Exploring friction stir welding for joining 316L steel pipes for industrial applications: Mechanical and metallurgical characterization of the joint and analysis of failure

Abstract

Efficient welding of pipelines is critical for industries like power plants. Friction stir welding (FSW) has shown the ability to provide higher strength joints and is envisaged to be suitable for joining industrial pipelines as well. Hence, the present study assesses the application of FSW for joining 316L stainless steel industrial grade pipes. The welding is carried out by varying the process parameters, viz., spindle speed (rpm) and welding speed (mm/min), within a suitable range under force control mode. Microhardness and tensile tests have been conducted to evaluate the mechanical strength of the joints, whereas microstructure evaluation by optical microscopy (OM) and field emission scanning electron microscopy (FE-SEM) has been done in order to comprehend the mechanical performance of the weldments. It is found that both the selected welding process parameters (spindle speed and welding speed) have significant influence on the metallurgical and mechanical properties of the welded joints. High grain refinement due to dynamic recrystallization (DRX) in the stir zone results in enhanced hardness. At the optimal combination of parameters, i.e., 400 rpm spindle speed and 50 mm/min welding speed, the weld zone displays higher tensile strength than that of base metal. Also, at the same combination of parameters, stir zone of the welded samples displayed maximum hardness (255HV0.5kg). Some of the weldments displayed early failure during tensile tests. The present study also explains the cause of these failures. Optimum amount of heat generation during FSW process is key to achieving defect free good quality weldments.