Mo Content Effect on Microstructures and Toughness of the Simulated Coarse-Grained Heat-Affected Zone of Weathering Bridge Steels

In this study, varying Mo contents were added in weathering bridge steels to clarify the role of Mo in governing toughness of the coarse-grained heat-affected zone (CGHAZ) in detail. The simulated CGHAZ containing Mo contents from 0.05 to 0.50 wt.% was prepared using a Gleeble 3500 simulator under welding heat input 20 kJ/cm. Optical microscopy, electron back-scattering diffraction, scanning electron microscopy, and transmission electron microscopy were performed to examine the microstructure. The Charpy V-notch impact test was carried out at -40 degrees C to measure low-temperature impact toughness. Results showed that the microstructure in the CGHAZ of all tested steels consisted of granular bainite and lath bainite (LB), in which martensite-austenite (M-A) constituents were found in various morphologies. With Mo content increased, LB increased and refined, while M-A constituents varied in morphology and fraction. The low-temperature impact toughness decreased from 271 to 106 J with Mo contents from 0.05 to 0.50 wt.%. Analyzing various factors affecting toughness one by one, slender M-A constituents severely deteriorated low-temperature impact toughness in the Mo-bearing CGHAZ.