Research On The Heat Resistance Of Ni-resist Ductile Iron And Numerical Simulation Of Exhaust Manifold

In recent years, the automotive industry is facing serious challenges because of world energy consumption, reducing emissions to protect the environment is recognized as one of the key issues for the automotive exhaust system. The automobile exhaust manifold is the closest device to the engine, but its working environment is very poor. Early automobile engines, gas temperature did not exceed 500℃, but as the efficiency of engine rised, the maximum exhaust gas temperature has been raised from 500℃to 900℃, and it is expected to rise in the future. Traditional materials could not meet their requirements, but Ni-resist ductile iron which has good heat resistance used in exhaust manifold has been widely recognized.The objective of this paper is to self-develop and manufacture the Ni-resist ductile iron used in exhaust manifold. It is divided into two parts: one is the research on the microstructure and heat resistance of Ni-resist ductile iron, the other is numerical simulation of casting process.The microstructure and heat resistance of Ni-resist ductile iron:The ESCALAB-250 X-ray diffraction (XRD), JSW-5310 and JXA-840 scanning electron microscope (SEM), the energy spectrum of British LinK-ISIS (EDS) and Olympus-PMG3 optical microscope (OM) were carried out to study the microstructure of Ni-resist ductile iron. The morphology of thermal fatigue cracks and the crack initiation and propagation characteristics of thermal fatigue tests were analysis by SEM, The high temperature oxidation process was also investigated by SEM and EDS. FM-70 Vickers microhardness (HV) was used to study the hardness. By analyzing the experimental data, some conclusions have been drawn as follows:The matrix of Ni-resist ductile iron is austenite, and intermetallics FeNi3 was found in it. Three typical areas were analyzed, Most of these areas contain large quantities of silicon and nickel, which form Ni-Si phase and Fe-Ni-Si phase, some carbides distributed in the grain boundary. Not only spherical graphite, but irregular shape graphite also exist in some regions.Thermal fatigue properties of Ni-resist ductile iron:In the thermal cycles,the pressure strain occurs while specimen heated and the tensile strain occurs while it cooled. When the heating temperature rised, the thermal stress of the specimen increased with the heating temperrature increased, and the expansion rate of themal fatigue crack accelerated. Thermal fatigue cracks formed along the edge of the specimen, and then they extended with the thermal cycle times increased. In the end, they formed"T"or"+"type thermal fatigue crack; Oxidation reaction occured when oxygen entered the cracks, which lead to the material occurs thermal growing. The material size increased with the number of cycle times, but because the coefficient of thermal expansion of Ni-resist ductile iron is low and the austenite is stable in room temperature and high temperature, thermal deformation is smaller than other materials; With the increase of thermal cycles, the hardness of the matrix of specimen decreased;In the thermal cycles,the surface of the sample was corroded by oxidation. Thermal fatigue cracks and large numbers of holes which areformed in the surface are caused by the effect of oxidation. The datas provide an experimental and theoretical basis to the exhaust manifold in the actual production and application.Numerical simulation of casting process:As the exhaust manifold materials,Ni-resist ductile iron is the complex material which has high nickel and low carbon. It has high melting and pouring temperature, Poor casting process, uneven thickness, high dimensional accuracy, especially the three dimensional surface measurements. If it is designed only through personal experience and manual operation, we will not only spend much time and cost, but also receive low accuracy and poor reliability results, and the quality of castings is hard to guarantee. The effective method could solve above problems through using CAE numerical simulation of casting process. The software can view filling and solidification of the whole cast processing, prediction and analysis of casting defects in the site. We can optimize the parameters to improve casting technology before its actual production, and help taking effective measures to avoid casting defects.In this paper, using FEM software ProCAST to do a dynamic simulation to observe temperature field, velocity field of the process of filling and solidification. We can predict the thermal section, shrinkage porosity and other defects, to determine the optimal casting process, reduce costs and improve the quality of the exhaust manifold. During the process of casting, when pouring temperature 1470℃, pouring speed 0.3m/s, the filling is stable and the defects less than other plans.