The grid
| ISO | Colour | Material | Dominant wear | Usual coating route |
|---|---|---|---|---|
| P | Blue | Steel, 160–330 HB | Flank wear, crater | CVD, alumina layer |
| M | Yellow | Stainless, austenitic and duplex | Work hardening, notch wear | PVD, or thin CVD |
| K | Red | Grey and ductile cast iron | Abrasion, comb cracks | Thick CVD |
| N | Green | Aluminium, copper, plastics | Built-up edge, adhesion | PVD, polished; sometimes uncoated |
| S | Brown | Titanium, nickel superalloys | Notching, depth-of-cut wear | PVD, fine grain substrate |
| H | Grey | Hardened, above 45 HRC | Plastic deformation, chipping | PVD, sharp edge, T-land |
Most grades are not pure. A grade described as P20–P35 covers a speed band within steel; a grade described as P/M covers steel and stainless with a performance compromise in each. The grid tells you which compromise you are buying.
Why P and K get CVD, while M and S get PVD
CVD runs at around 1,000 °C and deposits 5–15 µm, including an aluminium oxide layer that insulates the substrate from the heat of the cut. That is exactly what a steel or cast iron application wants: high speed, continuous cut, thermal barrier. The same process leaves a tensile stress in the coating and a slightly rounded edge — both unwelcome in stainless, where a sharp, tough edge matters and the cut is often interrupted.
PVD runs at 400–500 °C, deposits 2–4 µm, leaves compressive stress and preserves a sharp edge preparation. That is why stainless and superalloy grades are usually PVD, and why PVD is also the default for small-diameter solid carbide, where a thick coating would blunt the edge before it ever cut.
The practical consequence for a buyer: if a supplier offers a CVD grade for a stainless interrupted cut, ask why. There may be an answer, but there should be a reason.
The universal grade and what it really costs
A grade that claims both P and M coverage typically delivers around 85% of the life of the dedicated grade in each. That trade is worth making when you hold one grade for six materials and cannot control which one the operator fits. It is not worth making when a single material runs more than a few thousand parts a year — at that volume the missing 15% is the entire margin on the tooling.
Five questions that make any quote comparable
- Which ISO application area is this grade, and what secondary area does it cover?
- What cutting speed range do you recommend in my material, at my hardness, with my coolant?
- CVD or PVD, and what is the coating stack?
- Is this a first-choice grade for my operation, or a universal grade you keep in stock?
- What is the edge preparation — honed, T-land, or as-sintered — and to what tolerance?
With those five answers you can compare a quotation from any supplier without knowing their catalogue. Without them, comparing one supplier's grade code with another's is comparing two words that happen to look technical.
Where buyers get caught
- Price per insert instead of price per edge. A cheaper insert at 60% of the life is not cheaper, and the difference usually exceeds the price gap by a factor of two.
- Buying the hardest grade for roughing. Hardness and toughness trade directly. The hardest grade in the range is the wrong grade for an interrupted cut, however good the brochure is.
- Assuming a grade that works in one machine works in another. Stability changes the answer, and stability differs between machines even of the same model.
- Ignoring the edge preparation. Two inserts can share a grade name and differ in edge hone, which changes life by 20–30% in interrupted cutting. The hone is part of the specification, not a manufacturing detail.