Continuous and pulsed TIG welded joint of ASTM A105-AISI 316L steels: Characterization of microstructure, mechanical properties, and fracture analysis

The aim of the present study was to investigate the effect of continuous current gas tungsten arc welding (CC-GTAW) and pulsed current gas tungsten arc welding (PC-GTAW) on microstructural evolution, mechanical properties, and fracture mode of welded joint of ASTM A105-AISI 316L. For this purpose, welded joints of ASTM A105-AISI 316L with two current types of continuous and pulsed were prepared using ER309L. Optical microscopy, Field emission scanning electron microscopy (FE-SEM), Energy dispersive spectrometry (EDS), and X-ray diffraction (XRD) were used for microstructural and phase characterization. Tensile, Charpy impact and microhardness tests were employed to evaluate mechanical properties of the welded joints. In order to examine the fracture surfaces, fracture mode was analyzed using FE-SEMM. Microstructural analyses showed that the weld metal (WM) microstructure was austenitic-ferritic, containing columnar and equiaxed dendrites, so that changing the welding type from CC-GTAW to PC-GTAW leads to the transformation of dendrites from columnar to equiaxed ones and thus contributes to the presence of fewer dendrites in the WM, reducing the width of partially melted zone (PMZ) and heat-affected zone (HAZ). XRD analysis revealed that the dominant phase and the preferred crystallography plane in the WM were austenite and (111), respectively. In both types of welding modes, tensile samples were broken from the ASTM A105 side. The welded joints tested by the tensile test showed such mechanical properties as Y.S. of 247 ± 9 MPa, U.T.S. of 485 ± 8 MPa, and T.El. of 30 ± 2%. Microhardness and Charpy impact test results indicated that changing the welding type from CC-GTAW to the PC-GTAW one results in increasing microhardness (from 235 ± 7 HV to 261 ± 6 HV) and impact energy values (128 ± 3J to 157 ± 5J)in the WM. Fractography of the broken joints demonstrated that a ductile fracture occurred in mechanical tests.