Microstructural Changes of High-Chromium Ferritic Stainless Steel Subjected to Cyclic Loading in 475°C Embrittlement Region

When ferritic stainless steels are exposed to the temperature range of 400-550 °C, they undergo 475 °C embrittlement that could be attributed to the spinodal decomposition. In the present study, high-chromium ferritic stainless steel, type 447, was aged at 520 °C under both static and cyclic loading conditions and microstructure was analyzed using a transmission electron microscope (TEM). By aging under static condition, hardness and tensile strength were largely increased. TEM observation revealed the mottled contrast due to the typical spinodal decomposition of Cr phase. Under cyclic loading condition, increases in hardness and tensile strength were smaller than those under static condition. TEM micrographs of the specimens subjected to cyclic loading exhibited a lower contrast of the mottled pattern than those of the specimens aged under static condition, indicating that cyclic loading suppressed the spinodal decomposition, thus resulting in the lower increases in hardness and tensile strength.