With the development of the automobile industry, the exhaust temperature of the engine is increasing continuously, which leads to the continuous development of the exhaust manifold materials.Material of exhaust manifold for automobiles in developed countries in Europe and America has experienced the development process of stainless steel, including common grey cast iron, high strength grey cast iron, common, vermicular cast iron, common stainless steel, mesh steel, high silicon molybdenum ductile iron, high nickel ductile iron and high alloy stainless steel.The materials used for automobile exhaust manifold in our country have basically experienced the development process of common grey cast iron, high-strength grey cast iron, common ductile iron, vermicular cast iron, alloy cast iron and stainless steel.
The typical structure of grey cast iron is flaky graphite. The macroscopic observation results of its section are grey-black, which is different from white cast iron with silver-white section. It mainly contains three elements Fe, C and Si.Grey cast iron is widely used in equipment manufacturing in automobile, chemical industry, equipment manufacturing and other fields due to its good casting properties, relatively low tendency of shrinkage and low cost.
In the early days of the automotive industry, internal combustion engines were designed and manufactured with low power and low combustion efficiency. The exhaust air temperature discharged through the exhaust manifold was less than 500 degrees Celsius.Under such operating conditions, the requirements can be met by manufacturing the exhaust manifold with ordinary grey cast iron.China’s first and second automobile manufacturers used HT200 and HT250 to manufacture exhaust manifolds on automotive and diesel engines in the early years.HT250 has higher strength than HT200, but its thermal conductivity is poor.The reliable working temperature of both is not more than 500 C.The mechanical properties and oxidation resistance of grey cast iron can be improved by adding alloying elements such as Cr, Ni, Si and P into the material. However, thermal fatigue cracking is easy to occur due to the low thermal conductivity of grey cast iron and the large temperature difference between inside and outside during operation.As a result, the increase in operating temperature is very limited, generally not exceeding 550 C. Grey cast iron is mainly used in the manufacturing of exhaust manifolds under low temperature loads.Through optimizing the shape structure of the exhaust manifold, some vehicles still use grey cast iron to produce the exhaust manifold. For example, the intake and exhaust manifold of SY492 engine is made of grey cast iron. The SY492 engine belongs to the rake-shaped intake manifold. The length of the mixed air passage to each cylinder is basically the same. Therefore, the charge of each cylinder is more uniform, but the bending is more rapid and the intake resistance is large.With single outlet exhaust manifold, the structure is simple, but the exhaust resistance is high.Intake air preheating uses exhaust gas preheating, so intake and exhaust manifolds are joined together. This preheating method has compact structure, simple structure and good workmanship, but its preheating effect is not as good as water preheating.