Correct Answer - Option 2 : Isothermal quenching
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 martensite.
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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.