US2363736A - Stainless steel process - Google Patents

Description

' eral operational steps,

/ is indicated in the claims.

, equipment, and the like.

Patented Nov. 1944 John N. Lynn,

Iron and Steel Corporation, corporationfof Delaware Towson, MIL, assignor to Rustless Baltimore, Md., a

" No Drawing. Application July '31, 1,941, Serial No.404,919

The present invention relates to tents, and particularly to anartof heattreating and working the same. I

One of the objects of my invention "the pro- "vision of a simple, direct andpractical process for improving the cold-working, properties of high-chromium stainless steel alloys of high-carbon contents, which enables the employment Fof conventional equipment at maximum efilciency and with minimum.expense.

-. Another object of my invention is the coldreduction, of stainless steel bars, rods and wire, of the class indicated, inan economical'manner of the same to one described herein, the

To promote a clearer understanding of my invention, it is to be noted at ,thispointlthat the hardenable hypereutectoid, high-chromium' stainless steel alloys comprising" 16%"to 22% chromium, 0.75% to 1.5%carbon, and thebalance substantially iron, except for special purposes, small additions'of molybdenum and the like, are metals of great'strength, toughness and hardness, possessing high resistance to corrosion and abrasion. Because of excellentserviceability, thesealloys enjoy a wide range of utility,

applications as the procutlery, valves, valve being employed in such duction of ball bearing 1 seats, dental instruments, superheated steam parts, milk bottling equipment, oil ing machines, laundry pumps, sprayequipment, chemical Alloys of the class described, however, have, as the result of hot-working, objectionable brittleness, which condition persists even where the metal is annealed after hot-working. Before cold-drawing, a high-carbon, high-chromium stainless steel into rods or wire, for; example, it

is conventional practice to hot-roll the metal us and in after, the metal is exposed' alternately toa'nneal ing, pickling and cold-drawing foperatio'nsuntil the metal is drawn 'torods or '7 Claims. (01. 14s 12) high-chro-. mium stainless steel alloys of, high-carbon con-' through fanfinitial range of reduction." There- 1. Much difliculty. is, encounteredgin'the form of f breakage during. the cold-drawing. steps, because of metal brittleness incurredby hot-rolling. Cold-drawing of high -carbon,,,high-chromium stainless steel alloys in accordance with prior praotice,'-';therefore, is tedious.

often is found with finished metal'productscbe- -cause they contain flaws, have-irregular grain the' hotrolling temperature, to be obiectionably brittle.

ability. e

structure and lack strength, toughnessand dur- Although I do not care to be bound by an ex- 4 planation,'itappears that partof the carbon in hypereutectoid stainless steels is in solid solution with ironand chromium, while anotherpa'rtJis in the metal carbide form. When-Ian alloysteel of the class described isihot-rolled,-car-bide particles segregate in the metal'to disrupt uniformity ofthe metal grain" structure. I Upon coolingfrom the metaljis found previous practice, annealing of the hot-rolled metal at, 1500 to 1600 F. serves to soften the metal andto relieve stresses incurred by hotthe presence of carbide H culty is encountered in i into rodsand wire, as b rolling. Infs'pite of the annealing operatlomdifll- .cold-workin'g' the metal y drawing, becausefofits excessive brittleness and lack of uniform grain structure. Aftercold-working the metal, ,grain structureis not unifor segregation; The resultant rod or wire'products are not entirely uniform in strength, toughness, hardness, corrosion resistance and abrasion resistanc i r An object of my invention,- accordingly,'is the provision of a process for producing high-carbon,

high-chromium stainless steel products of im- 7 proved cold-working and cold-forming properties steel barsQrods and wire ofa and'especially of producing cold-drawn stainless ensuring a minimum amoun of breakage and "waste in the cold drawing.

