various characteristics were nnconsistent with each other and could not be carried over from grade to grade. To emphasize the foregoing, it should be kept in mind that in the chromiumnickel stainless steels it is the chromium and the chromium-like additives which impart, amongst other attributes, the high degree of corrosion-resistance which characterizes these steels. And it is the nickel and the nickellike substitutes which impart the softness to these steels in the pre-hardened stage.

Now, it is at once apparent that it is highly desirable, if possible of achievement, to maintain the steels in a soft, workable condition during the many types of working and fabrication to which they are subjected including cold-working into sheet, strip, plates, bars, wire, rounds and the like, as well as more intricate shapes, such as trim, structural members and the like, for the aircraft industries. Other illustrations are cold-headed bolts and screws requiring hard shanks, Valves, valve seats, surgical instruments, shafting and the like. With this processing advantage is taken of the excellent forming properties of the metal in its comparatively soft, ductile condition, during the course of cold-forming, upsetting, cold-drawing, machining, stamping, or other operations consistent with the properties of the metal. Particularly desirable is the fabrication of the metal in its soft, ductile condition to nal form within close dimensional tolerances.

It is only after working and fabricating, whatever the character thereof may be, that it is desirable to produce an ultimate hardness and the high strength required for the many uses to which these fabricated articles are put. In so doing, however, the close tolerances of the metal must be retained, and this without scaling, warping, or other physical or dimensional change from the fabricated condition.

Heretofore, it has been possible to employ say, a straight chromium quench-hardenable stainless steel such as the 12% to 18% chromium grade and as well, stainless steel having but small addition of nickel, of which the 16% chromium-2% nickel grade is illustrative. Following working and fabrication, these may be quenched in water, oil or the like so as to bring about a phasetransformation of the metal, accompanied by a direct heat hardening, and this preferably from temperatures in the vicinity of about 1800 F.

It is found, however, that with the quench-hardening operation the character of the surface of the fabricated steel suifers as well as its shape frequently sulers, due in large measure to the scaling and tendency toward heat-distortion in the hardening treatment. For all practical purposes, then, it is difficult to achieve close dimensional fabrication in the soft condition of the straight chromium or low nickel-chromium stainless steels, followed by a quench-hardening high temperature heattreatment.

It has further been found according to earlier practices that where larger quantities of nickel are employed, illustratively, the 18-8 chromium-nickel stainless steels, then there is a decided change from the magnetic, quenchhardenable ferritic-martensitic steel to an austenitic nonmagnetic condition, non-hardenable by the usual high temperature heat-treatment. In these steels the metal remains stably austenitic down to room temperature. Although it is readily fabricated in its soft, ductile condition, it cannot thereafter be hardened by heat-treatment so as to impart thereto the desired high strength and hardness required. Moreover, the substantial quantity of nickel employed in itself involves a substantial element of expense.

In more recent years it has been found that certain of the chromium-nickel stainless steels will respond to a hardening treatment at a comparatively low temperature when there is employed in the steels a strong carbide forming element such as columbium or titanium. Of course, these additions are rather expensive. But of even greater consequence, these additions introduce ferrite, with the result that the steel is harder in its annealed condition than is the austenitic steel without these additions. Workability and formability accordingly suffer.

And in other chromium-nickel stainless steels a precipitation-hardening or age-hardening eort is had at comparatively low temperatures by including in the steel a small amount of copper. Like results also are had by including in the steel the ingredient aluminum instead of copper. In both of these steels, however, substantial amounts of nickel are required, this at appreciableexpense. And in the copper-bearing steel the workabllity and forrnability inthe annealed condition leaves considerable to be desired.

Accordingly, an outstanding object of my mventlon 1s the provision of a precipitation-hardened, chromiumnickel stainless steel which not only is hardenable from an annealed condition wherein it is readily worked, formed and fabricated, but is readily hardened through low temperature heat treatment without substanual heatscaling or warping, and in which hardened condltlon the steel is strong, tough, and durable.

