Oil hardening
Oil hardening is quenching in oil instead of water. Oil cools slower and more evenly — lower hardness than water, but a lot less distortion and crack risk. Standard for most alloyed tool steels.
Physical-Chemical Mechanism of Action
When you quench a glowing part in oil, it goes through three steps: first a vapor blanket forms around it, then the oil boils at the surface and draws off the heat effectively, and finally only convection cools. Oil removes heat slower than water but more uniformly over the whole part. For alloyed steels, this is still enough to get because the alloying elements lower the required threshold.
Metallurgical Relevance & Welding Risks
Cooling more gently means less difference between surface and core, hence less residual stress, less , and significantly less risk of cracking than with . The trade-off is slightly lower surface hardness, and for unalloyed grades, the simply isn't enough. Right after quenching, the part is under full stress and still contains . In this state, it is most sensitive.
Workshop Practice & Instructions
Preheat the oil and keep it moving to break up the vapor blanket and avoid soft spots. Use enough bath volume, otherwise the oil heats up and loses its quenching power. Immerse the part vertically and quickly. Keep a lid or fire blanket within reach—hot oil can catch fire. And after hardening, don't leave the part overnight, but temper it promptly .