Microstructure Evolution and Optimization of Two-Stage Tempering Process for As-Forged 9Cr3W3Co Steel

The post-forging heat treatment process played a crucial role in the final microstructure and mechanical properties of 9Cr3W3Co martensitic heat-resistant steel. In this study, comparative experiments were systematically conducted to investigate the influence and underlying mechanisms of deformation, austenitizing temperature, and two-stage tempering processes on the microstructural evolution and hardness of 9Cr3W3Co steel. Microstructural characterization and property evaluation were performed using scanning electron microscopy (SEM), X-ray diffraction (XRD) with Rietveld refinement, energy-dispersive X-ray spectroscopy (EDS, including elemental line scans and maps), Vickers and Rockwell hardness tests. The results indicated that quenching at 1100 °C prevented the formation of harmful δ-ferrite, completely dissolved the initial M23C6 carbides and effectively inhibited the growth of martensite blocks, achieving the highest quenching hardness (47.1 HRC). The experimental XRD patterns of all five heat-treatment conditions confirmed that the final hardness was dictated not only by the total carbide content but also by the degree of carbide refinement and matrix recovery. The dispersed precipitation of M23C6, M7C3 and NbC-type MX was significantly facilitated during the second-stage tempering at 570 °C + 700 °C, enhancing the secondary hardening effect. This process effectively suppressed excessive recovery of martensite blocks and the over-coarsening of carbides, maintaining excellent microstructural stability and mechanical properties (29.0 HRC). The optimized deformation and post-forging heat treatment process achieved an ideal microstructure, providing an important theoretical foundation and practical guidance for the engineering application of 9Cr3W3Co in ultra-supercritical units.