Induction Hardening Gears and Shafts: What to Specify on Your Drawing
"Induction harden teeth" is not a specification. Here are the six things to put on a drawing so hardened gears, shafts and splines come back the way you need them.
"Induction harden teeth" is not a specification. Here are the six things to put on a drawing so hardened gears, shafts and splines come back the way you need them.
"Induction harden teeth" is a note we still see on many drawings. It tells the supplier what process to use, but not what result is needed. Two workshops can both follow that note and deliver parts that behave very differently in service. When a part then wears early or cracks, there is no clear standard to check it against.
A good hardening note removes that doubt. This article covers what to state on a drawing or purchase order when gear teeth, shaft journals, splines or sprocket teeth need induction hardening.
An induction coil heats only the surface layer of a steel part, very quickly, and the part is then quenched. The heated layer becomes hard, while the core keeps its original strength and toughness. Because the heat is local, only the areas that need it are hardened, and distortion is generally lower than with hardening the whole part in a furnace.
Induction hardening needs enough carbon in the steel to harden. Medium-carbon and alloy steels such as EN8, EN9, EN19 and EN24 are common choices. Also state the condition before hardening. For shafts and pinions that carry heavy torque, EN19 or EN24 is often supplied hardened and tempered first, so the core is strong before the surface is hardened.
Give a range, for example 50–55 HRC, rather than a single figure. A single number cannot be held exactly on every part, and a range makes inspection clear. The achievable hardness depends on the carbon content of the steel, so check the range against the grade you have chosen.
State the effective case depth as a range, such as 1.5–2.5 mm, and the hardness level at which the depth is measured. A common convention is to measure to a hardness equal to 80% of the minimum specified surface hardness. Without that second value, "2 mm case" can be read in more than one way.
Mark the hardened zones clearly on the drawing: which journals, which teeth, which length of spline. Just as important, mark the areas that must stay soft. Keyways, threads, centre holes, circlip grooves and areas that will be machined or drilled later should normally be left unhardened.
Freshly hardened steel is under high internal stress. A low-temperature temper after hardening reduces the risk of cracking in service with only a small drop in hardness. Write "temper after hardening" on the drawing so it is not skipped.
Agree the inspection before the job starts. Surface hardness is normally checked on each part or on a sample from each batch. Case depth is checked on a sectioned sample part, usually on the first-off part of a new job. For safety-critical parts, crack detection after hardening can also be specified.
| Steel | Type | Suitable for induction hardening? | Typical use |
|---|---|---|---|
| EN8 | Medium-carbon steel | Yes | General shafts, pins, lightly loaded gears |
| EN9 | Higher-carbon steel | Yes, reaches higher surface hardness | Sprocket teeth, wear parts |
| EN19 | Chromium-molybdenum alloy steel | Yes | Pinion shafts, gears, heavily loaded shafts |
| EN24 | Nickel-chromium-molybdenum alloy steel | Yes | High-strength shafts and gears |
| EN353, 20MnCr5 | Low-carbon case-carburising steel | No, normally case carburised instead | Carburised gears |
| Mild steel | Low-carbon steel | No, too little carbon | Not for hardened wear surfaces |
If you are unsure which grade suits a part, tell us the load, speed and application and we will suggest options.
For gear teeth, the hardened pattern matters as much as the depth. On small teeth, the whole tooth often hardens through to the root, which gives good wear resistance but a less tough tooth. On larger teeth, the aim is usually a hardened layer following the tooth flank and root, with a tough core in the middle of the tooth. The right pattern depends on the module, the steel and the load, so tell us the module and the duty and we will confirm what is achievable for that part.
The same applies to chain sprocket teeth, where the aim is wear resistance at the tooth flanks where the rollers seat, and to spline shafts, where the spline must be hard but the shaft must still take shock torque.
Induction hardening moves a part less than furnace hardening, but it does not remove movement completely. Long, slender shafts can bow slightly, and bores can close up a little. For tight tolerances on bearing journals or tooth geometry, leave a grinding allowance and grind after hardening. Cut keyways and threads in the soft zones, and ideally finish them before hardening, so they are not affected by the heat.
The note below is only an example of the level of detail that helps. The actual values must suit your part, steel and load.
We carry out induction hardening at our unit in Gandevi, Gujarat, on gears and pinion shafts we make, and as job work on parts machined elsewhere. For a quote, send the drawing or a photo of the part, the steel grade, the areas to be hardened, the required hardness and case depth, and the quantity. If you are ordering a copy of a worn part, our guide on measuring a gear from a sample covers the other details to include. You can reach us through the contact page.
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