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In order to improve the strength of the castings more effectively, the castings need to be heat treated. Xinyuanzhu Group combined the experience in the casting process and made the following summary on the heat treatment of ductile iron and cast iron. Malleable cast iron ductile iron annealing will make cementite In addition to decomposing into flocculent graphite, the purpose of heat treatment of cast iron is to change the matrix structure, improve the performance of cast iron, and eliminate the stress of castings. It is worth noting that the heat treatment of the casting can not change the original graphite shape and distribution of the casting, that is, the original graphite flake or spherical after heat treatment is still flake or spherical, and its size will not change, and the distribution status will not change.

1、Aging
During the casting process, the iron castings have different cooling rates from the surface to the inside, forming the casting internal stress. If it is not eliminated, it will deform or even crack the parts during cutting and use. To relieve stress, artificial aging and natural aging are often used. The casting is heated to about 500~560℃ for a certain period of time, and then the casting is taken out with the furnace to cool down. This aging is artificial aging; the natural aging is to store the iron castings outdoors for 6 to 18 months to allow the stress to be released naturally. Aging can partially relieve the stress, but because of the long use time and low efficiency, it has not been adopted.

2、Heat treatment for the purpose of improving the overall performance of iron castings
In order to improve the overall performance of iron castings, white-mouth annealing is often used, ductile iron annealing to improve toughness, normalizing and quenching to increase the strength of ductile iron.
2.1 Eliminate white mouth annealing
The surface or thin wall of ordinary gray cast iron or spheroidal graphite castings appears white in the casting process due to the excessive cooling speed, and the cast iron cannot be cut. In order to eliminate the white mouth and reduce the hardness, this kind of cast iron is often reheated to above the eutectoid temperature (usually 880~900℃), and kept for 1~2h (if the cast iron has a high Si content, the time can be short) for annealing, and the cementite is decomposed It is graphite, and then the iron castings are slowly cooled to 400℃-500℃ and air-cooled out of the furnace. At a temperature of 700-780℃, that is, near the eutectoid temperature, the cooling rate should not be too slow, so that too much cementite will be converted into graphite, which reduces the strength of the cast iron.
2.2 Annealing of ductile iron to improve toughness
In the casting process of ductile iron, this ordinary gray cast iron has a large tendency to white mouth and large internal stress. It is difficult to obtain pure ferrite or pearlite matrix for iron castings. In order to improve the ductility or toughness of iron castings, cast iron is often used. The pieces are reheated to 900-950°C and kept for sufficient time for high temperature annealing, and then cooled to 600°C and then released from the furnace. In the process, the cementite in the matrix decomposes graphite, and graphite is precipitated from austenite. These graphites are gathered around the original spherical graphite, and the matrix is ??completely converted into ferrite. If the as-cast structure is composed of (ferrite + pearlite) matrix and spheroidal graphite, in order to improve toughness, it is only necessary to decompose the cementite in the pearlite into ferrite and spheroidal graphite, and then reheat the cast iron After the eutectoid temperature reaches 700-760°C, the furnace is cooled to 600°C after heat preservation, and then it becomes cold.
2.3 Normalizing to improve the strength of ductile iron
The purpose of normalizing ductile iron is to transform the matrix structure into a fine pearlite structure. The process is to reheat the nodular cast iron with ferrite and pearlite matrix to a temperature of 850-900℃, the original ferrite and pearlite are converted into austenite, and some spheroidal graphite is dissolved in the austenite. After heat preservation, the air-cooled austenite transforms into fine pearlite, so the strength of the casting is improved.
2.4 Quenching and tempering treatment of ductile iron
Nodular graphite castings as bearings require higher hardness, and iron castings are often quenched and tempered at low temperatures. The process is: the casting is heated to a temperature of 860-900℃, the original matrix is ??completely austenitized, and then cooled in oil or molten salt to achieve quenching, and then heated at 250-350℃ for heat preservation and tempering, and the original matrix is ??converted to back Fire martensite and retained austenite structure, the original spherical graphite shape remains unchanged. The processed castings have high hardness and certain toughness, retain the lubricating properties of graphite, and improve wear resistance.
As shaft parts, ductile iron parts, such as crankshafts and connecting rods of diesel engines, require comprehensive mechanical properties with high strength and good toughness. The iron castings are quenched and tempered. The process is: the cast iron is heated to a temperature of 860-900°C to keep the matrix austenitized, then cooled in oil or molten salt to quench it, and then tempered at 500-600°C to obtain a tempered sorbite structure (Generally, there is still a small amount of lumpy ferrite), the shape of the original spheroidal graphite remains unchanged. After treatment, the strength and toughness are well matched, which is suitable for the working conditions of shaft parts.
2.5 Austempering treatment of ductile iron
The purpose of the austempering treatment of ductile iron is to transform the matrix structure of the cast iron into a tough lower bainite structure, the strength limit can exceed 1100MPa, and the impact toughness AK≥32J. The treatment process is: heating the ductile iron castings to a temperature of 830-870℃ to keep the matrix austenitized, and then put it in molten salt at 280-350℃ to keep the austenite partly transformed into lower bainite. The original spheroidal graphite is not change. Obtain high-strength ductile iron.
The above heat treatment of cast iron shows that the heat treatment of cast iron can only change the matrix structure, but not the shape and distribution of graphite. The change in mechanical properties is caused by the change in the matrix structure. The graphite flakes of ordinary gray cast iron (including inoculated cast iron) have a great influence on the mechanical properties (strength, ductility), and the mechanical properties of gray cast iron are not significantly improved by heat treatment. It should also be noted that the thermal conductivity of cast iron is worse than that of steel, and the presence of graphite leads to higher notch sensitivity than steel. Therefore, the cooling rate (especially quenching) in the heat treatment of cast iron should be strictly controlled.

