Metal blanks generally need to be heated before forging. During heating, the input of external heat will cause changes in the internal energy state of the blank. With the increase of temperature, the atomic kinetic energy will increase, and the possibility of leaving its equilibrium position will also increase. Compared with normal temperature, dislocation is easy to move, slip is easy to occur, and deformation resistance will be reduced; In addition, at high temperature, the activity of atoms is increased, the diffusion speed is accelerated, recovery and recrystallization are easy, and the plasticity of metal is improved; When materials with allotropy transformation are heated, phase transformation recrystallization and phase dissolution occur in a certain temperature range.
The purpose of pre-forging heating is to correctly use the microscopic evolution law in the process of metal heating, improve the plasticity of the metal, reduce the deformation resistance, make the blank easy to deform and flow, and achieve smooth forming, and obtain good post-forging structure and properties. Pre-forging heating is an important link in the whole forging process, which has a direct impact on improving forging productivity, ensuring forging quality and saving energy consumption.
According to the different heat sources used in the heating of metal blanks, the heating methods currently used in production include electric heating method and flame heating method.
Electric heating. This is a heating method that converts electric energy into heat energy to heat the blank, including resistance furnace heating, induction electric heating, contact electric heating and salt bath furnace heating.
The selection of heating method shall be determined according to specific forging requirements, local energy conditions, investment benefits, environmental protection and other factors. Generally, large forgings are often heated by flame; Flame heating and electric heating can be selected for medium and small forgings; Induction heating or other non-oxidation heating methods can be selected for precision die forging.
Flame heating. Flame heating is a heating method that burns fuel (coal, heavy oil and gas, etc.) in a flame heating furnace to produce high-temperature gas (flame) containing a large amount of heat energy, and transfers heat energy to the surface of the blank through convection and radiation, and then conducts heat transfer from the surface of the blank to the center, so as to heat the whole blank.
When the heating temperature is lower than 600~700 ℃, the heating of the blank mainly depends on convection heat transfer. The so-called convective heat transfer refers to the continuous flow of flame around the blank and the transfer of heat energy to the blank through the heat exchange between high-temperature gas and the surface of the blank. When the heating temperature reaches 700~800 ℃, the heating of the blank is dominated by radiation heat transfer. When ordinary forging heating furnace is heated at high temperature, radiation heat transfer accounts for about 90%, and convection heat transfer only accounts for about 10%.
The advantage of flame heating method is convenient fuel source; The furnace is easy to repair and low in cost; It has a wide range of adaptability to blanks. Therefore, in die forging production, flame heating method is widely used for heating various large, medium and small blanks. Its disadvantages are poor working conditions, slow heating speed, difficult to control heating quality and low thermal efficiency.