US2254328A - Material for and method of heat treating - Google Patents

Description

Patented Sept. 2, 1941 UNITED STATES PATENT OFFICE MATERIAL FOR AND METHOD OF HEAT TREATING Francis J. Steigerwald, Detroit, Mich., assignor Y to Park Chemical Company, Detroit, Micln, a

corporation of Michigan No Drawing. Application November 6, 1939, Serial No. 303,018

4 Claims.

The present invention relates to neutral salt heat treating baths and methods of heat treating steel in such baths.

In the heat treatment of iron and steel and their alloys for the purpose of hardening, the articles to be hardenedare heated to a point above their critical temperature and then quenched. Difliculties are encountered during the heating operation, particularly with high carbon steels, by reason of oxidation and consequent loss of carbon from the surface of the part being heated. This loss of carbon and consequent softening of the outersurface of the iron or steel part' has serious consequences, particularly in connection with the heat treatment of tool steel, high'speed steel cutting tools, carburized parts, or any parts in which surface hardness is important, and many expedientshave been attempted to prevent this surface decarburization. Among other things, attempts have been made to immerse the steelbeing treated in borax or boric acid, but this expedient is unsatisfactory because it is difllcult to remove the coating of borax or boric acid, which forms on the part being treated.

metal is stirred into the bath to render it nonoxidizingand then is removed from the surface where it forms as a scum. On difficulty with this expedient is that aluminum floats on top of the barium chloride with the result that it is necessary tostop the heat treating operations in order to stir the aluminum into the bath, and thereafter skim it from'the surface. Moreover, the aluminum bursts into flames as soon as it strikes the surface of the bath and it is, therefore, very diflicult to handle.

If aluminum is added to certain other salt baths such as sodium or potassium chloride, it will sink to the bottom but is still unsatisfactory because it melts at a temperature below that at which steel must be heat treated. Consequently; the molten aluminum forms in a ball or globule at the bottom and does not disperse sufficiently Moreover, aluminum in some manner attacks the steel pots which are used in the process causing them to burn out quickly.

It is the object of the present invention to provide a novel neutral salt heat treating bath and a novel method of maintaining it in a non-oxidizing condition during use.

In accordance with the present invention, a fused bath is formed from an alkaline or alkaline earth chloride, or mixture of such chlorides, to

which is added a small quantity of a suitable reducing agent. As is the usual practice, the bath is kept in a molten condition in a pot formed of an iron alloyand' the steel parts are simply immersed in the bath until they reach the desired temperature. Any alkaline or alkaline earth chloride, or any mixture of such chlorides, may be used depending upon the temperature at which it is desired to conduct the heat treating operation; One suitable mixture of such chlorides contains 56% potassium chloride and 44% sodium chloride. This mixture, which is the eutectic mixture of these materials, melts at about 1230 F.,

and may be successfully used for heat treatment at any temperature between 1300 F. and 1700 F. The temperature at which th heat treating operation is to be conducted controls the choice of the chloride or mixture of chlorides to be used, since it is necessary 'to have a bath which will 39 melt well below the temperature at which the heat treating process is to be conducted, but

' which will not vaporize excessively at the heat to react to the best advantage with the bath.

cost. Alkaline chlorides are also preferred over.

alkaline earth chlorides because the former are easier to clean off the treated parts.

Any substance which is not lighter in weight than the bath, which will not melt below about 1400 F., and which has a sufficiently strong affinity for oxygen to reduce any oxidizing materials that may form in the bath may be used as the reducing agent. Examples of oxidizing ma.- terials that may form in the bath and which must be reduced are carbonates or iron oxide. both of which if present cause decarburization of the steel. In general, it is found that materials which are used to deoxidize steel during the manufacture thereof are, if heavier than the bath, effective reducing agents in the neutral fused heat treating bath of the present invention. The preferred reducing agent is silicon carbide, but others which are satisfactory include silicon, ferro-silicon, manganese, zirconium, ferro-zirconium, titanium, ferro-titanium, and calcium silicide. These agents, being heavier than the bath,

sinkor disperse therein and react in the body of the bath without interfering with the heat treating operation.

Calcium, while exceedingly active as a reducing agent, cannot be used successfully in its pure state because it is so light that it floats and burns upon the surface without reacting sufficiently with the main body of the bath. The same is true of aluminum in certain of the heavier baths such as those containing barium chloride. It is necessary for successful operation that the reducing agent be sufllciently heavy to either sink to the bottom of or disperse through the bath in order to react with the oxidizing materials in the body of the bath itself rather than with the atmosphere to which the surface of the bath is exposed. While calcium is too light to satisfy this requirement, it is possible to alloy calciumwith heavier materials, as in the case of calcium silicide, and thus produce a successful reducing agent. The melting temperature of aluminum is too low for satisfactory use.

The amount of reducing agent required does not appear to be critical. Satisfactory results have been secured with a bath which initially contains 1% of a reducing agent of the type mentioned and the balance a mixture of chloride such as that indicated above.

During continued use of the molten bath, the reducing agent is used up, and, accordingly, it is necessary to add a small quantity at regular intervals. In one instance, it was found suflicient to add approximately silicon carbide daily to a neutral salt bath in continuous use. However, the amount requlred will undoubtedly vary in accordance with the conditions of use and it may be necessary to determine by experiment from time to time whether the bath is in an oxidizing condition and add the reducing agent in suflicient' quantities to correct any oxidizing condition found to exist.

During use, of the bath a sludge accumulates at the bottom of the pot and must be removed at regular intervals.

It is found that by the use of the salt bath herein described excellent results are secured in heat treating in that no decarburization is noted, and no difficulties are encountered in removing any of the salt which may adhere to the surface of the articles being treated. Moreover, the bath retains its superior characteristics indefinitely,

' so long as suitable additions of the reducing agent are made at regular intervals.

What is claimed is:

1. A neutral salt heat treating bath for treating steel at temperatures between 1300 F. and 1700 F., said bath containing a salt from a group consisting of alkaline and alkaline earth chlorides and a small quantity of silicon carbide.

2; A neutral salt heat treating bath for treating steel at temperatures between 1300 F. and

FRANCIS J. STEIGERWALD.