US3888119A

US3888119A - Process for cold-working and stress-relieving non-heat hardenable ferritic stainless steels

Info

Publication number: US3888119A
Application number: US434397A
Authority: US
Inventors: Harry Tanczyn
Original assignee: Armco Inc
Current assignee: BALTIMORE SPECIALTY STEELS Corp A CORP OF DE
Priority date: 1974-01-18
Filing date: 1974-01-18
Publication date: 1975-06-10
Anticipated expiration: 1992-06-10

Abstract

A method of cold-working and stress-relieving iron-chromium ferritic stainless steels of non-heat hardenable type in the AISI 400 series, thereby increasing the ultimate tensile strength while maintaining good tensile ductility. Cold-working is effected by peripherally contacting the steel, i.e., extruding and/or die drawing, to produce a ''''cellular structure'''' not attainable in cold rolling. Repetitive cold-working and stressrelief anneals between about 750* and 1200* F increase the ultimate tensile strength in increments and restore the tensile ductility substantially to that of the hot rolled and annealed material, while maintaining the tensile to yield ratio substantially constant. Ultimate tensile strength up to about 400 ksi in small diameter wire is attainable.

Description

United States Patent Tanczyn PROCESS FOR COLD-WORKING AND STEELS [75} inventor: Harry Tanczyn, Baltimore, Md.

1731 Assignee: Armco Steel Corpnration,

Middletown, Ohio [221 Filed: Jan. 18, 1974 [21] Appl. No: 434,397

[521 US. Cl. 72/364; 148/12 EA [51} int. Cl. B2lc 9/00 158] Field of Search 72/364. 700; 148/12 B, 148/12 EA 156] References Cited UNITED STATES PATENTS l 7326l5 10/1929 Pungel 1. 148/12 2767836 10/1956 Nachtman et a1v 148/12 2.816052 12/1957 Hoff et a1 1 l t 1 r r 4. 148/12 2,933,424 4/1960 Canney et a1 14 148/12 I Cu LU 1 120 g y LU no E y 5 IOO 1 1 June 10, 1975 3.230.118 1/1966 Tufts 148/12 Primary ExaminerL0well A. Larson Attorney Agent, or Firm-Mel ille, Strasser. Foster & Hoffman [57] ABSTRACT A method of cold-working and stress relieving iron chromium ferritic stainless steels of nonheat hardenable type in the A181 400 series, thereby increasing the ultimate tensile strength while maintaining good tensile ductility. Cold-working is effected by peripherally contacting the steel, i.e., extruding and/or die drawing, to produce a cellular structure" not attainable in cold rolling, Repetitive cold-working and stress-relief anneals between about 750 and 1200 F increase the ultimate tensile strength in increments and restore the tensile ductility substantially to that of the hot rolled and annealed material, while maintaining the tensile to yield ratio substantially constant. Ultimate tensile strength up to about 400 ksi in small diameter wire is attainable 7 Claims, 1 Drawing Figure PATENTEDJUH 10 r915 ll OO m: wZmC, mbSZ EDD KOO 050 H5 STRESS'RELI EF TEMPERAT RE F PROCESS FOR COLD-WORKING AND STRESS-RELIEVING NON-HEAT HARDENABLE FERRITIC STAINLESS STEELS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process of cold-working and stress-relieving ferritic stainless steels containing about 11% to about 3071 chromium of non-heat hard enable type by extrusion and/or die drawing to produce bar, rod, wire, strip and special shapes having acceptable ductility at tensile strength levels greater than that of the hot-worked and annealed starting material. Steels which may be treated by the process of the present invention include AlSl Types 400, 409, 4l0-low carbon low nitrogen. 429, 430, 433, 434, 435, 436, 442 and 446.

2. Description of the Prior Art Conventional annealing practices for fe rritic stainless steels are intended to condition the steel for additional reduction in thickness and/or cross-section, i.e., to develop a capability for further cold-work processing. The degree of cold reduction is based on starting thickness and/or cross-section relative to the final gauge, with no predetermined or desired level of maximum mechanical strength in any stage. In other words, prior art processing is intended to produce an annealed final product which can thereafter be shaped. The mechanical strength of the shaped product approximates that of the starting material in the annealed condition.

