Induction hardening: a steel part gets a glass-hard surface in seconds, while the core underneath stays tough and ductile. Most case hardening methods, like carburizing or nitriding, change the chemistry of the surface by diffusing carbon or nitrogen into it. Induction hardening works differently. It keeps the alloy’s chemistry the same and changes only its microstructure, using electromagnetism and a very fast quench. How it works: - A water-cooled copper coil carries high-frequency alternating current, typically about 1 kHz to 400 kHz - The coil’s oscillating magnetic field induces eddy currents in the steel part - Resistive (Joule) heating from those currents raises the surface above its austenitizing temperature, roughly 850 to 950 C - An immediate water or polymer spray quench turns that austenite into martensite, the hardest microstructure steel can form Why only the surface heats: - AC current crowds toward the outer surface of a conductor, which is called the skin effect -...