US3253966A - Stainless steel - Google Patents

Description

United States Patent 3,253,966 STAINLESS STEEL Frank A. Malagari, Jr., Freeport, and Adolph J. Lena,

This invention relates to stainless steel having a critical range of composition which is characterized by exhibiting a high degree of nondirectionality when utilized in its cold worked condition.

The ever-expanding market for stainless steels has witnessed the use of these steels in various structural components, especially Where a high degree of corrosion resistance is necessary together with the ability to withstand cyclical loads. In general, it is known that stainless steels in use today are those referred to as the austenitic stainless steels and those steels referred to as ferritic or martensitic and various hybrids thereof. Typical of the former class is the group of steels designated as the AISI Type 300 series, for example, Type 301. ample of the latter class is the martensitic stainless steels typified by the AISI Type 410 composition or the territic steels as typified by the AISI Type 430 composition. Each of these various types of compositions is dependent on the chemical composition for the microstructure exhibited by the steel and the microstructure in turn will have an influence on the mechanical properties exhibited by the steel. It is also well known that most of the austenitic-type stainless steels are stabilized to such a degree that they will not transform from the metastable austenite to martensite with a consequent increase in strength. Therefore, the only practical means of strengthening these types of austenitic stainless steels, as distinguished from those of the transformation hardening type, for example, those steels known as the semi-austenitic stainless steels, an example of which is AM-355, is dependent upon an operation which will work harden the steel to obtain a designated temper to thereby increase their strength characteristics. Typically, AISI Type 301, an austenitic stainless steel, is work hardened and marketed carrying a temper designation of A hard, /2 hard, full hard, extra full hard, etc. These temper designations Anex-- refer to a degree of cold working which has been applied to the steel and, consequently, also relate to the strength level exhibited by the steel to which the various temper designations are applied. These steels as thus marketed, characteristically exhibit excellent corrosion resistance and strength characteristics, thus making them useful in a myriad of applications including use as structural components. i

It is to be noted, however, that there is one serious "ice of stainless steel in an application which is subjected to a cyclical loading involving both tension and compression, or when used under compressive loading alone, the design criteria must be based on the longitudinal compressive yield strength. The yield strength, when measured transversely to the direction of cold working, does not show any great divergence whether such yield strength is measured in tension or compression. The hiatus which occurs between the tensile yield strength and the compressive yield strength, each measured in the longitudinal direction, can seriously detract from the use of this steel.

The stainless steel of the present invention, with its controlled compositional range, produces a close correlation between the tensile yield strength and the compressive yield strength, each measured in the longitudinal direction after the steel lias been cold worked to effect a reduction in the cross sectional area of between about 10% and about 40%. The steel, as thus cold worked, will exhibit a ratio of compressive yield strength to tensile strength, each measured in the longitudinal direction, of greater than about .8.

An object of the present invention is to provide a stainless steel which is suitable for use in application requiring corrosion resistance and strength.

Another object of the present invention is to provide a stainless steel which, after cold working up to 40%, will have a close correlation between the tensile yield strength andcompressive yield strength when each is measured in the longitudinal direction.

Another object of the present invention is to provide a stainless steel having a critically controlled composition and which is subjected to cold working in order to increase the strength characteristics of the steel without imparting a high degree of directionality thereto.

A more specific object of the present invention is to provide a stainless steel having a controlled chemical composition and which, after being subjected to a reduction in the cross sectional area of between about 10% and 40%, will exhibit a ratio of compressive yield strength to tensile yield strength measured in the longitudinal direction of greater than .8.

Other objects of the present invention will become apparent to one skilled in the art when taken in conjunction with the following description and claims.

The steel of the present invention contemplates a critical composition. This critical composition requires carbon within the range between 0.08% and 0.12%, up to about 1.25% manganese, up to about 1% silicon, from about 15.3% to about 16.6% chromium, from about 5.5% to about 6.2% nickel, from about 0.025% to about 0.06% nitrogen, and the balance substatnially iron with incidental impurities. Each of the elements performs the usual function normally associated wit-h'the element in austenitic-type stainless steels. In this respect, the balancing of the carbon and nitrogen and nickel is exceedingly critical as respects the stability of the microstructure of the steel. In this same vein, it is noted that the chromium content increases the stability of the steel and thereby is influential in decreasing the work hardening rate thereby affecting the sluggishness of the austenite to martensite transformation. This occurs despite the fact that chromium is a strong ferrite-forming element and the use thereof requires a critical balancing of the austenite-forming elements such as carbon, nitrogen and nickel.

