Forging cannot be called casting. Special die forging is called die forging, and forging without special die is called free forging. Large forgings are generally small in batch, and the cost of manufacturing large molds is too high. In addition, the use of die forging requires too large a press, even if there is a huge die, there is no such a large press. At present, the tonnage of hydraulic press in China seems to be only 50000 tons. The main reason is that China's die forging hammer is only 16 tons, and the standard of large forging equipment is too low. Generally speaking, China's large die forgings mainly rely on imports. Russia Mias has a 50-ton air hammer, which can forge a 5-meter long marine crankshaft at one time. However, due to the relatively old equipment, the forging accuracy is not high! There is also a method of forging with the die of a splint hammer, which has poor accuracy. Therefore, large forgings are generally free forged.
Roll forging is a plastic forming process. The material produces plastic deformation under the action of a pair of reverse rotating dies to obtain the required forging or forging blank. It is a special form of forming rolling (longitudinal rolling). Roll forging is applicable to deformation processes such as shaft elongation, slab rolling, material distribution along the length direction, etc.
Die forging of tire is a process method for producing forgings using tire on free forging equipment. The blank is generally made by free forging. Then it is finally formed in the tire mold. Die forging is suitable for medium and small batch production, mainly for small and medium-sized factories without die forging equipment.
Die forging is a forging method that uses dies to form blanks on special die forging equipment and obtain forgings. Forgings with complex shapes can be forged; It is applicable to batch production. However, due to the high cost of the die and the need for special die forging equipment, it is not suitable for single piece or small batch production, and is obviously not suitable for large forgings. Therefore, only free forging is suitable for processing heavy forgings.
The quality of heavy forgings is mainly reflected in three aspects: purity, uniformity and compactness. Any defect will form defects and affect the product quality. Common internal defects of large forgings include segregation, mixing, crack, looseness, coarse grain and uneven grain. The internal landscape requirements of heavy forgings are very high, requiring not only mechanical properties and grain size testing, but also ultrasonic testing. When the defect exceeds the limit and cannot meet the technical requirements, the heavy forgings will be scrapped.
Therefore, excessive control of defects is the key to ensure product quality. The goal of forging control is to scientifically control the thermoplastic processing process of heavy forgings, eliminate defects in forgings, ensure good organization and performance, and obtain high-quality products. It has achieved good results in eliminating internal cracks, improving segregation, mixing, loosening, coarse crystals, etc. The three elements of internal defects of welded forgings are temperature, stress state (reasonable distribution of internal strain of forgings) and pressure.