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How does induction heating harden only the surface of a steel automobile axle?

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Diagram of a zinc-lead blast furnace system showing ore and coke entering through charge hoppers, with labeled furnace, air preheater, blower, taphole for slag and lead bullion, and a connected lead-splash condenser using rotors to separate and cool lead and zinc, with pathways for molten metals and off-gas. — metallurgy A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

Induction hardening allows for the selective surface hardening of a steel automobile axle by leveraging the skin effect. When a high-frequency alternating current flows through an induction coil surrounding the axle, it creates an electromagnetic field that induces eddy currents within the steel.12

Due to the skin effect, these high-frequency currents are restricted to the outer layer of the conductor, causing intense, localized heating only at the surface. By controlling the frequency of the current, engineers can precisely determine the depth of this heat penetration. Once the surface reaches the required temperature, the axle is immediately quenched, resulting in a hard, wear-resistant outer layer while the core remains relatively soft and ductile. This combination of properties is essential for components like axles that must withstand significant stress and fatigue.213