Referring now more tice of my invention, I improve the cold-working properties of high-carbon, high-chromium stainless steels by a heat-treatment cofn'prising suc- {cessive quench-hardening and annealing operations, respectively; To effect ahardening of the metal, I heat it to a temperature high enough to T Dims v time-consuming, and consequently,'uneconomical. Moreover, fault In accordance with apparently because of uniform structure partieularlyto the prac remain dispersed in 2 aeeaase ensure, within practical limits, that a maximum breakdown of carbide striations occurs; after which,'I cool the metal rapidly in oil, wateror air.

:Following these operations, carbides are found to exist in well dispersed globular form interspersed in a uniform and refined grain structure. I then anneal the metal by heating it through the critical range and thereafter slowly cooling it back through the critical range. Annealing does not destroy the effect of thorough metal carbide distribution brought about by'the previous hardening operation. It does, however, considerably soften the steel to permit subsequent ready cold working or cold-forming. It can be cold-worked more readily than steels heat-treated in accordance with practices heretofore employed inthe,

art. Moreover, the grain is more uniform and there is very much less breakage in the coldworking operation.

As illustrative of the practice of my invention,;

I heat a coil of stainless steel rod material analyzing 16% to 22% chromium, 0.75% to 1.5% car-. bon, and the balance iron, to a temperature prefand of uniform quality. A minimum number of breaks occur lri'the drawing.

For example, in drawing hot-rolled rod stock of /uinch diameter, and analyzing 17.0% chro mium, 1% carbon and the remainder iron. into wire of 0.284 inch diameter, aflrst drawto 0.318 inch was employed with a subsequent annealing treatment. x In total reduction, the wire broke only three times. According to prior practice, as many as fifty breaks commonly are encountered. These, of course, result in delay and a product of inadequate length. Moreover, with prior methads, as many 'assix to eight intermediate annealing operations arerequired, in order to achieve a reduction of this consequence in such a stainless steel, while my process requires but one or two intermediate anneals. Moreover, my finished wire-is stronger, tougher, and more serviceable thanheretofore known'products because of imerably within the range of 1800' F.'to 1925' II. At

*such temperatures, carbides break up, recrystallization occurs.

A metal soaking period of short duration at temperature, encourages-a more complete readjustment of the metal crystalline structure. At the end of the heating operation, I re-v move the metal from the furnacewhile it is still at a temperature of 1800 F. to 1925 F. The metal is quench-hardened promptly, Preferably by rapidly cooling in oil.

. ing in air may be effected. The hardened metal possesses n Where desired, quenchilne and uniform grain structure wherein metal carbides are well dispersed and are in globular form.

While I do not wish to be limited specifically to the range of temperatures given, Iflnd the microstructures of metal pieces quench-hardened from temperatures somewhat below 1800 F. contain banded carbides to some degree, while those quench-hardened from temperatures substantially above 1925 1. show grain growth. g

Following the hardening treatment, I anneal the hardened metal in a heat-treatment furnace, by heating it to a temperature of 1500 F. to 1600' F. From this temperature, the metal is cooled slowly in the furnace, preferably at a rate of approximately F. per half-hour, to about 1200 F. from which it is air-cooled to room tempera- 'ture;

AnnealingQin this case, brings about a softening of the hard, uniform metal. Carbides globular form. It can be seen, therefore, that the hardening operation is carried out primarily to lend refinement to the invention -as applied proved grain structure.

Thus, it will be seen that'tbere has been provided in this invention-an art of working highchromium stainless steel alloys of high-carbon contents, by which the various objects hereinbefore noted, together with many thoroughly practicaladvantages, are'successfully achieved. Also, it will be seen that the heat-treatin aspect of my to high-carbon stainless steels of the class described, enables them to withstand various cold-working and cold-forming operationssuch as straightening, bending, drawing, sectional reduction or-the like.

Aside from the many practical and economical operational advantages of my invention, coldworked products fashioned in accordance with my process are of-superior quality. They possess fine and uniform grain structure; flaws, blemishes, tears andworking defects, accordingly, being-at a minimum. The products are tough. hard and strong, andpossess improved resistance to corrosion and abrasion.

