Effects of carbon content, deformation, and interfacial energetics on carbide precipitation and corrosion sensitization in 304 stainless steel

Abstract

Utilizing transmission electron microscopy analysis of (110)/(110) grain boundaries in 304 stainless steel thin films prepared from specimens having 0.011, 0.025, 0.05, and 0.07% C, the M 23C 6 precipitate density was measured for grain boundary misorientations, Θ, for aging times of 10 and 50 h at 670°C, and for unstrained and 20% strained conditions. The density of precipitates in grain boundaries increased with increasing carbon content above 0.011% C and with increasing angles of misorientation. The propensity for precipitation also increased with straining to 20% and with increasing aging time, i.e. at high misorientation angles the sizes of precipitates increased with aging time. Essentially no precipitates were observed on grain boundaries at 0.011% C and no precipitates were observed on coherent ( Σ3) twin boundaries. Average energies at 670°C were calculated for random, high-angle grain boundaries, non-coherent twin boundaries, and coherent twin boundaries from experimental data and corresponding temperature coefficients. Precipitation observations suggested a critical or threshold energy requirement for precipitation. Correspondingly, a carbon content threshold also seems to exist for precipitation. It appears that of the special ( Σ) boundaries, only coherent twin ( Σ3) boundaries possess sufficiently low interfacial free energy to deter precipitation and sensitization, leading to the conclusion that only a twin-dominant grain boundary character could assure a significantly reduced corrosion sensitivity.