The strength of gray cast iron is mainly determined by graphite shape, size, pearlite in matrix and ferrite proportion.
With the increase of exhaust temperature of the engine, the grey cast iron with high carbon equivalent can not meet the requirements of application because the oxide film will form on the surface of the grey cast iron. At the oxide film, not only cracking will occur from the oxide film, but also oxidation of flaky graphite in the structure will be brought about by the oxide film. In order to improve the strength of the grey cast iron, a lot of research work has been carried out in various countries and fruitful results have been obtained.Successful results have been achieved.Usually the improvement of the tensile strength of gray cast iron is achieved by reducing the carbon equivalent, e.g. Silar cast iron with a carbon equivalent of 4% – 6%.
Some foreign research results indicate that elements such as Cr, Mn, Cu, Sn and Mo are generally added in the production of hot metal to obtain fine graphite and high pearlite content structure, thus obtaining high-strength grey cast iron to a certain extent and high-strength grey cast iron to a certain extent.[28].Since 1970s, HJ25-47 alloy grey iron has been generally used to manufacture automotive exhaust manifold abroad. Some domestic automotive manufacturers are also looking for new alloying elements, such as Sn, Sb, Te, Ti, etc., with small amount of addition and obvious effect and relatively inexpensive price. Their strength can reach the level of HT300.
As the exhaust temperature increases further, high-strength grey cast iron gradually exits the stage where exhaust manifolds are used, such as the modified HT200.During the road test on the exhaust manifold of a commercial vehicle made of high-strength grey cast iron, the cracking problem of the exhaust manifold was found. It was found that the cementite in the pearlite around the flaky graphite in the exhaust manifold had been decomposed and diffused to the graphite flakes at the operating temperature, indicating that the high-strength grey cast iron had not met the service requirements.However, in some low-speed agricultural vehicles, HTRCr’s high-strength grey cast iron exhaust manifolds are still visible, operating at temperatures limited to 500 – 550 degrees Celsius or below.