INVESTIGATION OF FRACTURE-TOUGHNESS OF DEEP CARBURIZED M50NIL STEEL UNDER DIFFERENT TEMPERING TEMPERATURES

Fracture toughness of carburised M50NiL steel at different tempering t emperatures has been investigated using a new evaluation method involv ing the 'pop in' phenomenon which occurs when a crack propagates throu gh the carburised layer. An increase in tempering temperature resulted in an increase in fracture toughness, and the rate of increase of the fracture toughness of the core was much higher than that of the surfa ce. The changes in microstructure, residual sti ess, and retained aust enite through the carburised layer were found to be the main factors c ausing the occurrence of pop in and causing the fracture toughness of the core to be higher than that of the surface. (C) 1998 The Institute of Materials.