Immersion of steel and other metals in a salt solution for tempering or heat treating. Salt baths may be classified in three general types: neutral, reducing or oxidizing.
Neutral Baths
(1) Low-temperature baths which are operated at 150to 595°C (300 to 1100°F) may be used for tempering or for low-temperature heat treatments such as the solution treatment or aging of aluminum alloys.
(2) Medium temperature baths, operated at 675 to 900°C (1250 to 1650°F) are used principally for heating steel before quenching. High-temperature baths, higher than 925°C (1700°F), are used primarily for heat treatment of high-speed steel (tool steel alloys), but may also be used for copper brazing.
Among the precautions to be observed in using various types of salt bath: it is important to avoid contamination of neutral baths with cyanide salts. Another precaution is to avoid overheating the bath.
Reducing Baths
Reducing salt baths are used for carburizing or nitriding. A sufficient concentration of cyanide must be maintained in reducing salt baths for satisfactory results. A carbonaceous blanket on top of a bath of this type not only cuts down heat loss, but also helps to reduce the breakdown of cyanides in the bath.
Oxidizing Baths
Oxidizing baths are used for coloring steels or other metals and may also be used for annealing noble metals. Fused salt baths of this type may be used at 5 10°C (950°F) for blackening steel, and an aqueous solution of this type may be used at 150°C (300°F) for the same purpose.
The surface hardness of heat-treated tool steel alloys may be increased by nitriding them in a “high-speed case” salt bath at approximately 550°C (1025°F) for a relatively short period of time. The tendency of sharp edges of tools treated in this manner to chip can be reduced by a subsequent tempering operation at 540 to 565°C (1000to 1050°F).