US2553707A - Stainless steel spring - Google Patents

Description

EQZOmIO E 0 m m. .t 2 Q E wzamm JMmPw mwmJzzhw m6 mN ad I Zlrwwwbm GEORGE N. GOLLER 'IBMQIN W 9 d l atented May 22 1 951 UNITED EPA'EENT OFFICE STAINLESS STEEL SPRING George Goller, Baltimore, Md, assignor to Armco Steel Corporation, a corporation of Ohio Application January 31, 1947, Serial No. 725,713

3 Claims. 1

This application for patent is a, companion to my application, Serial No. 725,712 of even date herewith entitled Spring and Method, and my copending applications, Serial Nos. 695,215, 695,216 and 695,217 all of September 6, 1949 and all entitled Stainless Steel and Method, now Patents No. 2,505,762, 2,505,763 and 2,505,764 of May 2, 1950, and the present invention relates to stainless steel, more especially to stainless steel products and to a method for producing the same.

An object of my invention is the provision of a reliable and thoroughly practical method for producing stainless steel spring which have great strength and high utility.

Another object of my invention is that of providing a direct and efficient method for achieving a high torsion modulus and other high elastic properties in stainless steel springs with improvement of elastic limit and ultimate strength of the metal.

A further object of my invention is the provision of corrosion-resistant steel springs, and steel products which are useful for making springs, these having high elastic properties inclusive of excellent values in tension and compression, high yield strength and high proportional limit.

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

The invention accordingly consists in the composition Of materials, features of products, in

the several 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 which is indicated in the following claims.

The single figure of the accompanying drawing illustrates the relative proportions of chromium and nickel employed in the stainless steel products and method of my invention.

For a more thorough appreciation of certain features of my invention, it may be noted at this point that springs and other resilient metal products heretofore made of stainless steel such as of the better known 18% chromium-8% nickel steels, have the highly valuable asset of resisting corrosion in any of a great number of circumstances where materials are present which would attack and corrode plain carbon steel springs. This ability for resisting corrosion is one of the beneficial properties of all stainless steels as a class, these steels usually being defined as including 10% to 35% chromium, with or without nickel, and with or without supplemental additions of manganese, silicon, copper, cobalt, molybdenum, titanium, vanadium, columbium, boron, zirconium, sulphur, and the like, for special urposes, and iron substantially the remainder.

Some of the heretofore available stainless steel springs do not satisfactorily respond to hardening heat treatment for developing desired spring properties and thus are put into use in the relatively soft state or in the cold-work-hardened condition. The unhardened springs, however, frequently have a somewhat too low elastic limit and an ultimate strength also too low. There are many instances where the metal of the spring is not amenable to cold working and give rise to cracks and splits which lead to excessive losses in production. In relying exclusively upon coldworking treatment, there are times when the metal does not gain sufficiently high elastic properties, throughout Where desired, as a result of the treatment. The inferior properties of course tend to impose limitations on use.

Any springs which come to a substantially fully hardness in response to annealing and quenching for developing desired elastic properties sometimes develop heat scale and surface pits which for example come about by reason of the high annealing temperatures employed. In the hardened condition the metal is relatively difficult to form, sometimes so difficult that the springs advisedly are fabricated to substantially final shape and dimensions before the high temperature hardening heat treatment. The em'stence of heat scale and pits in the hardened metal usually is not tenable for such reasons as often being responsible for fatigue failure of the springs or poor appearance of the finished products. These surface imperfections, however, are more expensive and time consuming to remove after the metal has been highly hardened, especially should the problem of removal involve the performance of mechanical operations such as grinding, machining, or the like. The problem at times includes that of cleaning up the metal with minimum disturbance of dimensions which are established by fabricating the springs before the high temperature hardening operations.

An outstanding object of my invention accordingly is the provision of physically sound and strong chromium-nickel stainless steel springs of good surface quality, hardened at low temperatures, and having high strength in torsion together with other elastic properties including those of high elastic limit and great ultimate strength.

Referring now more particularly to the practice of my invention, I produce chromium-nickelaluminum stainless steel springs having high elastic properties this by subjecting the metal to conditioning treatment which includes cold Working and low temperature precipitation heat treatment wherein aluminum is precipitated in a critically dispersed fine form giving improvement to the elastic properties. The cold working and precipitation heat treatment are importantly effective at low enough temperatures to avoid the formation of any substantial amount of heat scale or the like on the steel surface.

