Salt bath hardening

Nickel thermocouple sheaths have been used satisfactorily in salt baths, e.g. in barium chloride at 1 000-1 100°C and in sodium and potassium chlorides at up to 980°C. If the salt is contaminated with sulphur then an Inconel sheath is preferred to one of nickel. The latter type is also satisfactory in cyanide baths used for case-hardening. 

Case Hardening Replacing Salt Baths with Gas Carburizing and Nitrating, and with Applied Energy methods SR Kirk-Othmer 1981... 

SALT BATH. A molten mixture of sodium, potassium, barium, and calcium chlorides or nitrates to which sodium carbonate and sodium cyanide are sometimes added. Used for hardening and temperature of metals and fox annealing both feuous and nonfeiious metals, Tempeiatuies used may be as high as 1,315°C for hardening high-speed steels. Commercial mixtures are available for a variety of specifications. 

Nitride layers are produced after treatment in a salt bath or in a gas atmosphere by inward diffusion, usually below 600°C. Unlike transformation hardening, which is effected by lattice distortion as a result of embedded carbon atoms, nitriding and boriding provide the surface with an exceptional increase in hardness by the formation of an intermetallic bonding layer. These bonding layers also increase the general corrosion resistance, but they are so thin that they can break by mechanical point loading. 

This process simulates the hot bath method of hardening in a salt bath, and has the advantage that temperature equalization between face and core takes place in the austenitic micro structure state (e.g. at about 550 °C in the case of 1.2343... 

Use Treatment of brines in chlorine-alkali cells to remove sulfates, rodenticide, production of barium salts, ceramic flux, optical glass, case-hardening baths, ferrites, in radiation-resistant glass for color television tubes. 

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