US2374351A - Thermally hardening steel - Google Patents

Description

Patented 24, 1945- John M Hodge,

No Drawing. Original Serial No. 330,798.

1 Claim. (01. lit-s1) This invention is a method of thermally hard- Munhail, and m: w. Liahtner, Pittsburgh, Pa.

application April a, 1m, Divided and this application December 13, 1941, Serial No. 422,928

.en'ing a steel article having a portion with a high-- er carbon content than another portion. An example of such an article is a steel plate having a carburised surface providing the portion of higher carbon content than another portion, the

latter being the uncarburized portion of the plate. I Other carburized steel articles also may be con- 1 sidered as examples.

When steel is quenched from atemperature where it is austenitic to within the temperature range of from 100 C. to 400C. .and held within this range until all or mostof its austenlte'trans forms, the steel possesses greater ductility and impact strength than steel of the same chemical composition hardened to the same hardness by other methods. Also, the austenitic transformation rateof steel varies within this temperature range in accordance with its carbon content, the

austenite of a higher carbon steel transforming more slowly than the austenite of a lower carbon steel.

With the above in mind, the present invention is-generally characterized by including the heating of an article having a portion with a higher carboncontnt than another portion, so as to render both these portions austenitic, the articlev then being quenched to the temperature range of from 100' C. to 400- C. and held within'this range until transformation of at least a majority of the austenite of the portion of lower carbon content is efiected, whereupon the article is quenched-to below the defined temperature range prior to transformation of more than minor amounts of the austenite of the higher carbon portion and at a rate producing martensite.

The result of the above is a steel article having an at least partially martensitic portion and another portion predominantly comprising products resulting from a'ustenite transforming in the products being termed bainito? by those skilled at one portion and will have another-portion that is hardened but possesses the ductility and impact strength provided by products resulting from austenitic transformation within the defined temperature range. It is to be hardened by quenching to andholding within the defined temperature range, may be recognized by studying its microstructure since this is an-acicular type as contrasted to a martensitic temperature range of from l00@. to 400 C, such in the art. Thus, the article will be very'hard understood that metal structure produced by other hardeningmethods.

Alloying elements other than carbon also have a decided effect in retarding the austenitic trans formation rate ofsteel at temperatures within the range or from 100 C. to 400 C. Molybdenum, chromium, vanadium,itungsten, silicon, manganese, boron, nickel, tantalum, etc., all have this tendency more or less.- In addition to exerting this effect, these elements also function to increase the hardenability of steel to which they are added over that of plain carbon steel of similar carbon content, and for this and other reasons elements of this type are'frequently used to produce alloy steels intended for carburization hardening. I

Since the austenitic transformation rate of plain carbon steel is comparatively rapid; the

present invention further includes the addition to the article previouslydiscussed, while 'the steel of the steel is molten, of one'or more alloying elemerits other than carbon, of such a character and used in such amounts: as'to materially retard the austenitic transformation rate of the steel in the defined range as compared to that of plain carbon steel of similar carbon;content. LThe element or elements used may be any of the broad class hereinbefore mentioned. When such a steel is carburized to produce an article having a portion of higher carbon content than another portlon, the. higher carbon portion has a slower austenitic transformation rate than that of the portion of comparatively lower carbon content, and the transformation rates of both portions are considerably retarded 'as compared to similar porthis in the case of plain carbon steel. Therefore, the carrying out of the method hereinbedescribed'is made easier in that a greater time is provided for-manipulation of the. work during the various steps involved. a During the time 'the steel isheld at the temperature range of from 100? C. to 400 'C., the austenite of the lower carbon portion is continuously transforming to the particular products producedwithin this range. The portionof higher carbon content during that time doesnot harden due to transformation of its austeniteand, therefore, isin a relatively malleable and ductile state, Conseduentlm: during the period while austenite transformation is coins on in thecase the higher carbon portion subsequently hardened to martensite with consequent expansion cannot usual, because this expansion is resisted bythe steel of the lower carbon portionwhich is completely hardened about the time austenitic transformation of the higher portion is initiated. In the case of certain articles such as aplate having one carburized surface, it is possible to plastically form the plate while its lower carbon portion is undergoing austenitic transformation, so as to cause its carburized face to be concave to a degree sufficient to result in subsequent hardening of this surface to a martensitic condition causing this surface to flatten. Also, the plate may be produced by such forming with its car 15 austenite of the carburized surface failing to burized surface convex soas to be'placed in compression during flattening of the plate Usual hardening of carburized plate may result in the carburized face being concave so that flattenin places this face in tension so as to involve the possibility-of rupturing it.

