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, I_p, mainly reflected the size of M₂₃C₆ carbide because two other variables showed no significant change. The I_p 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 I_p 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 I_p. 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.