US Pat. No. 3,141,800, issued July 21, 1964 to A. T. Reichenbach, discloses a method of producing dimensionally stable plates of ferritic stainless steel, wherein hot rolled, annealed and pickled blanks are cold reduced about 18% to about 35% in thickness to final gauge, annealed between about l,350 and 1,450 F, pickled, flattened by stretch leveling, roller leveling or temper rolling, and stress relieved by heating between 300 and 800 F. The final stress relief treatment is stated to be critical in order to avoid shrinkage resulting from cyclic high pressure loading at elevated temperatures. Apparently the stress relief removes some directionality in mechanical properties resulting from the flattening of the plates.

US. Pat. No. 3,490,956, issued Jan. 20, 1970 tol. W. Wilton, discloses a process for reducing ribbing and roping of ferritic stainless steels when subjected to a deep drawing operation. A hot rolled, annealed and pickled ferritic steel is initially cold reduced from 40% to 80% in thickness, annealed at l,600 to 2,100 F in a protective atmosphere, cold reduced to final thickness, and annealed at l,450 to l,600 F in a protective atmosphere. Typical ultimate tensile strengths for material processed in accordance with this patent ranged from 81.5 to 86.4 ksi, with elongations ranging from 23% to 25.5%. Typically the tensile strength of a hot rolled and annealed ferritic stainless steel of the type disclosed in this patent would be about 80 to 85 ksi. Hence, the ultimate tensile strength of the final product is substantially the same as that of the hot reduced and annealed material.

US. Pat. No. 3,694,271, issued Sept. 26, 1972 to L. O. Egnell discloses a method of producing a composite article consisting of a supporting layer of austenitic stainless steel bonded at least on one side to an outer layer of ferritic stainless steel. A composite billet is cold reduced, e.g., by drawing, rolling and/or bending, with a reduction in cross sectional area of 5% to and subjected to an anneal at 650 to 950 C (l,202 to l,742 F) in order to recrystallize the ferrite layer. The extent of cold reduction of each layer of the composite is not defined, and the heat treatment affects only the ferritic steel portion. It is stated that the cold working increases the yield strength and creep strength of the austenitic portion drastically, and the subsequent heat treatment is intentionally controlled in order to avoid reducing the increased mechanical strength imparted to the austenitic portion by cold working. Accordingly, the austenitic portion would not possess sufficient ductility to permit futher cold reduction under practicable processing conditionsv A number of earlier patents relate to the problem of preventing roping, ribbing and/or ridging in the deep drawing of ferritic steels. In general these patents disclose that composition and heat treatment of the hot reduced product are important and that annealing of the cold worked product at temperatures of about l,350 to about 1,550 F is necessary in order to obtain good ductility. Representative prior art disclosures of this type include US. Pat. Nov 2,772,992, issued Dec. 4,1956 to G.C. Kiefer, et al; No. 2,808,353, issued Oct. 1, 1957 to W. B. Leffingwell, et al', No. 2,851,384, issued Sept. 9, 1958 to J. H. Waxweiler; No. 3,067,072, issued Dec. 4, 1962 to W. B Leffingwell, et al; and No. 3,128,211, issued Apr. 7, 1964 to J. H. Waxweiler.

To the best of applicants knowledge, the prior are neither discloses nor suggests the possibility ofsubstantially increasing the mechanical strength of non-heat hardenable ferritic stainless steels while at the same time retaining sufficient ductility to permit subsequent cold-work forming, e.g., cold heading operations and production of spring-temper wire.

There is a definite need for ferritic stainless steels having an ultimate tensile strength up to about 310 ksi along with 18% tensile ductility, cold headability at ultimate tensile strengths of about to about ksi, and springtemper characteristics equivalent to those of AlSl Type 302 spring wire, for application in products such as automotive thermostat springs, windshield wiper arms, automotive fasteners and straight pins.

SUMMARY It is a principal object of the present invention to provide a process for increasing the ultimate tensile strength of non-heat hardenable ferritic stainless steels to any desired level between about 100 and about 400 ksi while at the same time retaining sufficient ductility to permit subsequent cold-forming operations thereon.

It is a father object of the invention to provide articles of manufacture such as cold-headed fasteners, spring-temper wire and the like composed of a nonheat hardenable ferritic stainless steel having high ultimate tensile strengths.