In some instances it may be desirable to tem-.

per the higher carbon portion to relieve some of the strain resulting from .its conversion to a martensitic condition by the second described quenching. This tempering may be done by heating the higher carbon portion to any temper.-

ature less than the temperature rangewherein transformation of the austenite of the -lower carbon portion'took place. In other words, the article should be tempered only by the use of temperatures less than from 100 C. to 400 C., the exact temperature depending on the temperature where the mentioned transformation of austenite occurred. v v 1 Although the principles. of this invention are applicable to the production of a great many carburized-articles, such as dies, gears, tools, shafts, bearings; rolls, etc., a specific example of the invention as it has been commercially applied to the hardening of light armor plate will now be disclosed.

The armor plate is rolled from steel to which alloys are added while it is molten so'it contains 25% carbon, :50% manganese, 4.5% nickel and 40% molybdenum, the balance being iron excepting for-immaterial'amounts of those elements normally classed as impurities in the case of they product. under discussion. .'I-he resulting plate is then carburized on one surface in .the prior art manner. Tests have shown that at 250' C.

' it. requires about two hours to eflect austenitic transformation producing the desired hardness in the case of the low carbon portion of this plate, whereas no austenitic transformation occurs in the case of the carburized surface over'a period of five hours; Spch informationmust be developed experimentall in the caseofsteels of different chemical compositions, but the desired retardation of the austenitic transformation rate of this heating is sumcient to assure complete carbide solution whereby the plate is rendered completely austenitic.

The completely austenitic plate is next 1 necessary, the hardened surface is tempered by quenched to a temperature within the stated range of from 100? C. to 400' C. This quaiching isaccomplishedbymeansofashortoilor waterquench or inth'e case of thin'sections', byimhold its temperature withinthe stated range. v Thisbath is composed of either lead or a lead alloy adapted to give the desired fluidity at the holding temperature used, but oil or salt baths may be used for this purpose. Care is taken in all events to control the temperature of the bath to keep it constant, it being preferable to-hold the temperature of the plate at a constant value within the stated range since this provides better control of the method.

During the time the temperature of the plate is being held, the austenite of the uncarburized portion is transforming to the particular products formed in the temperature range used, the

transform to any great extent during this period because of its higher carbon content and, therefore, its much slower rate of austenitic transformation. As soon as the austenitic transformation of the uncarburized portionis complete,

the plate is quenched by means of oil or water to approximately atmospheric temperatureand at a rate hardening the untransformed austenite of its carburized surface to martensite, this producing a very hard surface. As Just stated, the

second quenching is carried out after the lapsing of sufficient time to assure substantially complete transformation of the austenite of the uncarburized-portlon of the plate, and in all cases the 3 0 second quenching must be performed prior to transformation of more than 25% of the austenite of the carburized surface to obtain a properly hardened surface. The time period of the flrst quenching and the time for initiating the second quenching are easily determined factors.

If the plate is flat at the termination of the period effecting austenitic transformation of the uncarburized portion, the second quenching conve'rting the carburized surface to martensite,

40 causes a consequent expansion of this surface so that it becomes convex. This is contrary to what usually occurs in the case of conventional hardening methods and is important in that subsequent flattening of the plate throws the hardened sur- 5 face in compression so that it cannot rupture.

' way, a flat piece-of armor plate is produced which requires little or no subsequent flattening.

The plastic forming is done in various manners,

I a roller leveler, pressure jig, or other forming device being usabie.- The plate is removed from the bath for the forming operation or,'if desired, the

forming device itself may be immerseddirectly in the bath. Any such forming. of course, is done prior to complete hardening of the imcarburised portion of theplate.

"The armor plate produced bythis method has a properly hardened or martensitic carburised surface with its remainder comprising those products recognined as resulting from austenitic transformation practically entirelywithin the tcmperatm crlnle'of from C. to400 C. If

rehestingtotunperatures less than the transformation temper-shire for suitable periods of As previously mentioned, transformation prodnctsofausteniteproducedintherangestated mei-singthepistsdirectbintothebathusedto arc microscopically recogniad. Ihrthermole,

physical tests show greater ductility and impact strength than can be obtained by conventional hardening methods in the case of steel of similar chemical composition.

This application is a division of copending ab- 5 nantly bainitic.

plication Serial No. 330,798, flied April 20, 1940, ythe same inventors.

We claim:

A steel article having a portion with a higher carbon content than another portion, the firstnamed portion being at least partially martensitic and the second-named portion being predomi- JOHN M. HODGE. MAX W. LIGHTNER-