While as illustrative of thepractice of my invention, high-chromium, high-carbon stainless I 22% chromium and 0.75% to 1.5 carbon are specifically described, good results are achieved in similar stainless steels, even where the chromium and carbon con. tents are extended somewhat.

' As many possible embodiments may be made of my invention and as many changes may be made in the embodiments hereinbefore set forth, it will -be understood that all matter described herein isto be interpreted as illustrative, and not in a limiting sense.

metal, while the annealing oper'ationserves to soften the refined metal. I find the metal'to be far less brittle than if the hardening operation had been omitted.

After my sequential hardening and annealing steps, I find the metal to be in ideal conditionfor cold-working. In cold-drawing high-carbon stainless steel rods of the type'described into rods and wire, I subject the metal to alternate colddrawing and annealing operations until the metal is reducedto final form. Preferably, the annealing is conducted in a controlled non-oxidizing atmosphere; otherwise, the annealing treatment is followed by a pickling operation to remove the anneal scale prior to a further-cold-drawing op.- eration. The annealing operations relieve stresses inthe metal incurred by cold-working. The finished metal then is ready for additional treatment orready for particular use. It is of good length Y I claim: v

1. In preparing for cold-working a previously hot-worked hypereutectoid stainless steel containing at least 16% chromium and 0.75% carbon' which is hardenable by' heat-treatment, the art which includes, quench-hardening the same from a temperature of l800 F. to 1925 F., and then annealing the hardened metal at a temperature of 1500 F. to 1600" F.

2. In the cold-working of hot-worked highchromium hypereutectoid alloy stainless steel of high-carbon content, comprising 16% to 22% chromium and 0.75% to 1.5% carbon, the art which includes, quench-hardening said metal from a temperature of at least 1800 F., annealing the hardened metal at a temperature of 1500" F. to 1600 F., pickling the same, and cold-working said annealed andpickled metal.

3. In the production of a cold-drawn, high- .chromium hypereutectoid stainless steel alloy of high-carbon content comprising 6% to 22% F., annealing the hardened 4. In the cold-working of hypereutectoid hotworked stainless steel comprising 16% to 22% chromium and 0.75% to 1.5% carbon content, the steps which include, heatin the same in a suitable furnace to a temperature of 1800" F. to 1925 F. to recrystallize the metal, rapidly cooling the metal to harden the same, softening the metal by annealing it at a temperature of at least 1500 F., and then cold-working the same.

5. In the cold-working of a hypereutectoid previously hot-worked stainless steel comprising 16% to. 22% chromium and 0.75% to 1.5% carbon, the art which includes, heating the metal in a suitable furnace to a temperature of .1800 F. to 1925 F. and then rapidly cooling the metal to harden the same, heating the hardened metal in a suitable furnace to a temperature of 1500 F. to 1600 F. and slowly cooling said metal to sub- 3 stantially 1200 F. to anneal the same, and coldworking the annealed metal.

6. In the production of cold-drawn hypereutectoid rod or wire-comprising 16% to 22% chromium and 0.75% to 1.5% carbon, the art whichincludes, heating hot-rolled bar or rod stock in a suitable furnace to a temperature of 1800 F. to 1925 F., and rapidly cooling said stock to harden the same, reheating the hardened stock in a suitable furnace at atemperature of 1500 F. to 1600 F. and slowly cooling said stock to substantially 1200 F. to anneal the same, and

finally cold-drawing the annealed stock into rod or wire.

7. In the production of cold-drawn hypereutectoid rod or wire comprising 16% to 22% chromium and 0.75% to 1.5% carbon, the art which includes, quenching hot-rolled bar or rod stock from a temperature of 1800 F. to 1925 F., re-heating and annealing the stock at 1500 F. to 1600 F., and thereafter subjecting the annealed metal alternately to cold-drawing and annealing operations. I

JOHN N. LYNN.