Microstructural evolution and mechanical enhancement of WAAM-fabricated Haynes 230 via cryogenic treatment

Thin-wall Haynes 230 structures with dimensions of 130 × 9 × 45 mm (length × width × height) were fabricated using Pulsed Gas Metal Arc Welding (P-GMAW)-based Wire Arc Additive Manufacturing (WAAM), followed by shallow cryogenic treatment and deep cryogenic treatment. The influence of cryogenic processing on microstructural evolution, elemental segregation, crystallographic characteristics, hardness, tensile properties, and fracture behavior was systematically investigated. The as-built condition exhibited elongated dendritic structures aligned along the build direction due to directional solidification during WAAM deposition. Pronounced microsegregation of refractory elements, particularly W and Mo, was observed in interdendritic regions. SCT induced moderate grain refinement and partial homogenization, with slight reduction in segregation. In contrast, DCT produced significant transformation to fine, uniformly distributed equiaxed cellular grains and substantially reduced elemental partitioning. X-ray diffraction confirmed that the γ-FCC matrix remained stable in all conditions, while peak broadening analysis revealed progressive reduction in crystallite size and increased dislocation density after cryogenic treatment, especially in the DCT condition. Mechanical characterization demonstrated a systematic improvement with increasing cryogenic severity. The average microhardness increased from 262 HV in the as-built state to 276 HV after SCT and 286 HV after DCT. Deep cryogenic treatment enhanced ultimate tensile strength by approximately 8% and yield strength by about 11% compared to the as-built condition, while maintaining comparable elongation. Fractographic examination revealed a predominantly ductile fracture mechanism in all samples, with refined and uniformly distributed dimples observed after DCT. The DCT significantly improves microstructural uniformity and mechanical performance of WAAM-fabricated Haynes 230 thin-wall structures.