Demagnetisation steps and residual field

Ferromagnetic steel keeps some of the field it was given. That remanence is what holds the particles in place during examination, but afterwards it attracts swarf to machined surfaces, deflects instruments, disturbs arc welding, contaminates bearings and can upset later magnetic particle or ultrasonic work. Demagnetisation removes it by walking the material repeatedly around its hysteresis loop with steadily shrinking amplitude, so the loop spirals in towards the origin and the remanence is left close to zero.

Each reversal must both flip the direction and reduce the peak. If the amplitude is cut without reversing, the material simply relaxes back onto the same branch of the loop and the remanence barely moves. The practical arrangement is either a decaying AC field – which reverses at line frequency while the current is ramped down – or a reversing DC supply that steps the current down and flips polarity at every step.

Each step leaves a fraction of the previous residual, so the field decays geometrically: H = H0 x r^n. Solving for the number of steps gives n = ln(H_limit/H0)/ln(r), rounded up. A per-step reduction ratio around 0.7 is typical of a reversing DC set; a smooth AC decay behaves like a very large number of very small steps. The first step must reach at least the peak current used to magnetise the part, otherwise the deepest part of the loop is never reversed.

The catch with AC demagnetisation is its shallow penetration. It works well on thin sections and on surface remanence, but it will not reach the core of a heavy forging – that needs a reversing DC cycle. The result is verified with a calibrated field indicator or Hall-effect gaussmeter, with a residual limit commonly set at 3 G and tightened to 2 G on aerospace work.