3、Surface heat treatment of cast iron
The purpose of surface heat treatment of iron castings is to improve the wear resistance of iron castings. Induction heating quenching, laser heating quenching, soft nitriding and other processes in steel are all suitable for cast iron. At present, the cylinder liners of diesel engines and internal combustion engines often adopt laser heating quenching or soft nitriding treatment. The laser heating cast iron parts have a very fast heating speed. After air cooling, the workpiece can form a hardened layer of high-carbon martensite structure, so the wear resistance is greatly enhanced. After the iron casting is soft nitriding treatment, a layer of e-phase compound (Fe2-3N) high hardening layer is formed on the surface, which not only has high hardness, but also has a small friction coefficient, so the wear resistance of ductile casting is greatly improved.
The heat treatment methods of nodular cast iron are as follows:
3.1 Annealing
Purpose: Generally to obtain ferrite matrix, improve its plasticity and toughness, improve machinability, and eliminate casting stress.
3.11 High-temperature annealing: The high-temperature annealing process is to heat the casting to above the eutectoid temperature range, that is, 900-950℃, hold for 2-4 hours, so that the first stage graphitization of the casting occurs, and then slowly cool to 600℃ with the furnace to make the casting Intermediate and second stage graphitization occurs, and then air-cooled out of the furnace.
3.12 Low-temperature annealing: The low-temperature annealing process is to heat the casting to near the eutectoid temperature range, that is, 720-760°C, hold for 2-8 hours, to cause the casting to undergo the second stage of graphitization, and then slowly cool to 600°C with the furnace, and then out of the furnace Air cooling.
3.2 Normalizing Purpose: To change the mixed structure of the matrix into a pearlite matrix in the as-cast state, thereby improving its strength and wear resistance.
3.21 High-temperature normalizing: heating the casting to above the eutectoid temperature range, generally 880-920℃, holding it for 1-3 hours, so that the matrix structure is completely austenitized, and then air-cooled out of the furnace to make it within the eutectoid temperature range The pearlite matrix is ??obtained due to rapid cooling.
3.22 Low-temperature normalizing: The casting is heated to the eutectoid temperature range, that is, 840-860℃, kept for 1-4 hours, so that the matrix structure is partially austenitized, and then air-cooled out of the furnace. Improve the toughness and plasticity of castings, but the strength is lower.
3.3 Austempering Although the normalizing treatment of ductile iron is widely used, when the shape of the casting is complicated and high comprehensive mechanical properties are required, the normalizing treatment has been difficult to meet the technical requirements, and austempering is often used.
The process is to heat the casting to 860-920°C, hold it for a certain period of time (about twice that of steel), and then quickly put it in an isothermal salt bath at a temperature of 250-350°C for half an hour to one and a half hours of isothermal treatment. Then take out the air cooler.
3.4 Quenching and tempering treatment For castings with complex forces, large cross-section dimensions, and high requirements for comprehensive mechanical properties, normalizing or austempering can no longer meet the above requirements, so quenching and tempering (quenching + high temperature tempering) is often used.
The quenching time and holding time of quenching and tempering treatment are basically the same as austempering, that is, the heating temperature is 860-920℃. In order to avoid cracking during quenching and cooling, oil cooling is generally used except for simple castings using water cooling. The structure after quenching is flake martensite and spheroidal graphite. Then heat to 550-600°C and temper for 2-6 hours.
Ductile cast iron can be subjected to a variety of heat treatments, such as QT400 can be annealed, QT700 can be normalized and tempered, but it is best to perform normalizing treatment before induction heating surface quenching.

Xinyuanzhu Group specializes in the production of Ductile iron castings, Pig iron gray iron casting, Steel casting, Brass Casting, Malleable Iron Casting, and undertakes batch casting processing of castings. Welcome guests to cooperate with pictures or samples.