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How can we reduce the heating time for the heat treatment of forged blanks?
How can we reduce the heating time for the heat treatment of forged blanks?

09

2022

-

12

How can we reduce the heating time for the heat treatment of forged blanks?

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The ratio of the durations of the two stages—surface temperature equalization (referred to as “uniform heating”) and core temperature equalization (holding at temperature)—to the total heating time varies depending on the heating temperature. At lower heating temperatures, the proportion of time spent on uniform heating is relatively smaller, while the proportion of time spent on holding at temperature is larger. Thus, the key to shortening the overall heating time lies in reducing the duration of the stage that accounts for the greater proportion of the total heating time. According to the principles of heat transfer, there are several ways to shorten the heating time: 1. Ensure uniform heating: When a workpiece is heated, if the temperature distribution is uneven and the surface heats unevenly, it can lead to an eccentricity in the heating center, effectively increasing the cross-sectional dimensions—for example, by adjusting the dimension to 1.4 times the diameter (D).
　　The ratio of the durations of the two stages—surface temperature equalization (commonly referred to as “uniform heating”) and core temperature stabilization (or holding) after the furnace reaches the desired temperature—to the total heating time varies depending on the heating temperature. At lower heating temperatures, the proportion of time spent on surface temperature equalization is relatively larger, while the proportion of time spent on holding increases as the heating temperature rises. Thus, the key to shortening the overall heating time lies in reducing the duration of the stage that accounts for the greater proportion of the total heating time. According to the principles of heat transfer, what are the possible approaches for shortening the heating time?
　　1. Even heating
　　When the workpiece is heated, if the temperature distribution is uneven and the surface heats up non-uniformly, it can lead to an eccentricity in the heating center—effectively increasing the cross-sectional dimensions. In such cases, if correction is made based on 1.4 times the diameter (D), the heating time should be extended by 30% to 50%. Therefore, homogeneous heating is the first factor that should be considered when shortening the heating time. Here, the working environment of the heating furnace must be kept in good condition; each temperature zone within the furnace should be accurately monitored and controlled; and the method of loading the workpieces into the furnace must take into account the conditions necessary for effective furnace gas circulation.
　　2. Reasonably select the temperature difference across the section.
　　After the surface of a forging is heated to temperature, due to limitations imposed by thermal conduction, the core temperature will take a considerable amount of time to match the surface temperature. Therefore, the heating time required in the process should be determined based on the actual needs of the workpiece’s core—allowing for a certain temperature difference across its cross-section. For example, in forging heating, for ordinary structural steels, given deformation requirements, it is not necessary to heat the workpiece up to a temperature close to the surface. However, when defects are present at the center of the forging, the core must be heated to a sufficiently high temperature. An analysis of foreign forging heating standards reveals that, in the case of high-temperature, wide-top, strong-pressure processes, the temperature difference across the cross-section is kept within 10℃. By contrast, for conventional forgings, the allowable cross-sectional temperature difference can approach 100℃, yet the heating time required is only about 60% of that needed for the high-temperature process. A similar issue arises during heat treatment: whether it’s the solid solution of alloy elements, grain refinement, microstructural transformation, or tempering parameters—all these processes have appropriate temperature ranges, regardless of their specific purpose. Thus, studying the optimal temperature ranges for different steel grades and varying process requirements, and determining reasonable allowable cross-sectional temperature differences during heating, can help shorten heating times effectively while ensuring product quality.
　　3. Appropriately increase the furnace temperature.
　　When billets are heated, the surface temperature always lags behind the furnace temperature. After the furnace temperature rises to the desired level, it takes a considerable amount of time for the billet’s surface to reach that same temperature. The duration of this lag is closely related to the required heating temperature. As the heating temperature decreases, the surface heat-transfer coefficient diminishes, and consequently, the time needed for the surface to reach the target temperature also increases. Therefore, before the billet’s surface temperature reaches the target level, appropriately raising the furnace temperature can significantly shorten the time required for the billet’s surface to achieve the desired temperature. This method is particularly suitable for workpieces with simple geometries. By performing temperature-field calculations, one can determine the appropriate degree to which the furnace temperature should be raised and how long it should be maintained, based on the workpiece dimensions, the required heating temperature, and the desired heating rate.
　　4. Strengthen convection
　　At high temperatures, heat exchange at the surface is primarily convective. When the furnace gas temperature remains constant, the velocity of the furnace gas flowing in contact with the surface of the charge and the forging billet determines the surface heat transfer coefficient and, consequently, affects the time required for the surface to reach the desired temperature. Currently, most gas and heavy-oil heating furnaces rely on forced convection driven by combustion gas flows. However, at low temperatures ranging from 200 to 400°C, the burner openings are partially closed, leading to a significant reduction in air volume. This decrease in air volume inevitably has a substantial impact on the heating efficiency in the low-temperature zone. Therefore, for low-temperature heating furnaces, improving the furnace structure and enhancing gas circulation can boost heating efficiency and shorten the heating time.

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