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A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 8 5 8 FEATURE 8 E lectromagnetic induction is used for heating ferro- magnetic and nonmagnetic metallic alloys, as well as certain composite materials. Induction heating (IH) generates heat internally at well-defined regions on the workpiece, which shortens process cycle times and results in high production rates. Heat intensity can range from high (exceeding 800°C/s for gear hardening) to moderate (as low as 2°C/s for tempering and stress-relieving). Highly control- lable heat intensity enables optimizing a wide variety of pro- cesses (Fig. 1). Due to the electromagnetic skin effect in induction heating, approximately 86% of induced power (heat gener- ation) is concentrated within the surface layer of the work- piece. This is often called current penetration depth ( δ ), which is proportional to the square root of electrical resis- tivity ( ρ ) and inversely proportional to the square root of frequency and relative magnetic permeability ( μ r ) of the ma- terial being heated. Lower frequencies promote deeper heat generation, while higher frequencies produce more shallow heat generation. Heat generation from induction heating of electrically conductive materials occurs by two mechanisms [1] . The pri- mary mechanism is associated with the Joule effect, which is often referred to as I 2 R heating; themagnitude of heat gen- eration is proportional to the product of the material’s elec- trical resistance and the square of the total current induced within it. The second mechanism occurs in heating ferro- magnetic materials (e.g., carbon steel), and is associated with magnetic hysteresis (magnetization-demagnetization cycles). Thermal energy is dissipated during the reversal of magnetic domains due to internal friction. Magnetic hyster- esis heat generation is proportional to the applied frequency and the area of the hysteresis loop, which is a complex func- tion of chemical composition, grain size, temperature, mag- netic field intensity, and frequency. Intrinsic characteristics of induction heating that make it attractive for use in aerospace applications include: • Repeatable quality with piece-by-piece processing capabilities and individual component traceability. Highly accurate process monitoring systems are available including profile/ signature monitoring. • Selective heat generation capability, which is advantageous in applications including band annealing and bend- ing, end heating, brazing, and selective hardening. • More energy efficient and environ- mentally friendly than other heating methods including gas-fired furnaces, salt and lead baths, and carburizing and nitriding systems. • Advantages in safety (no combustion or environmental contaminants), reduced labor cost for machine opera- tors, and automation capability. • Shorter startup and shutdown times, eliminating or significantly reducing idle periods of unproductive heating. No energy is needed to build or main- tain heat in non-operating conditions. INDUCTION HEATING AND HEAT TREATING FOR AEROSPACE APPLICATIONS Induction heating is used to produce high quality, reliable aerospace components as well as unique combinations of engineering characteristics. Valery Rudnev, FASM* Inductoheat Inc., Madison Heights, Mich. *Member of ASM International Fig. 1 — The highly controllable heat intensity of induction heating enables optimizing a wide variety of heating and hardening processes.