| The core technique of electric storage water heaters is the manufacture of tanks,and the welding technique is the most important technology of tank manufacture.A company producing water heater received feedback from its after-sales that a large amount of water leakage tanks were found in the market.The failure rates of both seepage detection and pulsed pressure test in company's production line were at an all-time high.Follow-up researches from manufacturing site showed that the position of water leakage defects mostly happens in welds of tanks.In addition,many other defects such as burn-through,incomplete fusion,deformation were also easily discovered in welds of tanks.These imperfections had a negative effect on production quality of water heater.And for this a study aimed at improving welding quality of water heater tanks is needed.Using optical microscopy(OM)and scanning electron microscope(SEM),this paper analyzed the microstructures and fracture morphologies of cracks generated in pulsed pressure test,and determined type of the cracks.To analyze the causes of cracks,several aspects were focused,namely,weld component and metallography structure analysis,welding residual stress analysis,welding temperature fields and thermal cycle analysis.This paper also tested the mechanical properties of longitudinal welds and adjusted the welding parameters of longitudinal welds according to results of numerical analysis.Results of OM and SEM analysis show that the microscopic structure of BTC330 R is ferrite and a little pearlite,the average grain diameter of ferrite is about 12?m.The microscopic structures of both longitudinal welds and girth welds are common weld microstructures,columnar grain structures are obviously distributed in weld zone.The crack type of longitudinal welds is fatigue and quasi-cleavage cracks,and their initial cracking zone is located at fusion line which is in root of welds near enamel coating.Numerical analysis results based on SYSWELD show that temperature fields of longitudinal welds and girth welds vary drastic with high peak temperature,their T8/5 are 3.69 s and 3.19 s.Bulky columnar grain is caused by exorbitant thermal input and drastic welding temperature change.Results of mechanical properties testing show that both the tensile strength and the micro-hardness of two welds are higher than base metal.There are higher residual compressive stresses in weld center and adjacent-weld zone.Vertical residual compressive stress is decrease first and then increase(149 MPa~28 MPa~134 MPa)from ends to the middle,transversal stress becomes smaller and smaller(332 MPa~240 MPa)from weld center to heat affected zone.After enameling,longitudinal welds' peak micro-hardness of coarse grained region reduces by 30.9 HV1,and their average micro-hardness of HAZ reduces by 5 HV1.The average vertical residual compressive stress difference of same test point reduces from 270.6 MPa to 17.2 MPa.It proves that enameling can improve the mechanical property of longitudinal welds.The present study indicates that the reasons which caused cracks in longitudinal welds are bulky columnar grain and higher residual stress.Based upon the interaction of these factors,the fatigue strength of longitudinal welds is reduced and the cracks are generated.By means of adjusting welding parameters,improving manufacturing processes and production sites,the fatigue strength of longitudinal welds has improved.But the fatigue strength of girth weld,magnesium bar seat weld and water pipe seat weld still need to improve. |
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