Study On Controlling Welding Cold Crack With Hybrid Electromagnetic Force And Heating

High strength low alloy (HSLA) steels are sensitive to welding cold crack due to high carbon equivalent. In general, preheat and post-heat treatments are adopted to control welding cold crack, but these processes have lowered the efficiency of welding and deteriorated working environment for welders. According to the characteristic of HSLA welding, a new process, welding with hybrid electromagnetic induction heating and electromagnetic impacting (WEHI) has been proposed in this paper. This process synthesizes the effects of electromagnetic induction heating and electromagnetic impacting, which is used to improve the property of joint's structure, reduce the residual stress and decrease the hydrogen contents of the joint. Therefore, the welding cold crack can be avoided.The distribution of welding temperature field, stress and residual hydrogen in the conventional joint and WEHI joint are studied by finite element analysis based on ABAQUS. The model of slit type cracking test for finite element analysis is established by HYPERMESH, the composite function of welding heating source and induction heating source is defined by FORTRAN program. Results show that the induction heating source can be taken as the assisted heat source to decrease the cooling rate so that improve the property of joint's structural. This process can be adjusted according to CCT curve of the welded material. Simultaneously, the heterogeneity of weld temperature field is improved and the transverse tensile plastic of joint root is increased during the WEHI, thus the transverse residual stress of joint root is reduced from tensile stress to pressure stress.The residual hydrogen of conventional joint and WEHI joint is measured by alcohol method. Results show that the residual hydrogen in WEHI joint is obviously lower than that of conventional joint. The measured result is set as the initial hydrogen concentration in mass diffusion-finite element analysis. The simulated results show that residual hydrogen contents near fusion line in HAZ of WEHI falls significantly compared with conventional welding.The cold cracking sensibility of HSLA 40CrNiMo which are welded by different welding methods is verified through slit type cracking test. These methods include conventional welding, welding with electromagnetic induction heating (WEH), welding with electromagnetic impacting (WEI) and WEHI. Results show that WEH, WEI and WEHI can decrease the joint surface crack rate and the cross section crack rate to some degree, but WEHI has more remarkable effect. The WEHI reduces the surface crack rate from 100% of conventional welding to 2.98%, and the cross section rate from 100% to 1.5%.