Modular Induction System Offers Billet-Heating Advantages
While the coil and power module are the basic components of the system, there are several others that complete it. On the in-feed end, a cabinet houses the PLC and other controls. A water system for cooling just the power-supply portion of the modules (the coils are cooled separately) can be included in smaller systems. The HMI (Human Machine Interface) is mounted on a pendulum so that the screen can be positioned for easy viewing by the operator. A tractor or pinch-roll drive system is mounted on top of this cabinet for pushing the billets through the induction coils. A weld-breaking extractor with accept/reject is mounted at the exit of the heater.
Many of the benefits of the modular construction result from the ability to control each coil individually. These benefits include:
- Temperature control
- Power distribution to match production rate
- High efficiency
- Standby and Rapid Start
Temperature ControlThe theory behind using individual coil control to control the final billet temperature profile follows a few simple rules. First, if you put more power in the first coils in the induction-heating line, the billet center will get hotter because the energy has more time to conduct into it (Figure 2). Second, if you put more power in the end coils of the induction-coil line, the billet’s surface will be hotter because it does not have as much time to soak into the center (Figure 3).
Therefore, if the line speed is fast or the billet diameter is large and you need to rapidly get the heat into the middle of the billet, you should put more power in the first coils. If the production rate is slow, you should put more power in the end coils so that the center temperature does not get too hot.
Power Distribution to Match Production RateThe original induction-heating design for forging used a single power supply and several coils, normally with the same number of turns on each coil. The temperature profile of this type of heating system is shown in figure 4, which is split into four segments – a, b, c and d.
In section “A,” the temperature of the entire billet is below Curie. The heating efficiency is in excess of 90% when the billet is magnetic. Since the energy is being induced into the “depth-of-penetration” area near the surface of the billet, the surface temperature increases with a very high slope.
In section “B,” the billet surface has reached the Curie temperature (it is not magnetic anymore). But, at some point below the surface though not deeper than the “depth-of-current” penetration, the temperature is still below Curie and the billet is still magnetic. Some of the induced current bypasses the surface and goes to the magnetic portion under the surface. Therefore, the surface temperature does not increase very rapidly.
In section “C,” the entire depth-of-current penetration is above Curie and therefore not magnetic. The current stays in this area near the surface. The surface continues to heat, but at a slower rate than when it was below Curie. Energy is conducting towards the center of the billet.
In section “D,” the induction-coil line has to be designed to be long enough to allow the energy to conduct into the center of the billet, producing temperature uniformity as required by the application.
Some older induction systems use a single power supply to power the line but change the turns on each coil to adjust the power distribution along its length. They do this by designing the induction coils to provide a graded heating pattern, putting more power into the coils at the beginning of the induction heater. This forces more energy into the billet at the front of the line, giving it more time to soak into the center of the billet. The temperature in the center of the billet can reach the forging temperature in a shorter period of time, reducing the length of the coil line.
The problem with this type of design is the power distribution along the coil line cannot be changed if the production rate changes. If the production rate is reduced, a center-overheating problem occurs. It is very common to find billet-sticking problems with graded coil induction systems. This occurs when the system is run at a rate slower than the maximum for which it is designed. Since the system puts more energy into the billet in the first coils, too much energy soaks down into the billets when the line runs slow. When the pyrometer measures the desired forging temperature on the surface of the billet, the temperature inside the billet is actually much higher. In a lot of cases, it can be hot enough to cause the billets to fuse together (Figure 6).
The system offers the benefits of high power in the first coils at high production rates while maintaining good temperature uniformity when running at slower rates. This means the temperature profile can be maintained when new parts are being developed, a new automation system is being set up or when new operators are being trained on a system.
High EfficiencyThe new billet heater can be 20% more efficient than older induction-heating systems for the normal design production rate of the system. It can be many times higher than this when the billet heater is run at reduced production rates. While most induction-heating systems can only achieve a rate of 2.5-2.7 kg/kWh, the system has been measured to exceed 3 kg/kWh.
The induction coil sits on top of the power module and is less than 1 meter away from the power-supply inverter. There are virtually no transmission losses between the coil and power supply. This increases efficiency by 5-10% over typical induction units in which the power supply is separate from the coil stand.
The billet-heater coil utilizes a removable liner with a backing of low thermal-conductivity paper to reduce the thermal losses from the coils. This gains an additional 3-5% in efficiency. The coil layout (using countersunk skid rails) adds an additional 7-10% in efficiency (Figure 9).
Standby and Rapid StartSince we can control the power going into each coil along the length of the induction-coil line, we can control the temperature in each coil when the line is stopped for any reason. The benefits of standby include:
- Energy savings (do not have to reheat billets)
- Material (fewer rejects)
- Time (do not have to wait for the billets to get back to forging temperature)
- More parts produced (the heater has parts ready for forging immediately after the press is fixed or adjusted)
For automatic press lines where all die positions must be filled, dynamic standby can be used. As in static standby, the billets are stopped when the system is put into standby. A billet is then pushed out every two minutes. This eliminates the cold billets between the induction coils. When the line is restarted, all the billets can be forged.
Dynamic standby is different from the old method used by induction suppliers in which the line speed and power were reduced. Here, the line is actually stopped. Periodically, a billet gets pushed out of the heater so that billets are not stuck between coils for an extended period of time. No hot temperature hump develops like it does for the old style of hold.
Using the same coil control technology as used with standby, we have also developed a system for rapidly starting heaters with a line full of cold steel. As heaters get larger, it becomes more difficult to push billets out before restarting. The problem with trying to restart the heater with a line full of cold steel is that a line full of billets at a temperature below Curie – and therefore magnetic – creates an electrical condition that is problematic for the traditional power-supply coil arrangement. The power supply will hit a very hard limit condition when this occurs. Without some sort of special control, it commonly takes two or three coil lines of billets to get the billets up to forging temperature.
With Rapid Start, the billets are heated to a specific level in each coil to simulate the normal temperature profile of the billets along the coil line. This is done prior to pushing the billets through the coil line.
FlexibilityToday’s forge shop must often adjust to a rapidly changing business environment. The modular billet-heater system was designed to meet these requirements. It is easy to add or remove modules to the billet-heating line to match changes in production. The frequency can be easily modified to match changes in billet size or material. The induction coil has been designed to match into a wider range of billet diameters, so less coil changes are required.
Author Doug Brown is president of the Forging Division of the Inductotherm Heating and Welding Group and Inductoheat, Inc., Madison Heights, Mich. He is the vice president of the FIERF board that works with universities and other researchers to bring new technologies to the forging industry. He may be reached at (248) 629-5046; or at firstname.lastname@example.org.