There are several heat treatment methods commonly used in ductile iron
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In the ductile iron structure, graphite is spherical, and its weakening and damaging effect on the matrix is weaker than that of flake graphite. The properties of ductile iron mainly depend on the matrix structure, and the influence of graphite is secondary. The matrix structure of ductile iron can be improved by various heat treatment methods, and its mechanical properties can be improved to varying degrees. Due to the influence of chemical composition, cooling rate, nodularizing agent and other factors, the mixed structure of ferrite + pearlite + cementite + graphite often appears in the cast structure, especially at the thin wall of the casting. The purpose of heat treatment is to obtain the desired structure and thus improve the mechanical properties.
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The commonly used heat treatment methods for ductile iron are as follows.
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(1) Low temperature graphitization annealing heating temperature 720~760 °C. Cooled to below 500°C with the furnace and air-cooled. The eutectic cementite is decomposed to obtain ductile iron in a ferrite matrix to improve toughness.
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(2) High temperature graphitization annealing 880~930 °C, turn to 720~760 °C for heat preservation, and cool to below 500 °C with the furnace for air cooling. Eliminate the white mouth tissue, obtain ductile iron with ferrite matrix, improve plasticity, reduce hardness, and increase toughness.
(3) Complete austenitizing normalizing 880~930 °C, cooling method: fog cooling, air cooling or air cooling, in order to reduce stress, increase tempering process: 500~600 °C, obtain pearlite + a small amount of ferrite + spheroidal graphite, improve strength, increase hardness and wear resistance.
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(4) Incomplete austenitizing normalizing 820~860 °C heating, cooling mode: fog cooling, air cooling or air cooling, in order to reduce stress, increase tempering process: 500~600 °C, obtain pearlite + a small amount of dispersed ferrite structure, and obtain better comprehensive mechanical properties.
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(5) Quenching and tempering treatment 840~880 °C heating, cooling method: oil or water cooling, tempering temperature after quenching: 550~600 °C, to obtain tempered sostenite structure, improve comprehensive mechanical properties.
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(6) Isothermal quenching is heated at 840~880 °C and quenched in a salt bath at 250~350 °C to obtain comprehensive mechanical properties, especially to improve strength, toughness and wear resistance.
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When heat treatment is heated, the furnace temperature of the casting is generally less than 350 °C, and the heating speed depends on the size and complexity of the casting, and it is selected between 30~120 °C/h. The furnace temperature of large and complex parts should be low, and the heating rate should be slow. The heating temperature depends on the matrix structure and chemical composition. The holding time is determined by the wall thickness of the casting.
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In addition, ductile iron castings can also be surface quenched by high-frequency, medium-frequency, flame and other methods to obtain high hardness, wear resistance and fatigue resistance. It can also be treated with soft nitriding to improve the wear resistance of castings.
Post time: May-19-2024