The Microstructure And Properties Of Alloy Ductile Iron For Automobile Exhaust Manifold

Recently, the automobile industries develop very fast, but with the energy andenvironmental problem have become increasingly severe, the automotive industry is alsofacing challenges. Which increase the temperature of fuel combustion is one of theimportant means to reply the energy and environmental problem. Of course, withincreasing the temperature of the fuel combustion, the engine materials and the exhaustsystem materials should be designed stickly. The complexity of the exhaust system wouldput forward strict requirements of the exhaust manifold, and exhaust manifold castingmolding is the primary means. Increase the service temperature of the exhaust manifoldmaterial will be significance to the increase of the temperature of the engine fuelcombustion.By controlling the raw material, smelting spheroidization and inoculation we got thesamples of high SiMo ductile iron and Ni-resist ductile iron. The matrix structure of thehigh SiMo ductile iron is ferrite, and spheroidal graphite distributes, moreover there are asmall amount of molybdenum carbon compounds in the grain boundary. By joining alarge number of austenite stabilizing elements, Ni-resist ductile iron has the austeniticmatrix at the room temperature; but adding a large number of alloying elements wouldlead to the super-cooling of liquid iron, so its nodularity only reachs grade3-4; at thesame time there are some iron and nickel compounds as well as Cr and Mn carbide. Thehigh SiMo ductile iron would have solid phase transformation at about860℃, namely theferrite phase transformation into austenite phase, which leading to dramatic changes ofthe coefficient of thermal expansion of the cast iron, then after the phase transformationthe coefficient of thermal expansion will become very large. Ni-resist ductile iron is notinvolved in the ferritic-austenitic solid-state phase transformation; thermal expansioncoefficient remained relatively stable.The thermal fatigue principle of the two kinds ofductile iron is similar, mainly in the hot and cold cycles with the accumulation of thermalstress, due to the inconsistent volume changes in the process stress concentrate betweenthe matrix phase and the second phase, then reaching the limiting emerge cracks. In theprocess of thermal fatigue, thermal stress is greater, and the material oxidation is moresevere with the service temperature is bigger; becease of thermal stress and oxidation, thefragile material phases (graphite phase or grain boundary second phase) would generatemicrocracks, with the actions of stress and oxidation, microcrack propagate and formmacroscopic cracks. Then oxygen would invade into material across the macro-cracks，and the oxide is easily break away from the matrix, making cracks propagate on thevertically and horizontally. Finally, fatigue cracks go through the material leading to thematerial failure. To the two kinds of ductile iron, by the circulation temperature increased,the fatigue cracks generate sooner and propagate faster. Compared with the Ni-resist ductile iron, the high SiMo ductile iron has the limit servicetemperature, namely phase transformation temperature, becease after phasetransformation the coefficient of thermal expansion will become large, and theperformance of thermal fatigue will decline in severe. Ni-resist ductile iron doesn’t havethe soild phase transformation process, so its service temperature would increase.However, we should take into account that adding lots of expensive alloy elements willraise the cost of production.Overmore, the paper make some basic research of the two kinds of ductile iron about theperformance of electrochemical corrosion in the solution of3.5NaCl (PH=7). Thecorrosion potential of high SiMo ductile is-1.002V, and the corrosion current is2.73×10(-4)A/cm~2, and passivation phenomenon occur in the anodic polarization process,the passive potential is-0.328V. Compared with high SiMo ductile, the performance ofelectrochemical corrosion of Ni-resist ductile iron improve more, corrosion potential is-0.523V, and corrosion current is0.36×10(-4)A/cm~2. In the corrosion process of two kindsof ductile iron was general corrosion process, and localized corrosion behaviour doesn’toccur.By the reseach of the microstructure and properties of the two kinds of ductile iron, hopeto guide the production in practice.