Annealing heat treatment process of nodular cast iron

The annealing heat treatment process of nodular cast iron includes internal stress relief annealing, high temperature annealing and low temperature annealing.

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The internal stress relief annealing of nodular cast iron is generally heated to 500-600 ℃ at the speed of 75 – 100 ℃ / h. according to the casting wall thickness, it can be calculated by holding for 1H every 25mm, and then air cooling. This method can eliminate 90% – 95% of the stress of the casting and improve the plasticity and toughness of the casting, but the microstructure has not changed significantly.

High temperature annealing refers to heating the casting to 900-950 ℃, holding for 1 – 4h, carrying out the first stage of graphitization, and then furnace cooling to 720-780 ℃, holding for 2-8h, carrying out the second stage of graphitization. If the furnace is cooled to 600 ℃ air cooling after holding at 900-950 ℃, pearlitic nodular cast iron will be obtained because the second stage of graphitization is not carried out. Because the nodular cast iron treated with Mg tends to form white cast iron, ledeburite and free cementite often appear in the as cast structure, which increases the brittleness, hardness and cutting performance of the casting. Especially when the thickness of the casting is uneven, the white mouth is easy to appear at the thin wall, which makes it brittle and inconvenient to process. In order to eliminate white cast iron and obtain ferritic nodular cast iron with high toughness, high-temperature graphitization annealing is required. The specific process is shown in Figure A. at this time, only the first stage of graphitization is completed and pearlite is obtained.

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High temperature annealing process for eliminating white cast iron of nodular cast iron

a) Annealing process of Pearlite Nodular Cast Iron b) annealing process of ferrite nodular cast iron
If the heat treatment is carried out according to the process shown in Figure B, that is, after the completion of the first stage of graphitization, the second stage of graphitization is carried out. Different proportions of ferrite and pearlite can be obtained according to the holding time of this stage. If carried out completely, the nodular cast iron with ferrite as the matrix can be obtained. This kind of cast iron has high toughness and the elongation can reach 5% – 25%. This kind of High Toughness Nodular Cast iron is mostly used to replace malleable cast iron and low carbon steel to make parts. Low temperature annealing is to heat the casting to 720 – 760 ℃, hold it for 3 – 6h, and then slowly cool it to 60 ℃ with the furnace, and then discharge it for air cooling, so as to graphitize and decompose the cementite in pearlite. When there is only ferrite, pearlite and spheroidal graphite but no free cementite in the structure of as cast nodular cast iron, low temperature annealing can be used to obtain high toughness ferrite nodular cast iron.

Microstructure of malleable cast iron

Malleable cast iron is a kind of high-strength cast iron obtained by heat treatment of white cast iron. Its graphite is in the form of flocculent, which weakens the cleavage effect and stress concentration effect of graphite on the matrix. Therefore, malleable cast iron has high strength, plasticity and toughness. Because its plasticity is better than that of gray cast iron, it is also called malleable cast iron or ductile cast iron. Malleable cast iron is especially suitable for mass production of small thin-walled parts with complex shape, and even castings with mass of tens of grams and wall thickness of less than 2mm. Malleable cast iron can partly replace cast steel, alloy steel and non-ferrous metal materials, and is widely used in many industrial sectors.

General range of chemical composition of malleable cast iron: the mass fraction of carbon and silicon is low. Carbon and silicon have basically similar effects in promoting graphitization and improving casting properties. They are elements that strongly promote graphitization, which can ensure that pure white cast iron can be obtained after pouring, reduce the amount of graphite after annealing, and help to obtain high strength and toughness. However, the content of carbon and silicon should not be too low. If the content of carbon and silicon is too low, it will make graphitization difficult during annealing, prolong the annealing cycle, and make the molten iron have poor fluidity, large shrinkage, easy loosening and cracking. Therefore, the carbon and silicon content of malleable cast iron should be controlled. Because manganese can hinder graphitization, its mass fraction can not be high, especially the mass fraction of manganese in ferritic matrix malleable cast iron should be lower than that in pearlitic matrix malleable cast iron. The mass fraction of phosphorus and sulfur should be strictly controlled and kept as low as possible.

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Malleable cast iron can obtain ferrite or pearlite matrix according to different annealing methods, and flocculent graphite is distributed on the matrix. The ferrite and flocculent cast iron can be obtained in the first stage of annealing above 738 ℃, that is, the ferrite and flocculent cast iron can be obtained in the second stage of annealing above 738 ℃. Because there is a lot of graphite in the structure and the fracture is black, it is also called black core malleable cast iron. If the first stage of graphitization is completed and the second stage of graphitization is not carried out or partially carried out, the structure of pearlite matrix + group flocculent graphite or pearlite and a small amount of ferrite matrix + group flocculent graphite can be obtained. Because pearlite exists in the structure and the fracture is bright, it is also called white core malleable iron. Malleable cast iron with pearlite matrix + flocculent graphite structure is called pearlite malleable cast iron.

Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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