Summary of Casting Process for Medium Manganese Ductile Iron

The chemical composition control of medium manganese ductile iron includes the following key points for controlling each major element:

The range of carbon (C) content is generally controlled between 3.0% and 3.8%. Control purpose and impact: Increasing carbon content can improve the fluidity and graphitization ability of cast iron, promote the formation of graphite balls, and improve hardness and wear resistance. However, excessive carbon content can cause graphite to float and reduce the mechanical properties of castings; If the carbon content is too low, it is easy to produce white cast structure, making the casting brittle.

The range of silicon (Si) content is usually between 3.0% and 4.5%. Control purpose and impact: Silicon is a strong graphitizing element that can refine graphite balls and improve the strength and toughness of cast iron. Moderate silicon content can reduce the tendency of white casting, but excessive silicon content can decrease the toughness and increase the brittleness of castings.

Manganese (Mn) content range: Manganese content is relatively high, generally between 5% and 9%. Control purpose and impact: Manganese can improve the strength, hardness, and wear resistance of cast iron, stabilize the austenite structure, and increase hardenability. However, excessive manganese content can lead to the presence of more carbides in the structure, reduce toughness, and increase the crack sensitivity of castings.

The range of phosphorus (P) and sulfur (S) content: The phosphorus content should be as low as possible, generally controlled below 0.05% to 0.1%; The sulfur content is usually controlled below 0.02% to 0.03%. Control purpose and impact: Phosphorus increases the cold brittleness of cast iron, reduces toughness and impact performance; Sulfur easily forms sulfide manganese inclusions with manganese, reducing the mechanical properties of cast iron and increasing the tendency for hot cracking.

The content range of rare earth elements (RE) and magnesium (Mg): The content of rare earth elements is generally between 0.02% and 0.05%, and the content of magnesium is between 0.03% and 0.06%. Control purpose and influence: Rare earth elements and magnesium are key elements in spheroidization treatment, which can spheroidize graphite and improve the mechanical properties of cast iron. However, excessive or insufficient content can affect the spheroidization effect, leading to irregular morphology of graphite balls or a decrease in spheroidization rate.

Metallographic Structure of Medium Manganese Ductile Iron

Graphite morphology - good spheroidization: After spheroidization treatment, graphite is uniformly distributed in a spherical shape in the matrix, which is a typical feature of medium manganese ductile iron. Graphite with good spheroidization can effectively reduce stress concentration, improve the toughness and mechanical properties of the material. Graphite size: The size of graphite spheres is usually relatively uniform, typically between 20 and 80 μ m. Smaller graphite spheres can be more evenly distributed in the matrix, refine the structure, and improve strength and toughness.

Matrix organization-

Martensite: In the as cast state, medium manganese ductile iron often contains a certain amount of martensite in the matrix structure. Martensite has the characteristics of high hardness and high strength, which can improve the wear resistance and compressive strength of castings. Its content is generally between 20% and 50%, and the content of martensite can be controlled by adjusting the chemical composition and heat treatment process.

Austenite: Austenite also accounts for a certain proportion in medium manganese ductile iron, usually between 30% and 60%. Austenite has good toughness and plasticity, can absorb impact energy, and improve the impact resistance of castings.

Carbides: There may also be some carbides in the matrix structure, such as carbides, alloy carbides, etc. Carbides have high hardness and are distributed in small particles or blocks in the matrix, which can significantly improve the wear resistance of castings. However, excessive carbide content can reduce the toughness of the matrix, and its content is generally controlled between 5% and 15%.

Organizational Uniformity - The ideal metallographic structure of medium manganese ductile iron should have good uniformity, that is, the distribution of graphite balls, the type and proportion of matrix structure should be relatively consistent throughout the casting. Uneven organization can cause fluctuations in the performance of castings, reducing their reliability and service life.

What factors affect the metallographic structure of medium manganese ductile iron

Chemical composition-

Carbon content: An increase in carbon content promotes graphitization, resulting in an increase in the number and size of graphite spheres. But if the carbon content is too high, graphite floating phenomenon may occur; If the carbon content is too low, it is easy to produce white cast structure, which affects the morphology of metallographic structure.

Manganese content: Manganese is the main alloying element of medium manganese nodular cast iron. Increasing the manganese content can increase austenite stability, promote martensite formation, improve hardness and wear resistance, but too high can lead to an increase in carbides and a decrease in toughness.

Silicon content: Silicon is a graphitizing element, and an appropriate amount of silicon can refine graphite balls and reduce the tendency for white spots. But if the silicon content is too high, it will increase the pearlite content in the matrix and reduce toughness.

Rare earth elements and magnesium content: Rare earth elements and magnesium are key elements in spheroidization treatment, and their content affects the graphite spheroidization effect. When the content is appropriate, graphite spheroidization is good; Insufficient content and incomplete spheroidization; Excessive content may result in casting defects.

Melting process

Melting equipment: Different melting equipment has different controls on the temperature and composition uniformity of molten iron. Accurate temperature control and good composition uniformity in electric furnace melting are beneficial for obtaining a good metallographic structure; The melting process in a blast furnace requires strict control of the furnace charge ratio and melting parameters. Spheroidization and inoculation treatment: The types, amounts, and treatment methods of spheroidizing and inoculation agents have a significant impact on the metallographic structure. Suitable spheroidizing agents and inoculants can ensure good graphite spheroidization, fine graphite spheroidization, and improve the matrix structure.

Cooling rate of casting materials: Different casting materials have different thermal conductivity. For example, metal molds have fast thermal conductivity and cooling rates, which can easily form white or martensitic structures in castings; Sand molds have slow thermal conductivity and cooling rate, which is conducive to graphitization and can obtain a relatively stable pearlite or ferrite matrix structure. Casting wall thickness: The cooling rate varies depending on the casting wall thickness. Thin walled areas cool quickly and are prone to forming white or martensitic structures; The cooling at thick walls is slow, graphitization is sufficient, and the matrix structure may be more inclined towards pearlite or ferrite. Heat treatment process, quenching temperature and time: Quenching temperature and time affect the transformation of austenite to martensite. Excessive quenching temperature or time can cause martensite to coarsen and reduce toughness; Insufficient quenching temperature or time can result in incomplete martensitic transformation, affecting hardness and wear resistance. Tempering temperature and time: Tempering can eliminate quenching stress, stabilize the structure, and adjust hardness and toughness. High tempering temperature and long time will cause martensite decomposition, reduce hardness, and improve toughness.