A Quick Overview of Heat Treatment in Salt Baths

25 October 2022

Workpieces made from metals are often processed by manufacturers to ensure that they boast the needed properties of various industries. One process that they carry out is heat treatment.

Heat treatment is comprised of various industrial, thermal, and metalworking techniques to change the physical or chemical properties of metal workpieces. Some forms of heat treatments that manufacturers apply to metal workpieces are hardening, annealing, tempering, and case hardening. These heat treatment forms require various tools, elements, or apparatus to ensure that the changes expected to metal workpieces will be achieved. 

One element that can be used to conduct these heat treatment processes is a salt bath. Any heat treatment process that has been carried out with a salt bath is called salt bath heat treatment.

Benefits of Salt Bath Heat Treatment

Salt bath heat treatment is a heat treatment technique that immerses treated metal workpieces into a molten salt or salt mixture. Tons of benefits are associated with salt bath heat treatment.

Quick Heating: One of the benefits of salt bath heat treatment is it can provide quick heating to workpieces. Metal workpieces that have been immersed into molten salt or salt mixtures can be heated through conduction. And since heat will be transferred by the liquid media, the heat transfer rate is expected to be much speedier than other heating methods like radiation or convection through a compatible gas.

Low Surface Oxidation: Another benefit of salt bath heat treatment is it can promise low surface oxidation to the treated parts. Whenever the workpieces are immersed in a salt bath, their exposure to the atmosphere is already minimised. This minimal exposure makes them protected from oxidation and damages associated with it.

Controlled Cooling: One more benefit of salt bath heat treatment is it can control the cooling conditions of the heat-treated parts during quenching. Water or oil are often used as the quenching media for conventional quenching operations, which may cause cracks and distortions due to their high cooling rate. Cooling in molten salt, alternatively, can make the cooling slower. It even stops at low temperatures, avoiding any damage.

Salt Bath Heat Treatment Processes

Many heat treatment processes can be done in salt baths. Some of them are as follows.

Quenching: Quenching is the process of cooling workpieces rapidly from their upper critical temperatures. Since molten salts can provide controlled cooling, they can save the parts from obtaining cracks and distortions.

Martempering: Martempering is conducted by quenching a part in a quenching medium or molten salt. The part will then be left to reach a uniform temperature distribution before its bainite formation. This process is carried out in parts with temperatures above their martensite formation or austenite-martensite transformation.

Austempering: Austempering is the isothermal hardening process wherein the workpiece is quenched in molten salt and is left to reach uniform temperature distribution. The workpiece will then be removed after the bainite formation has been completed.

Carburising: Carburising is another heat treatment process that involves the diffusion enrichment of a part’s surface layer with carbon. This process is normally carried out at temperatures of 850 to 950 degrees Celsius.

Other salt bath heat treatment processes that can be useful for many industries are hardening, nitriding, carbonitriding, and deep brazing. For your salt bath heat treatment needs, feel free to contact us at Alpha Detroit Heat Treatment.

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