Control Of Welding Cold Crack With Trailing Electromagnetic Induction Heating During Welding

Based on the electromagnetic induction principle, a new technique with electromagnetic induction heating during welding has been proposed to control welding cold craking in welded joint of high-strength steel. Based on the welding CCT cycle, induction heating behind welding electric arc will reduce the cooling rate, that it can reduce the amount of quenched structure and increase the resistance to welding cold cracking. The new technique could increase the production effeciency due to avoiding preheating and post weld heat treatment.With the finite element, the welding thermal cycle was simulated for various minimum distance between the coil and welding pool (CPD). The results indicated that the reasonable CPD should range from the DA (the distance between maximum temperature and austenitic decomposition temperature at weld junction) to the DM( the distance between maximum temperature and martensitic transformation temperature at welding pool. When the CPD is samller than the DA, the part of inductive heating coil above the metal whose temperature is higher than austenite transition temperature (800℃) takes no contribution to extend the t8/3; When the CPD is larger than the DM, the induction heating fail to prevent the formation of quenched structures. The two methods for selecting key temperature point were put forward: point at weld junction ( weak criterion) and point at slack quenching line (strong criterion).The trailing induction heating experiments for controlling welding cold cracking were carried out by the oblique Y groove method. The results showed that the surface crack ratio could be lower below 20% if the coil is located in the reasonable CPD. When the CPD was closer to DM, the crack ratio was lower. The CPD was beyound the reasonable CPD,trailing induction heating fail to obviously reduce welding cold cracking. The measured temperature cycle showed that the cooling curve would go through the bainite transformation zone if the coil is located in the reasonable CPD. The amount of quenched martensite were obviously reduced and amount of bainite were increased. When the position of coil is beyound the reasonable CPD, austenite stabilization would be occur, which prevents bainite transformation although the cooling cycle pass the bainite transformation zone. Therefore, there still was quenched martensite in the HAZ. The microhardness analysis showed that the hardness in welded joint with trailing induction heating was obviously smaller than that in conventional welded joint. The maximum hardness was reduced from 727 to 515.