The ratio of the surface temperature (average temperature for short) and the central temperature (insulation) of the forging to the total heating time varies with the heating temperature. The heating temperature is low, the specific force heating temperature is high, and the holding time accounts for a large proportion. It can be seen that the key to shortening the heating time is the stage in which the total heating time accounts for a large proportion. According to the principle of heat transfer, what are the ways to shorten the heating time:
1. Uniform heating
When the workpiece is heated, if the temperature is uneven and the surface is heated unevenly, resulting in the eccentricity of the heating center, it is equal to increasing the section size. If corrected according to 1.4d (diameter), the heating time shall be extended by 30 ~ 50%. Therefore, homogeneous heating is the first consideration to shorten the heating time. Here, the working environment of the heating furnace should be kept good, the temperatures of the heating furnace should be affected, and the furnace gas circulation conditions should be considered for the loading method of workpieces.
2. Reasonably select the section temperature difference
After the forging surface is heated to temperature, due to the limitation of heat conduction, the central temperature is consistent with the surface temperature, which takes a long time. Therefore, the heating time in the process should be determined according to the actual needs of the workpiece center, that is, a certain section temperature difference is allowed. For example, forging heating, on ordinary structural steel, from deformation requirements. Heating does not need to be heated to near surface temperature; When the forging center is defective, the center must be heated to a high enough temperature. By analyzing the heating standards of foreign forgings, it can be seen that the section temperature difference of high temperature wide top strong pressure process is within 10 ℃, the allowable section temperature difference of ordinary forgings is close to 100 ℃, and the heating time is only 60% of the former. The same problems also exist in the process of heat treatment, that is, the solid solution of alloy elements, grain refinement, microstructure transformation and tempering parameters have an appropriate temperature range for any purpose. Therefore, studying the suitable temperature range of different steel grades and different process requirements and determining the reasonable allowable temperature difference of heating section can better shorten the heating time on the premise of ensuring the quality.
3. Properly increase the furnace temperature
When the forging blank is heated, the surface temperature always lags behind the furnace temperature. After the temperature in the furnace rises to the required temperature, the forging surface can reach the temperature for a considerable period of time. The length of this period is closely related to the required heating temperature. With the decrease of heating temperature, the surface heat exchange coefficient decreases and the time required increases. Therefore, properly increasing the furnace temperature before the forging surface temperature reaches the temperature can greatly shorten the forging surface to temperature time. This method is especially suitable for workpieces with simple shape. Through the calculation of temperature field, the degree of furnace temperature and holding time can be improved according to the workpiece size, required heating temperature and heating speed.
4. Strengthen convection
At high temperature, the surface heat exchange is mainly convective heat transfer. When the furnace gas temperature is constant, the furnace gas velocity in contact with the forging blank surface determines the surface heat transfer coefficient and affects the surface temperature arrival time. The existing gas and heavy oil heating furnaces mainly rely on the combustion air flow for forced convection, and the burner is turned down under the low-temperature heating of 200 ~ 400 ℃. The reduction of air volume will greatly affect the heating efficiency in low temperature area. Therefore, for the low-temperature heating furnace, if the furnace structure is improved and the gas circulation is strengthened, the heating efficiency can be improved and the heating time can be shortened.