Abstract: Submerged arc welding technology was used to weld a steel plate with low susceptibility to weld cracking, and microstructure observation, conventional Vickers microhardness and nanoindentation examination were conducted in the heat affected zone (HAZ). The results show that a coarse grained HAZ mainly consisted of coarse granular bainite. Owing to a large number of nanoprecipitates and higher dislocation density, the higher microhardness and nanohardness were obtained in the coarse grained HAZ. The weld zone and fine grained HAZ mainly consisted of acicular ferrite plus proeutectoid ferrite and quasi-polygonal ferrite, respectively, and their nanohardness and modulus of elasticity were both less than that of the coarse grained HAZ. The greatest strength was observed in the coarse grained HAZ. The coarse granular bainite may diminish impact toughness, causing localized brittle fractures in the head of the coarse grained HAZ.