The Study On Heat-resisiting Cast Iron For Exhaust Manifold

Exhaust manifold is connected directly to the engine in the exhaust system. The hightemperature flue gases which are given off in the engine are fed and accomplished throughexhaust manifold. In the course of driving cars, the exhaust manifold has been working athigh temperature. And external automobile exhaust manifold directly contact with the naturalenvironment. At last, exhaust manifold internal temperature is up to several hundred degrees,especially, beyond one thousand degrees, but external temperature of the exhaust manifold isbetween100℃and200℃. So that, the exhaust manifold worked in the large temperaturedifference between inside and outside. The exhaust manifold usually works on alternativethermal cycle of quick-cooling and quick-heating during start-up or stop. Considering thecomplex working conditions, the exhaust manifold material requirement is very high. Theexhaust manifold shape depends on the engine’s structure. With the rapid development ofautomobile industry, the structure of the engine becomes more and more complex. In result,the structure of exhaust manifold also changes. Manufacturing the exhaust manifoldbecomes more and more difficult. With attention focused on global environmental problems,reduced pollutant emissions and increased engine efficiency will increase the exhaust gastemperature. In developed countries of Europe and America, the exhaust manifold materialschanged from gray iron to high strength gray iron, to ductile cast iron, to compacted graphiteiron, to Stainless Steel, to high Si-Mo ductile cast iron, to ductile Ni-resist cast iron, to highalloy stainless steel. In contrast, the automobile industry development of our time is short.The exhaust manifold materials changed from gray iron to high strength gray iron, to ductilecast iron, to compacted graphite iron, to alloy cast iron, to Stainless Steel. China’s autoindustry is still in a stage of stable development. In the12th Five Year Plan Period, Jilinprovince will take FAW as the center, and construct the system of vehicle R&D andmanufacturing and supporting parts and service. Especially, the project of three million carsin Changchun and one million cars in Jilin will help the rapid development of the automobileindustry of Jilin Province. According to the Jilin provincial government planning, to2015,the ambitious goal of four milion cars and2000million Yuan of automotive industrial addedvalue will be achieved. Considering the complex structure of automobile exhaust manifold,exhaust manifold selected materials are mainly iron because their main manufacturing arecast. For a long time, the automobile exhaust manifold material has been using the Germanautomotive standards. Sothat, automobile manufacturing cost is high in First Automotive Works （FAW）. Considerating of China’s reality, the target of exhaust manifold materialslocalization is set up by FAW. In order to achieve this goal, the Jilin University and R&DCenter of FAW Foundry Co., Ltd research new high Si-Mo ductile cast iron （DCI） andductile Ni-resist cast iron （DNCI） to meet the need of new car. The results of the paper arebased on the newly developed high Si-Mo DCI and DNCI. Oxidation resistance and thermalfatigue resistance and mechanical properties and corrosion resistance which the exhaustmanifold materials required were systematically researched. The actual production situationwas simulated. And Service behavior and failure time were presented and analyzed in orderto produce and applicate better exhaust manifold materials. To hope to reaearch the project,the reliable experimental references will be provided to develop new performancerequirements material for exhaust manifold of FAW automobile. The produced resultsprovide the therretical basis for the naxt work.The main conclusions and the results obtained are listed as following:1. To refer with GGG-SiMo51and GGG-NiSiCr3552, high Si-Mo ductile cast iron andductile Ni-resist cast iron which are researched in the study are designed and melted. Duringsmelting operation,75SiFe was used in spheroidizing operation. Finally, the melts werepoured into Y-shaped sand molds. Analysis and measuring instruments of OLYMPUS-PMG3light optical microscopy （LOM） and OLYPUS LEXT OLS3000laser microscope andJSM-5310scanning electron microscopy （SEM） and D/max2500pc X-ray diffractometerand energy dispersive spectroscopy （EDS） were used to observe and analys test samples. Inhigh Si-Mo ductile cast iron, its organization is made of ferrite and