The present invention constitutes a discovery that a non-heat hardenable stainless steel can be drastically cold reduced by extrusion and/or die drawing followed by a stress relief treatment at temperatures between about 750 and 1,200 F for a period of time of from two minutes to three hours, to effect a rapid rate of recovery of tensile ductility and a slow rate of decrease in ultimate tensile strength. Repetitive cold-working and stress-relieving make it possible to increase the ultimate tensile strength in increments, from the ultimate tensile strength of the hot-reduced and annealed start- 3 4 ing material upwardly to any desired level between DESCRIPTION OF THE PREFERRED about 100 ksi to about 400 ksi or higher. EMBODlMENTS The steels to which the process of the present invention is applicable include those non-heat hardenable ferritic stainless steels containing from about ll7r to about 30% chromium, up to about 01% carbon. up to about We maganese, with optional additions of ferriteforming elements which do not induce heat-hardening, e.g. about 1.5% molybdenum (AlSI 434); about 0.25% aluminum, (AISI 405); about 0.50% titanium, (AISI 409); about 0.03% columbium (AISI 439); and other elements such as tungsten, vanadium, zirconium. sili- A series of tests has been conducted on AISI Type 430 and Type 431 Modified steels, the starting material being rod stock varying in diameter from 0.250 inch to 0.800 inch. The condition of the hot reduced starting material is not significant in the process of the present invention. The structure thereof is a mechanically strained ferrite.

The compositions of the steels subjected to testing are set forth in Table I below.

TABLE I Compositions in Weight Percent AlSI Type Heat No. C Mn P S Si Cr Ni 430 762 .061 .52 .03 .02 .60 I688 .55 43] (Modif) 373 v056 .34 .02 .01 .45 V1.82 L36 430 902 .096 .81 .02 .02 .33 17.54 .32

con and the like. Austeniteforming elements such as nickel, cobalt, carbon, manganese and nitrogen should be restricted to relatively low levels, such as a maximum of 2% nickel as in AISI 43].

The balance is iron together with incidental impurities.

Molybdenum may be added in amounts up to about 4% by weight, for enhanced corrosion resistance.

Accordingly, the invention is broadly applicable to the AISI Type 400 series of non-heat hardenable stainless steels having a maximum l900F quenchedhardness of Brinell 250. This definition is intended to exclude AISI Types 4l0, 420, 440 and quasi-ferritic and balance iron, except for incidental impurities.

The test conditions and strength and ductility properties are set forth in Tables ll V below. It will be noted that a reduction in diameter of about 50% resulted in an increase in ultimate tensile strength ranging between about 50 and 70 ksi, and each cold reduction drastically reduced the tensile ductility as measured in percent elongation (Sample length 4 times diameter). However, when subjected to a stressrelieving anneal in accordance with the process of the invention, the percent elongation was at least partially restored and was in no instance less than about 15% after stressrelieving. In most instances, the percent elongation compositions. after the stress-relieving anneal was at least about 20%.

The minimum degree of reduction in thickness and- This provided adequate ductility for subsequent cold- /or cross-sectional area effected by extrusion and/or forming operations. die-drawing does not constitute a limitation on the In addition, it will be observed that the tensile-yield practice of the invention and is selected on the basis of ratio was substantially equivalent to that after the first the desired strength of the final product and the start- Cold drawinging size of the hot reduced and annealed material. Ordi- Helical Springs ware Wound from the 0-050 inch narily a reduction ofgreater than 50% up to about 95% ameter Wire of p 5 in Table and these springs in thickness will be effected. Similarly, the selection of hiblted room temperature elastic Properties equivalent a stress-relieving temperature and time at temperature to those of cold drawn stress-relieved springs fabricated will be predicated on the service requirements for the from AIS] yp -1 which had an ultimate tensile final product. Broadly, the temperature ranges from Strength 0f 280 ksi, and Slightly lnferlm Properties about 750 to l,200F (preferably from 850 to l,15( springs made from a precipitation hardenable stainless F) and the time from about two minutes to about three Steel 501d Undfrl the Registered Trad mark ARMCO hours (preferably hour to two hours), 17-7 P H, in the CH 900 condition with an ultimate tensile strength of 295 ksi.

