An important process before hot forging. When the metal is heated to a certain temperature, the plasticity increases and the denaturation resistance decreases. It is the high temperature strength change curve of carbon steel containing 0.45% carbon and alloy steel containing nickel, chromium and tungsten. According to the curve, the strength of the metal decreases with the increase of temperature.    

The forging body is generally heated to the allowable initial forging temperature of the metal. In order to ensure the uniform temperature inside and outside, the surface of the forging blank should be kept warm for a certain time after heating to the required temperature. The holding time is related to the thermal conductivity of the metal, the cross-section size of the forging blank and the state of placement in the furnace. The heating rate of cold billet heating should not be too high to prevent excessive temperature difference between the surface and the heart and large thermal stress in the heart. The thermal stress in the core is easy to cause cracks. Common thermometers include thermocouples for measuring furnace temperature and optical pyrometers for measuring metal surface temperature.    

Heating method In ancient forging, the forging blank is heated directly with an open flame. Modern forging billet heating uses a variety of coal, oil, gas and electric thermal industrial furnaces, including intermittent chamber furnaces, trolley furnaces, resistance furnaces, induction furnaces and continuous furnaces. Induction furnace has the advantages of fast heating speed, uniform temperature, small footprint and easy automatic control, and has been widely used in medium and small die forging production line. Forging blank heating consumes a lot of energy, so it is necessary to improve the thermal efficiency of industrial furnace and improve the heating management and operation.    

At high temperatures, the iron in the steel oxidizes with the furnace gas to form oxides such as FeO, Fe3O4 and Fe2O3, which are called oxide skins. The formation of oxide will increase the loss of metal. The oxidation loss rate of the general intermittent flame heating furnace is 2 ~ 3%, and the induction heating is less than 0.5%. In addition, the oxide will also increase the wear of the die, reduce the accuracy of the forging and lead to surface roughness, thereby increasing the machining allowance and increasing the material consumption. The oxide also impedes the conduction of heat, prolongs the heating time, and affects the life of the furnace bottom and the mechanized operation of industrial furnaces. In addition to producing oxide skin, oxidation will also reduce the carbon content of the surface layer of steel, form a decarbonized layer, and reduce the hardness and strength of the surface layer of forging. The production of oxide is more detrimental to precision forging. In order to avoid or reduce various problems and losses caused by oxidation, many studies have been done on forging blank heating with less oxidation since the 20th century, and the results have been used in industrial production.