Hot forgings
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 forging is in contact with the die for a long time. Therefore, the die material is required to have high thermal stability, high temperature strength and hardness, impact toughness, thermal fatigue resistance and wear resistance and be easy to process. Hot forging dies with lighter workloads 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 deformation resistance, make it easy to flow and form, and obtain a good post-forging structure. 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 two categories: flame heating and electric heating.
Flame heating
Flame heating is the process of using fuel (coal, coke, heavy oil, diesel and gas) to burn in a flame heating furnace to generate high-temperature gas (flame) containing a large amount of heat energy, which is then transferred to the surface of the blank through convection and radiation, and then the metal blank is heated by heat conduction from the surface to the center.
When the heating temperature is lower than 600-700℃, the blank is heated mainly by convection heat transfer. The so-called convection heat transfer is that the flame flows continuously around the blank, and the heat energy is transferred to the metal blank by the heat exchange between the high-temperature gas and the blank surface. When the heating temperature exceeds 700-800℃, the blank is heated mainly by radiation heat transfer. The so-called radiation heat transfer is that the heat energy is converted into radiation energy through the high-temperature gas and the furnace. After the radiation energy transmitted in the form of electric microwaves is absorbed by the metal blank, it is converted from radiation energy into heat energy to heat the blank. In general, when ordinary forging heating furnaces are heated at high temperatures, radiation heat transfer accounts for more than 90%, and convection heat transfer accounts for only 8%-10%. The advantages of the flame heating method are convenient fuel sources, simple furnace repair, low heating costs, and a wide range of blanks. 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 forgings
A general term for plastic processing such as cold die forging, cold extrusion, and cold heading. Cold forging is a forming process below the recrystallization temperature of the material, and it is forging below the recovery temperature. In production, forging without heating the blank is usually called 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, and can replace some cutting processes. Cold forging can strengthen metals and improve the strength of parts.
Continuous process innovation has promoted the development of cold extrusion technology. Since the 1980s, precision forging experts at home and abroad have begun to apply the split forging theory to the cold forging of spur gears and helical gears. The main principle of diversion forging is to establish a diversion cavity or diversion channel of the material in the forming part of the blank or the die. During the forging process, while the material fills the cavity, part of the material flows to the diversion cavity or diversion channel. The application of diversion forging technology has enabled the low-cutting and no-cutting processing of high-precision gears to quickly reach industrial scale. For extrusion parts with an aspect ratio of 5, such as piston pins, the use of axial excess blocks can achieve one-time cold extrusion forming through axial diversion, and the stability of the punch is very good; for the forming of flat spur gears, the use of radial excess blocks can also achieve cold extrusion forming of the product.