Study On The Macro-micro Deformation Rules During The Extrusion Of Large-scale Thick-walled Inconel 625 Pipe

The large-scale thick-walled pipe of Inconel 625 alloy have been widely used in the oil,chemical and energy industries due to its high performance,high corrosion resistance and high fatigue resistance at high temperature.However,hot deformation behaviors of Inconel 625 alloy are especially sensitive to strain rate,strain and deformation temperature and are thus extremely complicated during hot extrusion of the pipes.Moreover,distinctive thermal-mechanical coupling effect exists in superalloy pipes extrusion in virtue of the high extrusion speed,high extrusion temperature,and the large temperature gradient between billet and tools.Hence,it is in urgent need to reveal the influence rules of the key extrusion parameters on the deformation behaviors of the pipes in order to robustly control the forming quality of the pipes.To this end,the evolution rules of the macro variables and dynamic recrystallization（DRX）microstructure were deeply investigated,extrusion forming limit and optimized forming parameters were then obtained by means of extrusion experiments and finite element method（FEM）simulation.The mainresults are shown as follows.The macro-micro coupled FEM model for the extrusion process of large scale thick walled Inconel 625 pipe was developed based on the DEFORM-2D platform,with key issues including meshing,boundary conditions and rigidizing of the extrusion pad solved.Subsequently,the reliability of developed FEM model was validated by extrusion experiments.By implementing present FEM model,systematic investigations of macro-micro deformation behaviors of large-scale thick-walled Inconel 625 pipe were conducted.The evolution rules and key influence factors of exit temperature during the entire extrusion process were revealed.The results show that isothermal layer exists in the pipe thickness direction due to the combined effects of the friction heat and heat transfer.The exit temperature inside the isothermal layer exhibits a tendency of continuous decrease,while inverse rule is obtained outside the isothermal layer.The exit temperature uniformity firstly becomes better and then goes worse as the increasing initial billet temperature,extrusion speed and friction.Larger extrusion ratio results in a more uniform distributed exit temperature.Exit temperature uniformity is barely affected by die taper angle.The characteristics of DRX microstructure evolution and the influence rules of the key extrusion parameters on the average grain size and grain uniformity after DRX were studiedduring steady-state extrusion of pipes.The results show that with the increase of initial billet temperature and extrusion speed,the average grain size first decreases and then increases.Additionally,the increasing extrusion ratio results in a significant decrease in average grain size.The die taper angle and the friction factor have less significant effects on the average grain size.With the increase of initial billet temperature,extrusion speed and friction coefficient,the grain uniformity is firstly improved and then deteriorated.Larger extrusion ratio leads to a more uniform grain distribution.In summary,the extrusion ratio is the most significant influence factor on the grain size and grain uniformity,followed by initial billet temperature and extrusion speed.A new optimization method was put forward on the basis of response surface method（RSM）to obtain extrusion forming limit.Firstly,taking F_max<250 MN,T_max<1240 ℃,davg<28.3 μm as constraint conditions,the appropriate ranges of the key extrusion parameters,i.e.initial billet temperature,extrusion speed,extrusion ratio,were determined.Subsequently,the forming limit map for the extrusion process was developed.Finally,aiming at achieving best grain size uniformity within above mentioned limit ranges of forming parameters,the optimal extrusion parameter combination was obtained by established extrusion parameters-forming quality RSM model.