Effects of pre- and post-repair heat treatments on microstructure and tensile behaviors of 630 stainless steel repaired by metal additive manufacturing

Abstract In this study, we report a method for the repair of damaged SUS 630 parts by directed energy deposition (DED). Herein, we aim to analyze the variations in the mechanical properties of such parts caused by post-repair heat treatment performed to regain the pre-repair mechanical properties. Before repair, substrates were subjected to solution annealing (SA-wrought) or SA treatment followed by precipitation hardening (SA + PH-wrought). Each substrate was repaired using SUS 630 powder. The repaired region was dominated by BCC-structured ferrite (96.7%). It also contained FCC-structured retained austenite (3.3%). The martensite fraction of the samples treated with SA after repair increased by 3.1%. Although BCC remained dominant after the SA + PH post-repair heat treatment, reverted austenite and a large amount of precipitate were generated along the grain boundaries after sufficient aging time of PH treatment. The repaired region displayed lower hardness than the substrate. However, the post-repair heat treatment increased the hardness. Meanwhile, the tensile test results revealed that the tensile strength decreased from that of the wrought material because of the cracks generated at the interface between the repaired region and substrate. Although the tensile strength was improved by SA and SA + PH post-repair heat treatments, the elongation decreased because of the cracks at the interface and reduction in the FCC content of the repaired part. Although cracks occurred at the interface, tensile characteristics almost similar to those of SA-wrought and SA + PH-wrought were observed owing to SA + PH post-repair heat treatment.