Heat Treatment of Fasteners in a Salt Bath Furnace

Uses

• Salt bath furnaces change the physical and chemical properties of metals. During a salt bath, metal parts are dipped into a molten mixture of salt for varying amounts of time, depending on the type of metal and the desired effect. The temperature, chemical composition of the salt and the duration of the dip determine how the metal is changed in the salt bath. Salt baths are often used to improve the hardness or ductility of the metal or improve the regularity of its structure.

Salt Bath Furnaces

• Oil- and gas-fired salt bath furnaces heat salt and metal parts with combustion while electric furnaces use a series of electrodes to pass high voltage current through the salt. The electrical resistance of the salt generates heat until the salt reaches its melting point. Electric salt bath furnaces typically cost less to operate than oil- or gas-fired models. Unlike oil- and gas-powered furnaces, electric models do not require mixing equipment to ensure that the salt is uniformly heated. The electrodes and molten salt in the furnace create an electromagnetic field that pulls the molten salt downward so that it mixes with the metal.

Heat Treatments

• Salt baths are used in a variety of common metal treatments, including tempering, hardening, brazing and annealing. Brazening treatments are used to connect metal parts with another metal, such as silver solder, copper or aluminum. Salt bath furnaces are also used to increase the hardness of fasteners through an isothermal quench. This process submerges hot steel fasteners in a molten salt bath heated between 400 and 750 degrees Fahrenheit. Salt baths are also used to anneal metal fasteners. This process heats fasteners to temperatures in excess of 1,000 degrees to make them more ductile and less susceptible to breakage.

Benefits

• Fasteners that are heat-treated in a salt bath furnace are less likely to develop structural flaws due to how the molten salt interacts with metal. Fasteners that are placed into a salt bath are immediately coated with a protective layer of salt. This layer prevents the metal from heating too quickly and ensures that each fastener is heated evenly. The interaction between salt and fasteners reduces the heat stress on the metal and prevents it from warping or cracking during the treatment process.