Effect of thermal aging on metallurgical, tensile and impact toughness performance of electron beam welded AISI 316 SS joints

Electron beam welding was used to fabricate a fully penetrated joint of 18 mm thick AISI 316 stainless steel plates. The effects of carbide precipitation on the metallurgical and mechanical properties of the weld were evaluated after aging at 750 degrees C for 300 h. The optical microstructure analysis showed that the weld zone consists of skeletal, vermicular, lathy, and lacy ferrite which further transformed into carbides after thermal aging. The weld zone was weaker in terms of ultimate tensile strength as compared to the base metal, as the presence of ferrite content (varied from 8% to 2.9 %) in the weld zone. Analysis along the thickness of the welded joint revealed that the bottom of the weld bead possessed 6% higher ultimate tensile strength as compared to the top of the weld bead. The microhardness and impact toughness in as-welded specimens got increased by 12.5 % and 49.5 % respectively while traversing from top to root of the weld bead. Thermal aging promoted the formation of Cr rich carbides (Cr23C6) on the grain boundaries and reduced the delta ferrite content near to zero. Further, it led to a reduction in the impact toughness by 62 % at the bottom of WZ and increased the ductility and the microhardness of the weld bead.