Effect of phase separation on tensile and Charpy impact properties of MA956 ODS steel

Tensile and Charpy tests were carried out to investigate the effect of phase separation on the mechanical properties of MA956 oxide dispersion strengthened high chromium steel. The ferritic matrix of the investigated material separates into Fe-rich α and Cr-rich α′ phases during thermal ageing at 475 °C. The yield stress and tensile strength increased in the aged material due to the formation of the nanometric sized precipitates while the ductility was decreasing with ageing time. The effect of Cr-rich precipitates on the strengthening was estimated using analytical models based on different mechanisms. Modulus strengthening can be considered the most important contribution to the yield strength increase. Charpy properties showed orientation dependence reflecting the anisotropy in the microstructure. The thermally aged MA956 alloy exhibited inferior Charpy impact properties compared to the as-received material. After ageing for 1000 h the ductile-brittle transition temperature (DBTT) was shifted by 200 °C to higher temperatures, while the upper shelf energy decreased dramatically. In samples with L-R orientation, crack arrester type delamination was observed which is considered to increase the upper shelf energy as compared to the other orientations.