It is also noted that molybdenum cannot be used as a direct substitute for chromium on a 1:1 ratio basis in the steel of the present invention. While molybdenum, a strong ferrite-forming element, is effective for increasing the yield strength and the hardness of the steel in the annealed condition, nonetheless, it is apparently effective when used in a 1:1 ratio for decreasing the stability of the steel, thus permitting the steel to transform to martensite With a consequential effect that the steel exhibits a high degree ofdirectionality. While nickel materially contributes to the stability of the alloy, it must be limited together with the carbon and nitrogen in order to maintain the proper degree of balance.

In order to more clearly define the limits of the steel of the present invention reference may be had to Table I which tabulates the general and optimum ranges of composit-ion.

TABLE I.OHEMICAL COMPOSITION (PERCENT BY WT.)

Element General Range Optimum Range About 0.1. .0.

As was stated hereinbefore, AISI Type 300 series stainless steels, for example, Type 301, derive enhanced mechanical properties through the application of specific amounts of cold Work. It has been postulated that cold Work effects a Work hardening of these stainless steels, the strengthening occurring through the work hardening of the face centered cubic structure of the austenite, the transformation of austenite to martensite and the further work hardening of the martensite. The amount of transformation which can occur as a result of cold working is dependent upon the chemical composition and should be greatest in those austenitic stainless steels which contain the lowest amount of alloying components. Thus, it has been observed that for the AISI Type 300 stainless steels, Type 301, which contains the lowest amount of alloying components and which has a nominal composition of about 17% chromium and 7% nickel, exhibits the highest work hardening rate and, in addition, this steel also exhibits a transformation product when sufficiently cold worked. This steel can be cold worked to develop desirable high strengths for structuralapplications. Where the amount of the alloying elements increases, the austenite may attain such a high degree of stability that the only strengthening possible is the work hardening of the face centered cubic lattice structure of the austenite. Further cold working is insufficient to impart sufiicient energy to transform the metastable austenite to martensite. As noted previously, AISI Type 301 stainless steel in the cold worked condition exhibits a high degree of directionality when these steels are tested in the direction which is longitudinal to the direction of the cold working. In particular, this directionality is manifested by a lower compressive yield strength than the tensile yield strength when measured in the longitudinal direction. This discrepancy, however, is not noted when the same cold worked steel is tested in tension and in compression in a direction transverse to the direction of cold working. Thus, in Type 301 having a temper designation of full hard, the longitudinal tensile yield strength may be 140K s.i. but the .taining less than 50% ferromagnetic component.

longitudinal compressive yield strength may only be K s.i., yet the transverse tensile and compressive yield strengths are equivalent and of a magnitude similar to the longitudinal tensile yield strength. While the directionality of this cold worked austenitic stainless steel can be minimized or eliminated in some instances by a sub-critical heat treatment, other adverse effects are noted in the measured mechanical properties of the steel when so-heat treated. This is particularly true with respect to the ductility exhibited by the alloy. While the steel of the present invention contemplates a usage where strength criteria are of paramount importance, it becomes necessary for the steel to exhibit a yield strength measured in the longitudinal direction both in tension and compression of K s.i. minimum after the steel has been cold worked sufficiently to effect a reduction in the cross sectional area ranging between about 30% and about 40%.

In order to more clearly illustrate the critical nature of the steel of the present invention, reference is specifically directed to Table II which sets forth the chemical composition of a series of steels which were made and tested to illustrate the critical nature of the chemical com position.

TABLE II.-OHEMICAL ANALYSES Heat 0 Mn Si Cr Ni N Other 10 99 64 16. 3 6.0 03 .09 .85 .62 16.2 5.4 .04 10 83 60 16.6 5. 8 .04 12 77 53 16.3 5. 5 04 16 82 61 16.1 5. 4 04 19 .84 56 16.3 5. 4 04 09 78 57 15. 6 6.0 04 .10 .77 54 15.8 6.0 .04 10 82 60 16.8 6.0 05 10 85 67 17. 9 5. 8 04 09 87 61 14. 6 6.2 04 1.0 M0. 09 86 56 13. 4 6. 1 04 1.6 M0. 11 88 58 12. 4 6. 2 04 2.0 M0. 09 83 60 11.5 6. 2 04 2.6 M0. 10 82 56 16.2 5. 5 O5 11 82 61 16.7 6. 4 .04 10 80 .69 16. 7 6. 8 04 08 97 72 16. 5 6. l 03 09 93 67 15. 6 5. 5 05 1O .90 69 15.3 6.0 .06 10 99 73 16, 4 6. 2 03 09 91 .68 16.6 5. 5 03 are set forth hereinafter in Table III.

