Study On Microstructure And Mechanical Properties Of High Manganese Malleable Cast Iron

Malleable Cast Iron is a commonly used structural material, its wear-resistance and shockproof properties being better than steel. Its comprehensive mechanical properties are also better than grey cast iron, but a little lower than that of nodular cast iron. Malleable Cast Iron is widely used for the thinner parts of all kinds of machines such as cars, tractors and farming tools. Although the annealing time for the cast iron is longer, but its advantages, such as being easy to produce, requiring simpler production tools, as well as being cheap and consistent in quality, make it still very useful and applicable.Because manganese can prevent carbon from being graphitised and can easily segregate on the boundary of eutectic cell structure, its content in malleable cast iron is usually kept at around 0.4wt% to 1.2wt%. For the purpose of applying the cast iron in wear-resistance situation, this paper has come up with a new kind of malleable cast iron. Without adding any other alloying elements, the Manganese content is increased to 2.5%. And the silicon content is 2.2wt%, 2.5wt%, 2.8wt% respectively, which ensures the completion of graphitising annealing and decreases the annealing time. On the basis, this paper has further made a research on the high temperature graphitising annealing, the microstructure, as well as the mechanical properties of the high manganese malleable cast iron.According to the research, though the silicon content is high, white Cast Iron whose rupture surface is completely white can still be obtained through inoculation treatment with small amount of bismuth and aluminum and permanent-mould cast process. The graphitizing annealing of the cast iron with different Silicon content can accomplish at 880℃, 920℃and 960℃. The respective graphitizing annealing time of samples with a silicon content of 2.8wt%, 2.5wt% and 2.2wt% are 24.5, 26 and 28 hours at 880℃, 22, 24 and 26 hours at 920℃, and 18.5, 19.5 and 21 hours at 960℃. After the annealing, the samples are processed with furnance–cooling, air-cooling, oil-cooling and water-cooling respectively at 780℃, 820℃and 860℃. The experimental results are as follows: the hardness of the samples with the same silicon content are almost the same after either oil-cooling or water-cooling, while it increases a little as the silicon content increases after both oil-cooling and water-cooling. A combination of quenching martensite and the remains of austenite can be obtained after the cast iron is water-cooled. When the quenching temperature grows up, the carbon content of martensite increase and the needle-shaped martensite structure grows thicker, but it becomes thinner when the silicon content rises. The hardness of cast iron with different silicon content after the process of water-cooling at 860℃ranges from 53 to 55HRC, and that of the tempered martensite obtained after the stress-relief tempering at 200℃still stands high (about 3HRC lower) . Hence, it can be inferred that the high manganese cast iron (with a manganese content of 2.5wt%) is a potential choice for the application in wear-resistance situation.