Effect of Aging Isothermal Time on the Microstructure and Room-Temperature Impact Toughness of Fe-24.8Mn-7.3Al-1.2C Austenitic Steel with kappa-Carbides Precipitation

The microstructure and impact toughness of the as-cast Fe-24.8Mn-7.3Al-1.2C austenitic steel after solution treatment and subsequent aging treatment were investigated in the present work. Research on the kappa-carbides precipitation behavior was carried out by transmission electron microscope. The results show that nano-sized coherent kappa-carbides were obtained in the as-solutionized steel after aging treatment, which produced precipitation hardening. After being aging treated at 550 A degrees C for 1 h, the steel with regular hexagonal grain structure exhibited a good combination of yield strength (similar to 574 MPa) and room-temperature impact toughness (similar to 168 J). In the present steel, the typical cube-on-cube orientation relationship between austenite and kappa-carbides was observed. However, due to the long aging isothermal time and high C content, the coarse intergranular kappa'-carbide was formed and grew along the austenite grain boundary, which caused this orientation relationship to be destroyed and a dramatical increase of the coherency strain energy at grain boundary. Furthermore, serious embrittlement of grain boundaries caused that cleavage cracks trend to propagate along the grain boundaries. Accordingly, the room-temperature impact toughness decreased sharply. After aging isothermal time prolonging to 13 h, the Charpy V-notch impact toughness was only similar to 5 J and fracture mode turned to fully brittle fracture accompanied with flat facets, shear cracks and well-developed secondary crack.