Effect of cooling time t8/5 on microstructure and toughness of Nb-Ti-Mo microalloyed C-Mn steel

In order to further optimize welding process of Nb-Ti-Mo microalloyed steel, welding thermal cycles on coarse-grained heat-affected zone (CGHAZ) of welded joints were simulated using Gleeble 1500. The microstructure and low-temperature impact fracture were investigated using a scanning electron microscope and a pendulum impact machine, respectively. Moreover, the relationship between cooling time t(8/5) and the microstructure of CGHAZ was discussed, and the effect of microstructure on impact toughness was also studied. As cooling time increased, martensite fraction decreased from 97.8% (3 s) to 3.0% (60 s). The fraction of martensite/austenite (M/A) constituent increased from 2.2% (3 s) to 39.0% (60 s), its shape changed from granular to strip, and the maximum length increased from 2.4 mu m (3 s) to 7.0 mu m (60 s). As cooling time increased, the prior austenite grain size increased from 34.0 mu m (3 s) to 49.0 mu m (60 s), the impact absorption energy reduced from 101.8 J (5 s) to 7.2 J (60 s), and the fracture mechanism changed from quasi-cleavage fracture to cleavage fracture. The decreased toughness of CGHAZ was due to the reduction of lath martensite-content, coarsening of original austenite grain,and increase and coarsening of M/A constituent. The heat input was controlled under 7 kJ cm(-1) during actual welding for these steels.