Microstructural characterization of the physical simulated and welded heat-affected zone of 9% Ni steel pipe

Abstract

The microstructure, Vickers microhardness, and prior austenite grain size (PAGS) of the heat-affected zone (HAZ) in a circumferential welded joint of a 9% Ni steel pipe were compared with those of a physically simulated (Gleeble machine) HAZ to validate a developed methodology for evaluating the HAZ via finite element method simulation. The welding of the cap pass was computationally simulated; subsequently, the thermal cycles of the HAZ subzones were reproduced (thermomechanical simulator), and the HAZ (physically simulated and of the welded joint) microstructures were compared. The HAZs in these conditions showed similar microstructures, microhardnesses, and PAGSs, thereby validating the developed methodology. The subcritical (SCHAZ), intercritical (ICHAZ), fine-grain (FGHAZ), and coarse-grain HAZs (CGHAZ) exhibited base metal unaltered microstructure, dual-phase microstructures (martensite and ferrite), refined martensite, and coarse martensite with coalesced bainite, respectively. The physically simulated HAZ was investigated using X-ray diffraction spectroscopy and transmission electron microscopy, and it exhibited globular and thin-film austenite morphologies, with minimum and maximum contents in the CGHAZ and ICHAZ, respectively. The developed computational and physical simulation methodology can be reliably reproduced and can evaluate the HAZ microstructure of the 9% Ni steel welded joint.