Creep damage evaluation of CrMoV turbine rotor steel by carbide structure analysis

Abstract

CrMoV turbine rotor steel is strengthened by dispersed fine carbides, however carbides precipitated and coarsened at grain boundary during creep exposure. The carbide structure also changes from Fe_3C to M_<23>C_6 type carbide as coarsening. The carbide structure was electrochemically isolated by solving the matrix and analyzed with X-ray diffraction analysis. The X-ray peak intensity ratio of M_<23>C_6 to Fe_3C is tentatively defined as J-parameter. In order to evaluate the efficacy of J-parameter for creep damage evaluation, creep interruption test was conducted. The J-parameter increased with increasing creep and also correlated well to mean interparticle spacing. The change in J-parameter was affected by the chemical composition of the steel.