Cyclic deformation property and microstructure study of HT-9 ferritic steel at elevated temperatures

In this study, cyclic deformation tests were carried out on HT-9 steel at room temperature, 300, 400, 500 and 600°C with a diametral strain range of ± 0.5%, ± 0.25% respectively. Transmission electron microscopy was presented to describe the micromechanisms of HT-9 steel under cyclic straining. The mechanical test results showed that cyclic softening behavior was observed at each testing temperature. The softening was attributed to the rearrangement of the high dislocation density structure into subgrain. In cyclically strained HT-9 specimens, carbides distributed along the subgrain boundaries. These carbides were considered to be the barriers against the subgrain to grow. The resistance of dislocation slip by carbides decreased to a less or extent at higher testing temperatures. The sharp subgrain boundaries observed in the specimens tested at higher temperatures showed a large decreasing rate in internal stress during cycling. HT-9 steel cyclically softened rapidly at testing temperatures higher than 500°C. In the case of HT-9 steel cyclically deformed at lower strain range, cyclic plastic strain was essentially accommodated by the ferrite region and strain cycling did not have obvious effects on the substructure of lath martensite. © 1994.