The difference between ductile iron and gray iron

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1. Analyze why ductile iron is more sensitive to cuts than gray cast iron, while having poorer shock absorption and thermal conductivity? The sensitivity, damping capacity, and thermal conductivity of cast iron depend on the microstructure of the metal matrix and graphite. Gray cast iron contains a large amount of flaky graphite, which is equivalent to having numerous internal notches; this reduces the sensitivity of its mechanical properties to external notches. Similarly, the large amount of flaky graphite divides the matrix, hindering the propagation of vibrations, and it can be converted into heat energy that is dissipated, thereby providing good shock absorption properties. The microstructure of ductile iron consists of a metal matrix along with fine, round graphite particles; since this graphite does not cause any damage to the metal matrix, ductile iron is more sensitive to notches and has poorer shock absorption properties compared to gray cast iron. Similarly, due to graphite’s good thermal conductivity, gray cast iron contains numerous flaky graphite particles that facilitate heat transfer, whereas ductile iron has spherical graphite particles; the spherical shape does not facilitate heat transfer as well, so ductile iron has lower thermal conductivity than gray cast iron. 2. What are the principles for selecting chemical compositions in the production of ductile iron? What is the difference from gray cast iron? The selection should facilitate the spheroidization of graphite to obtain a satisfactory matrix, while also ensuring that the cast iron has good casting properties. For gray cast iron, it is appropriate to increase the Si/C ratio (for example, from 0.5–0.75) while keeping the carbon equivalent constant. 3. What are the main destinations of magnesium, the element used for spheroidization, during the spheroidization process? How to improve magnesium absorption? The fate of magnesium – desulfurization, deoxygenation – spheroidization of molten iron – burnout, floating, and vaporization. Methods: self-developed pressure-magnesium method, rotating bag method, and magnesium alloy method. 4. Analyze the effects of changing the heating temperature and isothermal quenching temperature on the microstructure and properties of austenite-bainite ductile iron during heat treatment. (1) To obtain a bainitic structure, ductile iron needs to be subjected to isothermal quenching. Low-temperature isothermal quenching yields lower bainite, while high-temperature isothermal quenching produces austenite and upper bainite structures. (2) The austenite-bainite microstructure is also affected by the isothermal temperature. When the isothermal temperature is above 330–350°C (usually 350–370°C), the matrix structure consists mainly of upper bainite and austenite; this results in a loss of strength and hardness. However, the wear resistance remains good. Additionally, different isothermal temperatures lead to varying amounts of residual austenite in the matrix. 5. Analyze the purpose of inoculation treatment for malleable cast iron, and explain how it differs from that in gray cast iron and ductile cast iron The purpose of inoculation treatment for gray cast iron is to promote graphitization, reduce the tendency toward white cast iron, lower section sensitivity, control the morphology of graphite, eliminate undercooled graphite, appropriately increase the number of eutectic colonies, and facilitate the formation of fine flake pearlite. The purpose of inoculation in ductile iron is to eliminate the tendency for supercooling, promote the spheroidization of graphite, and reduce intergranular segregation. The purpose of inoculation treatment for malleable cast iron is to promote the formation of cementite structure during the first crystallization of the molten iron, without having any effect on graphite formation or facilitating its development during the subsequent graphitization annealing process. Its biggest difference is that malleable cast iron is expected to yield cementite rather than graphite during the first crystallization stage. 6. Durable cast iron? Classification? Durable cast irons refer to a general term for ductile iron, nodular iron, and malleable iron. 7. What are the various microstructures of ductile iron? Production process? The normal structure of ductile iron consists of small, round graphite spheres along with a metallic matrix; in its cast state, the metallic matrix is typically a mixture of ferrite and pearlite. Production process: melting qualified iron melt, spheroidizing treatment (regarding composition and temperature), inoculation treatment, in-furnace inspection, casting of iron parts, cleaning and heat treatment, and quality inspection of castings. 