The springs which I produce are made of stainless steel more particularly containing chromium and nickel in amounts substantially in accord with the abscissa and ordinate of any given point of area ABCD in the accompanying diagram. The steel of the springs further contains anywhere from about 0.02% to 0.12% carbon, aluminum from about 0.50% to 2.50%, from incidental amounts up to about 8.0% manganese, from incidental amounts up to approximately 2.0% silicon, with or without molybdenum ranging up to about 3.0% illustratively to enhance corrosion resistance of the springs, and the remainder substantially all iron. In this, however, should the carbon, aluminum, silicon or manganese content be somewhat different from the amount upon which the accompanying diagram is based (the basis of the diagram being amounts of actual chrominum and nickel as called for, about 0.06% to 0.08% carbon, 0.60% to 1.0% aluminum, incidental amounts of manganese and silicon up to 1.0% each, sulphur and phosphorus up to 0.040% each, and the remainder susbtantially all iron) or should molybdenum be used, I find it preferable to modify the chromium 01' nickel content of the steel, or both of the contents as the case may be. This is to achieve chromium-like or nickel-like components in the steel which are substantially equivalent respectively in ferrite and austeniteforming relation to those amounts of chromium and nickel and of the other elements represented in the diagram. For example, I often replace a part of the chromium called for in the diagram with a quantity of aluminum, silicon or molybdenum, the replacement being approximately on a l to 1 ratio with respect to chromium as to maintain a proper balance between the austenite and ferrite forming components of the steel substantially as would be achieved by strict adherence to the diagrams. Similarly, I occasionally add several or all of the replacement elements in partial substitution for the chromium. The practice of substitution I find is particularly advantageous where either the aluminum or silicon is to exceed about 1.0 or where molybdenum is a constituent of the steel springs. Following substitution, the actual chromium content of the steel may at times be somewhat below those amounts prescribed by area ABCD in the diagram and still, in view of the effect of the substituted element or elements, be in substantial accord with the area.

Should the permissible quantity of carbon exceed about 0.08%, or where more than small amounts of manganese are present (say for example quantities in excess of about 1.0 I usually employ a proportionately decreased quantity of nickel in the steel of the springs as compared with the accompanying diagram. Ihe actual nickel content, however, importantly is not less than about 3.5% after substitution. Where the permissible quantity of carbon is on the low side, i. e. below about 0.06%, I usually employ an increased nickel content in the springs as compared with the diagram, or even at times increase the manganese content. For each part of nickel, I

add about 2 parts manganese or on the order of about /20 to /30 part carbon as the substantial equivalent. The actual nickel content of the steel, after partial substitution 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 effect thereof, be in substantial accord with the area.

A preferred practice of my invention includes the provision of springs from stainless steel containing chromium and nickel substantially in accord with the abscissa and ordinate respectively of any given point falling within the approximate limits of area abcd of the accompanying diagram, the amounts advantageously being about 16.65% to 17.5% actual chromium and approximately 6.75% to 7.6 70 actual nickel as represented by the area. The steel also preferably contains, on the basis of the diagram, aluminum in amounts be tween about 0.60% and 1.0%, carbon within the approximate range of 0.06% to 0.08%, and the remainder substantially all iron. I prefer small amounts of such elements as manganese, silicon, sulphur and phosphorus in this steel, as for example manganese and silicon each not exceeding about 1.0% and sulphur and phosphorus each up to about 0.040%.

In making springs I usually hot work the steel, as by rolling from the form of billets to a satisfactory size for cold working, and subject the steel, for example as rod, wire, plate, sheet or strip, to an annealing treatment before actually effecting the cold-working operations which serve to assist in developing the spring properties. The annealing itself advantageously includes heating the stainless steel at a sufficiently high temperature within the range of about 1300 F. to 2000 F. or more, for a long enough period of time to put the carbon and aluminum in solid solution and accordingly provide a structure which upon being quenched remains substantially fully austenitic. The exact nature of the annealed metal is dependent upon such factors as the relative amounts of austenite-forming constituents which are present.

I find it convenient to achieve the annealing in a suitable heat treating furnace by bringing the steel to annealing temperature and soaking the heated metal at temperature for a short period of time. This treatment serves to put the carbon and aluminum in solid solution, the usual period of soaking being about hour. The length of the treatment time, however, is not critical.

Following the annealing heat, I quench the steel as in air, oil or water to about room temperature, which serves to provide the steel in a soft, substantially fully austenitic annealed condition. The metal is reasonably ductile, has good directional properties and otherwise is susceptible to cold working and forming with satisfactory results. At this point I often resort to pickling or mechanical treatment such as grinding to remove any heat scale from the annealed metal surface.

In producing the springs I subject the stainless steel to cold-working treatment, for example, to at least one of the operations, rolling, drawing, straightening, or forming, and preferably by beginning these operations from the soft condition achieved by the annealing. The use of cold rolling in providing the necessary cold work, sometimes serves the additional function of reducing the metal to gauge which approaches or substantially corresponds to that of a desired finished spring product such as a flat spring. A standard rolling mill frequently is satisfactory for accomplishing the cold-rolling work, as by the use of several passes. Similarly, when relying, at least in part upon cold-drawing for the cold work required, it often is convenient to draw the metal as from rod or wire to substantially finished gauge of a wire or coil spring. Where desired, I use the coldworking treatment for producing cold-formed springs or like products, such as rods, sheet, strip and wire which are readily useful at some later time for making up stainless steel springs or other highly elastic metal products.

