Achieving ultra-high strength, good toughness and cost reduction in secondary hardening steel via dual precipitation

The dual precipitation of NiAl particles and M₂C carbides during ageing plays a critical role in achieving an optimal balance of strength and toughness in low-cobalt secondary hardening steel, yet this remains scarcely investigated. This work investigates the evolution of NiAl and M₂C during ageing at 482 °C for durations ranging from 1 h to 150 h, focusing on their influence on the strength and impact toughness of martensitic steel. It was found that after 5 h of ageing, NiAl particles nucleate rapidly with high number density and nearly reaches saturation, suppressing dislocation recovery. This leads to a high yield strength of 1875 MPa. When the ageing time is extended to 32 h, the NiAl particles coarsen and the volume fraction of M₂C carbides increases, while the dislocation recovery remains minimal. Consequently, the yield strength increases slightly to 1895 MPa. Notably, the impact toughness improves from 17 J at 5 h to 28 J at 32 h, reflecting to a 65 % improvement. This is predominantly due to the reduction in the number density of brittle NiAl particles which can induce cracks and due to the formation of film-like reversed austenite which can deflect cracks. After 32 h of ageing, the yield strength and impact toughness of the secondary hardening steel are comparable to those of commercial AerMet310 steel. Notably, the cobalt content is reduced from 15.0 wt% to 5.0 wt%, resulting in an approximate 36 % reduction in raw material costs.