US3194696A - Cyanate baths - Google Patents

Description

United States Patent "ice 3,194,696 CYANATE BATES Johannes lvliiller, Nen-l'senburg, Germany, assignor to Deutsehe Goldand Siiber-Scheideanstait vormals Roessler, ranltfnrt am Main, Germany No Drawing. Filed June 11, 1963, Ser. No. 286,954 Claims priority, application germany, June 20, 1962,

2 7 2 Claims. (Ci. 148--28) Cyanide baths for hardening steel have been known. For example, it has been known that increased hardness of steels with low or medium carbon content can be obtained by treating such steels in a cyanide-containing bath at temperatures of between 650 and 1000 C. (1200. and 1840 F.). Usually such baths are made up of 15-95 alkali cyanide, and a balance of chlorides and/or carbonates of alkalies or earth-alkalies. Carbon and a small amount of nitrogen penetrate the surface of the workpiece whereby the surface hardness is obtained by the formation of martensite resulting from a more or less severe cooling. Such cyanide-containing baths may, in certain cases, contain a maximum of 1% in cyanates.

This application relates to cyanate baths. For example, it has been found that surfaces with even better qualities can be obtained in a salt bath consisting of 3- 30% alkali cyanate, and a balance of chlorides and/or carbonates of alkalies or earth-alkalies. In this case, a relatively thin outer layer or zone of carbides and nitrides is formed on the steel surface (known as a compound layer) while beneath this zone a layer or zone of martensite is formed, causing a remarkable hardness increase on the surface of the workpiece. This outer thin compound layer or zone of carbides and nitrides is highly resistant to wear and seizure and, at the same time, the hard layer of martensite displays a high resistance to compressive stresses.

These qualities are particularly advantageous in the treatment of gears.

One important feature, in addition to those mentioned, is that due to the absence of cyanide the bath is nonpoisonous and, therefore, special protective measures during operation and sludge removal are not required. The bath is a cyanate, not a cyanide, bath.

It is advisable to avoid or prevent decomposition of the cyanate. This can be achieved by lining the bath container with a non-ferrous metal, such as titanium, nickel, cobalt, or their iron-free alloys. It is further recommended that other parts of the equipment, such as burner tubes, heating elements, etc., be built of the same metals.

The inevitable decomposition of cyanate takes place While carbonate is formed in the bath. Cyanide is formed in only insignificant quantities. The formation of martensite, resulting in the hardness increase, requires a more or less severe cooling from the operating temperature, depending upon the metal used.

REPLENISHMENT For replenishment purposes, salt mixes are used with a high percentage of cyanate, particularly above 40% or even pure cyanate.

Examples Example 1.A bath is made up of 10% NaCNO and 90% NaCl.

If a steel with a carbon content of 0.15% is treated in this bath at a temperature of 850 C. (.1560 F.) for 90 minutes, a compound layer or zone is formed, 20 microns in thickness, consisting of carbides and nitrides. Beneath this compound layer another layer is formed,

3,l9d,5% Eatented July 13, 1965 approximately 350 microns in thickness, penetrated by carbon and nitrogen, which after quenching, because of the formation of martensite, has a hardness of 800 Vickers-the original hardness of the material having been 150 Vickers.

The following mix is used for replenishment: 50% NaCNO and 50% NaCl.

This mix is added in such an amount as to result in a cyanate content of between 510% in the bath.

Example 2.A bath made up of 10% NaCNO, 50% NaCl, and 40% KCl.

A replenishment mix consisting of 50% NaCNO, 25% KCl, and 25 NaCl was used.

Replenishment is made in amounts sufiicient to maintain a cyanate content of 510%.

Compared to bath #1, this melt has the advantage of a lower melting point. It can be used under 800 C. 1620" F.) for the intended purpose.

Example 3.A bath is made up of 5% NaCNO, 50% BaCl and 45% KCl.

A replenishment mix of 40% NaCNO, 30% BaCl and 30% KCl was used.

As in Example 2, this mix has a lower melting point. During operation of this bath, alkali carbonate is formed by oxidation of the cyanate. This alkali carbonate may be added from the start if, for instance a bath is made up of 20% NaCNO, 40% 'Na CO 30% KCl.

If a lower melting point is not required, the addition of chlorides may be omitted and a mix may be made up containing 10 to 20% NaCNO and to Na CO Such a melt need only be replenishment with sodium cyanate or potassium cyanate to maintain the cyanate content necessary for a suflicient nitrogen and carbon concentration.

To eliminate heat radiation and fuming of the bath, particularly at temperatures of above 850 C. (1560 F.) or 900 C. (1660 F.), it is common to use a graphite or carbon cover.

The bath container may be heated externally by means of gas, oil, or electricity, or internally by immersed electrodes. In the latter case it would be necessary to build the electrodes of a material similar to that used for the container, or to coat them With such a material.

Now having described the cyanate bath hereof, reference should be had to the following claims.

I claim:

1. A fused bath for increasing the hardness of steel consisting of 3 to 30% of alkali metal cyanate and the remainder at least one member selected from the group consisting of alkali metal and alkaline earth metal chlorides, carbonates and oxides.

2. A fused bath for increasing the hardness of steel consisting of 3 to 30% of alkali metal cyanate and the remainder at least one member selected from the group consisting of alkali metal and alkaline earth metal chlorides, carbonate and oxides confined in a container having a surface of an iron free metal in contact with such fused bath selected from the group consisting of titanium, zirconium, nickel, cobalt and their alloys.

References Cited by the Examiner UNITED STATES PATENTS 2,007,136 7/35 Albrecht et al. 14828 X 2,927,875 3/60 Muller l48-15.5

OTHER REFERENCES Metals Handbook, published by the American Society for Metals, 1948 Edition, pp. 576, 577, 694.

BENJAMIN HENKIN, Primary Examiner.

WINSTON A. DOUGLAS, DAVID L. RECK,

Examiners.

Claims (1)

1. A FUSED BATH FOR INCREASING THE HARDNESS OF STEEL CONSISTING OF 3 TO 30% OF ALKALI METAL CYANATE AND THE REMAINDER AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL AND ALKALINE EARTH METAL CHLORIDES, CARBONATES AND OXIDES.