In　order　to　reduce　the　welding　residual　stress,　researchers　adopted　tempering　weld-pass　treatment　for　the　improvement　of　the　class　III　vessels　in　an　expansion　project.　Due　to　the　optimization　and　evaluation　of　welding　process,　the　researchers　conducted　a　comprehensive　test　and　evaluation　by　X-ray　method　on　six　Q345　steel　welding　test　plates　(600　mm×300　mm×30　mm),　with　three　kinds　of　processes　(welding　and　two　kinds　of　new　tempering　welding　treatment)　and　2　patterns　(flat　butt　and　T-joint　welded　plate)　and　considering　the　weld　metal　polished　(side　B)　while　the　other　side　(side　A)　not.　The　results　showed　that　the　welding　residual　stress　in　1　and　3　T-plate　decreased　by　46%　and　36%　respectively　than　which　is　not　with　tempering　welding　treatment.　Tempering　weld-pass　treatment　reduced　the　longitudinal　stress　(LS),　and　the　transverse　stress　(TS)　became　the　first　principal　stress.　The　residual　stress　of　A-side　was　25%　higher　than　that　of　B-side.　To　reduce　the　residual　stress,　the　weld　metal　can　be　removed　in　practical　engineering　applications.