Review of TTT and CCT diagrams of typical low alloy steels, their hardenability and requirements for preheat and use of low hydrogen processes. Alloy steel TTT diagram shows a region where the steel can form bainite during cooling. The nose is further to the right and hence it is easier for steels to form martensite and allow for deeper hardening compared to carbon steels. However, actual maximum hardness is still determined by the amount of carbon content.