Forgings can be divided into cold forging, warm forging, and hot forging based on the temperature of the billet during processing. Cold forging is generally processed at room temperature, while hot forging is processed at a recrystallization temperature higher than that of the metal billet.
Forgings use equipment to apply pressure to metal billets, causing them to undergo plastic deformation and changing their mechanical properties. Improve the mechanical properties of forgings. General industrial forgings refer to civil industries such as machine tool manufacturing, agricultural machinery, agricultural tool manufacturing, and bearing industry.
The characteristics of forgings are mainly reflected in:
1) Large weight range. Forgings can range from a few grams to several hundred tons
2) Higher quality than castings. The mechanical properties of forgings are better than those of castings, and they can withstand large impact forces and other heavy loads. Therefore, forgings are used for all important and heavily stressed parts.
For high carbide steel, forgings have better quality than rolled materials. High speed steel rolling materials can only meet the usage requirements after being modified and forged. Especially high-speed steel milling cutters must be forged.
3) The heaviest is the lightest. On the premise of ensuring design strength, forgings are lighter than castings, which reduces the weight of the machine itself and is of great significance for transportation vehicles, airplanes, vehicles, and aerospace equipment.
4) Save raw materials. For example, for a crankshaft with a static weight of 17kg used in automobiles, when using rolled material for cutting and forging, the chips account for 189% of the crankshaft weight, while when using die forging, the chips only account for 30%, which also reduces machining time by 1/6.
Precision forged forgings not only save more raw materials, but also save more machining hours.
5) High productivity. For example, using two hot forging presses to forge radial thrust bearings can replace 30 automatic cutting machines. When using a top forging automatic machine to produce M24 nuts, the productivity of a six axis automatic lathe is 17.5 times higher.
Of course, forgings also have some defects, such as common ones:
(1) Oxidation
The generation of oxide scale in forgings not only causes metal burning, but also reduces the surface quality and dimensional accuracy of forgings. When the depth of oxide skin pressed into the forging exceeds the machining allowance, it can cause the forging to be scrapped.
(2) Decarbonization
Decarbonization will cause a decrease in surface hardness and wear resistance. If the thickness of the decarburization layer is less than the machining allowance, it will not cause harm to the forging; On the contrary, it will affect the quality of forgings. Rapid heating, applying protective coatings on the surface of the billet, and heating in neutral or reducing media can all slow down decarburization.
(3) Overheating
Overheating can cause a decrease in the plasticity of the billet and a decrease in the mechanical properties of the forging. Therefore, it is necessary to strictly control the heating temperature and shorten the insulation time during the high-temperature stage as much as possible to prevent overheating.
(4) Overburn
After overburning, the strength of the material severely decreases, and its plasticity is poor. Once forged, it will break and become waste, which is irreparable. Therefore, it is necessary to strictly follow the correct operating procedures.
(5) Cracks
When heating large forgings, if the furnace temperature is too high or the heating speed is too fast, the temperature difference between the center and surface of the forging is too large, causing excessive internal stress and resulting in cracks. Therefore, when heating large forgings, it is necessary to prevent the furnace temperature from being too high and the heating speed from being too fast, and generally heat prevention measures should be taken.