Investigation of fracture toughness of deep carburised M50NiL steel under different tempering temperatures

Fracture toughness of carburised M50NiL steel at different tempering temperatures has been investigated using a new evaluation method involving the ‘pop in’ phenomenon which occurs when a crack propagates through the carburised layer. An increase in tempering temperature resulted in an increase infracture toughness, and the rate of increase of the fracture toughness of the core was much higher than that of the surface. The changes in microstructure, residual stress, and retained austenite through the carburised layer were found to be the main factors causing the occurrence of pop in and causing the fracture toughness of the core to be higher than that of the surface.