Qiuzhi Gao

Precipitates and Particles Coarsening of 9Cr-1.7W-0.4Mo-Co Ferritic Heat-Resistant Steel after Isothermal Aging

The precipitates obtained by EPE technology from the 9Cr-1.7W-0.4Mo-Co ferritic heat-resistant steel subject to isothermal aging were investigated using SEM, TEM and XRD. The particle size distribution and the coarsening kinetics of M23C6 with duration of isothermal aging were also analyzed with or without consideration of Laves phase. The results show that the isolated dislocations were detected in delta ferrite interior, and the precipitates on delta ferrite and martensite boundaries are obviously larger than other locations. Fe2W-Laves phase can only be found as duration of aging time to 2000 h, and is preferential to form adjacent to M23C6 particles. The small M23C6 particles firstly coarsen, but the large M23C6 are relatively stable during short aging. The total coarsening rate of M23C6 precipitates is 9.75 × 10−28m3s−1, and the coarsening of M23C6 depends on the formation of Laves phase.

Athermal martensite transformation of modified high Cr ferritic heat resistant steel undergoing different quenching temperatures

Abstract The thermal dilation behaviours of modified high Cr ferritic heat resistant steel quenched at different temperatures were employed to investigate the kinetics of martensite transformation. The martensite fraction and formation rate were obtained as a function of temperature. A model considering ‘spread’ martensites was introduced to explore the influence of quenching temperatures on martensite transformation. Both onset and offset temperatures of martensite formation decrease with the increase in quenching temperatures, and the reaction rate increases rapidly at the beginning of transformation and reaches a peak when a small quantity of martensite forms. The fitted data based on the proposed phase transformation model indicated that the width/length ratio of martensite laths decreases with the increase in quenching temperatures.