Abstract: Many casting defects were found in railway wagon side frame casted by level-B steel(ZG25MnNi),and the defects must be repaired by welding . But widmanstatten was not improved in heat affected zone of weld joint and sometimes granule bainite was found in weld joint zone. Through discussing the welding microstructure and heat treatment technology,the defects microstructure after heat treatment were analyzed and the preventive suggestions were proposed.