Salt bath heat treatment

Dr. Dmitri Kopeliovich

Salt bath heat treatment is a heat treatment process comprising an immersion of the treated part into a molten salt (or salts mixture).

• Benefits of heat treatment in salt baths
• Compositions of salt baths
• Heat treatments conducted in salt baths

• Fast heating. A work part immersed into a molten salt is heated by heat transferred by conduction (combined with convection) through the liquid media (salt bath). The heat transfer rate in a liquid media is much greater than that in other heating mechanisms: radiation, convection through a gas (e.g., air).
• Controlled cooling conditions during quenching. In conventional quenching operation either water or oil are used as the quenching media. High cooling rate provided by water/oil may cause cracks and distortions. Cooling in molten salt is slower and stops at lower temperature.
• Low surface oxidation and decarburization. The contact of the hot work part with the atmosphere is minimized when the part is treated in the salt bath.

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• Quenching of steels. Quenching is rapid cooling from the temperature above A₃ (upper critical temperature). Relatively slow cooling rate provided by molten salts prevents the work part from cracking and distortion.
• Austempering. Austempering is the isothermal hardening method in which a part is quenched in a quenching medium (molten salt) and is left in it reaching uniform temperature distribution. The part is removed from the quenching medium after the complete bainite formation. Tha austempering temperature range is 400-750°F (204-399°C). Nitrate salts No. 4-6 are used for austempering treatment.
• Martempering. Martempering is the isothermal hardening method in which a part is quenched in a quenching medium (molten salt) and is left in it reaching uniform temperature distribution. The part is removed from the quenching medium before the bainite formation. Martempering is performed at a temperature above the the temperature of martensite formation (austenite-martensite transformation), which is 400-480°F (200-250°C). Nitrate salts No. 4-6 are used for martempering treatment of most alloys. Sodium nitrate (No.7) a potassium nitrate (No.3) are used for martempering tool steels (hot-work and high speed steel).
• Hardening. Hardening is performed at 1400-2300°F (760-1260°C) in chloride salts (No.8-11).
• Nitriding. Liquid nitriding is the process of diffusion enrichment of the surface layer of a part with Nitrogen provided by a molten cyanide base salt (extremely toxic substance). The process is carried out at the temperatures 950-1075°F (510-580°C) for about 4 hour.
• Carbonitriding. Liquid carbonitriding is the process of diffusion enrichment of the surface layer of a part with carbon and nitrogen provided by a molten salt containing 20-25% of sodium cyanide (extremely toxic substance). The process is carried out at the temperatures 1500-1580°F (820-860°C).
• Carburizing. Liquid carburizing is the process of diffusion enrichment of the surface layer of a part with carbon provided by a molten salt containing 10-25% of sodium cyanide (extremely toxic substance). The process is carried out at the temperatures 1562-1742°F (850-950°C).
• Solution treatment of Aluminum alloys. Solution treatment is the operation of heating the work park to a temperature at which the hardening second phase particles dissolve in the matrix. Solution treatment of heat treatable aluminum alloys is carried out at 900-1025°F (482-551°C). Fast solution heat treatment may be achieved by heating an aluminum alloy part in a molten salt bath. Nitrate salts No.4,5,6 are used for solution treatment of aluminum alloys.
• Deep brazing. Brazing is a method of joining two metal work pieces by means of a filler material at a temperature above its melting point but below the melting point of either of the materials being joined. Dip brazing is a brazing method in which the work pieces together with the filler metal are immersed into a bath with a molten salt. The filler material melts and flows into the joint. Chloride salts with addition of reactive agents are used for deep brazing.
• Cleaning. Polymeric contamination on metal parts surfaces may be effectively removed by immersion of the part into a molten salt. Polymers decompose and burn at the temperature of the molten salt. Mixtures of hydroxides and nitrates at a temperature within 650-950°F (343-510°C) are used for cleaning operation.

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• Annealing and stress relief
• Normalizing
• Hardening
• Case hardening
• Ion nitriding
• Tempering
• Precipitation hardening
• Cryogenic treatment of steel
• Furnaces for heat treatment
• Basic principles of heat treatment

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