It is to be noted in Table III that one column is headed percent Mag. Res. This refers to the percent of magnetic response exhibited 'by each of the steels set forth in Table III as measured by the Magne-gage. As is well known, the Magne-gage measures the amount of ferromagnetic component in the steel and its accuracy is limited to a steel having an austenitic structure and con- Consequently, in Table III where the magnetic response is set forth therein as 50 it will be understood that this includes amounts greater than 50, since the magnetic response as measured by the Magne-gage is only accurate where up to a 50% maximum ferromagnetic component is present within the steel.

TABLE III.EFFECT OF COLD WORK ON MECHANICAL PRPERTIESContinued Tension Compres- Percent Percent sion, C.Y.S. Heat Cold Hardness Mag. 2% Y.S. T.Y.S.

Reduction Res. 2% Y S Ultimate Percent (K. s.i.)

(K. s 1 Str. (K. s.i.) El. 2"

FL-95 85 R 43 38. 2 152. 2 33. 39. 5 1. 03 5 38 R0--- 50 94. 2 190. 4 17. 5 91. 3 96 FL-88 0 27.5 R 50 37. 2 192. 2 19.0 53. 8 1. 44 5 44.5 R 50 153. 5 216.0 11.5 170. 2 1.10

10 46 Ru- 50 150. 9 218. 7 11. 5 166. 0 l. 09

FO-49 0 88.5 R 28-34 40. 3 166. 4 33. 5 48. 3 1. l9 5 43 Ru- 50 115. 7 210. 7 19. 0 113. 3 97 20 47.5 RP... 50 176.0 227. 2 l3. 0 158. 3 89 FL-96 0 86.5 R 4. 2 33. 7 152. 0 35.0 39.2 1. 16 5 38.5 Ba... 50 84.3 195. 9 21. 5 84. 8 1. 00

Fla-99 0 92 Rb--- 36 39. '4 174. 4 36. 0 43. 5 1.10 5 42 Re. 50 100. 1 210. 9 18. 5 109. 9 1.09

From the test results recorded in Table III for Heats FC-4, FE-26, FB-95, FB-93 and FB-92, it is apparent that increasing the carbon content is highly effective for stabilizing the austenite of the steel. Thus, in the annealed condition, while Heat FC-4 appears to be magnetic for work hardening the martensite a sufiicient degree to induce highly directional mechanical properties. This is substantiated by the fact that the Magne-gage readings, which only measure up to 50% ferromagnetic component, indicate a greater than 50% magnetic response throughout the working history of the heat. A comparison of the mechanical properties illustrates that with as little as 10% cold reduction, the ratio of the compressive yield strength to the tensile yield strength (each measured in the longitudinal direction ofthe cold reduction) effectively drops below .8 thus indicating the highly directional nature of the steel. While it appears that substantially all of the alloying elements except the nickel are within the range set forth in Table I it is clear that this deficiency in the nickel content in Heat FC-4, is suflicient to make the steel sufficiently unstable that the cold working is effective for inducing a high degree of directionality to this steel.

Reference is directed to Heat FA-22 which has a composition within the limits of the preferred range as set forth hereinbefore in Table I. This steel, in the annealed condition, exhibits a 40% magnetic response as measured by the Magne-gage and a ratio of compressive yield strength to tensile yield strength of greater than about .8. Cold working the subject steel various amounts up to about 40% is effective for transforming additional amounts of austenite to martensite so that the Magne-gage readings indicated a greater than 50% magnetic response after cold working only 5%. When this steel was tested both in tension and compression, it was noted that throughout the working range, i.e. up until 40% cold work was performed on the steel, the steel exhibited a ratio of compressive yield strength to tensile yield strength of greater than about .8 when measured in the longitudinal direction of cold working. While the steel had this excellent non-directionality, it also exhibited a compressive yield strength of about 209.9K s.i. and a tensile yield strength of about 241.9K s.i., after a 40% reduction in the cross sectional area thus indicating the high level of mechanical properties exhibited by this steel through the close control of the chemical composition, to be explained more fully hereinafter.