Now I have found that upon includlng both copper and aluminum in amounts critically proportioned to the chromium and nickel contents of the steel, there is realized a steel which can be preliminarily brought, through proper heat treatment, into an unstable austenitic condition persistent down to about room temperature and thls despite the relatively small amount of nlckel employed. And that the steel thus realized displays a high degree of ductility in its annealed condition, permittlng ready working and intricate fabrication in widely varled form and manner. As well, this preliminary treatment conditions the steel for a subsequent low temperature, precipitation-hardening treatment by which the steel achieves an extreme hardness with high strengths in the substantial absence ofdirectionality, all with retention of an adequate amount of ductility. Moreover, I find that by the inclusion of a moderate and proportioned quantity of columbium and titanium, certain enhancement is observed in the hardening process, and at the same time, the steel is better suited for subsequent high temperature processing such as welding.

In the more general practice of my invention, I employ chromium and nickel contents which are substantially in accord with the abscissa and ordinate of any given point of the area A B C D in the accompanying drawing. Illustratively, the chromium may range from about 14.00% to 19.00%, with nickel suitably proportioned as shown in the drawing, from about 3.0% to 7.0% depending upon chromium content, aluminum from about 0.80% to 1.75%, and copper from about 2.00% to 5.00%. In this general practice the carbon may range from traces up to 0.15% maximum, manganese up to about 2.00% maximum, both phosphorous and sulphur from traces up to about 0.050% each, silicon from traces up to about 2.00% maximum, and the remainder substantially all iron.

Actually I find that where the aluminum, copper, silicon or manganese are employed in amounts near their upper limits, some slight modification in the exact amounts of chromium and nickel called for by the diagram is desirable. This is for the reason that the diagram is based on a composition containing 3.00% to 4.00% copper, .85% to 1.00% aluminum, 1.00% maximum silicon, 1.00% maximum manganese, 0.05% to 0.08% carbon, with sulphur and phosphorous each not exceeding 0.030% maximum. And where there are employed greater amounts of either the austenitic forming elements, manganese and carbon on the one hand, or of the ferrite forming ingredients, aluminum, silicon and copper on the other, those respectively are viewed as partial substitutes for nickel and for chromium as called for by the diagram. In this regard, nickel may be replaced by carbon in the ratio of 1/0 to 1/30 part carbon for one of nickel and by manganese in the amount of 2 parts manganese for one of nickel. The chromium may be replaced by aluminum, silicon, and even molybdenum in an approximate l to 1 ratio. It will be understood, therefore, that the diagram ABCD gives the approximate amounts of chromium and nickel, or their substituted equivalents, for any specific composition.

In a preferred composition according to my practice, the range of the more important ingredients is somewhat more restricted. Here my new steel contains chromium and nickel in amounts substantially in accord with the abscissa and ordinate, respectively, of any given point substantially falling within the area abcd in the accompanying diagram. Therein chromium is advantageously present in an amount ranging from about 16.00% to 17.00%, with nickel ranging from about 3.50% to 5.00%. Copper is critically proportioned, ranging from about 3.00% to 4.00% and aluminum from about 0.85% to about 1.10%. Carbon ranges from 0.05% to 0.08%, with both manganese and silicon present in traces up to about 1.00%, and phosphorous and sulphur each in amount ranging from traces up to about 0.010%.

In carrying my process into effect, I heat the steel of the foregoing general composition at a temperature not lower than about 1600 F. and extending up to about 2000 F. or more, for such period of time as is required to bring the aluminum, copper and carbon into solid solution and then quench in air or water. This gives an austenitic structure which is unbalanced and unstable, and which is transformable above the usual prevailing room temperature. This preliminary heat treatment may well be likened to an annealing treatment, and is usually carried out in a heat-treating furnace. The denotion of this heat treatment is not exceptionally critical, and it may vary either way a moderate extent without detrimental effect. Usually, l find that a holding period of one-half hour is entirely satisfactory, both from the standpoint of efficiency and of insuring adequate solubility of the aluminum, copper and carbon.

