Evaluation of Crack Arrest Toughness (K IA) of P91 Steel in Various Cold Worked and Thermally Aged Conditions

Abstract

KIA is increasingly being regarded as a characteristic fracture toughness below which cleavage fracture does not occur. Its evaluation from small-sized Charpy specimens would be advantageous for applications in power plant industries. In this study, KIA has been evaluated for P91 steel in various cold worked and thermally aged conditions. Evaluation of KIA requires determination of crack arrest load(Parrest) and crack arrest length(aarrest). The main challenge is in the determination of aarrest due to the non-availability of standard methodologies and the absence of unequivocal microstructural signatures on the fracture surface in this steel to identify crack arrest. aarrest has been determined using the analytical Key-Curve methodology which has proven successful for this steel in unaged condition. The applicability of the Key-Curve method is validated by the good agreement of the determined final crack length with that measured optically on unbroken specimens of N&T and subsequently 15% cold-worked P91 steel which had been previously aged at 650 °C for 5000 h. Mean KIA varies from 47.46 MPa√m (NT steel aged at 600 °C for 5000 h) to 69.85 MPa√m(NT + 15% cw steel aged at 650 °C for 10000 h) for the various cold worked and aged datasets. KIA is found to be an average property unlike initiation toughness (KJd) which shows statistical scatter. Mean KIA is found to be in reasonable agreement with the lower bound values of cleavage initiation toughness (KJd) for the datasets in this study.