Hardening of Steel in a Salt Bath
Salt bath hardening is a fast and efficient process that delivers visible results. By design, salt baths can be divided into baths with internal or external heating and electrode baths.
Salt Baths with Internal or External Heating
This type of bath is designed for cooling high-speed steel during step hardening, as well as for the heat treatment of various metal sheets and profiles at temperatures of 450–525 degrees Celsius. The baths are 6 to 8 metres long and can reach a power of hundreds of kilowatts.
Baths with external heating are round or rectangular steel vessels placed in a shaft with heating elements. In units with internal heating, tubular heaters are placed directly inside the bath. Baths with internal heating have much lower heat losses than baths with external heating; in addition, the consumption of heating elements is reduced almost tenfold. They are also safer to use.
Electrode Salt Baths
An electrode salt bath is a bath filled with salt into which electrodes are immersed. This design ensures uniform and rapid heating and is especially popular for hardening tools. Recently, electrode salt baths with side-mounted electrodes in the form of round or rectangular rods have become increasingly common. In the bath, the electrodes are placed 20–25 mm apart from each other.
The current passing between the electrodes heats the salt, creating intense thermal circulation: salt particles rise and spread along the upper edge. Hardening high-speed steel in a salt bath is 5 times faster than in a gas-fired or electric furnace, and the heat is evenly distributed over the entire surface of the product. Another advantage is that only part of a tool, for example a drill, can be heated to the required temperature.
The disadvantages of hardening high-speed steel in a salt bath include heavy spattering of salt when a wet tool is immersed in the bath. In addition, the fumes released by a salt bath are harmful to human health, so it is very important to follow safety rules when working with it. Today, single-phase and three-phase electrode salt baths with a power of 20–150 kilowatts, designed for temperatures of up to 1300 degrees, are available.
Salt bath hardening, like hardening in a barium chloride bath, is very common in Ukraine. As a result of hardening in molten salts, products are covered with a layer of salt, which provides good protection against oxidation. When hardening metal, the most important thing is to follow the technology: if everything is done correctly, the result is hard and strong hardened steel.