Intelligent Induction Boosts Productivity, Energy Efficiency

Figure 1 (inset): The 97% efficiency L-LC induction furnace converter with eight zones. Figure 2 (above): A high-performance induction furnace for heating large bars.

Agrowing number of forging operations are taking advantage of the improved energy efficiency, higher productivity, and eco-friendliness available thanks to a new generation of induction heating technologies. Specifically, the advent of an intelligent induction furnace zone control is a milestone in the pursuit of manufacturing excellence.

U.S. forging operations produce about 5.5 million short tons of forged parts annually, with an electricity consumption of about 2,900 MWh/year to heat these parts to the proper forging temperature. At a typical energy price of $0.08/kWh, the annual energy bill totals about $230 million, underscoring the potential benefits available to forging operations that deploy high-efficiency induction furnace technology.

In response to rising energy costs, and demonstrating a commitment to environmental stewardship, the iZone induction heating technology developed by SMS Elotherm GmbH sets new standards for efficiency, productivity, and resource conservation.

The critical factor that led to higher induction heating efficiencies was the development of an entirely new generation of converters featuring L-LC (inductor-inductor/ capacitor) resonance circuits, with switching at the inverter output (see Figure 1.) Comprised of an unregulated rectifier, intermediate circuit capacitor, IGBT inverter, and output choke, this converter has a real-world operating efficiency of 97% and a power factor (cos X) across its entire output power range. A successful integration of the iZone induction furnace with a hot shear demonstrates the greater production flexibility and productivity available to forgers producing large workpieces.

In the hot shear line, a magazine delivers individual bars (dimensions: Ø120 – 300 mm, or Ø4.75 – 12 in., with a maximum length of 12 m, or 40 feet) to a roll table. The table transports the bars to and through an induction furnace consisting of nine 1,100-mm (44-in.) long heating coils, followed by one 1,800-mm (71-in.) long soaking coil at the furnace exit. Then, the hot shear cuts the precisely and uniformly heated bar into billets ranging in length from 150 to 1,000 mm (6 to 40 in.) The required throughput ranges from 1.5 to 9 metric tons per hour.

Figure 8: Warm charging and soaking for immediate recovery after a production stoppage; green panels indicate a furnace zone is operating in the “holding” mode.

Two L-LC converters provide the 4,200-kW heating power required for the specified 9-mt/h throughput. Each converter drives four independent furnace zones (eight zones in total.) The first five coils comprising the furnace entry section are individually driven, making up the first five independent furnace zones. The sixth and seventh coils are grouped together in one zone, as are the eighth and ninth coils. The tenth coil (the soaking coil) makes up the eighth and final independently controlled furnace zone. All eight zones share the same control architecture, and all nine heating coils are identical and fully interchangeable for maximum flexibility and machine availability, and requiring only a minimal number of spares. Only two spare coils are needed to support the entire furnace, which saves money as well as floor space.

Large bars must be heated from room temperature to 1,250°C (2,280°F), so it may take as long to 50 minutes until the first billet can be hot sheared. The bar progresses from one heating coil to the next in about five minutes. The iZone controller tracks the bar location, activating each zone as the bar enters. As a result, energy and money are saved by driving only the loaded furnace zones (see Figure 3.) In a process that is analogous to the start sequence, the stop sequence turns off each furnace zone as the bar exits that zone (see Figure 4.)

The iZone furnace allows modern forging operations to capitalize on the trend toward smaller lot sizes and flexible production schedules. The intelligent zone controller automatically optimizes the heating recipe for efficient performance at any production rate, up to and including 100% of the rated heating power. In this example, 300-mm (12-in.) diameter bars are heated at a reduced throughput rate of 6 mt/h, with a net energy savings of about 20% compared to conventional furnaces without iZone technology (see Figure 5.)

iZone offers forging operations a variety of heating strategies, including options for reduced scale formation and "soft" heating (heating with reduced thermal gradients in the workplace.) The intelligent controller behind the operator-friendly GUI (graphical user interface) automatically optimizes the heating profile and furnace parameters in response to operator selection. At the press of a button, indvidual furance zones are energized at the right moment with the right power, frequency, and load matching capacitance (see figure 6.)

Certain bearing steels, such as 100Cr6, are prone to internal cracking induced by thermal stress during the heating process. Conventional induction furnaces use sets of custom “soft heating coils” to reduce the heating rate below the Curie temperatue, so as to avoid thermal cranking. Because the iZone controller can drive each furnace zone independently, it is able to create a soft heating profile with standard heating coils. No custom heating coils are required; iZone simply (and instantly) fine-tunes the furnace to match the requested throughput and heating profile. This eliminates the cost and delay of maintaining and swapping mulitple sets of custom heating coils (see Figure 7.)

In the past, disruptions to the pace of a forging process (faults, pauses, and outages) usually have required that the induction furnace be purged of all in-process material, resulting in wasted energy, time, and money. By contast, iZone can maintain partially heated bars at the appropriate intermediate temperature until production resumes, with no down and no loss of product. This "stop and go" operation allows warm charging, holding, and soaking of partially heated bars based on the workpiece heat content (ent halpy) in each of the furnace zones. Oscillation (contiuous forward/backward cycling motion of the bar) essentially eliminates residual thermal gradients (zebra-striping) along the length of the bar. Strategically located pyrometers verify the bar temperature and provide input to the iZone adaptive controller (see Figure 8.)

Modern forging operations have achieved significant productivity improvements through the benefits of iZone induction furnace technology, including:

• Peak efficiency at any throughput rate in all operating modes, including startup, soaking, shutdown, and continous production;
• Simple and fast (less than two minutes) product/process changeovers;
• Adaptable heating profiles for reduces scale formation, crack prevention, etc;
• Stop and Go operation for fast, efficient re-starts after production pauses; and • Reduced heating coil invertories and spares requirements

In Europe and North America, iZone has been particularly well received by forging operations seeking to increase productivity while reducing costs and emissions. There is also a strong response from forging operations in Asia that must work with limited power availability. Meanwhile, new technologies for even greater induction heating efficiency are under development, and soon will be incorporated into a new generation of high-performance furnaces for the forging and long products production.