Correct Answer - Option 3 : toughness
Explanation:
The process of changing the structure and thus changing the properties of steel, by heating and cooling, is called heat treatment of steel.
Austempering: In this process, steel is heated above the austenitizing temperature (923°C). It is then quenched in the bath maintained at the temperature above the martensite start temperature within the Bainitic range (200–400°C generally). The final structure is Bainite in austempering. Bainite has higher toughness and ductility than martensite but its hardness and strength are less compared to the martensite.
Tempering: Tempering consists of heating the hardened steel to some temperature below the critical temperature, holding it for some time, and then cooling slowly. In tempering residual stresses are relieved, brittleness and hardness reduced and toughness is increased.
Martempering: Similar to austempering it also involves heating steel above austempering temperature and then quenched at a constant temperature just above the martensite start temperature. The final structure is Martensite.
