US2395608A - Treating inherently precipitationhardenable chromium-nickel stainless steel - Google Patents

Description

- atented Feb. 26, 1946 TREATING INHER-ENTLY PRECIPITATION- HARDENABLE CHROMIUM-N ICKEL STAINr' LESS STEEL Robert H. Aborn, Short mus; N. J.,,assignor to United States Steel Corporation of Delaware, 7

a corporation of Delaware a Na Drawing. Application December 10, 1943,

Serial No. 513,748

.2 Claims. (Cl. 14s-21.55)

This invention is a method of treating chromium-nickel stainless steel containing carbon and at least one of the'elements, such as titanium, columbium, vanadium, etc., which form a carbide more stable than chromium carbide. This type of steel usually contains aluminum, and the amount of those alloying elements which are ferrite formers other than the named carbide formers is proportioned to the amount of those whichare austenite formers so as to 30% titanium and .15% aluminum. The steel was rendered completely austenitic except for some stable delta ferrite by heating it to 2050 F..

after which it was cooled in the furnace to 250 F. in forty-two hours, it thereafter being air cooled to room temperature, the steel having been in 18 gage sheet form so that air cooling provided bring about the formation of stress-laden martensite at a relatively low temperature such as room temperature; when the steel cools from a relatively high temperature at which it is austenitic. This stress-laden martensitic structure of the metal is inherently precipitationhardenable and increases in hardness to a level depending on the temperature to which it is reheated and the time at temperature; but prior to this reheating, the structure has suflicient ductility to permit some degree of cold forming of the metal.

The present invention is a method of treating steel of this type by converting it to austenite and then cooling it in such a way that it remains temporarily as an austenitic or austenitic-like structure when it reaches room temperature. This temporary structure is more ductile and permits a far greater degree of cold forming than the precipitation-hardenable martensitic-like condition to which "it ultimate transforms, but only after an interval long enough to permit the completion of the desired forming operations.

According to this invention the steel is heated to austenitize it and is then cooled slowly to temperatures slightly above those producing the stress-laden transformation product, whereupon it is cooled more rapidly to room temperature at a rate preventing any substantial amount of transformation, whereby the steel remains austenitic or in an austenitic-like condition for a substantial time at room temperature. During perature and the conditions of cooling or if desired, the transformation may be hurried by exposure to other temperatures.

As a specific example of the invention, the treatment has been applied to the steel when it contains 17% chromium,7% nickel, .06% carbon,

an adequately fast 'cooling'method. By such treatment test reports showed that a maximum Olsen value of 0.500" wasreadily obtained,'the normal Olsen value being 0.250" when the steel was austenitized at 1900 F. and air cooled to room temperature.

In making the commercial form of this steel, it will have only approximately the range of alloying elements noted, with the usual balance of iron and impurities, and in such instances the austenitizing temperature may range from 1700" to 2000 F., and the temperature to which the ing, or, in the case of sections having such mass as to render this inadequate, by water quenching. After the treatment the steel is substantially nonmagnetic, since it appears to be essentially austenitic, and at ordinary room temperature it may remain unchanged for a period of several hours. If the steel is water quenched to room temperature from the high temperature where it is'fully austenitic, except for some delta'fer rite which may be present, the resulting struc ture may then be mainly austenitic but it will remain in this condition only a few minutes at most. It follows that the slow cooling to a temperature slightly above that at which the austenite would normally transform to the marte nsitic or martensitic-like structure is necessary, the transformation then being prevented by adequately fast cooling to room temperature. 5

. I claim: I i

1. A method of treating chromium-nickel stainless steel containing carbon and at least one of the stronger-than-chromium carbide formers such as titanium, columbium, vanadium, etc.,

with its components that are ferrite formers other than said carbide formers proportioned to those that are austenite formers to cause the production of an inherently precipitation-hardenablestress-laden martensite or martensitic-like structure at low temperatures when the steel cools to room temperature from high temperatures where it is essentially austenitic, said method comprising heating the steel to austenitize it, cooling it slowly to temperatures slightly above those producing said martensite or martensitic like structure, and then rapidlycooling the steel' to'room' temperature at a rate preventing any substantial degree of transformation, whereby the steel remains essentially austenitic for a substantial time at room temperature.

2. A method of treating chromium-nickel stainless steel approximately containing 17% chromium, 7% nickel, .06% carbon, .8 0 titamum, and .15% aluminum, with its components that are ferrite formers proportioned to those that are austenite formers to causethe production or stress-laden martensite or martensiticaccuses like product,

ROBERT H. ABORN.