Hot forging
The forging process performed above the metal recrystallization temperature is called hot forging. Hot forging is also called hot die forging. During forging, the deformed metal flows violently, and the contact time between the forging and the mold is long. Therefore, mold materials are required to have high thermal stability, high temperature strength and hardness, impact toughness, thermal fatigue resistance and wear resistance, and easy processing. Hot forging dies with lighter working load can be made of low-alloy steel.
The purpose of heating the metal blank before forging is to improve the plasticity of the metal, reduce the deformation resistance, make it easy to flow and form and obtain a good structure after forging. Therefore, heating before forging has a direct impact on improving forging productivity, ensuring the quality of forgings and saving energy consumption. According to the different heat sources used, the heating methods of metal blanks can be divided into flame heating and electric heating.
Flame heating
Flame heating uses fuel (coal, coke, heavy oil, diesel and gas) to burn in a flame heating furnace to produce high-temperature gas (flame) containing a large amount of heat energy. The heat energy is transferred to the surface of the blank through convection and radiation, and then heat is transferred from the surface to the center. The metal blank is heated.
When the heating temperature is lower than 600-700℃, the heating of the blank is mainly by convection heat transfer. The so-called convective heat transfer is the continuous flow of flames around the blank, and the heat exchange between the high-temperature gas and the surface of the blank is used to transfer heat energy to the metal blank. When the heating temperature exceeds 700-800°C, the heating of the blank is dominated by radiation heat transfer. The so-called radiant heat transfer is the conversion of heat energy into radiant energy through the high-temperature gas and the furnace. The radiant energy propagated in the electric microwave mode is absorbed by the metal blank, and then the radiant energy is converted into heat energy to heat the blank. Generally, when the ordinary forging heating furnace is heated at high temperature, the radiation heat transfer accounts for more than 90%, and the convective heat transfer only accounts for 8% to 10%. The advantages of the flame heating method are that the fuel source is convenient, the furnace is simple to build, the heating cost is low, and the scope of application to the blank is wide. However, the working conditions are poor, the heating speed is slow, the efficiency is low, and the heating quality is difficult to control. This heating method is widely used for heating various blanks.
Cold forging
A general term for plastic processing such as cold forging, cold extrusion, and cold heading. Cold forging is the forming process below the recrystallization temperature of the material, and the forging performed below the recovery temperature. In production, it is customary to call forging without heating the blank as cold forging. Cold forging materials are mostly aluminum and some alloys, copper and some alloys, low-carbon steel, medium-carbon steel, and low-alloy structural steel with low deformation resistance and good plasticity at room temperature. Cold forgings have good surface quality and high dimensional accuracy, which can replace some cutting processes. Cold forging can strengthen the metal and increase the strength of the part.
Cold precision forging is a (near) net forming process. The parts formed by this method have high strength and precision, and good surface quality. At present, the total amount of cold forging parts used by an ordinary car abroad is 40~45kg, of which the total amount of tooth-shaped parts is more than 10kg. The weight of a single piece of cold-forged gear can reach more than 1kg, and the tooth profile accuracy can reach level 7.
Continuous technological innovation has promoted the development of cold extrusion technology. Since the 1980s, domestic and foreign precision forging experts have begun to apply split forging theory to cold forging forming of spur gears and helical gears. The main principle of shunt forging is to establish a material shunt cavity or shunt channel in the forming part of the blank or die. During the forging process, while the material fills the cavity, part of the material flows to the shunt cavity or shunt channel. The application of shunt forging technology enables the small and no-cutting processing of high-precision gears to quickly reach the industrial scale. For extruded parts with a length-to-diameter ratio of 5, such as piston pins, a cold extrusion can be realized by adopting a wide range of axial residual material through axial splitting, and the stability of the punch is very good; for flat spur gears Forming, the use of radial residual material block can also realize the cold extrusion forming of the product.
Blocked forging is to form one or two punches one-way or oppositely extruding metal in a closed die to obtain a near-net-shape precision forging without flash. Some car precision parts such as planetary and semi-axle gears, star sleeves, cross bearings, etc., if cutting processing methods are used, not only the material utilization rate is very low (less than 40% on average), but it also consumes a lot of man-hours and extremely high production costs. Foreign countries adopt occluded forging technology to produce these net-shaped forgings, which saves most of the cutting processing and greatly reduces the cost.