Recent developments of direct heat-treated steels for high strength forgings have led to some new steel grades with excellent service properties. Quenched and tempered steels are nowadays more and more replaced by precipitation hardened ferritic–pearlitic steels and by bainitic steels. Based on the experience on upper, lower and carbide-free bainite, a critical assessment is performed concerning the most important influencing parameters and processing routes. Steel designs and cooling concepts are discussed regarding their optimal chemical composition, achievement of a homogeneous microstructure and a low sensitivity to processing parameters. Some controversial aspects are illustrated and further need for research work is highlighted.
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