Study On Thermal Cycle Of Multi-wire Submerged Arc Welding And Its Influence On Hardness

As the core technology of high-strength thick wall pipe manufacturing,multi wire submerged arc welding can not only realize high-speed welding,but also obtain excellent welding joint.However,as the welding heat affected zone is the weak link of the whole weld,its performance mainly depends on the characteristic parameters of thermal cycle.Therefore,it is important to clarify the temperature field and welding thermal cycle for the study of HAZ performance.In this paper,a compound heat source model is developed to simulate the thermal process of multi wire submerged arc welding with four wire internal welding and five wire external welding,so as to obtain the thermal cycle curves of coarse grain zone and fine grain zone under welding conditions.Through the simulation method of adding heat sources one by one,the effect of each heat source on its temperature field and thermal cycle characteristics is clarified.Based on the rekalin temperature field and t_6/5 as the evaluation index,the equivalent line energy suitable for four wire internal welding and five wire external welding is established.The accurate thermal cycle curve can be obtained by using the rekalin formula.The thermal cycle curves of each zone of HAZ were obtained by numerical simulation,and the influence of welding thermal cycle on hardness was obtained by thermal simulation test.In the temperature field of four wire internal welding,the third heat source has the greatest influence on the penetration.On the basis of the first and second heat sources,the increase amplitude of penetration accounts for 20%of the total penetration,and the fourth heat source has the greatest influence on the width,which accounts for 36%of the total width.When the third heat source acts,the first t_8/5 value appears.When the fourth heat source acts,the t_8/5 value of coarse grain region is 35s.In the temperature field of five wire external welding,the fourth heat source has the greatest influence on the penetration.On the basis of the first,second and third heat sources,the increase amplitude of the penetration accounts for14%of the total penetration,and the fifth heat source has the greatest influence on the width,and the increase amplitude of the width accounts for 21%of the total width.When the fourth heat source acts,the first t_8/5 value appears.When the fifth heat source acts,the t_8/5 value of coarse grain region is 46s,and the t_6/5 value increases most.The results of the experiments on hardness and strength of the heat affected zone showed that there is obvious softening phenomenon in the fine grain region.The peak temperature is1000?in the fine grain zone of the heat affected zone of welding joint.The tensile strength and yield strength are divided into 674Mpa and 534Mpa,the hardness is 200Hv0.2,the yield strength is 27%lower than that of the parent metal,and the hardness is 13%lower than that of the base metal.The main reason is that the quantity and volume of quasi polygonal ferrite are more,and there are a lot of fine and small M-A components at the grain boundary.With the increase of M-A size,the strengthening effect is weakened,and the increase of quasi multilateral ferrite will further reduce the strength of the zone.When the peak temperature is 1300?,it is in the CGHAZ of the HAZ of the welding joint.With the increase of t_8/5,the hardness decreases,mainly due to the decrease of bainite content and the increase of ferrite structure.The higher the cooling rate V_6/5 is,the higher the hardness is.The smaller the V_6/5 is,the larger the grain size is,the larger the M-A size is,and the strengthening effect is weakened.As a result,the hardness is reduced,and the larger the V_6/5 is,a large number of fine M-A islands are dispersed in the structure,thus improving the hardness.