Research On Cross Wedge Rolling Forming And Microstructure Evolution Of 21-4N For Hollow Valve

Valves are important parts to control the intake and exhaust of engine.Their quality directly affects the engine fuel consumption,life and other important performance,as a result,affect the reliability and safety of the engine.Hollow valve has the advantages of weight loss,cooling,energy saving and environmental protection.At present,the mainly methods of hollow valve manufacturing are drilling method and reverse extrusion method,which have the disadvantages of low production efficiency and high cost.In this paper,a new technology of forming hollow valve with cross wedge rolling(CWR)is proposed,which has the advantages of high efficiency,low cost and stable quality.The deformation behavior of 21-4N at high temperature was studied by using the Gleeble-1500D thermal simulation machine.The stress-strain curves with different temperatures and different strain rates were obtained.Experimental study of the influence of different temperatures and holding time on the grain growth 21-4N steel was conducted.And the influence of temperature,strain rate and stain on 21-4N heat resistant steel austenite grain refinement was studied by unidirectional compression experiments.Moreover,the microstructure of unidirectional compressed specimens was observed by the OA,SEM and EBSD platform,which reveals microstructure evolution mechanism of the 21-4N heat resistant steel.The microstructure model was developed based on dislocation theory using internal state variables method.The material constants in the microstructure model were determined using genetic algorithm optimization techniques with experimental data.To validate the model,the comparison was conducted between predicted results and experimental data and they have a great agreement,which means that the developed micro structure model can accurately describe the characteristics of the viscoplastic flow and microstructure evolution rule of 21-4N heat resistant steel during uniaxial compression.Deform-3D software was used to simulate the forming process of CWR under different processing parameters,and the effect mechanism of processing parameters,like forming angle,stretching angle,relative thickness of billet and forming temperature on ovality of workpiece was analyzed.By revealing the reasons of spread hole generating in the wedging position on inner wall of rolled piece,a method of changing stretching angle in model design was proposed to work this out.To improve the thickness uniformity of 21-4N hollow valve,the rolling process was controlled by mandrel,then,the concept of critical diameter of the mandrel was advanced,and it can be found that the thickness of hollow valve evenly distributed when rolled with mandrel at critical diameter.By analyzing the effect law of mandrel on the distribution of thickness of rolled piece,the mechanism of mandrel in rolled piece during the rolling process was uncovered.Based on the unified viscoplastic constitutive model coupling thermal-mechanical field and the microstructure evolution,using Deform-3D software to simulate the microstructure evolution during the deformation of hollow valve by CWR process in different parameters.The simulation results reveal how the microstructure evolution of 21-4N steel is affected by the forming temperature and the velocity of the rolling die,which can provided a theoretical base for the parameters optimization and the prediction of the microstructure and quality of the formed valve.To validate the established FE model,the hollow valve defamation experiment by CWR process were conducted and according to the comparison of the experimental and simulation results,it can be found that the simulation results have a great agreement with the experimental results,which indicates that the FE model is valid and the simulation results are reliable.Moreover,when the forming temperature is 1060?,the outer diameter of the blank is 18 mm,the thickness is 4 mm,the forming angle is 35°,the stretching angle is 5° from the wedge tip to broadening 40 mm and from broadening 40 mm to the end of the wedge the stretching angle is 4°,the top of the mould cutting 0.1 mm can meet the demand of size and performance requirements of hollow valves.In this paper,the study of the deformation and microstructure of 21-4N hallow valve by combing CWR process and forging process has been conducted,which can provide some guidelines for the application of novel technology.Moreover,the quality tests into the formed hollow valve has been conducted,as a result,the formed hollow valve can meet the requirements of the engine.