Effects of cryomilling on the microstructures and high temperature mechanical properties of oxide dispersion strengthened steel

The effects of cryomilling on the microstructures and high temperature mechanical properties of oxide dispersion-strengthened (ODS) steel were examined. Cryomilling was newly tried on this ODS steel to control oxides, grains, and dislocation microstructures. Fe-14Cr-3W-0.4Ti (wt.%) alloy powder and 0.3 wt.%Y2O3 powder were mixed and were mechanically alloyed (MA) through ball milling at each of room temperature (RT) and −150 °C and then hot isostatic pressing (HIP), hot rolling, and annealing processes were implemented to manufacture two types of ODS ferritic steel, K1 (RT) and K4 (−150 °C). Oxide particles were shown to be finer and more uniformly distributed in K4 (5-10 nm size distribution) than in K1 (average size 30 nm). The two alloys were subjected to high temperature compression (RT ∼ 900 °C) tests. K4 represented higher yield strength under all temperature conditions. However, K4 showed rapid strength decreases at high temperatures exceeding 700 °C and showed similar levels of strengths to K1 at 900 °C. This is considered attributable to the fact that although cryomilling increased the number density of oxide particles, it simultaneously reduced grain sizes too much, so that grain boundary weakening at high temperatures could not be sufficiently prevented.