Strengthening mechanisms of stress rupture performance in additive-manufactured 17-4PH stainless steel

To obtain the desired microstructures in laser powder bed fusion 17-4PH stainless steel for multifunctional performance demands, this study optimised the microstructure and mechanical behaviour under extreme service conditions of elevated temperatures and high stresses. Solution temperatures were varied from 980 to 1220 °C to improve austenite and precipitate characterisation. The reverted austenite content initially increased and then decreased with increasing solution temperatures, peaking at 1040 °C, which corresponded to an optimal plasticity of 21.30% at room temperature. Cu-rich precipitates were slightly coarsened and preferentially distributed along grain boundaries at elevated solution temperatures. The increased internal stress enabled partial retention of the body-centred cubic structure, even in large-scale particles. The enhanced coherency promoted dislocation shearing mechanism, which provided a stronger strength contribution and overcompensated for the softening induced by coarsening. Consequently, samples with optimised solution temperature of 1160 °C exhibited an improved tensile strength of 1103.9 MPa at room temperature, and the high-temperature stress rupture life reached 630 h. The supersaturated matrix promoted the dynamic reprecipitation of dispersed fine needle-like coherent Cu-rich precipitates. The dual-scale strengthening structure and carbides enhanced deformation resistance, leading to a remarkable enhancement in the high-temperature stress rupture performance compared with the conventional heat treatment process.