Evaluation on high temperature fracture toughness of CrMoV cast steel by small punch testing

Toughness of CrMoV cast steel for steam turbine casings operated at elevated temperature can be degraded during the operation. It is important to measure the degradation under operation to estimate the damage and the maintenance interval. However, conventional testing methods for fracture toughness require large specimens and it is impractical for successive operation after sampling these specimens.In this report, a new small punch test method to evaluate the fracture toughness at high temperature is proposed. Small punch specimens with fatigue pre-cracks were used in this test method. The crack extension from the pre-crack was detected by electric potential drop method and the plastic strain energy required for crack extension is calculated by FEM analysis. The fracture toughness of degraded CrMoV cast steel was estimated using this method and the values showed a good correlation with those obtained by the conventional test method with CT specimen.