Question

Explain why the steel cooled in air becomes ductile and the steel quenched in water becomes brittle

Answer 1

The fast cooling process is otherwise known as quanching. quenching results in a metal that is very hard but also brittle. quenching of metal can reduce the crystal grain size of metallic materials, so it will case the increase of hardness. Gently heating a hardened metal and allowing it to cool slowly will produce a metal that is still hard but also less brittle. This process is known as tempering. It results in many small Fe₃C precipitates in the steel, which block dislocation motion which thereby provide the strengthening.