Atom probe examination of thermally ages CF8M cast stainless steel

Abstract

The precipitation reactions which occur in the ferrite phase of cast type CF8M stainless steels after low-temperature (300 °C to 400 °C) aging have been investigated using atom probe field ion microscopy (APFIM). Both spinodal decomposition and G-phase precipitation were found in two heats of material aged in this temperature range for times varying from 700 to 30,000 hours. The extent of spinodal decomposition and G-phase precipitation increased with both aging temperature and aging time. The activation energy for spinodal decomposition was estimated to be 260 ± 50 kJ/mole, consistent with the activation energy for chromium diffusion. The kinetics of G-phase precipitation showed less dependence on aging temperature than did the kinetics of spinodal decomposition. Also, the composition of the G-phase precipitates was observed to change with aging temperature but not with aging time. The faster embrittlement kinetics of these two heats in comparison to most other heats of CF8M can be attributed to the large extent of G-phase precipitation.