Improved performance of Ni-Co secondary hardening steels through rapid thermal processing

• Steel properties can be improved from rapid heat treatment with times < 2 min. • Rapid heating effects can be divided between tempering and reversion phenomena. • Austenite reversion from fast heating increases strength and toughness. Demand for high strength, high toughness secondary hardening steels has led to high alloy compositions that often require long tempering times and cryogenic treatments, leading to high costs and low throughput. Rapid tempering has demonstrated improved Charpy impact energy in low-alloy steels compared to traditional isothermal tempering while maintaining hardness. This work aims to explore rapid tempering as a method to improve toughness in a secondary hardening Ni-Co steel while maintaining similar hardnesses to isothermal tempering. Hardness and Charpy impact testing were conducted on isothermal and rapid tempered conditions. Rapid tempering achieved the highest Charpy energy (59 J) but a lower hardness relative to the peak 8-hour isothermal temper. Subsequently, two-step rapid heat treatments were designed, consisting of rapid heating to just above the A c1, quenching, and then rapid tempering. These led to a 40 % increase in toughness (49 vs 34 J) at the same hardness (∼493 HV) or a ∼ 30 HV increase in hardness (518 vs 493 HV) while maintaining the same toughness (∼34 J) when compared to the peak-aged isothermal condition. The mechanism behind these improved properties is attributed to carbide refinement and accelerated austenite reversion and nucleation leading to a reduction in the effective grain size.