Research On Welding Procedure Of Underwater-wet Welding Technology With Trailing Medium Frequency Induction Heating

Due to the effect of water environment, the microstructure and mechanicalproperties of the beads gained by regular underwater wet welding are unsatisfactory. Inorder to reduce the cooling rate of welded joints, this paper uses medium frequencyinduction heating equipment to heat the welding zone or give postweld heat treatment tothe joints. The main aim of this paper is to reduce weld defects such as quenchedstructure and cold cracks in underwater wet welding joints, and to explore a newmethod that can improve the microstructure and mechanical properties of underwaterwet welding joints. The experiment results are discussed and analysed in detail.The paper primarily studies the influence rules of different shapes and positions ofmedium frequency coil to arc shape and appearance of welds. The signal acquisitionsystem is used to contrast the results. When the elongated coil is used and the distancebetween heating center and molten bath is kept at20mm, the arc shape is stable and thepenetration of the beads increases. The formation of beads are good-looking, thewelding residual stress of joints reduce to a certain extent.Based on the former results, the same medium frequency heating pattern is chosen,through changing the heating power, several temperature variation curves are gained.The results show that when the medium frequency heating voltage changed from100Vto450V, the retention time during high temperature t8/5improves30%to160%. Steelswith different carbon equivalents are welded by underwater wet welding method withcompound heating fields to weld. When the medium frequency voltage is lower than250V, the medium frequency heating field promotes the self-tempering behavior of thejoints. While when the medium frequency voltage is higher than250V, t8/3and t8/5of theHAZ increase prominently, the microstructure of the joints have changed greatly, thecontents of quenched structure such as martensite and upper bainite reduce, at the sametime the contents of microstructure with excellent ductility and toughness such asferrolites and lower bainite increase, the granularity of granular bainite and the size ofM-A components become thinning. When the joints are treated by post heating ofmedium frequency heating system, the hardened martensites in the joints start totempering transformation, the contents of tempered martensite and tempered troostiteformed during tempering transformation process are different under different postheattemperature. Those microstructure formed during tempering transformation processhave excellent yield ratio and toughness. The bending property and toughness of thebeads have different degrees of improvement.Considering the specific conditions of medium frequency heating field, theexperiment parameters are optimized. The gas chromatographic method is used tomeasure the contents of diffusible hydrogen under different medium frequency heating patterns and heating power. The contents of diffusible hydrogen in deposited metaldepend on the synthetic action of medium frequency heating system to the initialhydrogen contents and hydrogen escape velocity. Based on the measuring results ofdiffusible hydrogen,16mm Q345steel is used to conduct the inclined Y groove coldcrack sensitivity test, through the statistics of cracking rate of the fracture surface、metallographic structure、SEM, we gain the conclusions that the addition of mediumfrequency heating system can efficiently reduce the hardened microstructure and thequenching stress of the welding beads.