Induction Hardening
What metallurgical change makes the surface hard?
The surface is heated to the austenitizing range and then quenched fast enough to form martensite, which is much harder than the original structure.
Does induction hardening change the whole part?
No; the process is designed to harden only the targeted surface zone while leaving the core largely unaffected, making it suitable for selectively loaded wear surfaces.
What parts are best suited to this process?
Parts with defined wear tracks or bearing surfaces such as shafts, stepped journals, discs, rails, rings, and some gear-related components are common candidates.
Induction hardening is a surface hardening heat-treatment process in which a metal part is heated by an alternating electromagnetic field and then immediately quenched, transforming the surface to martensite while the core remains tough. It is commonly used in CNC automated cells for shafts, bearings, and gear components, providing a hard, wear-resistant case with minimal distortion.
On the shop floor, parts are cleaned, degreased, and de-rusted before loading to ensure consistent heating and uncontaminated quench. The CNC program controls power, frequency, heating time, feed rate, and rotation or traverse speed, directly influencing case depth and uniformity. The coil generates heat only where the magnetic field couples, making the process ideal for localized hardening of functional zones while leaving surrounding areas machinable. After heating, the part is quenched with water, oil, or emulsion, followed by tempering to reduce brittleness while retaining hardness. The main advantage over bulk heat treating is selective hardness with lower distortion, as heating is rapid and confined to the required zone. This makes induction hardening preferred for parts like shafts, journals, discs, rails, and rotor journals that demand precise wear resistance without compromising core toughness or dimensional stability.
Wrong frequency for required case depth: Higher frequencies heat thinner layers; using too high a frequency on a shaft journal can produce a shallow case that wears through early, while too low a frequency risks deeper penetration and distortion.
Poor surface preparation before hardening: Oil, scale, or rust interfere with heat transfer and quench wetting, leading to soft spots, patchy hardness, or uneven case depth across the part.
Quench timing or coverage errors: Delayed or uneven quench, or incomplete nozzle coverage, allows overheating before martensite forms, causing distortion, cracking, or inconsistent hardness bands along the scan path.