US2664369A - Method of softening low-carbon medium-alloy steel - Google Patents

Description

UNITED STATES PATENT OFFICE METHOD OF SOFTENING LOW-CARBON MEDIUM-ALLOY STEEL John M. Kiefer, Jr., Bedford, Ohio, assignor to United States Steel Corporation, a corporation of New Jersey No Drawing. Application August 6, 1951; Serial No. 240,613

' 1 Claim. o1.1'4s- 21.5)

1 This invention is concerned with softening lowwhere the steels structure includes both ferrite carbon medium-alloy steel. Such steel has acarand austenite which are both relatively stable bon content of less than 20% and may contain for relatively long periods of time, the amount of from 3 to 10% of one or more alloys of the class v ferrite being greatest and the amount of austenite consisting of chromium, nickel, molybdenum, and being the smallest at a temperature just barely manganese, which elements are soluble in iron above the A61 and AC1 temperatures depending on in such amounts and which have the common the method of heating, and the austenite increasproperty of slowing down the decomposition of ing in proportion to the ferrite with increasing aust nit temperatures until the steel acquires its maxi- The preferred manner of softening similar mum possible amount of austenite and has little steels is to heat them to temperatures in the or no ferrite. In practicing the preesnt invenvicinity of 1850 F. followed by slow cooling. tion, the particular temperature to which the This is a rather expensive treatment and requires steel is heated theoretically depends upon the considerable equipment due to the long time specific amount of carbides it contains, the best required for cooling at a sufiiciently slow rate to t mp u being a producing J Sufficient obtain the desired effect. I have discovered that austenite to with time dissolve all of the carbides, steels of this type can be rendered as soft or softer while the remainder of the steels structure reby austenitizing at a low temperature as hereinmains ferritic and untransformed by the heating. after described and quenching as they can be by 00 While i is necessary to allow me t me for the the foregoing treatment. austenite to dissolve all of the carbides, in com- In softening such steel by the method of th mercial work this time is usually provided to an present invention, the steel is heated to a temadequate degree y the t me qu ed for the Steel perature slightly above its A61 temperature, i. e., to s ak at the mp ra u just a ove the Aer not more than about 50 F. above the Aei tempoint to assure its at a n g this temperature perature and soaking at such temperature for throughout p all the b de bei g disa sumoient time to equalize th temperature solved in the austenite, the steels structure is and dissolve substantially all the carbides. By predominantly a w -P e Structure consisting such heating, just about sufficient austenite largely of ferrite with the balance austenite, the is produced to dissolve therein about all the carbides being large y d so d in the latter, as carbides that are in the steel, while the re i 30 described. Furthermore, it is ordinarily sufiicient der of the structure remains untransformed to at Steel 0f the yp W Which s i ve and relatively stable as ferrite which is relatively tion is concerned to just as little above its Aei unhardenable by quenching. A81 temperature temperature as can be done while assuring that of the steel is determinable, in so far as this the steel has actually been heated above its Ael method is concerned, by austenitizing a sample temperature so that it has acquired at least some. of the steel under question, quenching it violently austenite. In such a steel this usually provides to render it martensitic, whereby it has a highly a structure consisting of from 10 to 40% austenite stressed metastable structure, and then reheating with the balance untransformed ferrite. The the steel to various temperatures, in the vicinity ratio of these phases does not change when comwhere the Aei temperature is anticipated, and mercially practicable soaking times are considquenching until the first appearance of untemered, it being possible to hold the steel at tempered martensite in a background of martensite perature for several hours without any great tempered by reheating. The temperature where change occurring in the ratio between the relauntempered martensite is first discernible may tively Smaller amou t of au n t nd h re t be taken as the Ael temperature of the steel. amount of ferrite prevailing when thetempera- Due to the accuracy of control required and nec-" ture was first attained. essary, it is preferred to the A01 point for obtain- The hot steel may now be cooled to permit its ing the limited transformation desired although handling, and any rate of cooling may be used as an alternative method the steel may be heated without making the steel hard and brittle. In to just above its AC1 point and immediately withthe case of many shapes ordinary air cooling is drawn from the furnace to achieve the desired satisfactory. However, quick cooling of some sort transformation. is generally desirable in the interest of saving Between such temperatures and the higher time and if desired, the steel may be quenched. temperatures where the steel becomes as wholly pm An actual quenohingis of advantage since it keeps eustenitic as t can be made, there is a range the elements from diffusing into the ferrite and it transforms the relatively small amount of austenite included in the structure of the steel so that this portion becomes a hard constituent such as fine pearlite or martensite which is hence more easily spheroidized by tempering. This does not harden the steel but in fact softens it, because the amount of ferrite is so large that it overbalances the amount of austenite resulting from the desired heating. This is the reason why it is desirable in the case of the present invention to select a temperature that is only very slightly above the A61 temperature, since this provides for maximum retention of the ferrite and the production of just sufficient austenite to dissolve most of the carbides. The type of steel to which this invention is applicable contains a sufficiently small amount of carbides to make it practicable to follow what has been discussed, but in the case of materially higher carbon contents it becomes necessary to produce so much austen-- its, by going to a suitable higher temperature, as to leave so little untransformed ferrite that the steel is actually hardened.

