Microstructures And Mechanical Propetries Of Laser-MAG Hybrid Welding On S355J2W Weathering Steel

S355J2W weathering steel is one kind of low alloy steels with high strength, which isobtained by adding a small amount of alloy elements such as Cu, P, Cr and Ni. It has been widely used in transport industry because of its high strength and excellent resistance to the corrosion environment. The successful application of any materials depends on not only its inherent properties, but also the development of joining technologies. Generally, MAG weldingis a common method for joining the weathering steel plate with medium or large thickness.However, the defects associated with MAG such as large heat input, high deformation, low efficiency and harmful gases, limit the broad application of the S355J2W weathering steel. Fortunately, due to the high stability, low heat input, large penetration, good adaptation to gap and high efficiency, laser-MAG hybrid welding process can effectively solve these problemsexhibiting in the traditional MAG welding for the S355J2W weathering steel.The microstructures, mechanical properties and residual stress distribution oflaser-MAG hybrid welded S355J2W weathering steel joints were systematically investigatedby means of optical microscopy, scan electron microscopy, transmission electronmicroscopy, microhardness tester, material tensile testing machine and measure system forresidual stress. The suitable welding parameters window was obtained and the effects ofwelding process on the microstructure and mechanical properties of joints were revealed.Metallurgical analysis shows that the laser-MAG hybrid welded joint consists of weldzone (WZ), base metal, incomplete recrystallization zone, fine grain zone, coarse grain zone.Further joint microstructure of each area by SEM and TEM, especially in the phasecomposition was studied. The weld metal is mainly for columnar, blocky, acicular and stripferrites，and it also contains a small amount of pearlite and bainite. The weld phasecomposition of laser-MAG welding and MAG is-Fe phase, the average grain size of twowelding methods being similar.Microhardness measurement shows that the hardness of the WZ and HAZ was higherthan that of BM which has the minimum hardness level. The maximum hardness was foundat the boundary of WZ and HAZ. All the joints fractured at BM during tensile tests, showingthat the weld has a higher strength than BM. The fracture behavior and fracture mechanismof joints were studied. Further, some microcracks were observed on the joint surface afterside bend testing with180degree. The residual stress of the laser-MAG hybrid welded joints was measured by means ofblind-hole drilling method. The results show that the maximum value of the residual stresswithin the joint with as-weld condition is approximate to the yield strength of the BM, whichcan be reduced by the multi-pass welding process.