spheroidal graphite. A lotof Si was integrated into the ferrite. There are some bone shaped carbides on grain boundary.In ductile Ni-resist cast iron, its organization is mainly made of austenite and intermetalliccompound FeNi3and carbide of Mn and Cr. At this time, Ni can produce segregation ongrain boundary. And compound of Fe and Ni and Si can be produced. Under the samemelting process, nodularity of high Si-Mo DCI is higher than that of DNCI. And sphericalgraphite diameter of DNCI is less than that of high Si-Mo DCI.2. During the thermal fatigue test, after metallographic analysis, the acceptable sampleswere put into a high temperature electric resistance furnaces and heated temperatures weredecided from700℃to1000℃on base of material, respectively. After holding time forabout10min, the heated samples were quenched with tap-water. The samples were put intothe furnace again after cool-down terminate. The cycle was done again. A low temperature（Tmin） is the tap-water temperature. The water temperature is always kept constant. It is about 20℃. So, the effect of test temperature gradient （ΔT） on fatigue is essentially superior limittemperature（Tmax）. ΔT is equal that Tmax subtract Tmin. In fact, changing test temperaturegradient is changing the maximum temperature. Number of the thermal cycles was recorded.In this study, it was defined that a crack whichever is longer than0.5mm as one crack, andthat the total length of the longest crack as the length of crack. During thermal cycles, moredistortions and more cracks and oxidations were created. With the maximum cycletemperature and the cycle number increased, the distortions would be bigger and bigger, andthe main crack length became longer and longer, and oxidation degree became more andmore massive. At the same messuring conditions, the distortions of ductile Ni-resist cast ironare less than that of high Si-Mo DCI. And with cycle number increased, and manymicro-cracks expanded into the main crack. The cracks were always found at the biggersurface, and appeared cross shaped budding in the end. Micro-cracks could not occurrenceimmediately when the thermal cycle initiated. Initiations of micro-cracks need some energyaccumulations. At the same conditions, the cycle number of ductile Ni-resist cast iron ishigher than that of high Si-Mo DCI. And the thermal cracks were zig-zag-shaped across thesample’s surface. During thermal cycles, the number of micro-crack varied from multi tolittle. The thermal fatigue life is governed by many material properties and cooling velocityand the working temperature gradient. Based on these results, it is thought that thermalfatigue resistance of DNCI is better than that of high Si-Mo DCI. The maximum reasonableworking temperature of high Si-Mo DCI and DNCI is840℃and950℃, respectively.3. To evaluate oxidation resistance properties during high temperature service, somesheet specimens of high Si-Mo DCI and DNCI were aged at between800°C and1000°C forthe maximum of80h in a high temperature electric resistance furnace. The oxide thicknessof both high Si-Mo DCI and DNCI vary larger and larger with the development of time. Thethickness of oxide layer of high Si-Mo DCI is much thicker than that of DNCI during thesame heating time. The oxide consists of Fe2O3at the gas/oxide interface, then Fe3O4andfinally FeO. The porous iron oxide allows fast diffusion of oxygen and thus internaloxidation of the alloy. Protective silica may form or transform into Fe2SiO4in presence ofFeO or Fe3O4. In the cases, protective nickelous oxides can protect DNCI from the oxidedeeper into the material. The relations of between the oxide added weight and heat time atdifferent temperature are obstained. The oxidation process of the two materials was up toparabolic curve law. It has been found that DNCI has an excellent property of the oxidationresistance. 4. To evaluate the corrosion resistance of high Si-Mo DCI and DNCI, the electro-chemical test is operated. By testing and analyzing, the corrosion potential of high Si-MoDCI and DNCI is-1.002V and-0.523V. And their corrosion current is2.73×10^-4A/cm²and0.36×10^-4A/cm². There is a transpassive region during anodic polarization.5. In order to keep exhaust manifold size from varying in operation, because it is relateto the seal condition of engine, it is important to measure growth resistance. To use thermalexpansion instrument, it is found that the size of both high Si-Mo DCI and DNCI can changeafter working at high temperature. The size growth of DNCI is less than that of high Si-MoDCI.