Effects of Microstructural Change and Oxidation on Creep Behavior of P23/T23 Steels

Effects of oxidation and microstructural change on creep behavior were investigated on ASME P23/T23 steels. Creep tests at 625 degrees C and 650 degrees C were performed not only in air but also in helium atmosphere to retard formation of oxidation scale on the surface of specimen. Creep strength degraded in the long-term at 625 degrees C and 650 degrees C even in helium atmosphere, indicating that not oxidation but microstructural change can cause the creep strength degradation. The stress exponent of minimum creep rate at 625 degrees C and 650 degrees C was 7.8-13 for higher stresses and 3.9-5.3 for lower stresses. The change of stress exponent with stress levels was consistent with the drop in creep rupture strength in the long-term. The creep ductility of T23 steel decreased with increasing time to rupture at 575 degrees C to 650 degrees C. Fracture mode changed from transgranular to intergranular fracture in the long-term at 625 degrees C and 650 degrees C. Bainitic structure was remarkably recovered after long-term creep exposure at 650 degrees C.