The copper-coated 0.l l0 inch diameter wire coils resulting from

process Steps

2,3 and 4 of Table IV were BRIEF DESCRIPTION OF THE DRAWING cold headed into Phillips recessed-head fastener blanks. Excellent headability was exhibited by the wire Reference is made to the accompanying drawing at each of the three levels of ultimate tensile strength, which constitutes a graphic illustration of the influence i.e.,l40.9, l3l and I22 ksi, respectively. No undue of time and temperature on the ultimate tensile loading of the heading machine was apparent, and strength of an AISI Type 430 ferritic stainless steel cold-shearing of the fastener blanks proceeded without treated in accordance with the process of the invention. incident.

TABLE II Type 430 Heat 762 Wire Process Room Temperature Mechanical Properties Diameter Step Condition U.T.S. 0.2% Tensile 7a Elongation Redn. Proportional Tensile/Yield (Inch) No. Of Wire (ksi) Y.S.(ksi) (4 D) in Area Limit (ksi) Ratio .250 1 Hot rolled (HR) 200 56.0 .I I0 2 Cold drawn (CD) I69 ISl l3.3 49.3 |.|2

from Step I TABLE II Continued Type 430 Heat 762 Room Temperature Mechanical Properties Wire Process Diameter Step Condition IITS. 0.2 7 Tensile f1 Elongation Z Red'n. Proportional Tensile/Yield (Inch) No. Of Wire lksi] Y.S.tksii (4 D] in Area Limit (ksi) Ratio .ll 3 Step 2 159.3 [46.5 24.4 55.6 ll 1.03

annealed (Ann) I000F. 2 hrs. air cooled (AC) .050 4 CD from Step 3 226 220 3.0 1.03 .050 5 Step 4 Ann 800F. I hr.A( 223 202.6 20.0 38.9 135 1.10 .050 6 Step 4 Ann 204 I7) 27.0 36.0 132 I.I4

850F. l hr.AC .050 7 Step 4 Ann 201 IR] 20.0 39.0 137 l.I l

900F. I hr.AC .050 8 Step 4 Ann I92 I74 30.0 40.4 130 1.10

950F. I hrAC .050 9 Step 4 Ann 136 173 32.5 43.3 I26 I08 1000F. I hr.AC .050 I0 Step 4 Ann l7l 165 35.0 53.4 l2] L04 I050F I hLAC .050 II Step 4 Ann 147 142 40.0 573 I09 I04 I I00F.l hLAC TABLE III Type 430 Heat 762 Room Temperature Mechanical Properties Wire Process Diameter Step Condition U.T.S. 0.2; Tensile 1 Elongation Redn. Proportional Tensile/Yield of Wife IkSi] Y.S. (kSi) (4XD) in Area Limit (ksi) Ratio .800 l HR-Ann 77 44 31.0 68.0 I .525 2 CD from Step 1 I12 102 16.0 56.0 l .525 3 Step 2 Ann 101 92 29.0 60.0 1.10

I025F,3 hrs,AC .262 4 CD from Step 3 151 135.8 17.0 48.0 1.11 .262 5 Step 4 Ann 147 I342 21.0 48.2 112 L09 800F.I hr,AC .262 6 Step 4 Ann I33 I20 24.0 55.0 I02 1.! I

I000F.I hr.AC .130 7 CD from Step 6 202 5.0 39.0 .130 8 Step 7 Ann 187 168 27.0 55.0 124 1.1 I

I000F,I hr..AC .062 9 CD from Step 8 260.8 3.0 40.0 .062 10 Step 9 Ann 246 224 24.0 52.0 140 1.10

I000F,1 hr,AC .030 11 CD from Step 10 314 3.0 38.0 .030 12 Step I I Ann 308 272 18.0 48.0 155 1.13

800F, Ih AC TABLE IV Type 430 Heat 762 Room Temperature Mechanical Properties Wire Process Diameter Step Condition U.T.S. 0.291 Tensile 5k Elongation 7n Redn. Proportional Tensile/Yield (Inch) No. Of Wire (ksi) YzStksi) (4XD) in Area Limit (ksi) Ratio .250 l HR 20.0 56.0 .110 2 CD from Step 1 140.9 I322 204 58.6 1.14

+ Ann [075F,

l 1% hrs. AC flash-pickled 8: Cu coat .110 3 CD from Step I I31 115.75 28.9 59.0 1.13

Ann I F. 1 k hrs, AC flash-pickled & Cu coat .III) 4 CD from Step I 122 108 34.0 61.0 1.13

. Ann 1 F.

