Study On Microstructures And Mechanical Properties Of BWELDY700QL4Low Content And High Strength Alloy Welded Joint

With the rapid development of modern industry,as the essential transportation machineryof industrial production crane has become more and more large-scale,high parameter andlightweight. The carrying capacity of crane is increase with the technical innovation.So thesafety of its lifting process is especially important.The arm is the most important structure of acrane, so the welded joint quality and performance would affect the crane safety and reliabledirectly.Therefore,it is significance to safety in production by taking reasonable weldingprocedure to ensure the quality of the crane arm.Microstructure and properties of BWELDY700QL4low content and high strength alloywelding joint were investigated by tensile,bending,impact,hardness test and metallographicanalysis. Influence of microstructure and mechanical properties of BWELD700QL4highstrength alloy were investigated by crackresistance test,welding thermal simulation,impacttest，hardness test and metallographic analysis. The results showed that tensile, bending andimpact properties of the BWELDY700QL4welding joint which is welded with YM80-Awelding wire are excellent；The microstructure in the weld metal contains vast proeutectoidferrite that locates at the grain boundary,carbon-free bainite in parallel arrangement whichstretch from grain boundary into the grain,and acicular ferrite and pearlite present within thegrain,also some granular bainite distributes at exceptional locations.The microstructure in thefusion zone of welding joint contains lath martensite and martensite-austensite group.Themicrostructure in the normalized zone of welding joint contains granular bainite and a littleferrite.The hardness of BWELDY700QL4welding joint is between264-380HV.The valve ofthe magnitude in the weld metal is48-76HV higher than HAZ and parent metal. The hardnessof BWELDY700QL4butt welding joint is between255-370HV.The valve of the magnitudein the weld metal is24-77HV higher than HAZ and parent metal.The distinction of them isnot obvious.There is softing phenomenon in HAZ， which is neither obvious.Thecrackresistance of BWELD700QL4is not well in low temperature.It is necessary to preheatafter welding；When t8/5becomes longer at room temperature and-40℃，the impact values ofBWELD700QL4overheated zone become higher gradually.When t8/5is7.2s to13.0s,themicrostructure of BWELD700QL4overheated zone is lath martensite.The level of themicrostructure grain size is5to6.The variations of BWELD700QL4overheated zonehardness is not obvious.We shouid preheating properly and control the welding energy small in order to avoidthe welding cold cracking before welding.The welding joint should be cooling slowly afterwelding to ensure toughness and strength.