When bottle with water is frozen through, it cracks. The same is often happening when steel parts are quenched through. To prevent crack formation, such steel parts are quenched very slowly in oils, polymers of high concentration, or gas quenching is used. When bottle with water is not frozen through, cracks do not appear. For real steel parts an optimal quenched layer exists which provides the best condition for intensive quenching and increasing service life of hardened steel parts. It means that hardened layer of steel parts should be optimized.
An optimal hardened layer is often considered as a hardened depth which provides maximal compressive residual stresses at the surface of quenched steel parts and improves significantly their service life. Maybe, it is better to say: an optimal hardened layer which provides maximal service life of quenched steel parts.
That's true. H value is equal to generalized Biot number Biv . Knowing Biv, one can calculate cooling rate of steel parts during quenching. If cooling rate and Jominy curves are available, one can predict hardenability. Interesting fact is: at small values of H at the surface of steel parts small tensile or neutral residual stresses appear. At very large values of H at the surface of steel parts after quenching compressive residual stresses are formed even in through hardened steel parts. That should be taken into account when developing recipes for quenching processes.
The developer of low-hardenability steels (Shepelyakovkij) assured, that the optimal hardened layer after quenching seems to be about 10...20 % of the part cross-section. He had some publications in Metal science and heat treatment journal. If you are interested, I can try to look for them.
Dear Maxim,
Thank you very much for your remarks. Yes, that's correct. 20-35% is almost optimal We developed a method of calculation which allow us to use this approach for large steel parts like rotors or rollers. According to our methodology, chemical composition of steel should be optimized depending on size and configuration of hardened product.
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