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 °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.