Heat treatment of ductile iron—you casting professionals absolutely need to know this!

In ductile iron, the graphite is spherical, which weakens and disrupts the matrix—though not as severely as in flake graphite. The performance of ductile iron primarily depends on the matrix structure, while the influence of graphite plays a secondary role. By employing various heat treatments to refine the matrix microstructure of ductile cast iron, its mechanical properties can be enhanced to varying degrees. Due to factors such as chemical composition, cooling rate, and spheroidizing agents, the as-cast microstructure often features a mixed organization of ferrite, pearlite, cementite, and graphite—particularly in the thin-walled sections of the casting. The purpose of heat treatment is to achieve the desired microstructure and improve the material's mechanical performance.

Common heat treatment methods for ductile iron include: (1) The heating temperature for low-temperature graphitization annealing is 720~760 Degree, Cool down in the furnace to 500 ° Following this, cool in air. Decompose the eutectoid cementite to obtain ductile iron with a ferritic matrix, thereby enhancing toughness. (2) 880-930 Degree High-temperature graphitization annealing, 720–760 Degree Heat preservation, followed by furnace cooling to 500 Degree Following this, allow the casting to cool in air. This process eliminates the cast iron structure, resulting in ductile iron with a ferritic matrix, thereby enhancing ductility, reducing hardness, and improving toughness. (3) 880 ~ 930 Degree Fully austenitize and normalize, with cooling methods including spray cooling, air cooling, or fan cooling. To reduce stress, incorporate a tempering process. 500 to 600 Degree By obtaining pearlite and a small amount of ferrite with spherical graphite, strength, hardness, and wear resistance are enhanced. (4) Austenitization is incomplete at 820–860 Degree Normalizing, with cooling methods including fog cooling, air cooling, or still-air cooling. To reduce stress and enhance 500 to 600 Degree The tempering process yields a ferritic microstructure with a small amount of dispersed pearlite, resulting in excellent overall mechanical properties. (5)840 ~ 880 Degree Quenching and tempering, oil cooling or water cooling, 550 ~ 600 Degree Tempering produces a tempered sorbite microstructure, enhancing the overall mechanical properties. (6)840 ~ 880 Degree Isothermal quenching and 250~350 Degree Salt-bath quenching is used to achieve comprehensive mechanical properties, particularly enhancing strength, toughness, and wear resistance.

Quenching and Tempering Treatment of Ductile Iron: Ductile iron As bearings require high hardness, cast iron parts are often subjected to low-temperature tempering. The process involves heating the castings to 860–900 Degree , after heat preservation, the original matrix is fully austenitized, and then quenched by cooling in oil or molten salt. After 250–350 Degree After tempering and holding at temperature, the original matrix transforms into tempered martensite and retained austenite, while the original spherical graphite morphology remains unchanged. The resulting castings exhibit high hardness combined with moderate toughness, retaining the lubricating properties of the graphite and significantly enhancing wear resistance.

Normalizing enhances the strength of ductile iron: The purpose of normalizing ductile iron is to transform the matrix structure into a fine pearlitic microstructure. ， This process involves combining a ferritic matrix with a pearlitic matrix. Ductile iron Reheat to 850–900 Degree , the original ferrite and pearlite transform into austenite, with some spherical graphite dissolving into the austenite. After holding at temperature, the air-cooled austenite reverts to fine-grained pearlite, thereby enhancing the strength of the ductile iron.