My cold-reducing treatment as applied to the steel is sufiiciently intense to give anywhere from a mild to a heavy reduction. In general, the cold reduction amounts to about 5% to 80% or possibly 90% measured in terms of cross-sectional area of such shapes as wire. This same range usually is satisfactory in reducing the area of sheet or strip. A more extensive reduction without first repeating the anneal, frequently results in sharp loss of ductility and also internal fissure or cupping of the metal and consequent fracture.

The diameter or thickness of the stainless steel shape at the outset of cold working has some influence upon the maximum permissible amount of cold work. For wire having a diameter greater than that of approximate 1.4 inch, or sheet or strip exceeding about /4 inch in thickness, I preone hour. The holding time, however, may vary from approximately fifteen minutes to two hours or more, usually without under treatment or over treatment. During the heating, aluminum precipitates in the form of a compound serving to increase the elastic properties. This I believe to be an aluminum-nickel compound.

My stainless steel springs and the like (illustratively wire, sheet or strip, as for making the springs) while heated to the aluminum-precipitated condition, advantageously are given a quenching as in air, oil or water to about usual room temperature. In the quenched condition the stainless steel springs display such elastic properties as high ultimate strength, high elastic limit, and a high torsion modulus. The products are quite resistant to corrosion. They emerge from the cold-working and precipitation heat treatment substantially free of heat scale and unwarped by heat at the lower temperatures employed. I sometimes fabricate the precipitation heat treated wire, sheet, strip, or other suitable products of the steel into springs or for example make the unused metal available for customer use.

In way of illustration a chromium-nickel-aluminum stainless steel wire having a composition falling within the hereinbefore noted general range, and more specifically containing approxifer some 20% (even better to 60% reduc- 39 mately 0.062% carbon, 17.36% chromium, 7.07% tion by cold working, this extending up to about c 9 alllmlnum, a l a u ts of man- 80% for smaller gauges. The small gauge shapes ganese, silicon, sulphur and phosphorus, and the usually develop even somewhat better elastic remainder iron, was found by treatment in acproperties as a result of the working than do cordance with my invention to have the approxthose of heavy gauge. By subjecting any of the imate physical properties presented below in stably austenitic stainless steel compositions to a Table I. All values tabulated for the annealed cold reduction of at least about 50% or the soft condition resulted from annealing the steel at martensitic steels to a reduction of at least 30%, 1950 F. followed by quenching in water and cold the precipitation heat treatment to be described drawing to the amount indicated. The annealed hereinafter serves to develop elastic properties 40 and cold drawn metal provided under the condisimilar to plain carbon steel music wire. The tions just noted then was subjected to precipitastraightening and/or bending operation which at tion heat treatment at about 900 F. for one hour times I employ for effecting the cold work, may and thereafter quenched for obtaining the values be slight or extensive and may replace or conin the table as directed to annealed, cold drawn tribute to other types of cold work. and precipitation treated wire.

' Table I Wire Size Ult.Tens 0.2% Yld. Prop. L mt., Red.ofArea, E1ong.2, Rockwell Per a (inches) Str., p. s. 1. Str., p. s. 1. p. s. 1. Per Cent Per Cent Hardness ANNEALED 1950 F.)+OOLD DRAWING During or preferably following the cold-working treatemnt, I usually give the metal a precipitation heat treatment for further enhancin the elastic properties. This heat treatment includes heating the chromium-nickel-aluminum stainless steel within a temperature range of approximately 700 F. to 1000 F.', preferably to about 900 F., and holding the same at temperature for about The close correlation of aluminum, a comparatively cheap material, with other alloy elements,

inclusive of chromium and nickel, and proper treatment of the steel from a soft workable condition, accordingly gives springs having highly satisfactory elastic properties. It will be appreciated that the aluminum contributes to heat re- 5 sistance and to preventing the formation of heat 7 'scale, as during the precipitation heat treatment and in the spring products after the treatment. Among the specific types of springs which I provide are for example those for precision instruments such as bombsights, speed indicators, weighing scales, and navigation instruments, in which high elastic properties are required for exact and proper readings; gun springs, lock springs, governor springs and the like which are to function while exposed to corrosive atmospheres. In a further illustrative category, I provide stainless steel coil springs or spring-motors, these for operatin While under high stress. A still further illustrative group of my springs and products includes wire, rod, or fiat shapes as in the form of windshield wiper shafts, radio aerials, fishing rods and tackle, and knives illustratively with long blades, which are to be exposed to flexing, pulling, twisting or the like.