The magnetic saturation level in these steels is a function of cold work. Consequently, the magnetic saturation is indicative of the amount of the ferromagnetic component present within the steel. When the magnetic saturation was measured in the annealed condition on Heat FC-4 preparatory to any cold working, the steel had a saturation induction of about 500 gausses at a magnetization level in the range between 200 and 4000 oersted. After cold reduction as much as 40% saturation induction merely increased to a value of less than about 13,000 gausses. Thus, the low slope of the curve clearly illustrates very small change in the ferromagnetic component of this steel. This further substantiates the proposition that substantially all of the cold work performed on this steel was effective for work hardening the martensitic constituent.

Reference to Heat FE26, which has a composition within the limits set forth in Table I, clearly illustrates the structure sensitivity of the steel. Thus, the steel had an initial magnetic response of about 25.6% and it required a working of up to about 10% in order to obtain a magnetic response of greater than 50%. Magnetic saturation data illustrated a low magnetic saturation in the annealed condition which progressively increased with the cold reduction. This increase continued until about 40% reduction was applied to the steel after which the magnetic saturation showed little increase. Measurement of the tensile properties on this heat which are set forth in Table III, clearly illustrate that cold working within the range between about 10% and about 40%, is effective for increasing the mechanical properties without adversely affecting the ratio of the compressive yield strength to the tensile yield strength. This ratio was maintained at greater than .8 throughout the working range and clearly illustrates that with as much as 30% cold reduction extremely high yield strengths are obtained when measured both in compression and tension. These yield strengths are accomplished by adequate ductilities of about 8.5%. Thus the increase in the nickel and the slight increase in the carbon content was effective for controlling the rate of the austenite to martensite transformation and the subsequent work-hardening of the martensitic phase without inducing highly directional characteristics to the excellent level of mechanical properties exhibited by the steel.

Heat FB-95 also illustrates the critical nature of the chemical composition of the steel of the present invention. The chemical analysis of this steel shows a carbon content of 0.12% and a nickel content of 5.5 such carbon content being at the upper limit and the nickel content at about the lower limit. In the annealed condition, this steel exhibited about a 20.2% magnetic response when measured on the Magne-gag'e. After the application of as little as cold work, the ferromagnetic component of the steel increased sufficiently so that the Magne-gage readings indicated a greater than 50% response. This steel was cold worked various amounts to effect a reduction in the cross sectional area of up to 40%. This cold working was not effective for conferring directionality to the steel since the ratio of the compressive yield strength to the tensile yield strength, each measured in the longitudinal direction, was greater than about .8. Thus, it is clear that the steel of this composition is on the borderline and clearly demonstrates the critical nature of the chemical composition. Increasing the carbon content to 0.16% and 0.19% as in Heats FB-93 and FB92, clearly illustrates the effect of the carbon in stabilizing the steel. Thus, from Magne-gage readings it is apparent that it requires reductions of greater than in order to obtain greater than a 50% response for Heat PB93. Heat FB- 92 requires greater than cold reduction in order to attain a greater than 50% magnetic response. The mechanical properties set forth in Table III clearly illustrate the highly directional nature of these steels. Magnetic saturation measurements also' effectively illustrate the stability of the steel. Thus, it is clear that with a small variation in the carbon content the stability of the steel can be changed sufiiciently so that the steel will exhiibt a high degree of directionality when cold worked up to about 40%. This indicates that the chemical composition of the steel of the present invention must be critically controlled in order to obtain a steel having a ratio of the compressive yield strength to the tensile yield strength, each measured in the longitudinal direction of greater than about .8. Variation in the carbon content which affects the stability of the steel, also affects the rate of work-hardening so that carbon contents outside the limits set forth in Table I affect the transformation of the austenite to martensite during cold working with the result that the steel will exhibit directional properties. The curves from the magnetic saturation data confirm this.

