Study Of Weld Formation And Microstructure By Double-side Arc Welding On The Flat-overhead Position For Thick Plate Of High Strength Steel

Thick plates of high strength steel are widly used in shipbuilding, highpressure vessel and so on, which involve all-position butt welding. For overheadwelding, the adverse effects of gravity leads to bad welding shape. How to takeinto account both welding efficiency and weld quality on the flat-overheadposition for thick plate of high strength is a key problem to solve. Double-sideddouble arc welding(DASW), that backing run adopts double-sided pulse gastungsten arc welding (GTAW), and other passes adopt double-sided gas metal arcwelding (GMAW), is one of the effective ways to solve the problem. In order todemonstrate the feasibility, the weld formation and microstructures areresearched in this paper.Because of bad welding shape on overhead positon, arc swing welding isprovided. The influence of weld parameters including welding current, weldingspeed, the swing width, and duration has been investigated. Optimum weldingparameter was acquired. On this basis, double-side arc welding on the flat-overhead position for thick plate of high strength stee l is implemented.Firstly, heating process of HAZ has been theoretically studied. It shows that,according to the distance from the fusion line, the number of thermal cycles andits peak temperature change for HAZ. As a result, microstructures there chang.There are two rules of variation:1) a ne-t like microstructure (NM) is formed inIRCGHAZ which experiences once or twice the cycle of the coarse grained zone,then the cycle at an peak temperature of AC1~AC3; Apart from IRCGHAZ,microstructures in other parts of HAZ depend on the final welding thermal cycleexperienced. Near the fusion line, HAZ is heated above the reversetransformation temperature once, twice and three times, which leads to complexmicrostructures.Secondly, the microstructure of welding joint have been analyzed throughOM, and SEM. Typical isometric crystals and columnar crystals lies in MAG capweld, which consist of acicular ferrite, side plate ferrite and lath martensite.After several times of tempering for backing welding, the features of acicularand lath are weakende. Its microstructure approach to the microstructure of basemetal,tempering sorbite. The hardness of CGHAZ and IRCGHAZ is higher thanthe others. XRD analysis showes that phase composition of the whole jointincluding base metal、HAZ and weld metal are Fe and intermetallic compounds.The net-like microstructure observed in HAZ is simulated and reproduced by using welding thermal simulation experiments to investigate its finemicrostructure、 formation mechanism and mechanical property. The sesultshows that there are two rules resulting in the formation of the net-likemicrostructure: coarse grains are the precondition; thermal cycle at an peaktemperature of AC1~AC3is needed. The net-like microstructure is predominantlycomposed of granular M-A islands in which there are martensite lath beam withdifferent orientation. The size and number of net-like microstructures inIRCGHAZ are the largest resulting in worst toughness. High temperaturetempering contributes to the improvement of toughness in IRCGHAZ.Themicrograph of impact and tensile fracture indicated that it was dimple fracture.