Effect Of Low Transformation Temperature Consumables On Welding Residual Stress In Low Alloy High Strength Steel Welded Joints

Under the guidance of eco-friendly development,low alloy high strength steel has been widely applied to the manufacturing of bridges,ships,buildings,engineering machinery and offshore platforms,which are closely connected to the national economy and people's livelihood.Fusion welding technology is the most important way to connect various components in the manufacturing process of these structures.The performance of welding joints has a vital influence on the safety of the structure during service.Hot cracks,cold cracks and laminar tearing is usually related to transient stress during welding.On the other hand,the tensile residual stress generated by welding promotes the initiation of fatigue cracks,the reduction of fatigue life,brittle fracture and stress corrosion.Furthermore,the compressive residual stress may lead to bulking deformation,especially in the thin plate and thin-walled structure.Therefore,it is helpful to improve the fatigue performance of the weld joints and prolong service life of welding structure by optimizing and actively controlling the welding residual stress.At present,post-weld treatment is the leading method in engineering to modify the residual stress of the welding joint,such as post-weld heat treatment,shot peening and hammering.However,for some complex welding structure,the application of these methods will not only be restricted by the operation process,but also bring about the negative impact including longering schedule delays and adding costs.The idea of using low transformation temperature welding consumables to actively control the residual stress of the joint during the welding process provides a new way to control the residual stress of the welded joint.At same time,with the development of numerical simulation,it helps to study the evolution and distribution of residual stress efficiently and inexpensively.In this study,a thermal-metallurgical-mechanical multi-field coupling nonlinear finite element method considering solid-state phase transformation(SSPT)was developed based on SYSWELD platform.The method was used to study the residual stress distribution characteristics of low alloy high strength steel weld joints welded with low transformation temperature welding consumables.Firstly,the developed method was verified by means of experiment and the residual stress evolution mechanism of the low transformation temperature welding by Satoh test.Then the effect of low transformation temperature welding material on residual stress in single-pass welding and multi-pass welding of different grades of low alloy high strength steels(Q345,Q690 and Q960)was studied.In medium-thickness plate,the influence of some process factors including the interpass temperature,the arrangement of filler materials and the arrangement of welding bead of top surface on the welding residual stress were discussed.Finally,the T-joint and tube-sheet joint were taken as the research objects.For T-joint,the influence of arrangement of weld filler materials on the residual stress was discussed,and for tube-sheet joint,the influence of welding sequence was discussed.The research result shows that for single-pass welded joints of low alloy high strength steels with different strength grades(Q345,Q690 and Q960),the longitudinal residual stress in the weld zone is mainly determined by the solid phase transformation characteristics and mechanical properties of the weld filler metal(low transformation temperature welding consumables),and longitudinal compressive residual stresses will generate in the weld zone after welding.In the area outside the weld zone,the residual stress of low alloy high strength steel is mainly determined by the solid-state phase transformation characteristics and mechanical properties of the base material itself.In the heat-affected zone where the peak temperature of the base metal exceeds Ac3,due to the different solid-state phase transformation products in different strength grades of steel,the degree of residual stress affected by the solid-state phase transformation is different.In the outside area adjacent to heat-affected zone,higher longitudinal tensile residual stress exists,and the magnitude of its peak value is positively correlated with the strength level of the base metal.The distribution of transverse residual stress is generally not affected by the strength grade of low alloy high strength steel,but the maximum stress is related to the characteristics of solid-state phase transformation and yield strength.For multi-pass weld joints of low alloy high strength steels with different strength grades,the longitudinal residual stress in the weld area is determined by the solid phase transition characteristics,mechanical properties of the weld filler metal and the multiple thermal cycles experienced during welding.Significant longitudinal tensile residual stress will be generated in the upper surface welding area,and a higher tensile stress will be generated in other welding area.In the heat-affected zone where the peak temperature of the base material exceeds Ac3,the difference in solid-state phase transformation products also leads to the difference in residual stress.In the area outside the adjacent heat-affected zone,the welded joint forms a higher longitudinal tensile residual stress,and its peak value is positively related to the strength level of the base metal.And due to different heating cycle characteristics at different positions in the thickness direction,certain difference the ultimate tensile stress peak in the thickness direction is also different.The transverse residual stress is mainly related to the solid-state phase transformation characteristics and the yield strength of the material.Q690 and Q960 welding joints have similar transverse residual stress,but that of Q345is different at certain extent.For the multi-layer and multi-pass welded joints of medium-thickness plates,when the interpass temperature is higher than the martensite transformation start temperature(M_S)of the low transformation temperature welding material(LTTW),the longitudinal compressive residual stress can be generated in the entire weld region,and transverse compressive residual stress generated in most of weld zone.But when the interpass temperature is lower than M_S,though longitudinal compressive residual stress generated in the upper surface welding area,tensile residual stress generated in the weld zone,and transverse tensile residual stress also appears.Compared with the joint with two LTTW welding beads in surface area,the overall residual stress of the joint with only one LTTW welding beads in surface area is better.For T-joints,the longitudinal and transverse residual stresses on the weld surface can be significantly improved when LTTW is added to the weld toe.For the tube-plate joint,three welding sequences are used to complete the welding,the numerical simulation result shows that the welding sequence has insignificant effect on the residual stress distribution in the stable region,but has a great influence on the welding residual stress near the start-end location.