The effect of a variation in the chromium content is illustrated by Heats FE25, FD-Sl, FD-86 and FE-30. These heats have a chromium content varying between 15.6% and about 17.9%. From the test results recorded in Table III it is clear that both Heats FE- and FD81 show non-directional characteristics when they are cold worked up to about The magnetic response measurements indicate that Heat FE-25 has a greater than 50% magnetic response in the annealed condition. Increasing the chromium -contenta strong ferrite-forming elementis effective for increasing the austenite stability and, as a result thereof, the steel of Heat FD-Sl in the annealed condition shows a magnetic response. When these same heats are tested for their mechanical properties it is apparent that up to a 40% cold reduction can be applied to these heats, while still maintaining a ratio compressive yield strength to tensile yield strength of greater than about .8. Thus, through the controlled chemical composition of these heats, it being noted that all of the alloying elements are within the range set forth in Table I, an outstanding combination of mechanical properties is readily obtained which exhibit the desired degree of non-directionality. Further increases of the chromium content as in Heats FD-86 and FE-30 are effective for stabilizing the steel so that a greater than 10% cold reduction is necessary in order to obt ain'a greater than magnetic response as measured by the 10 Magne-gage. Magnetic saturation measurements clearly indicate that the saturation induction decreases with increasing chromium and the steels apparently have not attained the highest degree of saturation induction with cold working up to 40%. Consequently, it is apparent that the steels FD-86 and FE-30 have an insufficient amount of transformation product. As a result thereof these steels show considerable directionality which is apparently caused by the stability of the work hardened austenite through the addition of these higher amounts of chromium. Thus, the ratio of the compressive yield strength to the tensile yield strength decreases below about .8 with the result that the steels show an increase in the directionality of the properties when the composition thereof is increased as respects the chromium content to a value in excess of about 16.6%. Replacement of part of the chromium with molybdenum (and in this respect, chromium contents partially substituted by molybdenum, show substantially similar results as will be set forth hereinafter.

Reference is now directed to Heats FB-90, FB-88, FB-87 and FB-86, in which the compositional dependence of the partially substituted chromium content is illustrated in this series of steels. The chromium content was varied through substitution of molybdenum, the substitution being in a ratio of about 1:1. At first glance, it is apparent that the effect of molybdenum in each of ,these steels is to provide steel with a high degree of instability thereby resulting in an early transformation of austenite to martensite with the result that, effectively, a single-phase alloy is subjected to the cold working and thus exhibits a high degree of directionality. Thus, after a cold reduction of up to 10%, these steels exhibit highly directional characteristics wherein the compressive yield strength to tensile yield strength ratio is less than about .8. The level of the yield strength in the annealed condition of each of these steels is high which is accompanied by high hardness. Magnetic sauration measurements reveal that increasing the molybdenum content at the expense of the chromium, results in a successively higher saturation induction of these steels in the annealed condition. Moreover, the slope of the curve with respect to cold reductions indicates that very little transformation is taking place. Consequently, all of the cold working is apparently accomplished on the substantially single-phase steels (probably martensitic in character), resulting in a high degree of directionality being imparted thereto. The data set forth in Table III indicates thatincreasing the molybdenum content also promotes higher room temperature strengths with increasing cold work. From these data it appears that molybdenum cannot be substituted for a portion of the chromium on a 1:1 basis at this level of chromium.

Heats FD-82, FD-lOO and FE-29 clearly show the effect of nickel on the steel of the present invention. Thus, an increase in the nickel content of from about 5.5% up to about 6.8% clearly illustrates the adverse effect on the directionality of the properties of these steels which is produced by increasing the nickel content to an amount in excess of about 6.2%. Thus, Heat FD-82 having a nickel content of about 5.5 and which had a 40% magnetic response in the annealed condition at room temperature, had a ratio of compressive yield strength to tensile yield strength in excess of about .8 after cold working up to 40%. On the other hand, Heat FD-lOO which had a nickel content of 6.4% and Heat FE-29 which had a nickel content of 6.8%, indicate that these amounts of nickel are effective for conferring a high degree of stability to the steel. This is shown by the percent magnetic response and was confirmed by the magnetic saturation measurements. Thus, the test results set forth in Table III clearly illustrate that increasing the nickel content to more than about 6.2% is effec tive for inducing a high amount of directionality to the 1 1 steel. With as little as 5% cold reduction, these high nickel-containing heats will usually have a ratio of com pressive yield strength to tensile yield strength of less than .8.

