Creep Damage Assessment of 12%Cr Ferritic Heat-Resistant Steam Turbine Steels by Electrochemical Method

Abstract

The change in electrochemical property of 12%Cr ferritic steam turbine steels with creep was investigated to examine an applicability of electrochemical material characterization method as a procedure for creep damage assessment. The anodic polarization curve measurement was applied to several creep test specimens with various degrees of damage. Experimental results revealed that a single current peak appeared at around +150-200 mV during the measurement. The electrical charge of the peak (F) could be expressed as a function of the volume fraction (V), the average size (r) and the Cr content (a) of M23C6 carbide, i.e., F = 70890×r×V×a. This result suggested that the peak current density, Ip, mainly reflected the size of M23C6 carbide because two other variables showed no significant change. The Ip increased with accumulating the creep damage, and this increment was more pronounced in the long-term creep region. There was a good correlation between the Ip and the Vickers hardness. It was also found that the applied stress and the creep life fraction of creep damaged samples could be predicted based on the measured Ip. Additionally, the electrochemical property of actual turbine components (high middle pressure rotor and casing) was measured using the portable electrochemical cell for on-site measurement. The results obtained in this study indicated that this electrochemical material characterization method was a potential procedure for creep damage assessment for turbine components.