電気化学的手法によるＷ強化型９％Ｃｒフェライト系耐熱鋼のクリープ損傷評価

Abstract

In order to develop a methodology for creep damage evaluation of W alloyed 9%Cr ferritic steel KA-STBA29/KA-STPA29 (ASME T92/P92), which is the candidate material for an ultra-super-critical fossil power plant, changes in electrochemical property of the steel caused by creep have been investigated. Experimental results on electrochemical polarization measurements revealed that the peak current density “Ip”, which appeared at a specific potential during potentiodynamic polarization measurement in 1N-KOH solution, increased with thermal aging and creep damage and reflected not only the thermal effect but also the stress effect on creep damage. The Ip value corresponded to the selective dissolution volume of precipitates (M23C6 and Laves phase) and the increase in the Ip due to creep damage reflected the increase in amount of chromium precipitated as Laves phase. It was also found that the difference in the Ip value between the as-tempered material and the creep damaged materials (ΔIp) were uniquely correlated with a newly proposed parameter “t·F(T)·exp(-Q(σ)/RT)” irrespective of test conditions, which is the parameter based on Laves phase precipitation kinetics. On the basis of such correlation between ΔIp and t·F(T)·exp(-Q(σ)/RT), the test temperature or the applied stress of the creep damaged materials can be nondestructively estimated by electrochemical polarization measurements.