IMPLICATION OF ELASTIC COHERENCY IN SECONDARY HARDENING OF HIGH CO-NI MARTENSITIC STEELS

Composition and stoichiometry of coherent M2C carbide precipitates, (M = Cr, Mo, Fe), in two high Co-Ni martensitic steels have been calculated using thermodynamic conditions for mechanical, chemical and interfacial equilibrium between the ferrite (tempered martensite) and the M2C phase. Compared to the corresponding incoherent equilibrium compositions, our calculations at the standard secondary hardening temperature of 783 K predict a substantial carbon deficiency and a measureable solubility of iron in the M2C phase, both of which have been verified experimentally. These deviations from the incoherent equilibrium compositions are found to be in line with the influence of these constituents in lowering the strain energy by reducing the principal strains of the M2C stress-free transformation strain.