I follow the practice when the austenite-formers, particularly nickel, tend towards the low side in the general alloy composition range given, of employing temperasoi tures in the higher end of the range, say from about 1800 F. to 2000 F. to insure the maximum solubility of the copper and aluminum in the austenitic condition, with subsequent transformation of the metal upon quenching. And with a maximum of the austenite-formers the lower temperatures are permissible.

Following heat-treatment within the range of 1600 F. to 2000 F. I quench the steel as in air, oil, or water carrying the same down to about room temperature. This effects a transformation of the austenitic constituent without substantial precipitation of copper, aluminum, or copperor aluminum-compounds. Such steels, thus subjected to a preliminary heat-treatment, thereupon displays a reasonable degree of ductility, and range in hardness from about C28 to C32 Rockwell. The steel is readily formable and machinable and may be fabricated at this point into any of a wide variety of products which are susceptible to subsequent hardening. Illustratively, I convert the annealed and transformed steel into sheet, bars, strip, plates, or wire. And where desired, I fashion the chromium-nickel-aluminum-copper steel into more intricate shapes, illustratively, structural members for airplanes. The use of such steels is particularly advantageous and the fabrication of those parts requiring great strength both in tension and compression is coupled with corrosion-resistance and toughness.

I lind the combination of aluminum and copper as critical additives to the steel to be highly advantageous in this regard; for not only does this combination result in increased workability in the pre-hardened or annealed condition, but as well, I find that they contribute to the achievement of higher strength qualities and a greater degree of hardness when in the precipitation-hardened condition than has heretofore been the case, all with the retention of a required degree of ductility, close adherence to the dimensional tolerances in the substantial absence of heat-scaling and heat-warping, and under comparatively low temperature hardening conditions.

Accordingly, following the annealing heat-treatment and subsequent to fabrication while in the soft condition, I thereupon subject the metal to a comparatively low-temperature precipitation-hardening heat-treatment, this at temperatures ranging from approximately 800 F. to approximately 1l00 F. Any suitable furnace may be employed for this treatment, and illustratively, I use the same furnace that was employed for the preliminary annealing heat-treatment. In this furnace I maintain the heat-treatment for a period of up to say ten hours, although again, this duration is not critical and I find that a duration of treatment from about 3A hour to about 2 hours may be employed advantageously.

Following this heating, the steel is brought to room temperature either quickly or slowly. The precipitation-hardened product displays a Rockwell hardness of approximately C46 to C50, and an ultimate tensile strength of about 215,000 pounds per square inch. The original dimensions are retained within the close tolerances permitted, and the surfaces are found unaffected y by heat; that is without scaling, and without distortion. 60

Thus, I have effectively combined ready working in a soft, pre-hardened condition with final hardnesses and physical strengths of exceptionally important values.

Apparently my treatment serves to precipitate the aluminum and the copper through the metal and thus imparts a material gain in hardness and strength. While it is reasonably well established that these metals are precipitated during the hardening treatment, it is not fully clear as to the exact changes which take place. Certain it is that no appreciable or recognizable changes in volume of the metal are to be noted during the hardening action. Perhaps there occurs a rearrangement or ordering of precipitated aluminum-nickel and coppernickel compounds or maybe an aluminum-copper-nickel compound, within the lattice structure of the matrix, and which thereafter interposes an interference hardening effect. l have not, however, fully confirmed this theory, and accordingly do not desire to be bound by the suggested explanation.

illustrating certain properties of a chromium-nickel stainless steel provided and treated in accordance with my invention, I refer to the following table, presenting physical values experimentally determined for a steel analyzing about 16.73% chromium, 4.00% nickel,

. 3.56% copper, 0.91% aluminum, and 0.069% carbon,

and the remainder substantially all iron, there being TABLE I Physical properties of chromium-nickel stainless steel containing copper and aluminum 0.2 Percent Yield Str., p. s. l.

Ult. Tens. Str., p. s. i.

