Effect of deep cryogenic treatment on the microstructure and mechanical behaviour of additively manufactured SS347 stainless steel

This study investigates the microstructural evolution and mechanical performance of SS347 stainless steel fabricated through wire arc additive manufacturing (WAAM) using a dual-pulsed gas metal arc welding process. The as-built specimens were compared with those subjected to deep cryogenic treatment (DCT) to assess the influence of post-processing. Optical and scanning electron microscopy, along with energy-dispersive X-ray spectroscopy (EDS), revealed the coexistence of δ-ferrite (BCC) and γ-austenite (FCC) phases. DCT enhanced grain refinement and phase uniformity while maintaining structural integrity. EDS mapping indicated nickel enrichment in austenite and chromium enrichment in ferrite, contributing to improved phase stability and compositional homogeneity. X-ray diffraction analysis showed a decrease in crystallite size and an increase in dislocation density in DCT-treated samples, indicating enhanced lattice strain. Mechanical testing demonstrated a 12.1% rise in ultimate tensile strength, a 16.6% increase in yield strength, and a slight improvement in elongation. Impact toughness improved marginally from 128 J to 130 J, with fractography revealing a transition to finer, ductile dimples. Microhardness increased from 243 HV to 263 HV. The combined effect of dual-pulsed WAAM and DCT resulted in a refined microstructure and improved mechanical properties suitable for high-performance applications.