07
2022
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07
How should the forging be cooled?
Author:
The key to make the forging cooling specification is to choose the appropriate cooling speed, so as not to produce all kinds of defects. Generally, the post-forging cooling specification is determined according to the chemical composition, structure characteristics, raw material status and section size of the billet, with reference to the relevant manual data.
Generally speaking, the simpler the chemical composition of the billet, the faster the cooling rate after forging; The opposite is slower. According to this, air cooling is adopted after forging of carbon steel and low alloy steel forgings with simple composition. For the medium and high alloy steel forgings with complex alloy composition, pit cooling or furnace cooling should be adopted after forging.
For steels with high carbon content (such as carbon tool steel, alloy tool steel and bearing steel, etc.), if slow cooling is adopted after forging, network carbide will precipitate in the grain boundary, which will seriously affect the service performance of forgings. Therefore, such forgings are cooled quickly to 700°C by air cooling, blast or spray after forging, and then slowly cooled in a pit or furnace.
For steels without phase change (such as austenitic steel, ferritic steel, etc.), rapid cooling can be adopted because there is no phase change in the cooling process after forging. In addition, rapid cooling is also required to obtain single-phase microstructure and prevent brittleness of the ferritic steel at 475°C. Therefore, such forgings are usually air-cooled after forging.
For self-quenched steels (such as high speed steel, martensitic stainless steel, high alloy tool steel, etc.), martensitic phase transformation will occur due to air cooling, which will cause greater structural stress and easy to produce cooling cracks. Therefore, such forgings must be cooled slowly after forging. For steel with sensitive white spots, in order to prevent white spots in the cooling process, furnace cooling should be carried out according to certain cooling specifications.
Forgings forged with steel will cool faster after forging, while forgings forged with ingot will cool slower after forging. In addition, for forgings with large section size, due to large cooling temperature stress, they should be cooled slowly after forging. For forgings with small section size, they can be cooled quickly after forging.
Sometimes, the intermediate billet or forging is also cooled locally to room temperature during the forging process, which is called intermediate cooling. For example, intermediate cooling is required for blank inspection or defect cleaning. For example, when large crankshaft is forged with multiple fires, the middle part is forged first and then the two ends. When the middle part is forged, the middle cooling should be carried out so as not to affect the quality when the two ends are heated again. The intermediate cooling specification is determined in the same way as the post-forging cooling specification.
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