1 /2 hrs. AC flash-pickled & Cu coat TABLE V Type 431 Modified Heat 373 Room Temperature Mechanical Properties Wire Process Diameter Step Condition Ll TS 0.2% Tensile 7r Elongation 9? Red'n. Proportional Tensile/Yield (Inch) No. Of Wire lksi) Y.S.(ksil [4XD1 in Area Limit (ksi) Ratio .375 l HRAnn 85.5 33.0

187 2 (D from Step I 151 l35 17.0 49.0 1,12 .187 3 Step 2 Ann 1. .3.7 121 25.0 56.0 1.10

l025lfi2 hrs-.AC .080 4 (D from Step 3 W8 174 8.0 42.0 l.l3 .081 5 Step 4 Ann 196.4 184.3 15.6 44.9 129 1,06

8U1lF l hr,AC .080 6 Step 4 Ann 184.3 175.3 18.8 48.6 129 1.05

900%. l hr.AC

From the data of the preceding Tables, it has been found that optimum results are obtained under the following Conditions:

A Stress-relief at 950 to l,l50F for l to 2 hours to produce the following properties;

single reduction ultimate tensile strength 100 ksi elongation (4 X D) multiple reductions ultimate tensile strength 130 ksi elongation (4 X D) 24% B Stress-relief at l,075 F to l,l50 F for l to 2 hours to produce the following properties:

single reduction ultimate tensile strength 120 ksi elongation (4 X D) 28% multiple reductions ultimate tensile strength 145 ksi elongation (4 X D) 40% The above conditions are set forth by way of example as preferred but non-limiting procedures.

The foregoing Tables also set forth proportional limit values for a number of samples. Proportional or elastic limit is a measure of the capacity ofa fabricated article, such as a spring, to be mechanically loaded by service stresses without undergoing permanent damage which would destroy serviceability. For spring 4 temper applications a higher proprotional limit is thus associated with greater efficiency in service and design. AlSl Type 302 spring temper wire at 280 ksi ultimate tensile strength exhibits a proportional limit of 20 to 30 ksi; Armco 17-7 PH at 280 ksi ultimate tensile strength exhibits a proportional limit of about 105 ksi; AlSI Type 430 processed in accordance with the present invention to 280 ksi ultimate tensile strength exhibits a proportional limit of 125 ksi, and at 300 ksi ultimate tensile strength exhibits a proportional limit of 135 ksi. The proportional limit increases directly with ultimate tensile strength in Type 430, which is not true of Type 302 or Armco 17-7 PH.

Ordinarily, AISI Type 430 is cold headed from an ul timate tensile strength of 80 ksi into fastener blanks such as the Phillips-type recessed-head fastener. It has therefore been necessary in the past to utilize an austenitic chromium-nickel stainless steel in order to obtain cold headed fasteners having ultimate tensile strengths ofgreater than about 120 ksi. It will be apparent that the present process projects non-heat hardenable ferritic stainless steels into applications now fullfilled only by the much more expensive austenitic chromium-nickel stainless steels.

It has been found that conventional cold rolling mill reduction of ferritic stainless steels in strip and sheet form results in a product which does not respond to stress-relief annealing in the same manner as a starting material which has been extruded and/or drawn through peripherally contacted dies. Although not wishing to be bound by theory, it is believed that the extrusion and/or die drawing develop a cellular structure" within the crosssection of the cold worked product which is not obtained in cold rolled strip and sheet having a width many times its thickness. The metallurgical reactions operative in stress-relief are believed to include partial recovery from the prior cold work by annealing out of vacancies and/or rearrangement of dislocation pile-ups (without complete relaxation of the prior cold worked structure), slow growth of the cells of sub-grains formed during cold work, and recrystallization.

The data of Table V show that a modified Type 431 containing 1.36% nickel responded to the process of the present invention in the same manner as regular AISI Type 430. Here again, it was observed that the percent elongation was restored to an adequate level by reason of the stress-relief anneal.

By way of comparison, an AISl Type 302 wire was cold drawn to 0.262 inch diameter from a 0.5 inch starting material, annealed at 850F for one hour and air cooled. The 0.262 inch diameter wire exhibited an ultimate tensile strength of 175 ksi, a 0.2 tensile yield strength of 143 ksi, a percent elongation (4XD) of 9.0, a percent reduction in area of 52.0, a proportional limit of ksi and a tensile/yield ratio of 1.22.

