Service induced degradation of creep and fatigue properties for steam turbine component materials

In order to characterize the service induced material degradation of steam turbine components, and to reflect it's effect on life assessment, retired components from fossil power plants after long time operation at elevated temperatures were cut into test specimens and metallurgical and mechanical properties were investigated. It was clarified that the softening was the typical and common phenomenon of material degradation for chromium and molybdenum contained low alloy steels, and that the softening was caused by a coarsening of carbide and a recovery of dislocation structures. The mechanical properties, such as creep and fatigue, were affected by softening and were degraded. Quantitative relationships were established between the material hardness and mechanical properties from which degraded mechanical properties were predicted by hardness measurement on the actual components. The statistical nature of the degraded mechanical properties were investigated making use of the quantitative correlation above, and the creep and fatigue properties estimated by the hardness measurement showed Weibul distribution. A new procedure to evaluate creep and fatigue damage was proposed, based on the hardness measurement for the degraded steam turbine components