Pulsed current gas tungsten arc welded joint of ASTM A105-AISI 316L steels: Failure analysis and susceptibility to hydrogen embrittlement under slow strain rate tensile tests, and hydrogen sulfide corrosion
Masoud Sabzi, H.R. Jafarian, Amin Abdollahzadeh, S.H. Mousavi Anijdan, A.R. Eivani
Abstract
Highlights • By increasing I b and decreasing I p in the PCGTAW process, the WM of the welded joint altered its microstructure from columnar dendritic to coaxial dendritic. • The change in current mode in the PCGTAW process resulted in a decrease in the HAZ width and a decrease in the ferrite grain size in the HAZ region of ASTM A105 steel. • By increasing I b and decreasing I p in the PCGTAW process, the amount of hydrogen diffusioned and trapped in the dissimilar welded joint is increased. • By increasing I b and decreasing I p , the susceptibility to hydrogen embrittlement with the characteristics of hydrogen blistering and hydrogen cracking increases sharply. • By increasing I b and decreasing I p in the PCGTAW process, the corrosion resistance of the WM zone of the dissimilar welded joints has increased. However, none of the WMs showed susceptibility to pitting corrosion. The present study aims to investigate the failure analysis of Pulsed Current Gas Tungsten Arc Welded (PCGTAW) ASTM A105-AISI 316 L under Slow Strain Rate Tensile Tests (SSRT) and hydrogen sulfide corrosion. For this purpose, 316 L stainless steel and ASTM A105 steel sheets were joined together by PCGTAW process, and also by conditions of background current (I b )= 70, 110, 150A and peak current (I p )= 180, 220, 260A. The formed microstructure and phases were characterized by optical microscopy, Field Emission Scanning Electron Microscopy (FE-SEM), Energy-Dispersive X-Ray Spectroscopy (EDS) and X-ray Diffraction (XRD). To explore the hydrogen sulfide corrosion behavior of welded joints, open circuit potential (OCP), potentiodynamic polarization and cyclic polarization methods were employed in sour oil environment saturated with H 2 S and CO 2 . Moreover, the susceptibility to hydrogen embrittlement of welded joints was evaluated by immersing in NACE TM0284-Solution A and instantaneously in SSRT. After testing corrosion and SSRT, hydrogen damage and corrosion morphology were analyzed by FE-SEM and EDS. Microscopic studies revealed that with increasing I b and decreasing I p , the weld metal (WM) changes its microstructure from columnar dendritic to coaxial dendritic. Likewise, the changed current state in PCGTAW process has led to a decrease in the width of heat affected zone (HAZ) and thus in ferrite grain size in the HAZ region of ASTM A105 steel. The SSRT results indicated that with increasing I b and decreasing I p in the PCGTAW process, the susceptibility to hydrogen embrittlement of welded joints is developed by an increased amount of diffusioned and trapped hydrogen from 3.5 ± 0.1 to 5.9 ± 0.1 ppm, a decreased yield strength from 146±4 to 102±5 MPa, formation of hydrogen blisters and cracks as well as the growth of hydrogen cracks and brittle fracture occurred in the HAZ region of ASTM A105 steel. Although the results of the sour corrosion showed that the WM of welded joints is insensitive to pitting corrosion with good resistance, its corrosion resistance had been reduced with increasing I p and decreasing I b .