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2 2023

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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.

Kumar Amit, Pandey Shailesh M., Pandey Chandan

Archives of Civil and Mechanical Engineering

2023

Vol. 23, no. 1

art. no. e54, 2023

A dissimilar joint of martensitic grade P92 and Ni-based Inconel 617 (IN617) alloy are employed commonly in advanced ultra-supercritical (AUSC) units to fabricate thick section components such as steam pipes and turbine rotors. This study investigated the weldability of the IN617 alloy and P92 steel dissimilar welds. Ni-based superalloy ERNiCrMo-3 filler was used to attempt the dissimilar joining for conventional V groove and narrow groove design by employing the gas tungsten arc welding (GTAW) process. The weld metal for the capping pass, backing pass, and near the interface showed the columnar and cellular grains while equiaxed grains are observed corresponding to root and filling passes. The energy dispersive spectroscopy and Electron probe micro-analyzer (EMPA) study confirmed the segregation of the Nb and Mo particles in inter-dendritic spaces and resulted in the formation of the Nb-rich NbC and laves phases and Mo-rich phases. The EDS line map and EPMA study of the P92 interface showed a sharp increase in Cr, Mo, and Ni concentration and a steep decrease in Fe concentration as moving from P92 base metal to weld metal. A negligible diffusion from filler weld to IN617 or vice versa across the IN617 interface was detected. The microhardness gradient along the weldments indicated a sharp rise in hardness value near the P92 fusion boundary due to the formation of the unmixed zone of lower hardness and the P92 coarse-grained heat-affected zone of higher hardness. The weld metal hardness results showed a great variation with an average hardness value for V groove and narrow groove welds of 227 and 262 HV, respectively. The mechanical tests were conducted at the ambient temperatures and data obtained for the weldments were compared with the base metals. The room temperature tensile tests showed the failure from the region of the P92 BM or the interface of P92 BM/weld metal, with joint strength of 646 ± 6 MPa and 747 ± 4 MPa in AW conditions for V groove and narrow groove, respectively. The Charpy impact test (CIT) also showed the variation in impact toughness along the weldments, and the ERNiCrMo-3 filler weld was identified as the weakest region of the welded joint in terms of impact toughness for both the groove designs. The residual stress variation along the thickness of the weld plate was measured using the deep hole drilling (DHT) methods, and the results indicated the peak magnitude of the residual stress for the V groove welded joint. The test results indicated that welded joint produced using ERNiCrMo-3 filler was safe for AUSC power plants' boiler applications for both the groove weld while optimum mechanical properties were measured for narrow groove weld.