Correct Answer - Option 2 : Isothermal quenching
The process of changing the structure and thus changing the properties of steel, by heating and cooling, is called heat treatment of steel.
- 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 martensite.
In isothermal quenching or austempering, the steel is heated to the required hardening temperature in the same manner as in martempering but the quenching time in the salt bath is longer.
- Another difference in this method is that the salt bath temperature for austempering is above the martensite point to ensure a sufficiently complete austenite decomposition into bainite.