Effect of Heat Input on Microstructure of Heat Affected Zone in Submerged Arc Welded Joints of X70 Pipeline Steel

X70 pipeline steel was welded by submerged arc welding. The grain size of bainite in the heat affected zone was evaluated with OLYCIA m3 software. The microstructure of heat affected zone was investigated by OLYMPUS-PMG3 optical microscope. The results show that the original acicular ferrite and perlite structure in the base metal transforms to coarse grain bainite structure after welding. X70 pipeline steel is a kind of steel with fine grain structure, and the microstructure has a tendency of coarsening under the effect of high temperature. During welding, with heat input increasing,the microstructure became coarsening. The grain size of grain bainite under the condition of high heat input with 30.6 kJ/cm is almost 1.2 times higher than that of grain bainite under the condition of lower input with 19.04 kJ/cm.