Microstructure and Notched Fracture Resistance of 0.56% C Steels After Simulated Induction Hardening

Historically, steels with carbon contents above about 0.45% C that are quenched to attain hardness above about 53 HRC (560 HV) are prone to premature brittle fracture when stressed in uniaxial or cyclic tension. In this study, six laboratory melted steels containing nominally 0.56% C and no grain refining additions (Ti, Al, V, or Nb) were heat treated on a Gleeble 3500® simulator to emulate thermal heating and quenching cycles for induction hardening. Limiting the peak heating temperatures and times produced a very fine grained austenite with final hardness above 60HRC (700HV). Fracture resistance measured by notched bend tests increased by up to 3 fold for the short low temperature heating cycles as compared to longer higher temperature cycles. Fracture surfaces showed trans-granular crack propagation for the short low temperature cycles as compared to inter-granular propagation for the longer higher temperature cycles.