With the continuous progress of society, atmospheric warming and environmental pollution have attracted more and more attention around the world. In order to protect the environment, automobile manufacturers have continuously improved technology to improve the burning rate of fuel and the purifying rate of exhaust gas. As a result, the temperature of exhaust gas is getting higher and higher. The exhaust temperature of some vehicles in China even exceeds 800 degrees Celsius.As the closest part to the engine, the exhaust manifold operates in a high temperature environment for a long time and is in direct contact with the atmosphere outside, so good oxidation resistance is very necessary for the exhaust manifold of an automobile.
High temperature oxidation is a process in which metal materials react with oxidizing gas media in the environment to form oxide scale at high temperature and oxygen atoms invade the matrix continuously, which leads to material deterioration and subsequent destruction of its service performance.With the continuous demand of designers for heat resistance of materials, the research on high temperature oxidation of metals is also developing.
A large number of studies have shown that the micro-structure characteristics of materials affect their oxidation resistance. Therefore, in order to obtain good oxidation resistance, some alloying elements are often added to the materials. So far, the better addition elements are niobium, chromium, aluminium, silicon, molybdenum, manganese, etc.
F. Tholence and other scholars believe that the type and size of secondary phases produced in the alloy will influence the oxidation process and mode of the alloy.The phase transition and segregation of the alloy can also regulate or inhibit the oxidation process to a certain extent under high temperature operating conditions.
Manufacturing cost has always been one of the concerns of automotive manufacturers during the general decline in automotive manufacturing prices. As for the complex automotive exhaust manifolds, casting technology has always been the ideal method and method for the production of automotive exhaust manifolds and therefore tends to be cast iron in material use.
By analyzing the structure transformation of high silicon molybdenum nodular cast iron and high nickel nodular cast iron in high temperature working environment, the oxidation progress of high silicon molybdenum nodular cast iron and high nickel nodular cast iron was studied, and the oxidation kinetics of high silicon molybdenum nodular cast iron and high nickel nodular cast iron was studied and discussed. The following conclusions were drawn:
1) Under the working environment of high temperature thermal cycle, the spherical graphite on the surface disappears due to oxidation, but the SiO2 formed on the surface layer is lost, but the SiO2 formed on the surface layer limits the oxidation to a certain extent, and the degree of oxidation at the thermal fatigue crack is significantly higher than that in other areas; at the same time, the formation of oxide at the crack tip can promote the crack propagation.
2) The spherical graphite on the surface of high nickel ductile iron decreases or disappears due to oxidation under the working environment of high temperature thermal cycle; the formation of silicon-rich oxides and chromium-rich oxides on the surface limits the occurrence of oxidation to a certain extent; at the same time, when Fe2S is formed with iron, Fe2SiO4 and Fe2SiO4 are formed with iron.Fe2CrO4 is a denser oxide film; the intermetallic compound FeN I3 in the matrix structure does not oxidize FeN I3 nor FeN i3.
3) Under high temperature working environment, the oxidation weight per unit area of high silicon molybdenum ductile iron and high nickel ductile iron increases with the working time, and the thickness of oxide scale increases with the working time. After curve fitting, the stable change stage basically conforms to the parabolic law.