Studies on mechanical properties, microstructure and fracture morphology details of laser beam welded thick SS304L plates for fusion reactor applications

• CO2 laser welding of 8 mm thick SS304L plates has been carried out and full penetration welds fabricated and characterized for mechanical properties and microstructure details.
• Welded samples have shown tensile properties comparable to base indicating good weld quality joints.
• Impact fracture tests of weld zone and heat affected zone samples have shown poor toughness compared to the base metal.
• SEM analysis of fracture samples of tensile and impact specimens indicated the complex microstructure features in weld zone and combined ductile and brittle fracture features.
• Combined features of dendrite and cellular structures are observed in weld microstructures with narrow HAZ and delta ferrite is found in the welds and further confirmed by higher Ferrite Number data.

Austenitic stainless steel is widely used structural material for the fabrication of the fusion reactor components. Laser welding is high power density process which offers several advantages over the other conventional processes like Tungsten Inert Gas welding. The features like low distortion, narrow heat affected zone, deep penetration in single pass, good mechanical properties are some of the advantages of laser welding process. The laser weld process parameters optimization has several challenges in terms of overcoming the weld defects like voids due to lack of penetration over depth, undercuts and porosity. The present paper reports the studies carried out with CO2 laser welding of 8 mm thick austenitic stainless steel SS304L plates and their characterization of mechanical properties, microstructure and fracture morphology details. The weld process parameter optimization towards defect free welds with full penetration welding has been carried out. The welded samples have shown tensile properties comparable to base metal, bend tests are successfully passed. The hardness measurements have shown slightly higher for weld zone compared to base metal and the impact fracture tests have shown lower values for the weld zone (WZ) and heat affected zone (HAZ) compared to base metal (BM). Microstructure studies revealed the combined dendrite and columnar type features attributed to the higher cooling rates involved with laser welding process. Delta ferrite is observed in small residual contents in the weld zone and confirmed with Ferrite Number (FN) data. Scanning electron microscopy (SEM) studies have been carried out for the surface fracture morphology analysis for the tensile and impact tested samples to understand the ductile and brittle fracture details.