In this process, the casting to be hardened is temporarily heated near the surface by submerging it into a high-temperature liquid bath (metal or salt bath).
The minimum bath temperature must be at least 150 to 200 °C above the Ac3 temperature (see A1-A4) of the steel to be hardened. The heating process must be accomplished fast enough that the heat flow into the surface is higher than the heat flow dissipated to the core by thermal conduction. Thus, heat is accumulated in the outer edge zone.
Immersion hardening is always used as a process for rim layer hardening if torch hardening and induction hardening or other processes cannot be used for technical or economical reasons. This process is primarily used for hardening the shell or the face of cylindrical parts.
Electron beam hardening
Salt bath furnace