The test results set forth in Table HI for Heat FL-95 clearly illustrate the effect of the extreme ends of the range as respects the carbon and nitrogen contents. It is noted that the nickel and chromium contents are near the upper limit in this steel in order to obtain the proper degree of balance in the chemical composition. As annealed, the steel had a magnetic response of 43% as measured by the Magne-gage and was in a relatively soft condition, the hardness being measured at 85 R This steel after cold working up to 40%, had a compressive yield strength to tensile yield strength ratio of greater than .8. Magnetic saturation measurements indicated a low saturation of the steel prior to cold working; thereafter, the slope of the curve indicates an initial rapid rise in the magnetic saturation followed by decrease in the rate of saturation, yet the level increases for cold reductions of up to about 40%. Thus, it is clear that at least 0.08% carbon and 0.025% nitrogen are necessary in the steel of the present invention in order to obtain an adequate strength level without adversely affecting the ratio of the compressive yield strength to tensile yield strength.

Heat FL-88 clearly shows the effect of chromium and nickel near the lower limit where the carbon and nitrogen are near the mid-point of the range set forth in Table I. Cold working the steel of Heat FL-88 to effect re-' ductions of up to about 40%, increased the strength levels yet the ratio of compressive to tensile yield strengths exceeds .8. While the initial magnetic saturation was higher than that of Heat FL-95 as would be expected since Magne-gage readings indicated Heat FL-88 possessed a greater than 50% magnetic response, yet the slope and the shape of the curves are quite similar, the only difference being a displacement upwardly for the latter heat as compared to Heat FL-95.

Reference to the test results recorded in Table III for Heat FO-49 clearly illustrates the effect of chromium near the lower limit of the General Range. Thus, with a chromium content of about 15.3% it is noted that cold reductions of up to about 40% are effective for showing an outstanding increase in the attainable mechanical properties and without any directionality occurring in this steel. The compressive yield strength to tensile yield strength ratio exceeds .8 and magnetic saturation measurements indicate :a curve substantially similar to that for Heats FL-95 and FL-88.

Heat FL-96 illustrates the effect of a nickel content of 6.2% which approaches the upper limit of the nickel range. This steel showed an initial magnetic response of 4.2% tested with a Magne-gage. Magnetic saturation measurements when plotted against the percent cold reduction indicate that for a small amount of cold work there is a fast rise in the magnetic saturation. This would correspond to the amount of ferromagnetic material present and indicates the relative degree of stability of the steel. Thus, as would be expected, while this steel is slightly more stable than that of Heat FL-95 and FL- 88, it quickly transforms to martensite with small amounts of cold work as shown by the slope of the magnetic saturation curve as well as the percentage magnetic response measured by the Magne-gage readings. Cold working up to 40% is effective for increasing the level of the mechanical properties and does not adversely affect the non-directionality of the steel, it being noted that this steel had a compressive yield strength to tensile yield strength ratio of greater than .8 after as much as 40% cold reduction. Thus, it would appear that the nickel content should not exceed about 6.2% in the steel of the present invention.

Heat FL-99 which contains a chromium content at the upper limit, anickel content near the lower limit and a nitrogen content near the lower limit, also favorably responds to produce outstanding mechanical properties which are non-directional. While this steel had an initial magnetic response of 36% as measured by the Magnegage, cold working as little as 5% was sufficient to provide a greater than 50% response as measured by the Magnegage. Magnetic saturation measurements follow the same general pattern as that for Heats FL9-5 and FL-88. After cold working up to 40% the compressive yield strength to tensile yield strength ratio exceeded .8, thus illustrating the non-directionality of this steel.

From the foregoing, it is apparent that the steel of the present invention must be critically controlled within the limits set forth hereinbefore in Table I. Variations from the ranges set forth in Table I apparently cause a variation in the stability of the steel which will affect the rate of transformation as well as the rate of work-hardening of the steel. By controlling the stability of the steel it is possible to control the directionality thereof so that when these steels are cold worked up to 40%, it is possible to obtain a minimum compressive yield strength of K s.i. and a minimum tensile yield strength of K s.i., these levels being achieved without incurring directionality to the steel such that the compressive yield strength to tensile yield strength ratio will exceed about .8.

It is to be noted from the data set forth in Table III that the steels falling within the scope of the subject invention which are cold worked from about 30% to 40% may, in some instances, exhibit what appears to be a ductility which in some applications may be considered to be on the low side. While the amount of ductility itself is not objectionable, said design criteria may require that the steel have or exhibit a higher ductility without seriously adversely affecting the level of the mechanical properties as well as ductility. The ductility exhibited bythe steel of the present invention can be improved by the application of a sub-critical anneal or a stress relief anneal which consists of heating the steel to a temperature within the range between about 750 F. and 900 F. for a time period ranging between about 1 and about 16 hours. This stress relief anneal will have the effect ofincreasing the ductility as measured by the percentage elongation without adversely affecting the ratio of the compressive yield'strength to the tensile yield strength. While some drop is noted in the attainable level of the tensile yield strength, the drop is relatively minor.

