Bainitic Forging Steels for Cyclic Loading

Light-weight design is one of the main drivers for material development in the automotive industry. For optimum weight reduction new materials and their fatigue behavior under real cyclic service loads have to be taken into account (Gassner test). Currently the casted components made from Austempered Ductile Iron (ADI) show better service fatigue life for variable load cases than some traditional forging steels because of it’s inherent retained austenite. The traditional forging steels are the precipitation hardening ferritic-pearlitic steels (PHFP steel) and the martensitic quenched and tempered (Q&T) steels. The next steel generation for forged components in the drive train might be bainitic steels with an optimized microstructure with respect to cyclic behavior. Depending on the chemical composition and the heat treatment it includes a ferritic primary phase and a secondary phase, which consists of either carbides, martensite, retained austenite or M/A constituents. By alloying of more than 1% Si the formation of cementite will be suppressed and a carbide free bainite (CFB) will be formed. The secondary phase of this CFB contains retained austenite, which has the possibility to close crack tips by local compression stresses due to the transformation to martensite. As a result of this CFB exhibits better cyclic properties than the commonly used forging steels. The materials and process design as well as results of the fatigue behavior will be presented.