At times, for producing the springs, or p-rod ucts such as sheet, strip and wire, I subject the steel to annealing at solution temperature and quenching to the soft annealed austenitic condition as hereinbefore described, and thereafter reheat the metal, this reheat being to with the approximate temperature range of 1200 F. to 1600 F., and preferably to about 1400 F. I hold the steel at temperature for say about hour or more, during which period of time there results a precipitation of carbides (and possibly a mild precipitation of aluminum) and a raising of the transformation point of the austenitic matrix. Then, by quenching the reheated steel as in air, oil or water, I achieve transformation and a preliminary hardening of the metal, this before introducing the cold reducing operations for developing the elastic properties. In particular, the cold reduction to which I subject the transformed steel, usually ranges from a slight amount up to about 60% reduction in cross-sectional area. After the cold reduction I subject the steel to the precipitation heat treatment as hereinbefore described, this advantageously being at a temperature of about 800 F. to 950 F.

and preferably around 900 F. The resulting products have many highly desirable elastic properties, though perhaps somewhat at the sacrifice of the tensile strength achieved where the preliminary hardening heat treatment is omitted from my process.

Thus it will be seen that there is provided in this invention, a method of producing chromium-nickel-aluminum stainless steel springs, and products such as wire, sheet, strip, and the like for use for fabricating into springs, in which the various objects hereinbefore noted together with many thoroughly practical advantages are successfully achieved. It will be observed that the method is capable of giving rapid and enicient production and is well suited for the achievement of highly elastic springs by low temperature treatment of the cold-worked metal. It will also be seen that the resulting products are strong, durable and reliable in any of a great number of uses.

As many possible embodiments may be made 8 of my invention, and as many changes may be" made in the embodiments 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. Cold-worked and precipitation-hardened stainless steel Wire, sheet and strip springs, having a proportional limit of about 40,000 to 230,- 000 p. s. i. or more, said steel containing chromium and nickel in amounts substantially in accordance with area ABCD in the accompanying diagram, carbon between about 0.02% and 0.12%, about 0.50% to 2.50% aluminum, from incidental amounts up to approximately 8.0% manganese, from incidental amounts up to about silicon, and the remainder substantially all iron. and the steel having been subjected to annealing, slight to extensive cold working, and to precipitation heat treatment at about 700 F. to 1000 F. for long enough time to increase the elastic properties.

2. Cold-Worked and precipitation-hardened stainless steel springs having a proportional limit of about 40,000 to 230,000 p. s. i. or more containing chromium and nickel in amounts substantially in accordance with area a bcd in the accompanying diagram, carbon between about 0.02% and 0.12%, about 0.50% to 2.50% aluminum, from incidental amounts up to approximately 8.0% manganese, from incidental amounts up to about 2.0% silicon, and the remainder substantially all iron, the steel having been subjected to annealing, slight to extensive cold working, and to precipitation heat treatment at about 700 F. to 1000 F. for long enough time to increase the elastic properties.

3. Cold-worked and precipitation-hardened stainless steel wire, sheet and strip springs having a proportional limit of about 40,000 to 230,- 000 p. s. i. or more, said steel containing about 0.06% to 0.08% carbon, about 16.65% to 17.5% chromium, from approximately 6.75% to 7.6% nickel, aluminum in amounts between about 0.60% and 1.0%, and the remainder substantially all iron, and the steel having been subjected to annealing, cold working to reduction in crosssectional area of about 5% to 90%, and to precipitation heat treatment at about 700 F. to 1000 for long enough time to increase the elastic properties.

GEORGE N. GOLLER.

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

UNITED STATES PATENTS Number Name Date 1,943,595 Foley Jan. 16, 1934 2,390,023 Wyche Nov. 27, 1945 FOREIGN PATENTS Number Country Date 495,562 Great Britain Nov. 14, 1938

Claims (1)

1. COLD-WORKED AND PRECIPITATION-HARDENED STAINLESS STEEL WIRE, SHEET AND STRIP SPRINGS, HAVIONG A PROPORTIONAL LIMIT OF ABOUT 40,000 TO 230,000 P. S. I. OR MORE, SAID STEEL CONTAINING CHROMIUM AND NICKEL IN AMOUNTS SUBSTANTIALLY IN ACCORDANCE WITH AREA ABCD IN THE ACCOMPANYING DIAGRAM, CARBON BETWEEN ABOUT 0.02% AND 0.12%, ABOUT 0.50% TO 2.50% ALUMINUM, FROM INCIDENTAL AMOUNTS UP TO APPROXIMATELY 8.0% MANGANESE, FROM INCIDENTAL AMOUNTS UP TO

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