Part preparation
Last updated 2026-09-29 · note revision 4
Heat treatment changes size and surface, so a part has to arrive with the right allowance and no stress raisers. What is set before the furnace cannot be fixed after it.
Per-side allowance
The allowance exists so the finishing cut can take off distortion, scale and the damaged surface layer. How much depends on how hard the operation heats and warps the part.
| Operation | Per-side allowance, mm |
|---|---|
| Quenching | 0.3–0.5 |
| Carburising | 0.5–0.8 |
| Normalising, annealing | 0.2–0.3 |
| Stress relief | 0.1–0.2 |
The decarburised layer
During a long soak in the air atmosphere of a chamber furnace, carbon burns off the surface and leaves soft ferrite 0.1–0.3 mm deep. That layer is built into the allowance and ground off afterwards; this is why the carburising allowance is larger.
Stress raisers and geometry
A sharp corner, an unradiused fillet, a hole edge and a deep tool mark are where a quench crack starts. We ask for fillets no smaller than R 0.5 mm and for coarse tool marks to be removed before the furnace.
- We do not quench a finished thread - it warps and cracks in the roots.
- Put centre-punch marks outside the working zone: under a quench they turn into a stress raiser.
- Long parts arrive with room for straightening - residual bow is corrected after tempering.
If you are unsure of the allowance for a particular part, send the drawing to order@ironquench.cloud and we will size it from the section. What happens when the allowance was too small is on the defects page.