Predicting crack initiation under the effects of ingot defects in P92 pipes based on a ductile fracture model

Abstract The ingot defects of P92 martensitic steel could induce crack initiation during the critical normalization-tempering treatment, which deteriorates the properties of P92 pipe used in ultra-supercritical (USC) power plants greatly. Thus, this study conducted numerical simulation based on a modified Johnson-Cook fracture model to thoroughly explore the effects of ingot defects on crack initiation in P92 pipes under common cooling approaches. Tensile tests of smooth and notched P92 steel specimens were executed at wide range of temperatures and strain rates to assist in calibrating the parameters for Johnson-Cook constitutive and fracture models. The fracture criterion for P92 steel as a function of temperatures, strain rates, and stress states simultaneously was thus put forward. Based on calibrated fracture model, validation simulation and experiment were conducted with small P92 CT specimens, showing good consistency between predictions and experiments. Subsequently, cooling simulations of a large P92 pipe with four types of defects placed at various positions of the pipe were performed. Largest strain concentrations were observed around the tips of radial pre-cracks followed by axial pre-crack, void, and inclusion at all positions and cooling rates. Only radial pre-cracks located at inside and outside walls were predicted to initiate under water-spray cooling. Other pre-defects under both air and water-spray cooling pose no risk of crack initiation. The direction of the pre-crack proves to be a crucial factor inducing crack initiation.