Rockwell Hardness Percent Elong. in 2l] Percent Condition Red. Area Annealed 155, 000 Hardened 19o, ooo

It should be noted that in the stainless steel of my invention, the aluminum and copper are closely correlated with the other elements in the steel, and particularly with respect to the nickel. Moreover, the nickel in my new steel is present in relatively small amounts, a feature advantageous from a cost standpoint. This critical proportioning of ingredients give a commercially valuable precipitation-hardenable steel, which becomes effectively hardened upon proper treatment from the soft condition, in which it either has been worked or is ready for working. Such steels are fully as soft, and in some instances softer and more workable in the pre-hardened condition, than are similar steels containing either copper or aluminum alone as the precipitation-hardening element.

It has been suggested hereinbefore that columbium and titanium may advantageously be added to the cornposition of steel, this likewise in critically proportioned amounts. Illustratively, columbium may be included up to about 8 times the carbon content thereof. I find that this quantity of columbium and titanium suppress the tendency of the carbon to occasion any substantial chromium loss at the grain boundaries during welding and other processes carried out at detrimentally critical temperatures. Any such loss of chromium not only decreases the corrosion-resistant qualities of the steel, but at the same time, results in some embrittlement of the metal with consequent loss of strength.

The metal tantalum, to a certain extent, is akin to columbium and l find that as a practical matter, tantalum in part may be substituted for columbium and titanium.

It is noteworthy in connection with the columbium, titanium and tantalum additives, that the precipitation of carbides from the matrix at about 1400 F. is not in itself harmful, probably because at this temperature the chromium is suticiently active that it diffuses fast enough to replenish those regions depleted by carbide formation. At lower temperatures, however, say 1100 F. to 1200 F., such carbide precipitation is harmful, in the manner pointed out hereinbefore, the chromium apparently being so sluggish at these lower temperatures that it will not remedy the loss of chromium to the carbide. Any harmful formation of carbides is averted by the addition of any one of columbium, titanium and tantalum. More than this, it appears that these additives, although not in sufficient amounts to detract from the working or forming properties in the annealed condition, actually attribute something toward the physical properties of the steel in the precipitation-hardened condition, particularly in matters of strength.

Thus, my invention provides a stainless steel suited to low temperature hardening treatment, and as well, a method of precipitation-hardening such steels, in which the manifold objects hereinbefore noted are successfully achieved, along with many thoroughly practical advantages. In this pre-hardened condition, the steel is fully as workable, if not more so, than has heretofore been the case with prior precipitation-hardening stainless steels, permitting any one or more of a number of machining, forming or fabricating operations. to close dimensional tolerances. These products can thereupon be precipitation-hardened, at low temperatures, without substantial warping, discoloring, scaling or variance in such dimensional tolerances through heat effects. Moreover, the nished products display high yield and ultimate strengths, and are comparatively free from directional qualities.

It is apparent from the foregoing that many possible embodiments may be made of the basic teachings of my invention, and that as well, many changes may be made in the embodiment hereinbefore set forth. Accordingly, it is to be understood that the foregoing disclosure is to be interpreted as illustrative, and not by way of limitation.

I claim as my invention:

1. A chromium-nickel-aluminum-copper stainless steel susceptible to annealing and quenching through phase transformation to a substantially fully aluminum-soluble and copper-soluble condition, and thereafter to precipitation-hardening by single heat-treatment to a Rockwell hardness of at least about C46, said steel consisting essentially of chromium and nickel in amounts substantially in accordance with the area ABCD in the accompanying diagram, carbon up to about 0.15% maximum, about 0.80% to 1.75% aluminum, about 2.00% to about 5.00% copper, both manganese and silicon from incidental amounts up to approximately 2.00% maximum of each, and the remainder substantially all iron.

2. A chromium-nickel stainless steel having precipitation-hardenable properties by single heat-treatment to a Rockwell hardness of at least about C46, and consisting essentially of about 16.00% to 17.00% chromium, about 3.50% to about 5.00% nickel, copper ranging from about 3.00% to about 4.00%, aluminum about 0.85% to about 1.10%, carbon not exceeding about 0.15%, and the remainder substantially all iron.

