Microstructure and properties of simulated heat affected zones of weathering heavy steel plate for bridge

Advanced heavy steel plates(60 mm thick) for bridge with room temperature yield-strength greater than 500 MPa and assured low temperature impact toughness(Charpy V notch impact energy(CVN) of 200 J at -40℃) have been produced via thermomechanical control process(TMCP). The dependence of microstructure and the impact toughness at -40℃ of the coarse-grained heat-affected zone(CGHAZ) generated by single-pass simulated welding upon the heat input energy(E) and peak temperature T_p² of simulated second-pass welding processes were revealed. Thermal cycles of the CGHAZ and the sub-regions(intercritically reheated coarse-grained zone(IRCGHAZ), supercritically reheated coarse-grained zone(SRCGHAZ)) of the subject steel plate were simulated employing a Gleeble 3 800 thermomechanical simulator. The microstructure of the CGHAZ consists of lathlike bainite(LB) under E less than 50 kJ/cm. The microstructure changes gradually into granular bainite(GB) associated with coarsened martensite/austenite(M/A) constituents in the cases of E greater than 100 kJ/cm. Ductile impact fracture behavior is observed under the CGHAZ conditions with E less than 100 kJ/cm, while brittle fracture is rendered with E greater than 100 kJ/cm. The IRCGHAZ is the so called local brittle zone(LBZ) under all the tested conditions. This is attributed to the formation of coarse M/A constituents. The SRCGHAZs with moderate and small E which is no greater than 50 kJ/cm show ductile fracture. These are LBZs with increased E which is once again attributed to the formation of GB. The increase of T_p² leads to improved impact toughness in the SRCGHAZ. This is attributed to increased hardenability of austenite resulting in transformed-microstructure consisting of fine LB and GB.