Research On The Crafts Of Low Alloy Steel CO₂ Gas Shielded Welding And The Quality Of Weldment

In this paper,the crafts of low alloy steel CO2 gas shielded welding and the quality of weldment was researched.Welded Q345C steel of different thickness by using Φ1.2mm ER50-6 type wire,drafted the welding craft and then assessed it.using this welding craft in key parts of the "da nao tian gong"project,and the quality of weldment was tested and analyzed.The preliminary of the common defect formation mechanism was researched.Through the experiments,determined 8mm thick plate vertical welding,horizontal welding,flat welding process.The groove was V type,welding current was 100-240A,welding voltage was 17～26V.Determined 40mm thick plate vertical welding,horizontal welding,flat welding process.The groove was X type,welding current was100～350A,welding voltage was17～35V,and it must be tempered after welding.By assessing the crafts,the tensile strength of 8mm thick plate was 525～540MPa,the impact absorbed energy was 33.6～47.7J.The tensile strength of 40mm thick plate was 490～495MPa,the impact absorbed energy was 150～224.5J.By mixing 85%C02 and 15%Ar gas together to weld the major parts.Controlled the gas flow between 15ml and 20ml per minutes,welding current was about 200A,welding voltage was 25～30V.Pre-welding and interlamination must be clean,clean up the oxide skin on the both sides of groove,the number of the welding plies was choosing the multi-pass welding.Using the thermocouple to preheat,the mini preheating temperature was 150degree,the max interlamination temperature was 200degree.After welding,by using ultrasonic testing to detect the column base,the main support column and the main support tube,the testing grade was II.Using magnetic particle testing to detect the main bearing bracket,the testing grade was I.The weld quality of the major parts were good.Through this welding craft,the appearance quality of weldment was good,effectively reduced occurrences of the surface stomas,splashes,undercuts and the craters.Statistical analyzed of all production weldment,found that internal defects was most common quality problems,the probability of occurrence up to 78.9%.After analyzed the formation mechanism of stomatal,the gas of the CO/H26/H2/N2 was most likely to occur,the most common stomas was CO.weld pores were easily formed because the gas bubble was easily formed in the molten pool of metal,and the bubble floating speed was slower than the speed of metal crystals.Add appropriate Ar gas,can effectively reduce the occurrence of Splash,Ar gas is added in an amount of usually 20%to 30%.