Evaluation of embrittlement of long-term used CrMoV rotors by small punch test.

By means of a small punch test technique, the possibility of evaluating embrittlement in the HP-IP steam turbine CrMoV rotors used in service for a long time was investigated. It was found that for the 5% NiCrMoV steel specimens subjected to various aging time, or various degrees of temper embrittlement, the SP energy of the small punch test at -196°C corresponds to the extent of embrittlement, especially the 50% FATT. By adopting this technique to the CrMoV rotors in service, it was recognized that their 50% FATT and Charpy energy at 100°C can be estimated from the SP energy at -196°C. The cause of embrittlement, especially temper embrittlement, can be determined from the observation of fracture surface of the small punch test specimens at -196°C by SEM. The results obtained suggest a possibility to estimate the extent of embrittlement of HP-IP CrMoV rotors in service by a quasi-non-destructive method.