Divergent regulatory mechanisms of nano-WC-Co on microstructural evolution and tribological behavior of austenitic and martensitic stainless steels via directed energy deposition

Additively manufactured stainless steels are increasingly applied in demanding environments, yet their insufficient wear resistance and unclear nano-reinforcement mechanisms limit further performance optimization. This study systematically investigates the divergent regulatory mechanisms of nano-WC-Co on the microstructural evolution and tribological behavior of austenitic 316L and martensitic 420 stainless steels fabricated via directed energy deposition (DED). Nano-WC-Co reinforced composite powders were prepared via high-energy ball milling and deposited under identical conditions to enable direct comparison. The results reveal strong matrix-dependent effects. In 316L, nanoparticles promote heterogeneous nucleation and suppress epitaxial growth, quantitatively refining the average grain size from 29.19 μm to 16.05 μm. This refinement is accompanied by enhanced localized lattice distortion, evidenced by the average Kernel Average Misorientation (KAM) increasing from 0.66° to 0.84°. In contrast, in 420 steel, partial dissolution of nano-WC-Co induces carbon enrichment and stabilizes retained austenite, altering the coupled solidification–phase transformation pathway. This results in a homogenized martensite–austenite dual-phase structure with alleviated local strain localization (average KAM decreasing from 1.01° to 0.96°). These distinct microstructural evolutions lead to different strengthening and wear-resistance mechanisms. The specific wear rates are significantly reduced in both systems; the 316L composite exhibits a 46.3% reduction, while the 420 composite demonstrates an exceptional reduction of 90.7%. Furthermore, nano-WC-Co shifts the wear characteristics of 316L from severe to mild adhesive-oxidative wear. Concurrently, in 420 steel, the failure mode fundamentally transitions from adhesive-abrasive-oxidative mechanisms to benign mild abrasive-oxidative wear.