Thick plates are used in the local weak position of the large oil filling equipment

and the welding process parameters have a great influence on the microstructure and mechanical properties of the material. The butt weld of oil-immersed transformer tank rim which is manufactured with 40 mm thick Q355B plate is taken as an example

and the effect of welding process with preheating before welding on the microstructure and mechanical properties of the material is studied in this paper. A series of tests of examining the mechanical properties of steel plates are conducted after utilizing gas-shielded arc welding on the basis of following four parameters: no preheating prior to welding

preheating at 60 ℃

preheating at 100 ℃

and preheating at 150 ℃. The test results show that

after preheating treatment

the internal defects are reduced

and the hardness mutation of the welded joint is reduced. The welded joint under tensile loading is broken at the base metal

and the bending test can be bent to 180°. The impact test results of specimens cut from different region of the welded joint show that the impact resistance of each region of the joint is improved and more uniform after preheating treatment before welding

and the comprehensive test results are the best one after preheating at 100 ℃. The research results of this paper can provide a theoretical basis for the explosion-proof design and welding process improvement of the oil tank of large oil filling equipment.