Study On Hot Extrusion Of GH3625 Alloy Pipe

GH3625 alloy is mainly used in aerospace,petrochemical and other piping systems,and the traditional hot extrusion pipe preparation forging open billet links difficult to control,It is easy to form microcracks or break.Therefore,this topic has made a research and improvement on the traditional pipe-making process,and provided a method of preparing short-process to hot-extruded GH3625 seamless tubes,which eliminates the need for forging.Directly from the esr ingot of hollow core to hot extrusion tubes and then cold rolled / cold drawn into the finished product of the pipeline.There must be some new key technical problems in the improved process.In this paper,the five key technologies of the short-process GH3625 alloy hot extrusion pipe preparation process was carried out research.such as solidification segregation,evaluation of homogenization process,thermal deformation behavior and pipe extrusion,hot extrusion pipe cracking problem and high temperature long term aging performance.For the alloy pipe hot extrusion process to provide a theoretical basis to guide practice,and improve the finished product rate.The solidification process of the GH3625 superalloy was in-situ observed at different cooling rates(30?·min-1,100?·min-1 and 200?·min-1)by means of confocal laser scanning microscope combined with an infrared image furnace.And observed the solidified microstructure and the regularity of precipitated phases.The results show that the liquidus temperature of GH3625 superalloy was 1356.5?,and the relationship between the variation of solid phase content at the free surface and temperature as well as time is in accordance with Avrami equation.With the temperature decreases,the precipitation sequence of phases were ? matrix phase,carbides and laves phase.In the solidification process of GH625 superalloy,with increasing the cooling rate,the dendritic structure refined,the dendrite arm spacing decreased,component segregation reduced,the laves phase distribution is more dispersed,and precipitated eutectic laves phase as its main.A large number of elements Nb segregation of final period of solidification in the interdendritic and grain boundaries,and this is the main reason for the formation of laves phase.The subject in front of a large number of experiments,developed a two-stage homogenization process: 1140 ? /10h+1210 ? /48 h.The homogenization process was evaluated by ClSM,DSC,SEM,EDS and homogenization kinetics equations.The results show that there are more serious Nb,Mo segregation and low melting laves phase in the GH3625 alloy,and the solution temperature of the laves phase was 1171?.After the first stage homogenization,the Laves phase was eliminated.After the second stage of homogenization treatment significantly reduced Nb,Mo and other elements of segregation,to achieve the purpose of homogenization.The thermal deformation behaviors of GH3625 alloy were investigated by Gleeble3800 during temperature at 800~1200? and strain rate at 0.01~10s-1.The results show that the thermal deformation activation energy of GH3625 alloy was Q = 512.97KJ/mol.There are six unstable deformation zones in the thermal processing of GH3625 alloy,and different regions correspond to different deformation mechanisms.The processing safety zone temperature was between 1100-1200?,The strain rate was 0.1-2s-1,the area corresponds to the dynamic recrystallization mechanism;the hot extrusion temperature of the alloy pipe was 1150?,the extrusion speed was 50mm/s,and then the GH3625 alloy pipe has extruded successfully,and the strength of pipe is 771 MPa.Aiming at the hot extruded cracking pipe of GH3625 alloy,the mechanism of crack formation was discussed by analyzing the organization,fracture,temperature distribution and stress distribution of the pipe.The results show that the stress in the radial contact between the blank and the mold is changed from compressive stress to tensile stress,and the drop is 517 MPa.When the additional tensile stress is greater than the breaking strength of the alloy at this temperature which may contribute to the generation of surface cracks in the pipe.This is the external factor.The excessive extrusion ratio(9.86)in the hot extrusion process produces relatively large strain concentration and stress concentration,which induced the pores and microcracks in the inclusion or brittle phase of the nucleation,the pores merged to form microcracks,which in turn lead to alloy instability,the fracture mechanism is mainly cleavage fracture,which is internal.The effect of different aging time on microstructure and properties of hot extruded GH3625 alloy tube at 720?was investigated The results indicated that the M23C6,? " phase and ? phase were mainly precipitated in the GH3625 alloy at 720? during long term aging,and the strength and hardness of GH3625 alloy were increased,but the plasticity decreased drastically.With the extension of aging time,some M23C6 were transformed into M6 C,and most of the ?" phase was transformed into acicular ? phase,after aging for 5000 h,the ? phase changed from long acicular to long rod-like,weakened the strengthening effect of ?" phase.The ? phase increased with aging time and the growth rate of average length was 5.9×108 nm3/h,which was in accordance with LSW theory,while the average width of coarsening deviates from the LSW theory.