A Method for Estimation of the Fracture Toughness of CrMoV Rotor Steels Based on Composition

Abstract

Assessment of the remaining life of steam turbine rotors in the presence of bore defects requires a knowledge of the fracture toughness (KIC) of the rotors. Current procedures for estimating the KIC involve two steps; as a first step, the fracture appearance transition temperature (FATT) at the critical location is determined; the FATT value is then used to estimate the KIC, based on published correlations between the excess temperature (T-FATT) and KIC. Some problems arise in implementing both of these steps. To determine the FATT of the material, large pieces of material have to be removed, machined into charpy specimens and tested; this procedure is often time consuming and expensive and sometimes not feasible. The excess temperatures versus the KIC correlation that is used to derive the KIC values from the FATT data is based on a variety of low alloy steels and is therefore characterized by a large scatter band, thus leading to considerable uncertainty in the estimated KIC. In this work, FATT and KIC data reported for a number of retired CrMoV rotors were gathered and analyzed and correlations specific to CrMoV rotors were developed. Based on these correlations, a method for estimating KIC with greater accuracy, based on a knowledge of the steel chemistry alone, is proposed. The method offers the advantage that very small samples removed from noncritical locations in the rotor would be sufficient to get the desired data.