Purpose of heat treatment of hottest cast iron

The purpose of heat treatment of cast iron

in addition to the malleable cast iron ductile iron annealing to decompose cementite into flocculent graphite, the purpose of heat treatment of cast iron lies in two aspects: one is to change the matrix structure and improve the properties of cast iron, and the other is to eliminate the stress of castings. It is worth noting that the heat treatment of the casting cannot change the original graphite morphology and distribution of the casting, that is, the graphite that was originally flake or spherical is still flake or spherical after heat treatment, and its size and distribution will not change

in addition to malleable ductile iron, which decomposes cementite into flocculent graphite after annealing, the purpose of heat treatment of cast iron is to change the matrix structure and improve the properties of cast iron, and to eliminate the stress of castings. It is worth noting that the heat treatment of the casting cannot change the original graphite morphology and distribution of the casting, that is, the graphite that was originally flake or spherical is still flake or spherical after heat treatment, and its size and distribution will not change

I. aging

during the casting process, the cooling rate of iron castings from the outside to the inside is different, forming casting internal stress. If it is not eliminated, it will deform or even crack the parts in the process of machining and use. Artificial aging and natural aging are often used to release stress. Heat the casting to about 500~560 ℃ for a certain period of time, and then take out the casting with furnace cooling for air cooling. This aging is artificial aging; Natural aging is to store the iron castings outdoors for 6-18 months to release the stress naturally. This aging can release the stress partially, but it is not used because of its long time and low efficiency

second, the purpose of heat treatment is to improve the overall performance of iron castings

in order to improve the overall performance of iron castings, there are often white cast iron annealing, ductile iron annealing, normalizing and quenching to improve the strength of ductile iron, etc

1. Eliminate white iron annealing

white iron appears on the surface or thin wall of ordinary gray iron or ductile iron castings during the casting process due to too fast cooling speed, and the iron castings cannot be machined. In order to eliminate the white cast iron and reduce the hardness, this kind of cast iron is often reheated to above the eutectoid temperature (usually 880 ~ 900 ℃), and maintained for 1 ~ 2H (if the cast iron has high Si content, the time can be short) for annealing. The cementite is decomposed into graphite, and then the cast iron is slowly cooled to 400 ℃ -500 ℃ and discharged from the furnace for air cooling. At the temperature of ℃, i.e. near the eutectoid temperature, the cooling speed should not be too slow, so that too much cementite can be transformed into graphite, which reduces the strength of iron castings

2. Annealing of ductile iron to improve toughness

during the casting process, this ordinary gray cast iron has a large tendency to white cast iron and a large internal stress. It is difficult to obtain a pure ferrite or pearlite matrix for iron castings. In order to improve the ductility or toughness of iron castings, the iron castings are often reheated to ℃ and maintained for enough time for high-temperature annealing, and then cooled to 600 ℃ and cooled out of the furnace. In the process, the cementite in the matrix decomposes into graphite, which precipitates from austenite. These graphite gather 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 the toughness, it is only necessary to decompose the cementite in pearlite JG158 ⑵ 004 "rubber powder polystyrene particle external wall external insulation system" into ferrite and spheroidal graphite. Therefore, the iron castings are reheated to the eutectoid temperature of ℃, and then cooled to 600 ℃ after insulation, and then discharged from the furnace for cooling

3. Normalizing to improve the strength of nodular cast iron

the purpose of normalizing nodular cast iron is to convert the matrix structure into fine pearlite structure. The technological process is to reheat the nodular iron castings whose matrix is ferrite and pearlite to the temperature of ℃. The original ferrite and pearlite are converted into austenite, and some spheroidal graphite is dissolved in austenite. After heat preservation, the air-cooled austenite is converted into fine pearlite, so the strength of the castings is improved

4. Quenching and tempering of nodular cast iron

nodular cast iron castings need higher hardness as bearings, and they are often quenched and tempered at low temperature. The process is as follows: the casting is heated to the temperature of ℃ by tai1 Yingtuo technology company in Hengli Town, and the original matrix is austenitized by heat preservation, and then cooled in oil or molten salt to realize quenching. After heating and tempering at ℃, the original matrix is transformed into tempered martensite and residual austenite, and the original spheroidal graphite morphology remains unchanged. The treated castings have high hardness and certain toughness, retain the lubricating property of graphite, and improve the wear resistance

as shaft parts, such as crankshaft and connecting rod of diesel engine, ductile iron castings require comprehensive mechanical properties with high strength and good toughness, and the cast iron castings are quenched and tempered. The process is: the cast iron is heated to a temperature of ℃, the matrix is austenitized by heat preservation, and then cooled in oil or molten salt to realize quenching, and then tempered at a high temperature of ℃, to obtain tempered sorbite structure (generally there is a small amount of pure massive ferrite), and the original spherical graphite morphology remains unchanged. After treatment, the strength and toughness match well, which is suitable for the working conditions of shaft parts

5. Austempering treatment of nodular cast iron

the purpose of austempering treatment of nodular cast iron is to convert the matrix structure of cast iron into a strong and tough lower bainite structure. The strength limit can exceed 1100mpa, and the impact toughness AK 32j. The treatment process is: heat the nodular iron castings to the temperature of ℃, austenitize the matrix, and put it into the molten salt of ℃, so that the austenite can be partially transformed into lower bainite, and the original spherical graphite remains unchanged. Obtain high strength ductile iron

the above cast iron heat treatment shows that the heat treatment of cast iron can only change the matrix structure, not the morphology and distribution of graphite, and the change of mechanical properties is caused by the change of matrix structure. The graphite flakes of ordinary gray cast iron (including inoculated cast iron) have a great impact on the mechanical properties (strength and ductility). The gray cast iron is heat treated to contain the pest epidemic. With the transportation of goods, it is dispersed all over the world, and the mechanical properties are not significantly improved. It should also be noted that the thermal conductivity of cast iron is poorer than that of steel, and the existence of graphite leads to higher notch sensitivity than that of steel. Therefore, the cooling rate (especially quenching) of cast iron in heat treatment should be strictly controlled