Synergistic effect of well/over-tempered hierarchical martensitic structure and carbides (M23C6 and MX) on austenite growth in 9Cr steel

To clarify the synergistic effect of well/over-tempered hierarchical martensitic structure and carbides (M23C6 and MX) on austenite growth, the austenite growth during step-heat austenitization for a selected 9Cr steel was investigated by dilatometry, multi-scale characterization and cellular automaton simulation. The austenite grain inherits the geometrical characteristics of the hierarchical martensite substructure to a significant extent through covariant and free growth. In particular, the initially heterogeneous grains with non-uniform martensitic substructure can easily produce mixed and heterogeneous grains. Step isothermal heating under AC1 can promote the evolution of martensitic structure to recrystallized ferrite and recovered equiaxed substructure, associated with coarsening of M23C6 and precipitation of MC. Apart from the recrystallized ferrite promoting the growth of equiaxed austenite grains, the recovered approximate equiaxed substructure decorated with coarse M23C6 and a large amount of MX can promote the subsequent development of fine spherical austenite grains by reducing the abnormal austenite growth and increasing the resistance to interfacial migration. The synergistic effect of the well/over tempered martensitic hierarchical structure and the carbides (M23C6, MX) is critical to the final austenite grain distribution. A window of critical temperature values for step heating austenitization to generate homogeneous austenite grain growth was established by exploiting the inheritance of the equiaxed substructure and reasonably reducing abnormal growth.