In this setup, alternating current (AC) is used to generate controlled resistive (Joule) heating directly within the steel. When AC flows through the metal, electrical resistance converts electrical energy into heat (P = I²R), raising the temperature uniformly from the inside out. Because the heating is internal rather than surface-applied, thermal gradients can be tightly managed—especially when frequency, current density, and geometry are carefully selected. At elevated temperatures, steel’s microstructure transforms. Austenite (face-centered cubic iron) becomes stable above the critical temperature (Ac₁–Ac₃ range depending on carbon content). Rapid water quenching then suppresses diffusion, trapping carbon in a distorted body-centered tetragonal lattice and forming martensite—a supersaturated, hard but brittle phase. Tempering follows to restore toughness. Reheating below the eutectoid temperature (typically 150–650 °C depending on target properties) allows controlled carbon diffu...