Controlling the MC and M2C carbide precipitation in Ferrium® M54® steel to achieve optimum ultimate tensile strength/fracture toughness balance

Ferrium® M54® exhibits an excellent UTS/K1C balance allowing its application in aeronautical structures. This steel belongs to the Co-Ni UHS steels family with M2C nanometer-size carbide precipitation during tempering. These steels provide very high strength with a very good fracture toughness thanks to the M2C fine precipitation during tempering, but also because coarse particles are dissolved during austenitizing without grain coarsening. The goal of this article is to identify the different carbide populations in M54®. A small addition of Ti in M54® forms a Ti-rich MC carbide precipitation that is stable at high temperature. Consequently, during austenitization at 1060 °C, all other types of coarse carbides are dissolved in the matrix without grain coarsening. As a very small part of the initial carbon content is needed to form MC carbides, efficient and intensive nanometric M2C carbide precipitation takes place during tempering, leading to very high final strength. Due to this double precipitation of carbides in M54®, the steel achieves an outstanding UTS/K1C balance. •Small and uniformly distributed undissolved Ti-rich MC carbides control the grain size during the austenitization•High austenitizing temperature (1060°C) allows the full dissolution of the coarse carbides (size > several of 100nm)•Intensive nanometer-sized carbides precipitate during tempering thanks to the high supersaturation of alloying elements•Coarse carbides dissolution and nanometer-size carbides precipitation is necessary to achieve high UTS/KIC balance