A new methodology for investigating the weld joint microstructure and micro-hardness of the austenitic stainless steel clad plate made by SMAW/GTAW multi-pass welding process: part II—transition zone case study

This study investigates the effects of repeated thermal cycling during welding on the microstructural transformations and microhardness of Austenitic Stainless Clad-Steel Plates (ASCSP), with a focus on the transition zone. A novel approach was employed by progressively shortening subsequent passes after initially filling the groove. Microstructural analysis using optical microscopy revealed predominantly Widmanstätten coarse grains in the Substrate Heat-Affected Zone (S-HAZ) after the fourth pass, which refined following the fifth pass due the lower heat input (9.5 kJ/cm) compared to the fourth pass (18.1 kJ/cm). The Weld Metal Transition Zone (WM-TZ) displayed a columnar and cellular structure indicative of a high solidification rate, with a planar grain solidification band near the fusion line. Successive passes from No. 4 to No. 9 influenced grain size, ferrite content, and structural regeneration, with annealing effects observed from pass No. 6 acting as a post-heat treatment. Additionally, decarburization in the S-HAZ following the ER316L deposition (pass No. 6) due to carbon diffusion into the WM-TZ, leading to progressive dark layer formation following each pass. Micro-hardness measurements indicated peak values in the carbon-rich layer and WM-TZ cellular structure, with a gradual decrease across passes due to carbide dissolution and microstructural refinement.