A Type 302 spring wire was cold drawn to 0.080 inch diameter from a 0.19 inch starting material, annealed at 850F for one hour and air cooled. It exhibited an n] timate tensile strength of 255 ksi, 0.2% tensile yield strength of 240 ksi, percent elongation (4XD) of 2.0, a proportional limit of 70 ksi and a tensile-yield ratio of 1.06.

The extremely low ductility of Type 302 when treated under similar conditions thus contrasts sharply with the good ductility values of the ferritic stainless steels when treated by the process of this invention.

The drawing illustrates graphically the influence of time and temperature in the stress-relief anneal between 900 and l,250 F. These curves were plotted from test data on heat 902 of Table l for a wire cold drawn to 0.051 inch diameter with an ultimate tensile strength of 153 ksi. it will be noted that a stress-relief temperature above 1,200F results in an ultimate ten sile strength of less than ksi even if the time at temperature is limited to less than five minutes. Accordingly, the maximum temperature of 1,200F is considered to be critical in the process of the present invention, It is further apparent that lower temperatures in the range of 900 to 1,100 F can be utilized even up to three hours without reducing the ultimate tensile strength to less than 100 ksi.

TABLE V1 l Type 430 Heat 762 Time and l empernturc Relation to Room Temperature Mechanical Properties U.T.S. 0.2% Tensile Elongation I? Redn. Tensile/Yield Condition (ksi) Y 5.1 ksi) (4XD) in Area Ratio .100 inch diani- 173 14! 7.5 48.0 1.18 etcr (D from .250 in diameter HR rod CD100 Ann 171 150 12.0 50.0 1.14 700F. hr.A(' CD101)" Ann 1702 148 211.11 511.1) 1.15 700F. 1 hr. AC (D .100" Ann 167.2 149 13.0 50.0 1.12 800F. V; hr.A( CD .100" Ann 165 149.4 22.5 50.0 1.10 800F, l hr.Af CD .100" Ann 165 148.6 15.0 50.0 1.10 900F. V2 hr,AC CD .100". Ann 163 146.8 25.0 52.8 1.11 900F. l hr.AC CD .100". Ann 160.5 146 19.0 51.0 1.10 1000F. rt hr.AC CD .100". Ann 1552 144.2 27.5 55.6 1.08

1000F. l hrAf Additional data showing the influence of time and temperature on other mechanical properties is set forth in Table VI above. From these data and those of the preceeding Tables II\/, it will be apparent that the process of the invention provides an increase in tensile strength of at least about 50 ksi for each 50% reduction in thickness, and that the stressrelieving treatment within a range of about 750 to l,200 F with a time at temperature of about 2 minutes to about 2 hours achieves an elongation value (at least about 15%) adequate to permit subsequent cold forming operations. The graph of the drawing indicates that time at temperature should be varied inversely with temperature.

Wire and rod sections of 0.262 inch diameter and greater were cold drawn with single-stand drawing arrangements. This type of processing required a relatively slower rate of cross-sectional reduction than that obtainable with multipledie, cold-drawing operations. Accordingly, the process of the invention appears to find greatest utility in the production of stainless steel wire sections and/or special shapes in final sizes less than 0.220 inch diameter.

As indicated above, the condition of the starting material does not constitute a limitation. The present pro cess can be applied to annealed, hot rolled or quenchhardened mill sections which have been melted, cast and hot reduced in accordance with conventional practice. Typical starting conditions include hot rolled; hot rolled and stress relieved at temperatures below about 1.300 F; hot rolled and annealed at temperatures above l,300 and below 1,700 F; and hot rolled and quench-hardened from temperatures higher than Novel products of the present invention include cold headed fastener blanks having an ultimate tensile strength of at least about 125 ksi. helical springs having an ultimate tensile strength of at least about 200 ksi, cold worked and stress-relieved bar, rod. wire, strip and special shapes having ultimate tensile strengths ranging from about 125 to about 300 ksi and sufficient ductility to permit subsequent cold forming operations, all fabricated from a non-heat hardenable ferritic stainless steel having a composition as hereinabove defined.

