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Structure-property relationships and corrosion behavior of laser‑welded X‑70/UNS S32750 dissimilar joint

Maurya Anup Kumar, Pandey Shailesh M., Chhibber Rahul, Pandey Chandan

Archives of Civil and Mechanical Engineering

2023

Vol. 23, no. 2

art. no. e81, 2023

This research aims to study the microstructure characteristics, mechanical properties, and corrosion behaviors of the dissimilar autogenous laser beam welded joint of pipeline steel (X-70) and super duplex stainless steel (sDSS 2507). Pipelines for the transmission of oil and gas and risers for offshore oil and gas drilling require this dissimilar joint. A dissimilar joint must maintain its properties and be defect-free under such challenging operating conditions. The microstructure of the interface, weld zone and heat-affected zone (HAZ) were all investigated thoroughly using optical microscopy (OM) and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). This dissimilar joint had signifcant microstructure anomalies in the weld and interfaces. Microstructure inhomogeneity’s effect on welded joint mechanical properties, including microhardness, tensile and impact strength, was also studied. The linear potentiodynamic polarisation test in neutral 3.5 wt.% NaCl solution was used to study this weldment’s corrosion behavior. The corroded surfaces were examined using an OM and SEM for the surface morphology investigation of corroded specimens. The macro-optical investigation has revealed full penetrations in the weld without any inclusions or porosities. The interface between the sDSS 2507 weld zone and the X-70 coarse grain heat-affected zone (CGHAZ) indicated a peak hardness of 418 Hv0.5. With an average of 345 Hv0.5, the WZ’s hardness variation was reported to be in the 298-420 Hv0.5 range. The hardness of the X-70/sDSS 2507 weld interface was assessed to be greater than that of the other region of weldments. An untempered martensitic region in WM and the CGHAZ of X-70, and the presence of M-A components are credited with the increase in hardness. The welded joint achieved reasonably excellent strength and ductility and met the marine and offshore standards requirements. The base metals and weldment for X-70 and sDSS 2507 have respective ultimate tensile strengths (UTS) of 610±6 MPa, 995±8 MPa, and 675±10 MPa. The tensile findings revealed that the fracture location for weldment was evident in the X-70 base metal, ensuring that the weld metal was of adequate strength for the laser-weld joints. It was observed that the weldment’s WM had the lowest impact strength. The Charpy impact toughness of the weld metal, however, was higher than both the ASME standard (>41 J) and the EN 1599:1997 standards (>47 J). The sDSS 2507 BM (310±4 J) clearly outperforms the weld zones (185±3 J) and X-70 base metal (295±2 J) in terms of impact strength. The electrochemical corrosion test shows the corrosion potential, and the weld zone's corrosion rate is between sDSS 25,070 (- 260±1.3 mV, 0187±0.002 mm/year) and X-70 base metal (- 454±1.8 mV, 0.321±0.017 mm/year). Additionally, the surface morphologies and the electrochemical measurements matched significantly.

Effect of filler metal composition on microstructural and mechanical characterization of dissimilar welded joint of nitronic steel and super duplex stainless steel

Maurya Anup Kumar, Pandey Chandan, Chhibber Rahul

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 2

art. no. e90, 1--28

The present paper experimentally investigates the effect of filler metal on the mechanical behavior, solidification, and microstructure of the super duplex stainless steel (sDSS2507) and nitronic steel (N50) dissimilar welded joint. This dissimilar joint is primarily applicable in the subsea control unit for high-pressure tubing and coupler assembly. For this investigation, the gas tungsten arc welding process (GTAW) employed the super duplex filler ER2594 and carbon steel grade ER70S-2 filler. The weld's structural integrity has been assessed to compare both the fillers through multiple investigations on the joint. The microstructure characterization of the base metal and as-welded specimen was carried out using an optical microscope (OM) and scanning electron microscope (SEM). Super duplex filler ER2594 weld solidified in primary ferritic mode with precipitation of several reformed austenite in the ferrite matrix, whereas ER70S-2 filler weld had long marten site laths embedded in ferrite matrix. The microstructural study reported the presence of microsegregation and Type II boundary formation. The type-II boundary is detected close to the fusion boundary at the N50 and the sDSS 2507 side of the ER70S-2 weldment. The Vickers microhardness test, Charpy impact test, and the tensile test were performed to obtain the mechanical properties of this joint. The microhardness investigation of the weld zone of ER2594 and ER70S-2 shows the average hardness of 287.34±10 Hv0.5 and 372.36±10 Hv0.5, respectively. The peak hardness of 410 Hv0.5 was observed in the weld zone of ER70S-2. The formation of large marten site laths in the ferrite matrix in the weld zone leads to higher hardness in ER70S-2 filler compared to the precipitation of softer reformed austenite in the ER2594 fller. The average impact toughness result of ER2594 and ER70S-2 is 165±5 J and 110±8 J, respectively. The Charpy impact trials showed the ductile fracture mode by employing ER2594 filler, while ER70S-2 showed the mixed fracture mode (ductile-brittle). The weldment tensile strength of filler ER2594 and ER70S-2 is 897 MPa and 873 MPa, respectively. The tensile test results indicate the ductile fracture mode for both fillers, and the failures were detected in sDSS2507.