| High Si-Mo ductile iron is widely used in automotive engine components such as exhaust manifold and turbine housing, because it has good high strength, excellent thermal fatigue resistance, oxidation resistance, growth resistance, and good high temperature creep resistance.Exhaust manifold for automobile engine is connected directly to the engine through the flange.The flue gases which are given off in the engine are exhausted through exhaust manifold. Large temperature difference between surfaces and inner cavities of exhaust manifold will cause thermal fatigue crack for the reason that the exhaust manifold usually works on alternative thermal cycle due to quick-cooling and quick-heating during start-up or stop. Therefore,improving the thermal fatigue resistance and oxidation resistance of high Si-Mo ductile iron is of great significance to service life of materials.This paper aims at improving micro structure and increasing mechanical property of high Si-Mo ductile iron by adding alloy elements such as chromium and vanadium. The effects of elements on high Si-Mo ductile iron was researched by means of tensile strength test, thermal fatigue experiment and oxidation resistance analysis. The results show that:The microstructure of high Si-Mo ductile iron consists of ferrite matrix, spheroidal graphite, Mo-riched carbide and a small amount of pearlite. The graphite is uniform in size and is embedded in the ferrite matrix. There are some carbides in grain boundary.For high Si-Mo ductile iron, both numbers of pearlite in matrix and carbides precipitation in grain boundary increases with adding chromium. The tensile strength and the hardness of the alloy increase by 8% and by 41% respectively. The amount of pearlite and size of fettite decrease with adding vanadium. The tensile strength and the hardness of the alloy increase by 15% and by 7% respectively. Moreover the number of pearlite and precipitated carbides size in grain boundary decrease with compound adding chromium and vanadium, the tensile strength of the alloy increase by 9% and the hardness basically unchanges.In the thermal fatigue testing, four kinds of high Si-Mo ductile iron specimens occur some degrees deformation firstly. The cracks form on the surface of the specimen when the deformation achieving to a certain extent. It will form totally on the surface of large area, and usually originate from the position of grain boundary carbides or disappearance of spheroidal graphite. During propagation of cracks, spheroidal graphite can change the direction of crack propagation. The crack growth rate increases with the increase of oxidation degree of matrix near the crack. To achieve the same length of crack, the high Si-Mo ductile iron adding chromium needs the maximum number of thermal cycles under the same thermal cycling conditions.At high temperature, the oxidation weight of four kinds of high Si-Mo ductile iron increase with time going on. The spheroidal graphite in specimen surface will disappear due to the oxidation. Meanwhile, the SiO2 oxidation film will form on the surface of the specimen. The film will resist the crack growth subsequently and improve the oxidation resistance of the alloy.Besides,the surface of the alloy adding chromium will also form Cr2O3oxidati on film,which will further improve the oxidation resistance of the alloy. In the conditions of given temperature and atmosphere in furnace,The weight gain per unit area and oxides thickness of the alloy adding chromium are the least than that of other alloys, which has the best oxidation resistance. |
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