Study On Corrosion Behavior Of SUS304/Q345 Laser-MAG Hybrid Welded Joint

Stainless steel materials are widely used in equipment manufacturing and engineering components due to their excellent and comprehensive mechanical properties and corrosion resistance.In actual production,in order to save costs,SUS304 stainless steel and carbon steel are often connected by welding.Compared with the base metal,the chemical composition of the weld seam is extremely inhomogeneous,and the microstructure is complicated,resulting in poor corrosion resistance.Therefore,corrosion has also become a common form of failure failure of dissimilar welded joints in petrochemical,transportation,and pressure vessels.Therefore,it has a great practical value to figure out the fracture behavior of dissimilar steel welded joints under the combined influence of load and corrosion.In this paper,the SUS304 stainless steel material was welded with low-alloy steel Q345 by laser-MAG hybrid welding to study the stress corrosion and corrosion fatigue crack growth behavior in 3.5NaCl%solution.A constant displacement stress corrosion testing was carried out using a three-point bending specimen by a self-made cycle-immersion device.The stress corrosion cracking critical strength factor for the weld metal is less than 30MPa?m^1/2,and the stress corrosion cracking critical strength factor for Q345 base metal and Q345 heat affected zone is less than 25MPa?m^1/2.however,304 base metal and heat affected zone are not sensitive to stress corrosion cracking.Different microstructures in welded joints lead to different growth rates of stress corrosion cracks in different regions.Through the analysis of microstructures and XRD phase,it is found that there are a large amount of martensite in weld metal.Cracks easily nucleate and propagate along the grain boundaries of martensite and austenite,which results in the rapid growth of stress corrosion cracks in weld metal.Because of the high overall strength and corrosion resistance of 304base metal and 304 heat-affected zone,the stress intensity factor applied in this test failed to cause stress corrosion cracking.For Q345 base metal and heat-affected zone,stress corrosion cracking also occurs because of its low strength and poor corrosion resistance,but because of its stable structure and fine grain,its crack growth rate is much lower than that in weld zone.The interaction between 3.5wt%NaCl solution and fatigue load accelerates the corrosion fatigue damage of joints,reduces the threshold value of corrosion fatigue crack growth in various regions of joints,and accelerates the corrosion fatigue crack propagation.The morphology of corrosion fatigue fracture is different within different regions of the joint.The fracture morphology of Q345 base metal and Q345 heat affected zone is beach-like,which belongs to transgranular quasi-cleavage fracture.However,a interaction mechanism of transgranular cracking and intergranular cracking is found in 304 base metal and 304 heat affected zone,and small intergranular cracking planes are found in some regions.The corrosion fatigue fracture morphology of weld zone is also transgranular cracking and intergranular cracking.In some areas,ice-sugar pattern can be observed.Under the influence of corrosion solution,crack is prone to propagate along the martensite interior or the grain boundary of martensite and austenite,which forms a fast path of corrosion fatigue crack propagation.