Effect of heat input on microstructure and impact toughness of on F69A4-ECM4-M4 metal-cored submerged arc welding wire deposited metal

Deposited metals were submerged arc welded with F69A4-ECM4-M4 metal-cored wire under different heat inputs 18.67，20.57，22.56，24.63 kJ/cm, respectively. Their microstructure and impact toughness were investigated by optical microscope, scanning electron microscope and impact testing .Results showed that the morphology of microstructure and their percentage of deposited metal had significant differences under different heat inputs. With the increase of heat input, the width of columnar crystal of deposited metal increased, impact toughness at -40 ℃ decreased, and radical zone ratio of fracture surface increased. When heat input was 18.67 kJ/cm, microstructure consisted mainly of fine acicular ferrite, impact absorbed energy reached the maximum value 139 J. The reason of decrease of impact toughness was that deposited metal stayed longer at high temperature, thus acicular ferrite grains grew bigger quickly with the increase of heat input.