The hardened work piece is extremely crunchy. By performing the return operation, hardened steel, stiffened, its hardness decreases slightly and the residual tensions are eliminated.
Depending on the purpose of the tool, for example, a high toughness and return low stiffness at high temperatures are performed. If the hardness and abrasion resistance are high, the return is performed at low temperature.
The work piece is reheated immediately after the quenching. The return temperature depends on the type of steel. Each steel has its own return graph. The effect of return operation can be estimated by maintaining long-term low temperature or short-term maintenance at high temperatures.
In the case of tool steels, double returns are very helpful. This is especially true in terms of toughness. Tool steels should be kept for about 2 hours at a return temperature.
At the surface of the work piece, hardened and tempered depending on the temperature, various oxide layers are produced in thickness. If white light shines on the hardened surface, it turns into color light depending on the thickness of the oxide layer. The color of the light created depends on the temperature. The determination of temperature with the help of return colors today has been replaced by modern measurement methods. This method is not used in alloy steels. Because these steels are not easy to rust.
Changing Dimensions in Hardness and Return:
Dimensional changes are unpredictable dimensions and shapes. The reason for this change in size is the difference between the mass for the ferrite-cementite structure, austenite, and martensite. The martensitic structure of hardened steel is generally larger than the original soft structure. The alloying elements, the amount of austenite remaining and the cooling rate are effective on the change of dimension. In addition, it is very important to have brain or surface hardened.
The largest increase in volumes is in non-alloy steels, and also in brain hardening. The volume increase is about 0.7-1%. This amount occurs in low-alloy and high alloy steels up to 0.5%
For tools – screw cutting and cutting, and also – accurate measurements, steel with a very small dimensional change is anticipated. The exact amount of resizing can be called from the steel manufacturer.
Resizing changes in hardening will change with return operation. In low-alloy steels, a low volatility reduction occurs at a return temperature below 200 ° C. This is due to the restructuring of martensite. Between 200 ° C and 300 ° C, a volume increase occurs. The size of the work piece is larger than the pre-return status. A further increase in the temperature of the return leads to a reduction in volume again.
Depending on the type of steel, different behaviors appear in high alloyed steels. Resizing in hardness and return should be combined.