Heating Time and Temperature Profile

The uniformity of final temperature is one of the main criteria for evaluating the quality of heating in forges. The temperature of the heated workpiece is in practice measured contactless by pyrometer. However the surface temperature tells little about the quality of the heating. The measurement of the temperature inside the workpiece is very complicated therefore it is very advantageous to use computer simulation which calculates the entire time development of the core-surface temperature profile.

Due to the skin effect the heat is generated more at the surface of the workpiece and reaches the center by conduction. The penetration depth at which most heat is generated depends on the frequency. High frequencies cause a low penetration depth and extend the required heating time. Low frequencies deepen the penetration depth and shorten the heating time. Too low frequencies cause transparency and reduce heating efficiency. Sufficient heating time should be chosen to heat up the core well. However, too long a time leads to low efficiency, greater scale formation and subsurface overheating.

A typical temperature development during heating is shown in Fig.1. Magnetic steel has a very low penetration depth at the start of heating and the thin surface layer heats up very quickly to 800°C where it loses its magnetic properties and the penetration depth increases. This is followed by a slow increase in surface temperature. Electromagnetic waves penetrate more deeply and a lot of energy is consumed for crystal structure changes. After overcoming phase changes the surface temperature rises faster. Towards the end of the heating, radiation losses gain significantly. The surface heats up more slowly and the temperature maximum moves below the surface of the workpiece. The core-surface temperature difference decreases. After the heating is finished the surface is naturally cooled and the temperature maximum moves to the core.

Fig. 1 – round billet heating, magnetic steel, diameter 80 mm

The graph below shows how the heating time affects the final temperature distribution of 60 mm round steel billet when the surface is heated to 1200 ° C. In short heating times the core is colder than the surface, in long heating times it is vice versa. The temperature maximum occurs below the surface due to large radiation heat losses. The transport of the hot billet to the forming machine usually takes a few seconds. During this while the surface cools down quickly. The dashed curves show the temperature after 8 seconds of natural cooling.

Fig. 2 – final temperature distribution, round steel billet, diameter 60 mm, f=2000 Hz

Optimal heating time before forming

At an optimum heating time the surface should be several tens of degrees warmer than the core at the end of heating. After the heating is finished the surface cools rapidly due to radiation and the core-surface temperature difference can inverse within a few seconds. Too short heating time causes an under-heated core which increases the deformation resistance and wear of the die. In practice is more often used too long heating time, which has following severe disadvantages:

1) Material overheating
Long heating times together with a high target temperature increase the risk of material overheating below the surface. The temperature below the surface can be up to several tens of degrees higher than the pyrometer shows. When the material overheats intergranular melting occurs which irreversibly worsens the mechanical properties (ductility, strength). Overheating can also occur during forming, when the temperature increases further from deformation.

2) Scale formation
Long heating times cause decarburization, internal oxidation and increased scale formation. Scale formation is associated with loss of metal and energy. Scales cause dimensional inaccuracy, clogging of device, reduced life of forming and cutting tools. Scales have a negative effect on the inductor's durability. The wear of the skid rails increases. Scale dust penetrates into the refractory micro-cracks and accelerates their enlargement resulting in a short circuit.

3) Efficiency drop
Radiation heat losses are proportional to the fourth power of the thermodynamic surface temperature and greatly affect the heating efficiency to forging temperatures around 1200 ° C. The longer the heating time, the more energy is emitted by the surface. Excessive heating time can reduce efficiency by several tens of percent.

4) Billet sticking
Undesirable sticking/fusing/welding of the billets occurs due to the high temperature in combination with the pressure from the feed mechanism. The higher the heating time is the higher is the maximum temperature below the surface. The pressure between the billets in the inductor decreases with the distance from the feed mechanism. If the heating time is long high temperatures are reached at the beginning of the inductor where the pressure between the billets is highest.

The recommended minimum heating times for heating round magnetic steel to a forging temperature of around 1200°C are given in the following table. For slab the heating time is approximately twice as high. The heating times are calculated for in-line continuous constant power heating where in the final state the surface is 50 degrees warmer than the core. Double this time can be considered acceptable. At longer heating times it is advisable to consider measures that could shorten the heating time: shortening the cycle, increasing the frequency, using an inductor with shorter coil, using a lower power heater with a shorter inductor.

U průběžného jednostupňového ohřívače s konstantním podélným rozložením výkonu doba ohřevu závisí na taktu a délce přířezu podle následujícího vztahu:

 doba ohřevu = (délka cívky) / (délka přířezu) ∙ takt

In order to optimize the heating quality ROBOTERM offers multi-stage heaters (Fig. 4), inductors for accelerated heating (Fig. 5) and inductors with shortened coil (Fig. 6). Multistage heaters have in line two or more inductors with separate power sources. In multistage heaters the power can be distributed arbitrarily along the heating line. This makes it possible to vary the cycle while maintaining an optimum final core-surface temperature profile. At low power the material is heated only in the last inductor. At high power, the first inductor heats to full power and the other inductors only maintain the surface temperature while the heat is soaking in the core.

Inductors for accelerated heating and inductors with short coils can be used on single power source heaters. The advantage is the rapid replacement of the inductor, especially on heaters with transversely movable inductors. The coil length can be designed for optimal heating time and required power. Inductors for accelerated heating have a coil with non-homogeneously distributed threads. The highest thread density is at the beginning of the inductor which allows minimizing the required heating time.

Table. 1 - Recommended minimum heating time of round shape magnetic steel

Fig. 3 - Standard inductor	Fig. 4 – Double stage heating
Fig. 5 - Inductor for accelerated heating	Fig. 6 - Short coil inductor

References:
[1] Rudnev, V.: Induction heating of steel billets: causes of billet sticking/fusing problem and its prevention, in heat processing 4, 2019, p. 57-60
[2] Rudnev, V.; Brown, D.; Van Tyne, Ch.; Clarke, K.: Intricacies for the successful induction heating of steels for modern forge shop. Proc. of the 19th Int'l Forging Congress, Chicago, IL, September 2008
[3] E. Rapoport, Y. Pleshivtseva, "Optimal Control of Induction Heating Processes", 2006, ISBN 9780849337543