Effect of Weld Thermal Cycles on Microstructure and Properties of Simulated Heat Affected Zone in Thick-Wall X80 Pipe Steels

Continuous cooling transformation behaviour of the single cycled grain coarsened heat affected zones (GCHAZs) produced with a peak temperature (Tp) = 1350°C and cooling times, Δt_800-500 = ∼ 1 to 100 s was evaluated for three different X80 pipe steels having various content of C, Mn, Ni, Cr, Mo and microalloying elements that include Nb, V and Ti. Optical microscopy was initially used to characterize the simulated GCHAZ, which consisted of a range of coarse prior austenite grains that transformed to different fractions of mainly low carbon lath martensite/fine bainite, mixtures of upper bainite and/or granular bainite as a function of increasing cooling time. A consistent trend of decreasing microhardness with increasing cooling time occurred for the range of GCHAZs formed in the pipe steels. The significant differences in GCHAZ microhardness for Δt_800-500 < 15 s is attributable to the respective pipe steel compositions and the resulting microstructures. The GCHAZ microstructures were further characterized by means of scanning electron microscopy with electron backscattered diffraction and transmission electron microscopy with focus to analyze features of the transformation products, fraction of high angle boundaries and the nature of microconstituents, including carbonitride precipitates and inclusions. The simulated GCHAZ Charpy-V-notch impact energy transition curves revealed a consistent upward shift towards higher temperatures with increasing cooling time (Δt_800-500 = 6, 15 and 30 s). The primary factors contributing to the variations in impact toughness of the respective GCHAZs were the differences in the microstructure, hardness and detailed features, including fraction of high angle boundaries (packet size), and the presence of various M-A microconstituents.