Ductile iron with ferrite matrix has certain strength, good impact toughness and plasticity, which can be obtained as-cast or by annealing.Due to the high content of Si, the hypoeutectic composition can be guaranteed and the floatation of graphite can be prevented. At the same time, with the addition of silicon element, SiO2 can be formed at high temperature, and SiO2 Oxygen 2 oxide film can be formed at high temperature to cover the surface of workpiece, thus preventing the continuation of oxidation to a certain extent; while elemental molybdenum can improve the recrystallization temperature of the matrix and form it.Fe3M and elemental molybdenum can improve the recrystallization temperature of the matrix and form Fe3M, while elemental molybdenum can improve the recrystallization temperature of the matrix and form Fe3Mo3C to improve the high temperature strength of ductile iron, and the high temperature characteristics of the material will be significantly improved.Therefore, this series of alloy ductile iron mainly refers to medium and high silicon molybdenum ductile iron.
During the 1960s and 1970s, in order to find high temperature materials suitable for internal combustion engines, it was found that ductile iron with ferrite matrix had excellent crack resistance and oxidation resistance. After research, Lena Magnusson Aberg and other scholars considered that ductile iron with 4-6% Si and 0.5-2% Mo could work steadily at 750-760 C.Ninety percent of the world-renowned Wescast turbocharger housings are made of Si-Mo ductile iron.The reason is that the content of W (Si) in Ferritic Ductile iron is about 4%, which can significantly improve its antioxidant ability; when the content is too high, it will worsen the toughness; the maximum is not more than 4.5%.Adding Mo to the high W (Si) content can further improve the high temperature strength and deformation resistance of cast iron.
In 1997, the American Foundry Association held a special discussion on high silicon molybdenum ductile iron for automotive exhaust manifolds at the 101st Annual Foundry Conference in Seattle.After that, the world’s major automobile companies and research institutes started the research and exploration of high silicon molybdenum ductile iron for automobile exhaust manifold.It is found that the number of graphite embedded in the ferrite matrix is mainly affected by the content of carbon and silicon, and the alloy ductile iron has good formability and is suitable for manufacturing complex-shaped automotive exhaust manifold manifolds.
After years of research and practice, great progress has been made in the research of melting process, chemical composition, mechanical properties, heat resistance, fatigue and forming properties of high silicon-molybdenum ductile iron in China.High silicon molybdenum ductile iron has been used by automotive companies all over the world.At present, Ford, Chryster, Glide-Lebert Automotive Company, Berlin Foundry Company, etc. have successively promoted the application of high silicon molybdenum ductile iron in automobile exhaust manifold. In China, Changchun No.1 Automotive Manufacturing Plant, Wuhan No.2 Automotive Manufacturing Plant, Nanjing Iveco Automotive Factory, etc. have been using high silicon molybdenum ductile iron to manufacture automobile exhaust manifold.Temperature is 750-800 degrees Celsius and is constantly updated according to actual needs.
The research on high silicon molybdenum nodular cast iron for automobile exhaust manifold has been deepened all over the world. For example, Aixinqiu Company of Japan added V, Nb, Ni, Cu and other alloying elements to improve the quality of exhaust manifold on the basis of Si-Mo nodular cast iron.The test shows that the best effect is to add V with mass fraction of about 0.3%.Aixinqiu has developed V-Si-Mo ductile iron and successfully introduced it to the market. There are two main brand standards: JSA/X Ni35Si5Cr2 in ISO 2861:2007 and D-5S in ASTM A439-83. They have excellent oxidation resistance at high temperature, enabling the working temperature of the exhaust manifold to reach 830 C.
At the beginning of this century, the introduction of Euro III emission standard led to the exhaust temperature of some automotive engines exceeding 900 C. This exhaust manifold of high silicon molybdenum ductile iron has undergone a lot of deformation, even cracking, and leakage.For the high exhaust temperature of the engine, the heat resistance of this Ferritic Ductile iron is not ideal, so it is possible to study and use high nickel ductile iron.
