The heat treatment of steels in the light of the principle sufficient hardenability
The risk of applying traditional hardenability measures  based on the so-called critical diameter Dk, ideal critical diameter Dik, basic diameter Do together with the hardenability coefficient f, which are worldwide popularised, among others by papers [2,3], is emphasised in this paper. The problem with traditional hardenability measures is that none of them indicates places, on the cross-section of the hardened rod, where the – so-called – upper bainite, the component of a very low crack resistance, is formed. Whereas a majority of steels for quenching and tempering has a high Bs temperature and – in practice – each cooling with a rate lower than the critical rate causes formation of the upper bainite zone on the hardened rod cross-section. The formal and practical ways of avoiding the upper bainite zone formation in the – so-called – reliable cross-section of the hardened rod , are presented in this paper.
Keywords: hardenability, sufficient hardenability, CCT diagrams, martensite, upper bainite, lower bainite