Materials that have not been heat treated show different hardness at room temperature, depending on the carbon content they contain. Some materials may not be easily machined due to their hardness. Especially for plastic deformation processes, it is desired that the materials have minimum hardness. For this reason, vacuum softening annealing is applied to the materials in order to soften the materials.
The structure of steel materials at room temperature is in the form of thin long plates, in the form of particles and in direct proportion to the carbon content in it, in the form of carbide precipitates in the form of rows. The frequency of carbide plates in this structure, which is called perlite, increases with the carbon content of the material, and this causes an increase in hardness.
By vacuum softening annealing, elongated carbide plates are transformed into a shorter and spherical structure. In this case, the steel has a softer and easily shaped structure compared to its original state.