Corrosion Fatigue Behavior And Life Prediction Method Of Welded Joints Of G20Mn5QT Cast Steel With Q355D Steel

Cast steel has good mechanical properties,machinability and weldability,and has been widely used in structural engineering,bridge engineering and ocean engineering in recent years.Due to the influence of complex environment such as corrosion,the structures and components of these projects are prone to corrosion fatigue damage.Corrosion has become a huge potential safety hazard in engineering structures.In this paper,corrosion fatigue tests of butt welded joint,non-load-carrying cruciform welded joint and load-carrying cruciform welded joint of G20Mn5 QT cast steel and Q355 D hot rolled steel in simulated seawater environment are carried out,and compared with the standard specimens of butt welded joints in simulated seawater environment.According to the fracture analysis of corrosion fatigue failure specimens obtained from corrosion fatigue test,combined with the corrosion fatigue S-N curve fitted by test data,the corrosion fatigue failure mechanism of three different types of welded joints was analyzed.The corrosion fatigue S-N curve model based on three-parameter Weibull distribution is established by using the data samples obtained from the test,and the applicability of Weibull model in corrosion fatigue life assessment of welded joints is verified.Thereby providing guidance for engineering design.The main conclusions are as follows:(1)Corrosion fatigue fracture occurs at the weld toe of butt welded joint of G20Mn5 QT cast steel.At higher stress level(> 220MPa),casting defects and welding defects are the main causes of crack nucleation,while at lower stress levels(≤220MPa),the surface corrosion pit and large size defect are the main causes of crack nucleation.The corrosion fatigue life of butt welded joint is lower than that of butt weld standard specimen at all test stress levels.(2)Corrosion fatigue fracture occurred in the welded toe of the non-load-carrying cruciform welded joint of G20Mn5 QT cast steel.At higher stress level(> 170MPa),casting defects near the surface and welding defects of the specimen are the causes of corrosion fatigue failure.When the stress level is small(≤170MPa),the surface corrosion pit is the main cause of crack nucleation.The stress concentration at the welding toe has a great influence on the life of the unloaded cross welded joint.(3)Corrosion fatigue fracture occurred in the welded toe or root of the load-carrying cruciform welded joint of G20Mn5 QT cast steel.When the specimen is broken at the weld toe,the casting defects near the surface and the stress concentration at the weld toe are the main reasons leading to the crack nucleation at a large stress level(>160MPa).However,at lower stress level(≤160MPa),the surface corrosion pit is the main causes of crack nucleation.When the specimen is broken at the weld root,the welding defect at the weld root of fillet weld is the main cause of crack nucleation.(4)The corrosion fatigue S-N curve based on the three-parameter Weibull distribution can well interpret the corrosion fatigue behaviors of three different types of welded joints of G20Mn5 QT cast steel.Compared with Stromeyer expression,the corrosion fatigue S-N curve with three-parameter Weibull distribution model has higher fitting accuracy,which is more applicable in fatigue life prediction of welded joints of cast steel and has guiding significance for practical engineering design.