How to Choose a Carbide Grade: P/M/K/N/S/H Explained

Geometry tells you what an insert fits. Grade tells you what it cuts. ISO 513 splits workpiece materials into six main groups — P, M, K, N, S, H — each with an identification colour, an application band and a coating family that suits it. This guide walks through all six groups, the CVD vs PVD choice, and why a grade swap deserves more review than a geometry swap.

Read the ISO 1832 designation of an insert and you know what holder it fits, what corner it presents and what depth of cut it can take. You do not yet know whether it is the right insert for your workpiece. That decision lives in a second label — the carbide grade — and the workpiece materials a grade is designed for are organised by ISO 513 into six main groups, each identified by a single letter: P, M, K, N, S and H.

This guide walks through all six groups, the application band that runs from finishing to roughing inside each one, the CVD-vs-PVD coating choice and why a grade swap on the same geometry should be reviewed more carefully than a geometry swap on the same grade. Every group, band and behaviour statement cited here is rendered the same way on InsertBase's own material group pages and grade records — no manufacturer catalogue, no entry.

What a carbide grade actually is

A carbide grade is not just "the carbide". It is the combination of three things: a cemented carbide substrate (a tungsten-carbide-cobalt powder metallurgy body, sometimes with cubic-carbide additives for hot hardness), a coating family deposited on that substrate, and a set of post-coating edge preparations that decide how sharp, how rounded or how reinforced the cutting edge is. Two inserts can share the same ISO 1832 geometry and the same nominal coating family and still behave differently because the substrate grain size, the cobalt content or the edge hone differs.

That is why a grade record on InsertBase lists five identification fields per manufacturer offer: the grade designation (for example 4325), the descriptive string (for example 4325 / HC / CVD TiCN+Al2O3+TiN), the ISO 513 groups the manufacturer files it under, the coating family and the substrate class. Sandvik's grade 4325 on InsertBase, for example, is filed under ISO 513 groups K and P, coating family CVD, substrate HC — facts pulled from the official catalogue record, not interpolated.

The six ISO 513 groups at a glance

ISO 513 (the classification of carbides by use, not by composition) splits workpiece materials into six main groups. Each group has an identification colour used on catalogue pages and shop-floor charts, a typical workpiece material family and a behaviour the carbide grade has to be designed around:

Group	Workpiece material	Identification colour	Decisive wear mode
P	Steel (plain, alloy, tool)	Blue	Crater and flank wear at high cutting speed
M	Stainless steel (austenitic, duplex, martensitic)	Yellow	Notch wear and built-up edge from work hardening
K	Cast iron (grey, nodular, malleable)	Red	Abrasive wear; thermal load on nodular iron
N	Non-ferrous metals and non-metallics	Green	Built-up edge on soft alloys; abrasive fibres
S	Heat-resistant superalloys and titanium	Brown	Notch wear and plastic deformation at edge
H	Hardened steel and hard materials	Grey	Micro-chipping and thermal cracking

Some grades cross multiple groups. Sandvik's grade 1125, catalogued on InsertBase with a verified source, is filed under P, M, K, N and S — a deliberately broad multi-group general-purpose grade. Multi-group grades cover a wider window of workpiece materials, but they are rarely the optimal choice at the edge of any single group: a dedicated K-grade will outlast a multi-group grade in pure grey-iron facing, and a dedicated S-grade will outlast it in Inconel turning.

Group P — Steel

P covers plain carbon steel, alloy steel, tool steel and cast steel. The chips come off long and continuous, the cutting speeds are high, and the dominant failure modes are crater wear (where the chip rubs the rake face) and flank wear (where the clearance face rubs the cut surface). The identification colour is blue and the application band runs P01 at the fine-finishing end to P50 at the heavy-roughing end.

P-grade carbides need hot hardness to resist crater wear at the temperatures steel allows. CVD multilayer coatings (TiCN/Al₂O₃/TiN) dominate general steel turning because the thick, thermally-stable Al₂O₃ layer is what the chip actually sees, and it survives the temperature. PVD-coated grades appear in finishing and interrupted cuts where edge sharpness matters more than thermal shielding. On InsertBase, the Sandvik 4325 and 4425 records are both filed under P (and K) with CVD coatings — a typical steel-and-cast-iron general-purpose pairing.

Selection rule for P: for stable continuous cuts pick wear-resistant mid-band grades (P15–P35); for interrupted cuts or slender setups where the edge sees shock, step toward the tougher end of the band (P35–P50). A grade that is too wear-resistant in an interrupted cut will chip; a grade that is too tough in a continuous cut will wear out before its body cost is recovered.

