Effect of cryomilling on the high temperature creep properties of oxide dispersion strengthened steels

This study investigated the effect of cryomilling on the creep properties of oxide dispersion strengthened steels. Milling temperatures during mechanical alloying (MA) were set as room temperature and −150 °C to manufacture ODS ferritic steels, K1 (room temperature) and K4 (−150 °C) with Fe-14Cr-3W-0.4Ti-0.3Y2O3 (wt%) composition. Microstructural observation identified large, inhomogeneous grains in K1 (700 nm), and a relatively fine, homogeneous grains in K4 (303 nm). Some grain boundaries featured secondary phases. The dispersion strengthened phase, fine oxide particles, was finer and more homogeneously distributed in K4 (7 nm) than in K1 (22 nm). A compressive creep test was performed at 650 °C. The steady-state creep rate in an identical stress range was lower in K4 than in K1. The stress exponent (n) was similar in the two alloys (n<1) at low stress range, but at a higher stress range, K1 had a slightly higher stress exponent (n=3-4). Microstructural observation identified that dislocation creep is relatively well controlled in K4, and this is assumed to be due to the homogeneous, fine grain size, narrow inter-spacing among oxide particles and appropriate particle size.