The heat treatment of H+T is the sum of quenching and tempering. Quenching and tempering is carried out to obtain a metal with a fine grain structure and characteristics suitable for future use.
When steels are hardened, martensite is formed, a structure with high hardness and considerable breaking load. However, its resilience is rather low and, following impacts, it can cause cracks or fractures of the material.
For this reason, steel is subjected to the tempering heat treatment and the part of martensite is transformed into tempered martensite. In the absence of binding elements, martensite is formed only with a percentage of carbon higher than 0.2%. The steels for this treatment are called tempering steels and their carbon content ranges from 0.4% to 0.6%. Below this limit, it does not make sense to temper the steel or carry out the remediation.
Martensite is a metastable phase, that is, it is formed only because the carbon atoms are unable to escape from the lattice due to the high cooling rate which prevents diffusive motions.