| As an important component of the nuclear energy system,the clad tube plays the role of isolating the core fuel and coolant.Ensuring the mechanical stability and integrity of cladding tube is an important prerequisite for normal operation of nuclear reactor.The clad tube in the harsh service environment for a long time,and the choice of clad material and design of structural have an extremely important impact on its stability in service.316L stainless steel(SS)is an ideal clad material at present because of its good high temperature resistance,irradiation resistance,easy processing,economical price and other advantages.In the past,clad tubes were isolated by welding metal steel wires outside the tubes,but the solder joints were easy to fall off under the scouring of high temperature and pressure liquids,leading to the failure of isolation among clad tubes.Therefore,a ribbed clad tube integrated structure was proposed,but there is no research on the forming of such tubes at present.Since the characteristic size is under millimeter level,the influence of size effect(SE)on forming needs to be considered.In addition,the height of the tube rib is the same as the wall thickness,which makes it difficult to fill the height of the pipe rib.And,the inner wall of the tube is prone to appear grooves during the forming process of the tube rib.The above problems cause difficulties for the processing and manufacturing of ribbed clad tube as well as accuracy control,this paper proposes an integrated forming process for ribbed clad tube and investigates the SE in the cold deformation behavior of 316L SS and the scientific problems and technical difficulties in the forming of ribbed clad tubes.Firstly,the influences of grain size(GS)on the macroscopic mechanical properties and deformation-induced twinning and martensitic transformaiton of 316L SS during cold deformation was investigated by uniaxial tensile experiments.The results show that the deformation behavior of 316L SS shows a strong sensitivity to GS,and the flow stress and yield strength decrease with the increase of GS.While the content of twins and martensite increases with GS,and the GS continues to refine with the increase of deformation.Based on the uniaxial tensile test results,a constitutive model of 316L SS was established by coupling the SE,dislocation slip,deformation twinning,martensite transformation and grain refinement.The parameters in the model were solved by genetic algorithm,and the prediction accuracy was evaluated.The results show that the developed constitutive model can accurately describe the deformation mechanism of 316L SS,and also accurately predict the macroscopic flow stresses,reflecting the relationship between macro/microscopic deformation of materials.The the established constitutive model of 316L SS was implied into the finite element software ABAQUS to establish a uniaxial tensile finite element model(FEM),and the accuracy of the model was verified by comparing with experiment results.It is found that the deformation-induced twinning is only sensitive to the initial GS of the material,while the martensitic transformation is not only sensitive to the initial GS,but also the grain refinement that occurs during the deformation of the material will have an inhibitory effect on the martensitic transformation,and the correctness of the model prediction is verified experimentally.A cold drawing FEM of ribbed clad tube was established and corresponding cold-drawing forming experiments were conducted to analyze the forming mechanism of the tube ribs and to study the influence of the billet size,drawing process,process parameters(semi die angle,rib groove angle,drawing speed and friction coefficient)on the forming of ribbed clad tube.The results show that the main difficulty of ribbed clad tube forming is the full filling of ribs,and the difference of deformation between two areas of ribs and round tubes is the main reason of rib forming.The rib height and depression depth increase with the increase of the outer diameter of the tube billet and decrease with the increase of the wall thickness.The height of the tube rib is smaller with the increase of semi die angle,friction coefficient and drawing speed,and it tends to increase and then decrease with the increase of mold rib groove angle.When the semi die angle is 6°,the rib groove angle is 10°,the drawing speed is 5mm/s,and the friction coefficient is 0.05,the tube rib filling effect is better.In addition,the ribbed clad tube with the required rib height could not be obtained by single pass drawing,while the ribbed clad tube with the required rib height could be obtained by double pass fixed mandrel drawing of 9.5-0.6mm billets,but the inner wall was depressed,and it was found that the fixed mandrel drawing process could effectively reduce the depression depth of the inner wall of the ribbed clad tubeBased on the FEM,the influence of different process parameters(semi die angle,rib groove angle,drawing speed and friction coefficient)on the micro structure distribution of ribbed clad tubes were analyzed.The results show that with the increase of die half angle and drawing speed,the content of twin and martensite in the ribbed clad tube decreases and the GS increases.When the die rib groove angle is 10°,the ribbed clad tube has the highest twin content and the GS of the inner and outer walls of the ribbed clad tube is smaller than that of the central area.When the die rib groove angle is 12°,the martensite content of the ribbed clad tube is the highest and the average GS is the smallest.The influence of friction coefficient on the microstructure distribution of ribbed clad tubes can be ignored.When the half die angle is 6°,the drawing speed is 5 mm/s,the rib groove angle is 10°,and the friction coefficient is 0.05,the high content of martensite and twin in the ribs and inner and outer walls of the ribbed clad tube can improve the strength of the ribbed clad tube,ensure the integrity and stability of the structure under the mechanical collision between components,while the finer grains of the inner and outer walls of the tube can improve the corrosion resistance and service life of the ribbed clad tube.The internal and external SEs in the forming process of ribbed clad tubes were studied.The mechanism of depression formation and elimination in cold drawing forming process of ribbed tubes was analyzed,and the methods of suppression and elimination were given.The ribbed clad tubes that meet the dimensional requirements and without defects on the inner wall were successfully produced.The results show that with the increase of the GS,the rib height of the ribbed tube decreases while the depression depth increases,and the rib of the formed ribbed tube appears bulges and burrs,and the surface morphology of the rib shows a wavy shape.Large diameter with thick-walled tube shows a different pattern from the small diameter with thin-walled tube,the height of the rib increases with the increase in the width of the die rib depression and the thickness of the billet,and the depth of depression increases with the increase in the width of the ribbed tube and decreases with the increase in the wall thickness.For rib forming,the more deformation grains are involved in the die cavity and in the wall thickness direction,the more favorable the filling of the ribs and the ability to suppress the defect generation.In this paper,the forming of ribbed clad tubes of 316L SS has been studied through material-based deformation experiments,finite element simulations and cold drawing experiments,which enriches the forming research of small diameter thin-walled shaped tube and provides a reference for the development and application of a new generation of clad tube. |
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