Induction hardening

The mechanic

A high-frequency alternating current runs through a copper coil shaped to match the target surface. The magnetic field heats the steel directly beneath the coil (via eddy currents) very rapidly — seconds, not hours. Per Gear Solutions Magazine and Wikipedia's process descriptions, the heated zone transforms from ferrite into austenite. Quench immediately (water spray, polymer spray, or oil) and the austenite locks into martensite. Hard surface, untouched core.

Key numbers

• Case depth is typically 0.030-0.120 inches (~0.75-3 mm), though large parts like axles can run up to 0.5 inch. The case depth is usually defined as the location where the microstructure is at least 50% martensite.
• Power and frequency are the two main control variables: higher frequency = shallower case (skin effect concentrates the current near the surface).
• Cycle time is measured in seconds for the heat, plus the quench. The whole operation on a single part can be under a minute for small features.

When induction makes sense

• You need a localized hardened zone, not full-surface hardening.
• The substrate already has enough carbon to harden (medium-carbon steels around 0.4-0.6% C, or tool steels). Induction doesn't ADD anything — it just rearranges what's already there. Low-carbon steel can't be induction-hardened without first carburizing.
• You need fast, clean, repeatable hardening with no gas atmosphere and no long furnace cycles.
• You need to limit distortion. Less material at high temperature = less thermal warp.

When induction doesn't make sense

• You need a uniform hard surface over a complex 3D geometry. Coil design gets difficult; coverage gets uneven.
• The substrate is low-carbon. You need to add carbon first (carburize, then quench).
• You need corrosion resistance or low-friction in addition to hardness. Induction only gets you hardness; it doesn't change the surface chemistry.

What this means on the shop floor

• Common applications you'll see: shafts and axles (selective hardening of bearing journals), bearing races, gear teeth (specifically the tooth flanks), cam lobes, hydraulic cylinder rods. Less common on the tooling itself, but very common on the parts the tooling acts on.
• Induction-hardened parts can be welded or re-machined in the untreated zones without damaging the hardened zones. That's a real advantage over through-hardening for parts that need post-treatment work.
• Cost-wise, induction is typically cheaper per part than nitriding or carburizing for the right application — fast cycle, low energy per part, automatable.

Next: the world of PVD coatings, which sits on top of heat-treated substrates and is what most people picture when they hear "surface coating."