The previous blogs discussed the general, basic principles of the tempering process. The following tempering characteristics were discussed:
- Reduced risk of cracking
- Acquiring the appropriate mechanical properties of tensile strength, impact strength and other properties
- Producing the required and desired metallurgical properties
- The principle of soaking time at temperature
This article will now discuss how tempering affects the mechanical and metallurgical properties of the steel being tempered.
Hardness is perhaps the first noticeable change that takes place during the tempering operation. Based on the selected tempering temperature and the time at that particular temperature, the final hardness will be determined.
There is generally no noticeable change in hardness to the steel (through hardened or surface treated by carburizing) at a temperature of, say, 375°F. The final tempered hardness will depend on: selected temperature, residence time of the steel at the tempering temperature, material chemistry (carbon content and carbide-forming elements), type of martensite present in the microstructure and retained austenite (if present after quenching).
The type of martensite that is present will be determined by the carbon content of the steel. It can be stated that there are two types of martensite that can form: lath martensite (steels with low carbon content) and plate martensite (steels with carbon content above 0.55%).