Group M — Stainless steel

M covers austenitic, ferritic, martensitic and duplex stainless steels. These materials work-harden as they cut: the chip rubs, the surface under the tool hardens, and the next pass meets a harder layer than the previous one. The identification colour is yellow and the band runs M10 (finishing) to M40 (roughing).

M-grade carbides need toughness more than peak hot hardness, and they need a sharp edge. A dull edge rubs, the surface work-hardens, the dull edge rubs more, and tool life collapses in a feedback loop. PVD-coated fine-grain substrates are typical because PVD layers are thin enough to preserve edge sharpness; sharp, positive geometries with moderate feeds stay under the work-hardened layer rather than fighting it. The Sandvik grade 2035 on InsertBase, a PVD TiAlN+TiN grade filed under M, is a typical stainless-steel-grade record.

Selection rule for M: use sharp, positive geometries with moderate feeds to stay under the work-hardened layer; avoid dwell and light passes that rub rather than cut. Coolant strategy is part of the grade decision — through-tool coolant or high-pressure coolant changes which grade is optimal.

Group K — Cast iron

K covers grey, nodular (ductile) and malleable cast iron. The chips come off short and abrasive, like small flakes of abrasive grit, and the dominant wear mode is pure abrasive wear on the flank face. The identification colour is red and the band runs K01 (fine finishing) to K40 (roughing).

Grey iron cuts at high speed with low cutting forces and responds well to dry cutting with a CVD multilayer grade. Nodular iron is tougher and heats the edge more — the abrasive chips plus higher cutting temperature push the grade toward the tougher end of the K band. CVD-coated grades are standard for dry grey-iron turning; uncoated fine-grain grades appear in finishing where coating thickness would round the edge too much.

On InsertBase, the Sandvik grade 4325 and grade 4425 records are both filed under K (and P) with CVD coatings — a typical cast-iron-and-steel pairing. The selection rule for K is to match grade hardness to the abrasive load: pure grey iron in stable cuts tolerates a harder grade; nodular iron in interrupted cuts wants a tougher one.

Group N — Non-ferrous metals and non-metallics

N covers aluminium alloys, copper, brass, bronze, and also plastics, composites and other non-metallics. Cutting temperatures are low — often very low — and the failure modes are built-up edge on soft aluminium and abrasive-fibre wear on composites. The identification colour is green and the band runs N01 (fine finishing) to N30 (roughing). Cutting speeds are commonly very high, especially on aluminium.

N-grade carbides are often uncoated or polished; where coatings are used they are thin PVD or DLC types, and diamond coatings appear in composite work. The substrate must be sharp and polished — a polished edge prevents the pressure-welded aluminium blob that built-up edge produces, and a high positive rake clears the soft chip without smearing it. A grade with cobalt additives that smear on aluminium is the wrong grade regardless of the application band.

Selection rule for N: choose sharp, polished edges and high-rake geometries; avoid grades with substrate additives that smear on aluminium; for composite work, move toward diamond-coated or fine-grain grades that resist abrasive-fibre wear.

Group S — Heat-resistant superalloys and titanium

S covers heat-resistant superalloys (nickel-base and cobalt-base, Inconel-type materials) and titanium alloys. These materials are hot, strong and abrasive at the cutting temperature, and the cutting speed has to stay moderate because the edge temperature runs close to the substrate's softening point. The identification colour is brown and the band runs S01 (finishing) to S30 (roughing).

The decisive wear modes are notch wear at the depth-of-cut line and plastic deformation of the cutting edge under thermal load. Thin PVD coatings with fine-grain substrates are typical because thick CVD multilayers can spall under the cyclic thermal and mechanical loads S workpieces produce. Edge preparation matters: a slightly rounded or honed edge survives the entry shock of an Inconel cut where a razor-sharp edge would chip on first contact. Generous coolant strategy — ideally through-tool or high-pressure coolant — changes which grade is optimal.

Selection rule for S: use strong edge preparations, moderate speeds and a generous coolant strategy; verify that the machine setup is rigid before committing to a long cut. Sandvik's grade 1125, filed under S among four other groups, is an example of a multi-group grade that lists S in its official record — but for production S work a dedicated S-grade will outperform it.

Group H — Hardened steel and hard materials

H covers hardened steel (typically 45–65 HRC), chilled and hardened cast iron. The chips come off red-hot and very short, the depth of cut is typically light, and the cutting speed is high relative to the hardness — the cut is essentially a finishing pass that generates heat and small chips. The identification colour is grey and the band runs H01 (fine finishing) to H30 (roughing).

H-grade carbides sit at the top of the hardness scale. Fine-grain hard carbide substrates are standard; CBN (cubic boron nitride) tipped solutions appear in the hardest H-band work. Coatings are thin and heat-resistant, and the geometry is typically negative with a strong edge preparation — a small nose radius or a wiper geometry is preferred because the depth of cut is small and the surface finish is the deliverable. The machine must be rigid; a worn slide or a flexible tailstock will produce chatter that chips the edge on the first pass.

