Effects of temperature on the low cycle fatigue and microstructures of HT-9 ferritic steel

Strain controlled low cycle fatigue tests of the HT-9 ferritic steel have been performed in the temperature range between room temperature and 600°C with ε{lunate} = 2×10^-3 s^-1. The stress ranges as a function of cycle were obtained at various temperatures. The corresponding microstructures of HT-9 specimens were also studied using transmission electron microscopy and correlated with the low cycle fatigue behavior of this material. A typical polygonization structure was observed in the specimens after fatigue testing at temperatures below 400°C. The influence of dynamic recovery increased with increasing temperature, and correspondingly led to a decrease of the stress range. The subgrain size of HT-9 specimens tested at 500°C was larger than after fatigue testing at 400°D and below. The martensitic lath structure evolved toward an equiaxed subgrain structure after cyclic loading at 600°C. The carbides redistributed along the subgrain boundaries. Control specimens were obtained from the underformed section of each specimen outside the gauge volume. These results confirmed that cyclic straining leads to significant temperature dependent effects on microstructural changes. © 1992.