The six material groups
Each group describes the workpiece and implies a coating direction. The colour code is a convention that most manufacturers follow, which makes it a useful cross-reference between brands.
| Group | Workpiece | Colour code | Typical coating direction |
|---|---|---|---|
| P | Carbon and alloy steel, ferritic and martensitic stainless, cast steel | Blue | CVD TiCN and Al2O3 for wear resistance |
| M | Austenitic and duplex stainless, manganese steel | Yellow | PVD TiAlN or AlCrN with a sharp edge |
| K | Grey, nodular and compacted graphite iron | Red | CVD or TiCN; toughness for casting skin |
| N | Aluminium, copper, brass, plastics, composites | Green | Polished, uncoated or DLC; sharp and non-sticking |
| S | Titanium, nickel and cobalt alloys | Brown | PVD AlTiN or AlCrN for hot hardness |
| H | Hardened steel, chilled cast iron | Grey or black | Ceramic, CBN, or PVD-coated carbide |
Moving within a group: wear against toughness
Inside a group the grades are graded on a single axis. A lower number is harder and resists abrasive and crater wear, which suits continuous cutting where the thermal and mechanical load is steady. A higher number is tougher and resists chipping and thermal cracking, which suits interrupted cuts, castings with a hard skin, and unstable setups.
The practical consequence is that a harder grade can wear longer in a stable cut and still fail earlier than a tougher one in a cut that is interrupted, because the failure mode changes from wear to chipping.
Where the CVD and PVD split falls
CVD coatings are applied at high temperature and are thicker, which gives excellent wear resistance in continuous turning of steel and cast iron. They are slightly thermally loaded and blunter, which is why they suit turning better than milling.
PVD coatings are applied cooler and are thinner with a sharper edge, which suits stainless, titanium, interrupted cuts, and most milling and threading. The general rule is CVD for continuous turning, PVD for anything interrupted or sharp-edged.
Grading beyond carbide
Above roughly 45 to 50 HRC the carbide grades run out of capability for turning and the choice moves to ceramic or CBN. Ceramics suit continuous high-speed turning; CBN suits hard turning where dimensional accuracy and finish both matter and the cost per edge can be justified.
For milling, coated carbide remains workable to about 60 HRC with an AlCrN coating and a rigid setup. Beyond that, grinding or EDM is generally the practical process rather than a different insert.
Frequently asked questions
How do I know which ISO group my material is in?
It follows from the material, not the application: P for steel and ferritic stainless, M for austenitic and duplex stainless, K for cast iron, N for aluminium and non-ferrous, S for titanium and nickel alloys, and H for hardened steel. If in doubt, tell us the material designation and we will place it.
Should I pick a harder or a tougher grade?
Harder for continuous cutting where the load is steady and wear is the failure mode. Tougher for interrupted cuts, castings with a hard skin, and setups that are not fully rigid, where chipping is the failure mode you are guarding against.
What do the coating colours mean?
They are a convention mapping the ISO groups onto coating families: blue for steel, yellow for stainless, red for cast iron, green for aluminium, brown for heat-resistant alloys, and grey or black for hardened materials. Most manufacturers follow it, which makes cross-referencing between brands easier.
When should I move from carbide to ceramic or CBN?
Around 45 to 50 HRC for turning. Below that, a tougher coated carbide grade is usually more economical. Above it, ceramic suits continuous turning and CBN suits hard turning where accuracy and finish both matter.
Can I use one grade for everything?
No, and trying to produces mediocre results across the range. A general-purpose grade will cut most materials acceptably but will be outperformed everywhere by the correct grade, and in difficult materials the difference is the difference between working and not working.