Effect of welding heat input on microstructural evolution and impact toughness of the simulated coarse-grained heat-affected zone of Q960 steel

In this study, the effect of welding heat input on the microstructural evolution and impact toughness of the simulated coarse-grained heat-affected zone (CGHAZ) of Q960 steel was investigated. Transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD) results revealed the martensite-bainite transition in the CGHAZ at different cooling times. The microstructure of the simulated CGHAZ was composed of martensite and bainite when the cooling time was less than 40 s. When the cooling time exceeded 40 s, the microstructure of the simulated CGHAZ consisted of only bainite. These microstructural variations resulted in fewer high-angle boundaries, which effectively arrested crack propagation and improved the toughness. High-angle boundaries and effective grain size had a positive linear proportional relationship with the impact energy. An investigation of the relationship between the martensite-austenite (M-A) constituents and the impact energy of the simulated CGHAZ at cooling times of 40, 60, and 150 s indicated that the impact energy decreased with an increase in the area fraction of the M-A constituent. The effect of large M-A constituents (L-max > 2 aspect ratio <4) on the impact energy was more severe than that of long strips of M-A (aspect ratio >4).