The improved results obtained by my improved method of softening low-carbon medium-alloy steel are aptly shown by the following tests conducted with specimens of hot-rolled and normalized SAE 3312 steel containing:

Percent Carbon 0.15 Manganese .38 Silicon .23 Nickel -i 3.95 Chromium 1.40

This steel had an Aei temperature of 124.0 F. and in the normalized condition a Brinell hardness of 382. Normalized specimens of the steel were heated to the temperatures tested in the following Table A, held at such temperatures for minutes and air cooled with the following results:

Table A Brinell Temperature: hardness 1,100 E 258 1.250" F 238 1,275 F; 255 1,300 F; 265

Similar results were obtained on AISI 2315 steel containing:

Percent Carbon -i 0.14 Manganese .45 Silicon .25 Nickel 3.75

This steel had a Brinell hardness in the hotrolled and normalized condition of 159 and an A61 temperature of 1215 F. Specimens thereof were heated to the following temperatures and held at the temperatures indicated for 2 hours and air cooled with the results shown in the following Table B:

i Table B Brinell Temperature: hardness 1,l F 143 1,220 F 1,2'I5 F 131 In both the foregoing cases the specimens were normalized prior to treatment to insure uniformity of results. .However, such prior treatment is not essential to attainment of the objects of this invention.

The foregoing data shows that instead of the expected hardening due to the formation of hard transformation products such as martensite or fine pearlite, the steel is actually softened if treated in accordance with this invention. As a result of such treatment, the steel has a structure consisting mainly of ferrite, so that the steel has the ductility and malleability imparted by this structure, which overbalances the minor amount Of hardened structure. In commercial work, this hardened structure will usually be a hardened metastable form of stressed ferrite and. carbides, such as martensite or fine pearlite. This structure can be spheroidized relatively easily by conventional spheroidizing treatment if further softening is desired.

This is a continuation-in-part of my copending application, Serial No. 107,377, filed July 28, 1949, now abandoned which was a continuation of my prior application, Serial No. 523,365 filed February 21, 1944, now abandoned.

I claim:

A method of softening low-carbon mediumalioy steel containing less than 20% carbon and from 3 to 10% of at least one of the alloying elements, chromium, nickel, molybdenum and manganese, which includes heating the steel to a temperature just above its A81 temperature to produce just enough austenite to dissolve therein about all its carbides while the remainder of its structure remains untransformed and relatively stable as ferrite; holding at such temperature for a suflicient time to dissolve substantially all of the carbides in said austenite, and then quenching the steel at a rate wherein the austenite with the dissolved carbides will transform to a hard constituent composed of a hardened metastable form of stressed ferrite and carbides, whereupon the cool steel is mainly unhardened due to its being composed principally of ferrite which overbalances the minor amount of the hard constituent.

JOHN M. KIEFER, JR.

References Cited in the file of this patent UNITED STATES PATENTS Iron and Steel Engineer, November 1947, pages 55 and 57.

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