The thermal cycle of coarse-grain heat affected zone (CGHAZ) of Q550qENH steel under different heat inputs was studied by a Gleeble-3800 thermal simulation tester. The effects of heat input on the microstructure and mechanical properties of CGHAZ of Q550qENH steel were analyzed using optical microscopy (OM)

scanning electron microscopy (SEM)

electron backscatter diffraction (EBSD)

a Vickers hardness tester and low temperature impact test. The results show that the CGHAZ of Q550qENH steel primarily consists of lath martensite (LM)

lath bainite (LB) and granular bainite (GB). As the heat input increases

the LM content decreases

while LB and GB contents increases

leading to a coarser microstructure. At a heat input of 10 kJ/cm

the CGHAZ consists of LM

LB

and a small amount of GB

whereas at 80 kJ/cm

the structure transforms into coarse GB. The microhardness of CGHAZ decreases from 338HV at 10 kJ/cm to 236HV at 80 kJ/cm. The impact energy of CGHAZ at -40 ℃ remains at 250-300 J for heat inputs between 10 and 40 kJ/cm but drops sharply to 13 J at 80 kJ/cm. The impact fracture at -40 ℃ shows an obvious river-like pattern

indicative of brittle fracture. According to the welding thermal simulation experiments

the optimal welding heat input range of Q550qENH bridge steel is below 30 kJ/cm

ensuring both good low temperature toughness and high hardness in the CGHAZ.