Microstructure and Notched Fracture Resistance of Low Alloy 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 brittle intergranular fracture when stressed in uniaxial or cyclic tension. In this study, five laboratory-melted steels containing nominally higher 0.56% C, additions of Mn, Mo, Ni, or W, 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 both induction surface hardening and conventional through thickness heat treatment. Rapid heating at 50 °C/s and limiting the peak heating temperatures and times to 950 °C and 10 s produced a very fine austenite grain size (AGS) as small as 10 µm with a final hardness above 60HRC (700HV) and additions of Mo. Ni or W further refined the AGS to as small as 5 µm for the rapid heating. Fracture resistance measured by the peak breaking load (PBL) in notched bend tests increased by up to threefold for the short low-temperature heating cycles as compared to longer time (1000 s) higher-temperature (1050 °C) cycles. Fracture surfaces showed transgranular crack propagation for the short low-temperature cycles as compared to intergranular fracture for the longer higher-temperature cycles. The addition of 0.56 wt. pct W was especially effective for reducing AGS and PBL, and the addition of 2.12 wt. pct Ni increased the PBL for most of the heat treatment conditions.