Metal Hardening (Induction Heat Treating)

Induction hardening that controls the heat pattern

Induction hardening is the most common form of induction heat treating. A coil carrying alternating current induces eddy currents in a steel part, heating a selected surface layer above its austenitizing temperature, and a quench follows immediately to reach the required case depth and hardness. The main advantage over other heat treatment methods is the ability to heat quickly and selectively, only where the part needs modified properties. Fluxtrol’s published work describes the results as lower distortion, a more favorable residual stress distribution, better microstructure and significant energy savings.

The difficulty is the heat pattern. Specifications call for hardness in a bearing race or fillet and none in the adjacent snap-ring groove, so the coil must deliver power to the right place and nowhere else. Fluxtrol designs and simulates hardening inductors and supplies the soft magnetic composite (SMC) flux controllers that shape that pattern.

What drives distortion in induction hardening

Distortion is a result of thermal gradients and phase transformation, so every part of the process contributes. Fluxtrol’s axle studies, which coupled electromagnetic, thermal, metallurgical and stress simulation, identify several levers:

• Heat in the core. On truck axle shafts longer than 1 meter, bowing and growth in length are the main concerns. Excessive heat internal to the shaft is the main contributor, and proper inductor design, process control and structural support help limit it.
• Quench severity and uniformity. A nonuniform or slow quench gives nonuniform hardness and possible overall distortion. In the axle model, higher spray cooling rates raised surface compressive stress and core tension together, so quench and heating must be designed as a pair.
• Heat pattern in geometry changes. Fillets, shoulders and splines need extra heat, which is where overheating above the fillet and added core heat originate.

A coil that puts the right power in the right place, with no more depth than required, is a distortion control tool.

How magnetic flux concentrators help

Flux concentrators give the magnetic field a low reluctance path, so the field is concentrated around the coil instead of spreading through space. Nearly all of the current in the copper is driven onto the heat face, and the current in the part is concentrated under it. The practical results are:

• A sharper, more controllable pattern, including in fillets, without overheating adjacent journals
• Lower coil current for the same power, which reduces copper temperature and losses in the busswork
• Less unintended heating of nearby machine components
• Longer coil life, since copper fatigue usually limits inductor lifetime

The ASM Handbook chapter on inductor design concludes that a flux controller is a stronger way to control the heat pattern than profiling the copper, and that profiling the concentrator poles smooths transitions between zones. See Principles of Induction Heat Treating for the physics.

Single-shot and scan hardening

Every induction hardening process falls into one of two groups. In single-shot hardening the coil stays in position relative to the heated length (the part typically rotates), as with crankshaft journals and spindles. In scan hardening the coil moves along the part with a quench following it, as with axles. Methods compares them, and Inductor Styles covers machined integral quench (MIQ) coils, quench in place, and encircling versus non-encircling designs.

Documented results

• Axle scan hardening. In a 48 mm full-float axle study, an optimized two-turn coil with Fluxtrol A on the lower turn scanned about 15% faster at 1 kHz and more than 35% faster at 3 kHz than a standard two-turn coil, and tolerated more variation in coil position.
• Crankshaft hardening. On a production crankshaft inductor, Fluxtrol A replaced iron-silicon laminations and copper keepers. The paper reports a 100% increase in coil lifetime and deeper hardening in the fillet.
• Wheel hub. Laminations replaced by Fluxtrol A lifted coil life from 8,000-13,000 cycles to 15,000-20,000. A simulation-optimized coil with Fluxtrol 50 reached more than 150,000 cycles.
• Camshaft and cam. Case studies report a 26% cycle time reduction with no back tempering of the adjacent lobe, and a 46% cycle time reduction with improved pattern.

More are in the case studies and the Technical Library .

Coil life and reliability

Hardening coils fail mainly in the copper. Each cycle heats the copper while power is on and cools it when power is off, and the resulting thermal cycling causes fatigue cracking, usually in nearly the same place on a consistently built inductor. The ASM chapter lists three ways to extend life: more cooling, lower local power density, or a different coil design. SMC concentrators help with the second, because they lower the coil current needed for a given power and so reduce copper temperature and busswork losses. In severe applications the flux controller can also limit life, typically through corrosion at elevated temperature, which is why Fluxtrol has published work on improving the corrosion resistance of SMCs for induction heat treating.

Designing a hardening process with simulation

Fluxtrol uses simulation to test coil geometry, frequency, scan speed and concentrator shape before building tooling, and can couple electromagnetic and thermal results with metallurgical and stress analysis to look at distortion. The approach is described in Induction Heating Computer Simulation and in the technical library paper Virtual Prototyping of Induction Heat Treating .

Talk to a heat treating engineer

Whether you are starting a new hardening process or fixing an existing one, contact Fluxtrol to discuss coil design, SMC selection and simulation. To buy flux controller material, visit Order SMC .