Experimental Investigation on the Influence of Deep Cryogenic Soaking of Additive Manufactured SS 316L on Hardness and Corrosion Resistance

The influence of deep cryogenic soaking of additive manufactured stainless steel 316L (SS 316L) parts on hardness and corrosion resistance is investigated. The fabrication of SS 316L was carried out using selective laser melting (SLM). A Gaussian beam for laser energy dissemination was employed in SLM process to produce SS 316L specimens characterised by distinctive curved boundaries within the melt pool, resulting in a unique grain morphology featuring semicircular melt pool boundaries and layered patterns. The deep cryogenic soaking (DCS) process treatment, conducted at an ultra-low temperature of − 196 °C for an extended duration of 120 h immersed in liquid nitrogen medium, led to a significant improvement in the microstructure. An increased amount of fine-cellular grain microstructure was achieved, with an average grain size reduced from 1.01 ± 0.5 μm to 0.78 ± 0.5 μm. X-ray diffraction (XRD) analysis revealed that the DCS treatment did not alter the crystal structures, with both SLM and DCS specimens exhibiting the presence of the FCC-austenite phase. Surface roughness analysis indicated a noteworthy reduction following DCS treatment, with a 3.23% decrease in the average surface roughness (Ra) from 9.155 μm over the SLM SS 316L surface to 8.868 μm post-DCS exposure. Moreover, the mechanical properties exhibited substantial improvement, with SLM SS 316L samples having an average microhardness value of 193.16 HV, while DCS-treated samples exhibited an average microhardness value of 222.6 HV, marking a 15.24% enhancement attributed to grain structure refinement. XRD analysis also revealed peak broadening in DCS-treated specimens, suggesting the possibility of a more refined grain structure. This fine grain structure was found to hinder ion movement, resulting in a reduction in the corrosion rate from 0.004695 to 0.003965 mm/year. Although the improvement in corrosion resistance was marginal, it underscores the potential of DCS treatment in enhancing the resistance of SS 316L to corrosion.