The suggested heat treatment of 420J2 stainless is to temper while it is still hand warm. I can't understand how it could improve mechanical properties and what exact temperature is that mean of hand warm?
My dear Samuel Sui;
Very good question , as a matter of fact it is not only question of improvement in mechanical properties it is a matter of saving the material from cracking.When the martensitic stainless steel JIS : 420J2/ASTM420/Din 1.4028 is austenitized and quenched it transform to martensite and some amount of retained austenite which is in meta stable condition.420J2 being 0.30% carbon 12.5 % chromium steel tend to have low Mf temperature and thus contains substantial amount of meta stable austenite .Thus quenched steel if not tempered immediately after quenching may result in cracking due to very large transformation stresses.Please refer attechment handbook of stainless steels to know more.
Regards
Shivraj Singh Kasana
As Shivraj Singh Kasana mentioned, 420J2 is martensitic stainless steel, indicating its martensite finish temperature is above room temperature. So Quenched to hand warm ( cool enough to touch with your hand, temperature about 50-60 C) is to ensure martensitic structure. If the quenched steel is not tempered right away, quench cracks would occur due to phase transformation-induced stress.
To avoid cracks due to residual stresses by retained austenite
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