Factors controlling ambient and high temperature yield strength of ferritic stainless steel susceptible to intermetallic phase formation

Abstract

The microstructure evolution in Ti-Nb dual stabilized ferritic steel at high service temperature was simulated with heat treatments at 600 °C for up to 120 h. Thermodynamic calculations indicated that, in addition to conventional MX type carbides and nitrides containing Nb and Ti, heat treatment at high temperature can promote the formation of intermetallic Laves, Chi (χ) and sigma (σ) phases. During the heat treatment, Laves (FeSi2NbMo) and σ (FeCrMo) phases formed. The effect of the intermetallic phases on the ambient and high temperature yield strength (σy) was investigated through a comprehensive breakdown of the mechanisms contributing to strengthening, i.e. grain boundary, dislocation, precipitation and solid solution strengthening, the last two of which are influenced by the precipitation of the Laves and σ phases. On the basis of a regression analysis of atomic radius and shear modulus misfit with respect to Fe, the solid solution strengthening coefficient of Nb in α-Fe was predicted to be 16 MPa/at%. The coefficient was experimentally validated by measuring the yield strengths for two conditions with different amounts of Nb in solution. The present value is considered more reasonable than the 4320 MPa/wt% (approximately 7187 MPa/at%) presented earlier in the literature. Heat treatment at 600 °C raised the yield strength at ambient temperature, due to intermetallic precipitation strengthening. However, the opposite was true regarding the high temperature yield strength, which suggests that the effects of precipitation strengthening are relatively smaller and solid solution strengthening greater at elevated temperatures.