8. What are the requirements for melting ductile iron? What are the commonly used ballizing agents and ballizing methods? Requirements for melting: High-quality molten iron should have a high temperature, low sulfur and phosphorus levels, as well as low levels of impurities (such as oxygen and de-spheroidizing elements). In China, rare earth magnesium alloys are commonly used as ballizing agents, while abroad, magnesium alloys and pure magnesium are mostly used for this purpose. Spheroidization methods: (1) Magnesium as a spheroidizing agent – self-pressurized magnesium injection method, rotating mold method, magnesium alloy method; (2) Rare earth magnesium alloy – injection method, in-mold spheroidization method. 9. Why is inoculation treatment necessary for producing ductile iron? Purpose: To eliminate the tendency for crystalline supercooling, promote graphitization, and reduce intergranular segregation. 10. Characteristics of ductile iron solidification: 1) A wide range of eutectic solidification temperatures; 2) Pasty solidification behavior; 3) Significant eutectic expansion. 11. Common defects in ductile iron: Common defects include shrinkage cavities, porosity, slag inclusions, subsurface pores, floating graphite, and degradation of spheroidization. 12. Main reasons for the decline in spheroidization and countermeasures. Reasons: Magnesium and rare earth elements continuously escape from the molten iron; this escape occurs through oxidation losses, sulfur reversion, combustion losses, etc. Additionally, it is related to the gradual decline in the inoculation effect. Measures: 1) Maintain an adequate level of spheroidizing elements in the molten iron. 2) Reduce the sulfur content in the raw molten iron and prevent its oxidation. 3) Shorten the residence time of the molten iron after spheroidization treatment. 4) After the molten iron has been spheroidized and the slag has been removed, to prevent magnesium and rare earth elements from escaping, the surface of the molten iron can be covered with a covering material to isolate it from air and thus reduce such escapes. 13. What are the performance characteristics of nodular cast iron and where is it commonly used? Performance characteristics: 1) Strength properties: The tensile strength of nodular cast iron is much less sensitive to changes in carbon equivalent compared to ordinary gray cast iron ； 2) Toughness and elongation: The impact toughness and elongation of nodular cast iron are lower than those of ductile cast iron but higher than those of gray cast iron; lower nodulization degree or higher ferrite content in the matrix result in higher toughness and elongation ； 3) The thermal conductivity of ductile cast iron depends primarily on the shape of the graphite; when the degree of ductilization is high, its thermal conductivity is roughly comparable to that of gray cast iron, while when it is lower, it approaches that of ductile iron ； 4) The casting properties exhibit good fluidity. Applications: 1) Due to its high strength, low sensitivity to section shape, and good castability, it can be used to manufacture complex large-scale parts such as gearbox housings ； 2) Due to its high mechanical properties and good thermal conductivity, nodular cast iron is often used to manufacture components that operate in heat exchange applications or under large temperature gradients, such as automobile brake discs. 14. Classification of malleable cast iron, principles for selecting the composition of malleable cast iron. Classified into ferritic malleable cast iron, pearlitic malleable cast iron, and white-heart malleable cast iron. Chemical composition is the main factor determining the mechanical properties of malleable cast iron and the heat treatment time. Selection principle: 1) To ensure that the entire cross-section of the casting achieves full white cast iron structure in its as-cast state, with no pores, as otherwise this would significantly reduce its mechanical properties. 2) The graphitization process must be fast to ensure that the graphitization annealing is completed in as short a time as possible, thereby reducing the production cycle. 3) It helps improve mechanical properties and ensures the production of high-quality products. 4) It possesses good castability while maintaining mechanical properties, which facilitates the production of sound castings. 15. Graphitization process of malleable cast iron and its influencing factors: 1) Graphite nuclei form at the austenite grain boundaries; 2) Cementite continuously dissolves into the austenite; 3) Carbon atoms diffuse from areas of high concentration to those of low concentration; 4) Carbon atoms deposit in the graphite nuclei, resulting in the growth of graphite. Factors affecting it: 1) The number of precipitated graphite cores 2) Carbon atom diffusion.