A gear tooth must not wear – so it has to be hard. The same tooth must not break under impact – so it has to be tough. One material cannot be both at once. The answer is to harden only the outside of the part and leave the inside tough. This article compares the three ways of doing that: case hardening, nitriding and induction hardening.
Why harden only the surface?
In steel, hardness and toughness pull against each other. Make a part 60 HRC all the way through and it will not wear – but it snaps like glass. Leave it at 30 HRC and it will not break – but it wears out quickly.
Surface hardening resolves the conflict: a hard skin outside, a tough core inside. The skin resists wear, the core carries impact and bending. There is a bonus too: because the hard layer grows in volume as it forms, it leaves the surface in compression, and that extends fatigue life. Why hardening a part right through is not always possible is explained in Quenching and Hardenability.
1. Case hardening (carburising)
A low-carbon steel is held for a long time at about 900-950 °C in a carbon-rich atmosphere. Carbon diffuses inward from the surface and raises the carbon content of the outer layer. The part is then quenched: the surface now has enough carbon to form martensite and hardens, while the core stays low-carbon and therefore tough.
- Case depth: typically 0.2-2 mm. Longer time means deeper case, but it follows a square root – roughly four times the time for twice the depth.
- Surface hardness: around 58-63 HRC.
- Advantage: a thick, durable case. The best method for heavily loaded gears.
- Drawback: high temperature followed by a quench means distortion. Precision surfaces are ground afterwards.
- Watch out: areas that must stay soft are masked (copper plating or a stop-off paste), or the case is machined away afterwards.
2. Nitriding
Here it is nitrogen, not carbon, that diffuses into the surface, and the process runs at a much lower temperature – around 500-550 °C. That is below the austenite range, which means no phase transformation and no quench.
The hardness comes from nitrogen combining with aluminium, chromium, molybdenum and vanadium in the steel to form very hard nitrides. So nitriding does not work on every steel – the material has to contain those elements. Die steels such as 1.2344 and 1.2367 nitride well. You can find those codes in the other systems, with their compositions, in the Steel Grade Cross-Reference tool.
- Case depth: very thin, typically 0.1-0.6 mm.
- Surface hardness: up to 900-1200 HV – harder than case hardening.
- Advantage: low temperature and no quench mean very little distortion. It can be applied to a finished part. It also improves corrosion resistance and fatigue life.
- Drawback: the thin case cannot carry heavy point loads; if the material beneath collapses, the hard skin cracks. Process times are long.
- Watch out: the nitriding temperature (500-550 °C) must be below the part’s tempering temperature, or the core will temper back and soften.
3. Induction and flame hardening
Nothing is added to the material here. The part already contains enough carbon to harden (medium carbon – 1040, 4140 and the like); the chosen area is simply heated fast and quenched immediately.
In induction hardening a coil induces eddy currents in the surface and takes just the outer layer to austenitising temperature within seconds, after which water is sprayed on. Flame hardening does the same job with an oxy-fuel torch.
- Case depth: 0.5-5 mm – adjustable, the most flexible of the three.
- Surface hardness: depends on the steel’s carbon, typically 50-60 HRC.
- Advantage: very fast, suits volume production, and can be applied to one area only – the bearing seat of a shaft, for instance. No furnace needed.
- Drawback: a coil has to be made for the part geometry. On complicated shapes the hardness may not distribute evenly.
The three side by side
| Case hardening | Nitriding | Induction | |
|---|---|---|---|
| Source of hardness | Added carbon | Added nitrogen (nitrides) | Carbon already present |
| Temperature | 900-950 °C | 500-550 °C | Austenitising, at the surface |
| Quench | Yes | No | Yes (surface only) |
| Starting material | Low carbon (8620, 1020) | Contains Al/Cr/Mo/V (1.2344) | Medium carbon (1040, 4140) |
| Case depth | 0.2-2 mm | 0.1-0.6 mm | 0.5-5 mm |
| Surface hardness | 58-63 HRC | up to 900-1200 HV | 50-60 HRC |
| Distortion | Considerable | Very little | Moderate, localised |
| Typical work | Gears, shafts, pins | Dies, extruder screws, crankshafts | Bearing seats, sliding surfaces, gear teeth |
How is case depth measured?
On this kind of work, surface hardness alone is not enough information. What matters is how deep the hardness reaches. The measurement goes like this:
- A section is cut perpendicular to the surface and the cut face is polished.
- Hardness is measured at set intervals inward from the surface using microhardness in Vickers.
- The depth at which hardness falls to a defined limit (550 HV, for example) is recorded as the effective case depth.
Frequently asked questions
Which method should I choose?
Three questions settle it. Is the part already at finished size? If yes, nitriding, because distortion is minimal. Are there heavy point loads? If yes, case hardening, because a thick case is needed. Does only one area need to be hard? Then induction.
I had a part nitrided and the hardness did not take. Why?
The material was probably unsuitable. Nitriding hardness comes from nitrogen combining with aluminium, chromium, molybdenum or vanadium in the steel. A plain carbon steel has none of these, so no meaningful hardness forms.
My part distorted after case hardening – can I correct it?
Only to a limited extent. The right approach is to leave a machining allowance before the process and grind to size afterwards. But calculate that grinding allowance against the case depth – grind too much and you remove the hard case altogether.
Can I machine a hardened surface afterwards?
Not with ordinary cutting tools. A surface at 58-63 HRC can only be worked by grinding, honing or with CBN inserts. That is why all holes, slots and threads are finished before hardening.
Does surface hardening really improve fatigue life?
Yes. Because the hard layer grows in volume as it forms, the surface is left in compression. Fatigue cracks usually start at the surface, and that compressive stress makes them harder to start.