3. A chromium-nickel stainless steel, having precipitation-hardenable properties by single heat-treatment to a Rockwell hardness of at least about C46, and consisting essentially of about 16.73% chromium, about 4.00% nickel, about 3.56% copper, about 0.91% aluminum, and about 0.069% carbon, and the remainder substantially all iron.

4. In the production of precipitation-hardened chromiuIn-nickel stainless steel, the art which includes providing a steel consisting essentially of about 16.00% to 17.00% chromium, about 3.50% to 5.00% nickel, about 3.00% to 4.00% copper, about 0.85% to 1.10% aluminum, and the remainder substantially all iron; then annealing the steel at about 1600 F. to 2000 F. and quenching the same to achieve phase transformation and a substantially fully aluminum-copper-carbonsoluble structure; and thereafter heating the transformed metal between about 800 F. and 1100 F. for a suticient period of time to precipitate the aluminum and copper, thereby achieving a Rockwell hardness of at least about C46.

5. In the production of precipitation-hardened chromium-nickel stainless steel articles, the art which includes providing a steel consisting essentially of about 16.00% to 17.00% chromium, about 3.50% to 5.00% nickel, about 3.00% to 4.00% copper, about 0.85% to 1.10% aluminum, and the remainder substantially all iron; then annealing the steel at about 1600 F. to 2000" F. and quenching the same to achieve phase transformation and a substantially fully aluminumcop per-carbon-soluble structure; fabricating the transformed steel into the desired articles; and thereafter heating the fabricated articles between about 800 F. and about 1100 F. for a sutlicient period of time to achieve a Rockwell hardness of at least about C46.

6. A precipitation-hardened-chromium-nickel stainless steel consisting essentially of about 16.00% to 17.00% chromium, about 3.50% to 5.00% 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, said aluminum and copper both being iinely precipitated as compounds within the matrix of the steel to impart a Rockwell hardness of at least about C46.

7. A precipitation-hardened chromium-nickel stainless steel having a Rockwell hardness of at least about C46 and consisting essentially of approximately 16.00% to about 17.00% chromium, about 3.50% to 5.00% nickel, about 3.00% to 4.00% copper, about 0.85% to 1.10% aluminum, about 0.05% to about 0.08% carbon, columbium and titanium present in amounts about 2,694,626 '7 '8 eight timesand about five times, respectively, said car- OTHER REFERENCES bon content, and the remainder substantially all iron.

Dispersion Hardening of Alloys of Nickel and Iron- References Cited in the le of this patent 5 NicllcelTitanium. Paper presented before the 21st Annua onvention of the American Society for Metals UNITED STATES PATENTS at Chicago, ocr. 23 to 27, 1939. Edited by Piuing and Number Name Date Talbot.

1,943,595 Foley Ian. 16, 1934 2,240,064 Allen Apr. 29, 1941 2,482,097 Clarke Sept. 20, 1949 10

Claims (1)

1. A CHROMIUM-NICKEL-ALUMIUM-COPPER STAINLESS STEEL SUSCEPTIBLE TO ANNEALING AND QUENCHING THROUGH PHASE TRANSFORMATION TO A SUBSTANTIALLY FULLY ALUMINUM-SOLUBLE AND COPPER-SOLUBLE CONDITION, AND THEREAFTER TO PRECIPITATION-HARDENING BY SINGLE HEAT-TEATMENT TO A ROCKWELL HARDNESS OF AT LEAST ABOUT C46, SAID STEEL CONSISTING ESSENTIALLY OF CHROMIUM AND NICKEL IN AMOUNTS SUBSTANTIALLY IN ACCORDANCE WITH THE AREA ABCD IN THE ACCOMPANYING DIAGRAM, CARBON UP TO ABOUT 0.15% MAXIMUM, ABOUT 0.80% TO 1.75% ALUMINUM, ABOUT 2.00% TO ABOUT 5.00% COPPER, BOTH MANGANESE AND SILICON FROM INCIDENTAL AMOUNTS UP TO APPROXIMATELY 2.00% MAXIMUM OF EACH, AND THE REMAINDER SUBSTANTIALLY ALL IRON.

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