MARTENSITIC/FERRITIC SUPER HEAT-RESISTANT 650°C STEELS - MICROSTRUCTURE

摘要

The model alloys DT4-1 (3%W), DT4-2 (4%W) and DT4-13 (3%W, 0.2%Ta) were microstructurally investigated after heat treatment and after compression creep at 650°C for up to 3000h with stepwise increase in stress. The subgrain size in the annealed martensite did not change significantly during creep in accordance with the laws of subgrain evolution. In the initial state after heat treatment the precipitates in DT4-1 and -2 consisted mainly of M{sub}23C{sub}6. In DT4-13 TaC was additionally present in a fine distribution. Creep resulted in massive precipitation of Laves phase. The evolution of the precipitate structure was consistent with the results of thermodynamic calculations of equilibrium phases using the program Thermocalc. The observed precipitates explain the high creep resistance of the model alloys which increases in the sequence DT4-1, -2 and -13.