The steel range and what it changes
ISO P covers carbon and alloy steels, ferritic and martensitic stainless, and cast steel. Within that group the practical differences are hardness and the tendency to form a built-up edge at low carbon content.
| Steel type | Behaviour | Tooling direction |
|---|---|---|
| Low-carbon and free-cutting | Soft, gummy, built-up edge | Sharp polished edge, higher speed, TiAlN |
| Medium-carbon structural | The baseline case | General-purpose TiAlN or CVD, square end mill, 3-flute drill |
| Alloy steel 4140 and similar | Higher strength, more heat | CVD turning grades, AlTiN milling for interrupted cuts |
| Tool steel up to 45 HRC | Abrasive | AlTiN or AlCrN, stronger edge, reduced speed |
| Hardened tool steel above 45 HRC | Very abrasive, high cutting temperature | AlCrN hard milling, ceramic or CBN turning |
Why low-carbon steel is not the easy option
Soft, ductile steel produces a continuous chip that welds to the cutting edge and forms a built-up edge. The effective geometry then changes several times a second, which produces inconsistent finish, size scatter and a tool that appears to wear unpredictably.
The remedy is the opposite of what intuition suggests: a sharp, polished edge run faster rather than a hard coating run slowly. Raising the speed reduces the built-up edge by getting the chip away before it can weld.
Turning steel: grades and coatings
CVD-coated P grades dominate steel turning because they resist the crater wear that continuous cutting produces on the rake face. The thick aluminium oxide layer is what provides the wear resistance, and it works best in a continuous cut where the thermal load is steady.
For interrupted cuts in steel, a tougher grade within the P range resists chipping better than a harder one, even though it wears faster in steady conditions. The choice is between wear resistance and edge security, and the operation decides which one governs.
Milling steel: the interrupted cut problem
A milling cutter enters and leaves the cut on every revolution, so the edge is thermally cycled and mechanically shocked many times a second. That is why milling coatings are PVD rather than CVD: a thinner coating with a sharper edge survives the cycling better than a thicker, more brittle one.
For alloy steel and interrupted cuts, AlTiN provides the hot hardness while AlCrN adds oxidation resistance. Where the steel is gummy rather than hard, a higher helix angle and a sharper edge matter more than the coating.
Frequently asked questions
What coating should I use for machining steel?
TiAlN for general milling and drilling under coolant, CVD-coated P grades for continuous turning, and AlTiN or AlCrN where the steel is hard or the cut is interrupted and the edge runs hot.
Why does low-carbon steel machine badly?
Because it is soft and ductile, so the chip welds to the cutting edge and forms a built-up edge that changes the effective geometry continuously. The fix is a sharp polished edge run at higher speed, not a harder coating run slowly.
When should I use a ceramic or CBN insert instead of carbide?
When the steel is hardened beyond roughly 45 to 50 HRC. Ceramics suit continuous turning of hard steel at high speed, and CBN suits hard turning where dimensional accuracy and surface finish both matter. Both are considerably more expensive per edge, so they need the volume to justify it.
Why do milling tools use PVD coatings rather than CVD?
Because a milling edge is thermally and mechanically cycled on every revolution. PVD coatings are thinner with a sharper edge and survive that cycling better, while the thicker CVD coating that excels in continuous turning is more prone to chipping.
How do I stop built-up edge on soft steel?
Raise the cutting speed rather than lowering it, use a sharp polished edge, and ensure the coating is smooth. A built-up edge is a symptom of cutting too slowly or rubbing rather than cutting.