Microstructure and toughness in reheated CGHAZ of ASTM4130 steel

The effect of different second peak temperatures on microstructure,impact toughness and fracture morphology in the reheated coarse-grained heat-affected zone(CGHAZ) of ASTM4130 steel was systematically investigated by using matallography,SEM,TEM and the welding thermal simulation test.The results show that low temperature impact toughness of the reheated CGHAZ is lower than the base metal,which indicates that the embrittlement of overall heat-affected zone(HAZ) occurs.The intercritically reheated coarse-grained HAZ(IRCGHAZ) becomes the local brittle zone with its toughness loss by 96.6% and 37.9% compared with that of the base metal and CGHAZ respectively.Meanwhile,the toughness of the supercritically reheated coarse-grained HAZ(SCCGHAZ) and the unaltered coarse-grained HAZ(UACGHAZ) decreases by 20.9% and 14.1% compared with that of CGHAZ respectively.The embrittlement of IRCGHAZ and SCCGHAZ is attributed to coarse grain caused by "structure heredity" as well as the formation of M-A constituent,and the fracture morphology is characteristic of quasi-cleavage with "fracture heredity" features.The reasons for the lower toughness of UACGHAZ are both the coarse grain and the existence of M-A constituent,and it fractures also by a quasi-cleavage mode.