The Research On The Molten Pool Forming And Welded Joint Properties Of Rail Ultra-narrow Gap Welding

Seamless Rails are the best choice of the high-speed?heavy-load track structure,Rail welding is the key technology for laying and maintaining seamless Rails.But the safety and stability is restricted by the low quality of in-situ welding.Therefore creating a new kind of rail welding method to improve the in-situ welding quality has an important significance and application value to improve the in-situ rail welding technology.Ultra-narrow gap welding with constrained arc by flux strips has the lower weld heat input?better quality of the joint?higher welding efficiency and more suitable for thick workpieces welding.This research uses ultra-narrow gap welding with constrained arc by flux strips to solve the in-situ problem.Firstly developing a welding test device which can achieve rail ultra-narrow gap welding,then aiming at the forming control of the molten pool and the mechanism and suppression of the hot crack are studied intensively,Combined with the above results,the control of the welding deformation and the technology parameters planning are systematically researched through rail welding technology test,finally the evaluation of the joint microstructure and mechanical properties are studied,the main results are as follows:(1)Designed and developed a set of rail ultra narrow gap welding equipment which can effectively guarantee the straightness of the rail welding joint.This equipment consists of three parts,according to the welding deformation characteristics designs a positioning and clamping mechanism wh ich can realize setting and clamping of the groove size;Aiming at the characteristics of the rail cross section,developing a constraint arc force forming device which c an extend the bead and guarantee the smooth weld bead;Combining with ultra-narrow gap groove size and rail thickness designs an ultra-narrow gap plate welding gun which can ensure the stability of wire feeding and conduction.The development of rail ultra-narrow gap welding equipment provides a necessary hardware support for the next test st udy and practical application.At the same time,developing a stable and reliable flux strips production process which can realize automatic production.and developing a placement method which can regulate the flux strips' positional accuracy in groove,effectively guarantee the ultra narrow gap welding process stability.(2)By using of method for interrupting arc quickly,the melting forms of flux strips,groove's bottom and sidewalls in the welding process are held and observed,the interaction mechanism of flux strips and arc during the welding process is analyzed.Results show that,the interaction area between flux strips and arc directly affect the degree of arc constrained,while the degree of arc constrained controls the arc heating area and molten pool forming.By changing the welding voltage,flux composition and the distance from flux strips to wire,it can obtain a relationship between the melting area of the flux strips and the area of the arc solid wall constraint which can accurately control the degree of arc constrained:When the melting ar ea of the flux strips was greater than the area of the arc solid wall constraint,the degree of arc constrained was too weak,the heating area of the arc extended to higher postion of groove's sidewalls,couldn't form a effective molten pool.When S<5mm2,the melting area of the flux strips(S)was smaller than the area of the arc solid wall constraint,the degree of arc constrained was too strong,the heating area of the arc was concentrated at the bottom of the groove and couldn't reach the side wall directly,thus forming a sidewall and sidewall root fusion bad weld.Just when 5mm2<S<45mm2,the heating area of the arc could be effectively restrained at the bottom of the groove and both sides of the wall,getting a good molten pool forming.(3)Based on the analysis of the micro-topography of "1-shape" and "herringbone" hot crack in the narrow gap weld,it is found that the formation of different forms of hot crack is due to the different shape of the weld section led to the molten pool solidification crystallization direction,thereby forming a different form of low melting eutectic film,finally making the film develop hot crack under the tensile stress.And it is influenced by weld form factor?heat input and gap width,as the weld form factor decreased,heat input increased and gap width decreased,the tendency of hot crack increased significantly,and the weld form factor is the main factor,when it increases to a certain value can avoid hot crack in the weld.Finally,obtaining a critical relationship between the weld form factor and the heat input under different gap width.(4)Angular distortion is recorded and analyed,rail joint angular deformation law can be got.Then the finite element analysis of stress and strain distribution and welding deformation during welding is carried out,optimizing the angular deformation law,and the calculated results agree well with the experimental results.With this law the appropriate anti-deformation angle and groove size can be estimated,Wherein the anti-deformation angle is 1.58 ° and the groove bottom gap width is 4.5 mm.At the same time,the adaptability of the welding process parameters is systematically studied according to the ever-changing groove size,finally the optimal value of the welding process parameters corresponding to the non-linear change of the gap width in the range of 3.5 ~ 5.3mm is obtained.Furthermore,The welding process of ultra-narrow gap is studied,and the single-sided welding double-sided molding is realized.(5)The effects of flux composition on the microstructure and mechanical properties of ultra-narrow gap welded joint are studied.When the flux is without alloying element,The crystallizing morphology on the columnar grain seam is pro-eutectoid ferrite?side plate ferrite?bar ferrite and small amount of pearlite.And it on the weld bead heat affect zone is uniform refined ferrite grains and little pearlite.and when the flux is with alloying element,the entire weld area are small and multi-directional distribution of acicular ferrite,and acicular ferrite become smaller with the amount of alloy increased.And using ultra-narrow gap welding method to weld rails makes the HAZ organization significantly refine.Ultra-narrow gap welding with constrained arc by flux strips applied in rail welding,the flux composition of flux strips by adding 19% of manganese,nickel,molybdenum,chromium.Can get the welded joint whose performance indicators are more than the level of flash welding.