Creep deformation process determining microstructure of type IV creep damage of the advanced ferritic heat resistant steel with high Cr content

Abstract

Type IV damage deteriorates the creep rupture strength at higher temperature than advanced Ultra Super Critical fossil power plant operation temperature, 600°C. Its metallurgical investigation is expected to avoid the creep damage at the outer edge of HAZ of welded joint. Metallographic analyses were carried out to clarify the micro-mechanism of Type IV failure for advanced ferritic creep resistant 9% Cr steel, ASME Gr.92. Assuming the Type IV failure arises at fine grain HAZ through the welding thermal cycle, the comparison of three candidate mechanisms, grain size refinement, dislocation sub-structure evolution and precipitation morphology development through the thermal cycle at HAZ, decided that the creep deformation process determining microstructure at HAZ is the globular sub-grain microstructure generated by thermal cycle of welding and PWHT. The creep test at 700°C of simulated globular sub-grain microstructure characterized the typical type IV phenomena on creep rupture curve. Precipitation morphology change by thermal cycle did not explain the creep time dependence of rupture strength difference between base metal and HAZ. Further works on the stress and temperature dependence of the creep life of Type IV phenomena is expected to subdue the creep life deterioration.