New bainitic steels for forgings

Steels with a bainitic microstructure offer great possibilities for highly stressed forged components. The variety of different bainitic morphologies requests for an aligned thermal treatment after forging in order to achieve the maximum performance. In dependence of the alloying concept and heat treatment bainite is composed of different microstructural components like the ferritic primary phase and the secondary phase, which consists of either carbides, martensite and/or austenite. Different combinations of mechanical properties can thereby be adjusted in these steels, dependent on the arrangement of the primary and secondary phase. Three steels have been investigated, which contain approximately 0.22% C, 1.5% Si, 1.5% Mn, 0.08% Mo, 0.003% B and 0.01 Ti. Their chromium contents vary between 0% for grade 1 and 1.3% for grades 2 and 3. The niobium content varies between 0% for grades 1 and 2 and 0.03% for grade 3. The Si addition is utilized to suppress the carbide formation, so a carbide free bainitic microstructure is expected to form, whereas Mo and B are employed in order to promote the bainite formation. The bainitic microstructure of these steels can be formed either after isothermal phase transformation or after continuous cooling. These two process routes lead to different results with regard to the mechanical properties, especially the Y/T-ratio. While after isothermal transformation the low chromium containing grade 1 exhibits a higher Y/T-ratio than grades 2 and 3, this fact is turned around for the case of the bainite formation after continuous cooling. In the latter case the high chromium containing steel exhibits both higher yield and tensile strength. These differences in mechanical properties can be correlated to characteristic features of the primary and secondary phases of the bainitic microstructures. The specific role of chromium is explained by its effect on the phase transformation kinetics.