Heat FA-22, having the composition set forth in Table II, was cold worked to effect a reduction in the cross-sectional area of 30%. This steel exhibited a tensile yield strength of 208.6K s.i. and a compressive yield strength of 2044K s.i. or a ratio of compressive to tensile yield strengths of 0.98. The steel also exhibited an elongation of 5.0%. Thereafter, the steel was annealed for 8 hours at a temperature of about 800 F. After heat treatment as set forth hereinabove, this steel exhibited a tensile yield strength of 189K s.i. and a compressive yield strength of 202K s.i. Thus, it will be noted that the compressive yield strength to tensile yield strength ratio is still greater than .8in this instance 1.08. It is also noted that the percentage elongation has increased from 5% to 12% as a result of the stress relief anneal. Thus, it is clear that where higher ductilities are required, the application of a stress relief anneal within the temperature range and within the times set forth hereinbefore is effective for imparting the requisite ductility to these steels. These ductilit-ies are obtained without seriously adversely afiecting the level of the mechanical properties and very little change is noted in the ratio of the compressive yield strength to the tensile yield strength.

The steels of this invention having a composition within the range set forth in Table I are made in the wellknown manners which are common in the stainless steel industry. No difficulty is encountered in hot working the steels and standard rnill equipment is used to supply the cold reductions to the steels of this invention in the forms in which they are used in regular commercial products sold to the industry today.

We claim:

1. A work hardened stainless steel cold reduced between about and about 40% and having a composition consisting essentially of from about 0.08% to about 0.12% carbon, up to about 1.25% manganese, up to about 1% silicon, from about 15.3% to about 16.6% chromium, from about 5.5 to about 6.2% nickel, from about 0.025% to about 0.06% nitrogen, and the balance essentially iron with incidental impurities, said stainless steel being characterized by exhibiting a ratio of compressive yield strength to tensile yield strength each measured in the longitudinal direction of greater than about 0.8.

2. A work hardened stainless steel cold reduced between about and and having a composition essentially consisting of from about 0.08% to about 0.12% carbon, from about 0.5% to about 1.25% manganese, up to about 1% silicon, from about 15.3% to about 16.6% chromium, from about 5.5% to about 6.2% nickel, from about 0.025 to about 0.06% nitrogen, and the balance essentially iron with incidental impurities, said stainless steel being characterized by exhibiting a minimum longitudinal tensile yield strength of 180K s.i. and a minimum longitudinal compressive yield strength of 160K s.i.

3. A work hardened stainless steel cold reduced between about 10% and about 40% and having a composition consisting essentially of about 0.1% carbon from about 0.8% to about 1.0% manganese, from about 0.5% to about 0.75% silicon, from about 15.9% to about 16.4% chromium, from about 5.8% to about 6.0% nickel, from about .03% to about 104%. nitrogen, and the balance essentially iron with incidental impurities, said stainless steel being characterized by exhibiting a ratio of compressive yield strength to tensile yield strength each measured in the longitudinal direction of greater than about 0.8.

4. Work hardened stainless steel sheets, strips, wire, bars and plates cold reduced between 10% and 40% and having a composition essentially consisting of from about 0.08% to about 0.12% carbon, up to about 1.25% manganese, up to about 1% silicon, from about 15.3% to about 16.6% chromium, from about 5.5% to about 6.2% nickel, from about 0.025% to about 0.06% nitrogen, and the balance essentially iron with incidental impurities, said stainless steel being characterized by exhibiting a ratio of compressive yield strength to tensile yield strength each measured in the longitudinal direction of greater than about 0.8.

5. A work hardened stainless steel article of manufacture characterized by exhibiting a ratio of longitudinal compressive yield strength to longitudinal tensile yield strength of greater than about 0.8 after the stainless steel from which said article is formed has been cold worked sufiicient to effect a reduction in the cross sectional area of the steel of between 10% and 40%, a ductility as measured by the percent elongation of at least 10% after the work hardened stainless steel has been stress relief annealed at a temperature within the range of about 750 F. to about 900 F. and a composition consisting essentially of from about 0.08% to about 0.12% carbon, up to about 1.25% manganese, up to about 1% silicon, from about 15.3% to about 16.6% chromium, from about 5.5% to about 6.2% nickel, from about 0.025 to about 0.06% nitrogen, and the balance essentially iron with incidental impurities.

