Microstructural characterization of Ni-based superalloy 625 clad welded on a 9% Ni steel pipe by plasma powder transferred arc

Abstract

The 9% Ni steel was for the first time cladded with Ni-based superalloy 625 by plasma powder transferred arc welding (PPTAW), and the clad microstructure was evaluated for application in the oil and gas industry. The microstructure of the clad was studied using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, electron backscatter diffraction, X-ray diffraction texture analysis and Vickers microhardness. In the transition zone (TZ), an abrupt variation in chemical composition, type-II boundary, macrosegregations, and intense epitaxial nucleation occurred. The crystallographic texture continuously increased from the TZ (randomly) to the clad surface with moderated rotated cube texture component. The clad showed low residual strain and high fraction of high-angle grain boundary. The segregation of Mo and Nb during the clad solidification generated interdendritic precipitates (MC-type carbide and Laves phase), and no hot cracks were found. The microhardness showed a peak in the heat- affected zone and continuously dropped from the TZ to the clad surface. The PPTAW one-layer clad reached the DNV-OS-F101 standards and has been approved for use in the oil and gas industry.