Usually, quenching of steel parts and tools are preformed in oils at the temperature 30oC - 60oC where distortion is high due to high viscosity of oils. In different surface areas of steel part viscosity of oil can be different depending on configuration of part. Temperature of oil should be optimized during quenching to provide smooth cooling.
Optimization can be done by use:
1. Optimal temperature of oils which provides maximum critical heat flux density for given oil.
2. Optimized agitation of oil.
3, Special additives which eliminate local film boiling.
4. Controlling cooling process to keep it the same.
Here are some useful data for mineral oil MK depending on its temperature. Prandtl number at temperature 10oC is 39000 and the temperature 150oC decreases to 113, almost 345 times..
When quenching in cold oil local film boiling appears due to high viscosity of oil. When quenching in very hot oil, local film boiling appears due to decrease underheat of oil. There is an optimal temperature where local film boiling is absent and distortion after quenching of steel parts is minimal. Every type of oil has its own optimal temperature.
Sir I think one way to minimize distortion is provide cooling rate just near to critical cooling rate. If one is using oil, then varion of characteristics of oils like viscosity, heat conductivity, latent heat of vaporization and specific heat of oil should be fundamental parameters, which one needs to consider in obtaining required cooling rate for that particular part. But if you are using another quenchant like some special polymer dissolved in water, they should minimizes risk of distortion, due to their inverse solubility, and hence by maintaining approximately constant viscosity.
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