Carburising vs nitriding: what a screw, nut or bolt actually gets
We case-harden with carbon on a continuous mesh-belt line and we do not run nitriding. This page shows why most small parts take the first route.
Carburising vs Nitriding
Short answer: almost every small fastener gets carburising; nitriding is a different line entirely
Carburising diffuses carbon at roughly 880–950 °C and then quenches to form martensite, giving about 58–62 HRC at the surface and a case you can set from roughly 0.1 mm to 1.5 mm and beyond — at the price of distortion you have to allow for. Nitriding works at roughly 500–570 °C with no quench, so distortion is very low and the surface is far harder at about 900–1100 HV, but the case is shallower, the cycle is much longer, and it needs a steel containing nitride-forming elements, which ordinary low-carbon fastener wire is not. We offer the carbon route only; a nitriding callout has to go to a specialist.
Carburising versus nitriding
| Decision factor | Carburising (our line) | Nitriding (not offered here) |
|---|---|---|
| What diffuses in | Carbon | Nitrogen, plus carbon in the nitrocarburising variants |
| Typical temperature | About 880–950 °C, in the austenitic range | About 500–570 °C, ferritic — no phase change |
| Quench needed? | Yes, followed by a low temper around 150–200 °C | No quench; the part cools from the furnace |
| Typical surface hardness | About 58–62 HRC (roughly 650–750 HV) | About 900–1100 HV in steels with nitride formers |
| Case depth | Roughly 0.1–1.5 mm and beyond, reported as case depth at an agreed hardness criterion | Roughly 0.05–0.6 mm, reported as nitriding depth referenced to core hardness |
| Distortion | Higher — high temperature plus a quench; allow for it on the drawing | Very low — low temperature and no quench |
| Steels that respond | Low-carbon cold-heading grades such as SWCH10A–22A or the SCM415 family | Needs chromium, aluminium, molybdenum or vanadium present, and the core is normally hardened and tempered above the nitriding temperature first |
| Cycle time and lot economics | Continuous flow on a mesh belt — suits small parts in volume | Hours to tens of hours per furnace load — suits specialised, low-volume work |
| Layer to watch | Control carbon overshoot, intergranular oxidation and retained austenite | A hard but brittle compound (white) layer can form; state whether it is accepted or must be removed |
| Fit for small fasteners | Yes — self-tapping screws, self-drilling screws, nuts and small parts in bulk take this route | Rarely — low-carbon fastener wire responds poorly and the cycle does not suit large lots |
| Who can run it | In-house on the continuous belt line, with hardness checks in our own lab | We have no gas, plasma or salt-bath nitriding line; this goes to a specialist |
How to choose
Carburising
Choose carburising when the part is a low-carbon steel that needs a wear-resistant surface over a tough core, needs enough case to carry contact load, and comes in volume where a planned distortion allowance is acceptable — self-tapping and self-drilling screws, nuts and rubbing pins.
Nitriding
Choose nitriding when the requirement is a very high surface hardness in HV with almost no distortion, the part is an alloy steel with nitride formers that has already been hardened and tempered, and a shallower case plus a long cycle are acceptable. That work belongs at a shop with a nitriding line.
Limits before this goes on the drawing
- We do not offer nitriding or nitrocarburising and will not silently substitute carburising for it — surface hardness, case depth, distortion and wear behaviour are not equivalent.
- HRC, HV and case-depth figures are typical guidance for small parts, not guarantees; confirm them with a hardness traverse on a real section.
- Case depth must be quoted together with the hardness criterion used to measure it, or the same number will mean different things to the maker and the inspector.
- On small, thin parts the case is only tens to hundreds of microns; grinding or polishing after hardening can remove it.
- If zinc plating follows hardening, assess hydrogen embrittlement risk and set the de-embrittlement bake against the hardness reached.
- High-carbon steel and parts already through-hardened do not need carburising on top; send the material data first so a furnace cycle is not wasted.
What to send us for review
- 1Part type, material and mill certificate if you have one
- 2Required surface hardness and case depth, with the criterion used to measure it
- 3Distortion limits and where they will be checked
- 4The standard or spec the drawing refers to, and the report you need with the parts
- 5Lot quantity or weight, and the finish that follows — zinc plating, phosphate or none
Frequently asked questions
Do you offer nitriding?
No. We have no gas, plasma or salt-bath nitriding line. A drawing that calls for nitriding or nitrocarburising has to go to a specialist shop. What we run is carbon case hardening for screws, nuts and bolts in bulk.
Which process will a small screw actually get?
In practice the carbon route — carburising or carbonitriding on a continuous mesh-belt furnace — because fastener wire is low-carbon steel, the core toughness comes from the quench, and the cycle has to be economic across large lots.
Why does nitriding distort less?
It runs far cooler and skips the quench, so there is no phase change to move the part. The trade-off is a shallower case and a much longer cycle.
Can carburising replace a nitriding callout?
It should not. The two give different surface hardness, different case depth and different distortion. If the change is genuinely needed, the drawing owner has to re-check the part function and approve it in writing.