Abstract
Creep strength degradation of Creep Strength Enhanced Ferritic (CSEF) steels was investigated, focusing on microstructural degradation. The creep strength of ASME Gr.91 steel remarkably decreased in the long-term at 600oC and 650oC. If the data in the long-term are selected for regression analysis, we can accurately evaluate long-term creep strength. The data under applied stresses larger than proportional limit stress should be omitted for the analysis. Creep deformation under a stress lower than proportional limit stress is strongly affected by microstructural changes due to diffusion. After long-term creep exposure at 600oC, the martensitic lath structure was collapsed by recovery and coarsening of precipitates occurred. The recovery of lath structure preferentially occurred around prior austenite grain boundaries. The Z-phase particles nucleated around prior austenite grain boundaries during creep exposure, consuming fine MX particles that were main strengthener.
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