Steels for induction hardening

The one requirement

Induction hardening works by heating steel above its austenitising temperature and quenching it fast enough that the carbon has no time to diffuse out. The trapped carbon distorts the crystal lattice, and that distorted structure is martensite. It is hard because the lattice is strained.

So the requirement is carbon. Below about 0.3 percent there is not enough of it to strain the lattice usefully, and you can heat and quench the part all day without meaningfully hardening it. Above about 0.6 percent you gain little further hardness but you gain a great deal of brittleness.

That gives a practical band of roughly 0.35 to 0.55 percent carbon for most induction hardened engineering components, with bearing steels sitting deliberately above it.

Grades commonly hardened

We commonly discuss medium and high carbon steels for induction hardening, including C45, EN8, EN19 and EN31. We confirm the material condition and acceptance criteria before developing a cycle.

Cast irons

We also develop induction hardening applications for nodular and grey cast irons. The graphite structure and matrix condition affect the response, so share the material specification and starting condition with us.

A predominantly pearlitic matrix hardens well. A ferritic one does not, because the carbon is not where it needs to be. Grey iron with coarse graphite flakes is more prone to cracking under a fast quench than nodular iron, where the graphite is spheroidal and does less to concentrate stress.

The practical applications are exactly where you would expect: cylinder liners, CI rings, and flywheel starter rings, all of which we build machines for.

What will not harden

• Low carbon and mild steel. Not enough carbon. It will heat beautifully, because it is magnetic and resistive, and then quench to almost the same hardness it started at. To harden it you carburise first.
• Austenitic stainless steels. The 300 series does not transform to martensite, so there is nothing to harden. It also has low magnetic permeability, so it couples poorly. Martensitic stainless grades such as 410 and 420 are a different matter and do harden.
• Aluminium, copper, brass and bronze. No martensitic transformation exists. They can be induction heated for forming, brazing or shrink fitting, but not hardened.

Why prior condition matters

Induction hardening gives the steel a very short time at temperature, often a matter of seconds. The carbon has to dissolve into austenite during that window, and how easily it does that depends on what the steel looked like beforehand.

A fine, uniform prior structure, such as a quenched and tempered condition, dissolves quickly and gives a consistent case. A coarse structure with large carbides does not; the carbon is still sitting in carbides when the quench arrives, so hardness comes out low and patchy.

This is why parts for induction hardening are frequently supplied in the quenched and tempered condition, and why a batch that suddenly starts failing hardness checks is more often a material condition problem than a machine problem.

Choosing a quench

The quench has to be fast enough to beat the diffusion, but no faster than necessary, because every increment of quench severity adds distortion and cracking risk.

• Water is the most severe. Cheap and effective on plain carbon steels in simple shapes, and the most likely to crack anything complex.
• Polymer solutions sit between water and oil, and the concentration is adjustable, which is why they are the usual choice on production induction lines.
• Oil is the mildest of the three. Suited to alloy steels with good hardenability, where you do not need severity.

We build our hardening machines with an inbuilt quenching water cooling and circulation system, separate from the demineralised water circuit that cools the coil and the power electronics.

Tempering afterwards

As-quenched martensite is at maximum hardness and minimum toughness. For most components that combination is unusable: the part will resist wear excellently right up until the first shock load cracks it.

Tempering reheats the part well below the austenitising range and holds it there. Internal stresses relax, some carbon precipitates out, and toughness rises substantially for a modest loss of hardness. The temperature chosen sets where on that trade you land, and it is dictated by the service condition, not by convenience.

Because induction tempering is fast and local, it can be built into the same machine as the hardening operation, so parts are hardened, quenched and tempered in one flow. That also closes the window between quench and temper, which is when a hardened part is most likely to crack.

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