Modifications may be made in the invention without departing from the spirit and scope thereof. Accordingly. no limitations are to be inferred or intended other than as set forth in the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A process for increasing the ultimate tensile strength of non-heat hardenable ferritic stainless steel bar, rod, wire, strip and special shapes while retaining good ductility therein, comprising the steps of cold working a non-heat hardenable ferritic stainless steel containing from about 11% to about 30% chromium and balance substantially iron by passage through a die peripherally contacting said steel, said cold working effecting a reduction in thickness sufficient to increase the ultimate tensile strength of said steel by at least about 50 ksi, stress relieving said cold worked steel by heating within the temperature range of about 750 to 1.200" F with a time at temperature of about 2 minutes to about 3 hours, whereby to obtain an elongation value adequate to permit subsequent cold forming operations.

2. The process claimed in claim 1, wherein said stress-relieving heat treatment is conducted at about 850 to about 1.150" F with a time at temperature of about '1; hour to about 2 hours.

3. The process claimed in claim 1, wherein the ultimate tensile strength of the cold worked and stressrelieved steel is at least about ksi and the elongation value is at least about 20% (4XD).

4. A process for increasing the ultimate tensile strength of non-heat hardenable ferritic stainless steel bar, rod, strip and special shapes while retaining adequate ductility therein to permit subsequent cold forming operations. comprising the steps of repetitively cold working and stress-relieving a non-heat liardenable ferritic stainless steel containing from about 1 1% to about 30% chromium and balanced substantially iron, each cold working stage involving passage of said steel through a die peripherally contacting said steel and effecting a reduction in thickness, each stress-relieving stage involving heating the cold worked steel within the temperature range of about 750 to 1.200 F with a time at temperature of about 2 minutes to about 3 hours, whereby to obtain an ultimate tensile strength greater than about ksi and an elongation value of at least about 15% (4XD).

5. The process claimed in claim 4, wherein each said stress-relieving stage is conducted at about 850 to about l,l50 F with a time at temperature of about V2 hour to about 2 hours. 6. The process claimed in claim 4, wherein the ultimate tensile strength of the repetitively cold worked 5 about 50%.

Claims

1. A PROCESS FOR INCREASING THE ULTIMATE TENSILE STRENGTH OF NON-HEAT HARDENABLE FERRITIC STAINLESS STEEL BAR, ROD, WIRE, STRIP AND SPECIAL SHAPES WHILE REATINING GOOD DUCTILITY THEREIN, COMPRISING THE STEPS OF COLD WORKING A NON-HEAT HARDENABLE FERRITIC STAINLESS STEEL CONTAINING FROM ABOUT 11% TO ABOUT 30% CHROMIUM AND BALANCE SUBSTANTIALLY IRON BY PASSAGE THROUGH A DIE PERIPHERALLY CONTACTING SAID STEEL, SAID COLD WORKING EFFECTING A REDUCTION IN THICKNESS SUFFICIENT TO INCREASE THE ULTIMATE TENSILE STRENGTH OF SAID STEEL BY AT LAST

2. The process claimed in claim 1, wherein said stress-relieving heat treatment is conducted at about 850* to about 1,150* F with a time at temperature of about 1/2 hour to about 2 hours.

3. The process claimed in claim 1, wherein the ultimate tensile strength of the cold worked and stress-relieved steel is at least about 100 ksi and the elongation value is at least about 20% (4 X D).

4. A process for increasing the ultimate tensile strength of non-heat hardenable ferritic stainless steel bar, rod, strip and special shapes while retaining adequate ductility therein to permit subsequent cold forming operations, comprising the steps of repetitively cold working and stress-relieving a non-heat hardenable ferritic stainless steel containing from about 11% to about 30% chromium and balanced substantially iron, each cold working stage involving passage of said steel through a die peripherally contacting said steel and effecting a reduction in thickness, each stress-relieving stage involving heating the cold worked steel within the temperature range of about 750* to 1, 200* F with a time at temperature of about 2 minutes to about 3 hours, whereby to obtain an ultimate tensile strength greater than about 125 ksi and an elongation value of at least about 15% (4 X D).

5. The process claimed in claim 4, wherein each said stress-relieving stage is conducted at about 850* to about 1,150* F with a time at temperature of about 1/2 hour to about 2 hours.

6. The process claimed in claim 4, wherein the ultimate tensile strength of the repetitively cold worked and stress-relieved steel is at least about 130 ksi and the elongation value is at least about 24% (4 X D).

7. THe process claimed in claim 4, wherein each said cold working stage effects a reduction in thickness of about 50%.