Carburising
Last updated 2026-09-28 · route revision 3
We carburise low-carbon grades to get a hard, wear-resistant case over a tough core - what gears, shafts and splines need under impact.
The case cycle
Carburising runs at 920–940 °C with a carbon potential of 0.8–1.2 % C. At 930 °C the case grows by about 0.1 mm an hour, and that is the basis of the sizing: we turn the ordered depth into a soak time, not the other way round.
| Case, mm | Soak at 930 °C |
|---|---|
| 0.3–0.5 | 3–5 h |
| 0.6–0.8 | 6–8 h |
| 0.9–1.2 | 9–14 h |
We do not go past 1.2 mm: holding a charge for a full day is wasteful and the grain has time to coarsen. A thicker case on the drawing is a job for someone else.
Quench and temper after the case
After the case the part is quenched from 820–840 °C into oil and given a low temper at 180 °C for two hours. The surface comes out at 58–62 HRC, and the core stays within 30–42 HRC.
| Grade | Case temp, °C | Case | Core |
|---|---|---|---|
| 20 | 920–940 | 58–62 HRC | 30–42 HRC |
| 20Kh | 920–940 | 58–62 HRC | 30–42 HRC |
| 18KhGT | 920–940 | 58–62 HRC | 30–42 HRC |
| 12KhN3A | 920–940 | 58–62 HRC | 30–42 HRC |
| 20KhGNM | 920–940 | 58–62 HRC | 30–42 HRC |
Grades we take
For carburising we run 20, 20Kh, 18KhGT, 12KhN3A and 20KhGNM - low starting carbon and the right core hardenability. We favour 18KhGT for its fine grain and 12KhN3A where impact work matters.
For every charge we cut a witness microsection and read case depth and structure under the microscope. A 0.9–1.2 mm case takes up to six working days. Measurement detail is on the control page.