6. A Work hardened stainless steel article of manufacture characterized in that the stainless steel from which said article is formed exhibits a minimum longitudinal tensile yield strength of about 180K s.i. and a minimum longitudinal compressive yield strength of about 160K s.i. after the stainless steel has been cold worked sufiicient- 1y to effect a reduction in the cross sectional area of between about 30% and about 40%, and a composition consisting essentially of from about 0.08% to about 0.12%

carbon, up .to about 1.25% manganese, up to about 1% silicon, from about 15.3% to about 16.6% chromium, from about 5.5 to about 6.2% nickel, from about 0.025 to about 0.06% nitrogen, and the balance essentially iron with incidental impurities.

7. A work hardened stainless steel article of manufacture characterized by exhibiting a ratio of longitudinal compressive yield strength to longitudinal tensile yield strength of greater than about 0.8 after the stainless steel from which said article is formed has been cold Worked sufiicient to effect a reduction in the cross sectional area of the steel of between 10% and 40% a ductility as measured by the percent elongation of at least 10% after the work hardened stainless steel has been stress relief anealed at a temperature in the range of about 750 F. to about 900 F., and a composition consisting essentially of about 0.1% carbon, from about 0.8% to about 1.0% manganese, from about 0.5% to about 0.75% silicon, from about 15.9% to about 16.4% chromium, from about 5.8% to about 6.0% nickel, from about .03% to about .04% nitrogen, and the balance essentially iron with incidental impurities.

8. A work hardened stainless steel cold reduced between about 10% and about 40%, stress relief annealed at a temperature within the range between about 0 F. and about 900 F., and having a composition consisting essentially of from about 0.08% to about 0.12% carbon, up to about 1.25% manganese, up to about 1% silicon, from about 15.3% to about 16.6% chromium, from about 5.5 to about 6.2% nickel, from about 0.025 to 0.06% nitrogen, and the balance essentially iron with incidental impurities, said stainless steel being characterized by exhibiting a ratio of compressive yield strength to tensile yield strength each measured in the longitudinal direction of greater than about 0.8, and a ductility as measured by the percent elongation of greater than about 10%.

9. A work hardened stainless steel cold reduced between about 10% and about 40%, stress relief annealed at a temperature within the range between about 750 F. and about 900 F., and having a-composition consisting essentally of about 0.1% carbon, from about 0.8% to about 1.0% manganese, from about 0.5 to about 0.75% silicon, from about 15.9% to about 16.4% chromium, from about 5.8% to about 6.0% nickel, from about 03% to about 04% nitrogen, and the balance essentially iron with incidental impurities, said stainless steel being characterized by exhibiting a ratio of compressive yield strength each measured in the longitudinal direction of greater than about 0.8, and a ductility as measured by the percent elongation of greater than about 10%.

10. A work hardened stainless steel cold reduced about 30% and having a composition consisting essentially of about 0.1% carbon, about 0.9% manganese, about 0.6% silicon, about 16.4% chromium, about 6% nickel, about 0.03% nitrogen, and the balance essentially iron with incidental impurities, said stainless steel being characterized by exhibiting a ratio of longitudinal compressive yield strength to longitudinal tensile yield strength of greater than about 0.8.

References Cited by the Examiner UNITED STATES PATENTS 1/1958 Waxweiler 75-128.5 9/1959 Waxweiler 75128.5

OTHER REFERENCES DAVID L. RECK, Primary Examiner.

Claims (1)

10. A WORK HARDENED STAINLESS STEEL COLD REDUCED ABOUT 30% AND HAVING A COMPOSITION CONSISTING ESSENTIALLY OF ABOUT 0.1% CARBON, ABOUT 0.9% MANGANESE, ABOUT 0.6% SILICON, ABOUT 16.4% CHROMIUM, ABOUT 6% NICKEL, ABOUT 0.03% NITROGEN, AND THE BALANCE ESSENTIALLY IRON WITH INCIDENTAL IMPURITIES, SAID STAINLESS STEEL BEING CHARACTERIZED BY EXHIBITING A RATIO OF LONGITUDINAL COMPRESSIVE YIELD STRENGTH TO LONGITUDINAL TENSILE YIELD STRENGTH OF GREATER THAN ABOUT 0.8.