Selection rule for H: rigid setups, small nose radii or wiper geometries, negative geometry with strong edge prep; verify machine stability before hard turning and consider CBN-tipped solutions for the upper H band.

The application band: P01 to P50, and why it matters

Each group has an application band — a numbered scale running from fine finishing (small numbers, hard wear-resistant grades) to heavy roughing (large numbers, tough grades). A grade designated P25 sits mid-band in steel turning: general purpose, balancing wear resistance and toughness. The band is not just a label — it encodes the toughness–wear trade-off a grade makes.

Small band numbers (P01, M10, K01...) — hard, wear-resistant grades for light cuts at high speeds. They survive crater and abrasive wear; they do not survive shock.
Mid band numbers (P25, M20, K20...) — general-purpose grades that balance wear resistance and toughness; the most common first choice when the cut is unknown.
Large band numbers (P50, M40, K40...) — tough grades for heavy roughing, interrupted cuts and unstable setups. They survive shock; they wear out faster in continuous finishing.

On a grade record, the band is part of what the manufacturer's catalogue assigns — not always explicit in the grade designation itself. The InsertBase grade records list the ISO 513 groups filed by the manufacturer; the application band within that group is read from the catalogue's application chart, which is why grade mappings on InsertBase are only made within the same group and band — a P25 steel grade is never mapped to a K10 cast-iron grade, even if both are nominally "general purpose".

CVD vs PVD: what changes for you

The two coating families a grade record names are CVD (chemical vapour deposited) and PVD (physical vapour deposited). They are not interchangeable. The difference is in how the coating is laid down and what that allows:

CVD coatings are deposited at high temperature (around 1000 °C) and grow thick — typically 5–15 µm of TiCN/Al₂O₃/TiN multilayer. The thick, thermally-stable Al₂O₃ layer is what makes CVD the standard choice for high-speed steel and cast-iron turning where the cutting temperature is high and crater wear is the failure mode. The cost of that thickness is a slightly rounded edge — CVD coatings grow on every surface, including the cutting edge, and that growth rounds it.
PVD coatings are deposited at low temperature (around 500 °C) and grow thin — typically 2–5 µm of TiAlN, TiN or DLC. The thin layer preserves edge sharpness, which is why PVD dominates stainless, finishing, interrupted cuts and non-ferrous work where a sharp edge is part of the grade's job. PVD coatings do not provide the thermal shielding CVD does, so a PVD grade in a high-temperature continuous steel cut will wear faster than its CVD counterpart.

The substrate also matters: an HC substrate (hard cemented carbide) is the standard turning body; an HM substrate (micro-grain) appears where edge sharpness is critical. The grade record's descriptive string carries all of this — for example, Sandvik grade 2035 on InsertBase reads 2035 / HC / PVD TiAlN+TiN, which tells you in one line that this is a sharp-edge PVD grade on a hard substrate, suited to stainless and finishing work.

Why geometry swaps are safe but grade swaps need review

A geometry swap — replacing CNMG120408 with CNMG120412 on the same holder — changes the nose radius from 0.8 mm to 1.2 mm and leaves everything else the same. The insert still fits the holder, the corner angle is the same, the chip-breaker family is the same. You adjust feed and depth of cut for the new nose radius; the ISO 1832 decoder confirms the geometry change is local and predictable.

A grade swap on the same geometry is a different decision. Replacing Sandvik's grade 2035 (PVD, M-group, sharp-edge finishing) with grade 4425 (CVD, P/K-group, wear-resistant roughing) on the same CNMG120408 geometry changes the substrate, the coating, the edge preparation and the workpiece the insert is designed to cut. It is not a like-for-like swap; it is a different cutting tool on the same body.

That is why cross-brand grade equivalents on InsertBase follow a strict three-level framework rather than a flat "equivalent" table:

FULL — same ISO 513 group, same application band and same coating family in both official catalogues. A safe substitution with normal cutting-data review.
PARTIAL — same application band but different coating or edge preparation. Usable, but the cutting data and edge-prep implications need review before the swap goes into production.
VERIFY — quoted in aftermarket cross-reference charts but not confirmed in both official catalogues. Treat as a hint, not as a verified equivalent; confirm against the manufacturer's catalogue before relying on it.

If no reliable equivalent exists, the grade record on InsertBase says so directly rather than guessing. The cross-reference finder applies the same framework: enter a code, see what InsertBase has catalogued, and request a verified mapping through the inquiry box on any model page if the answer is "no direct equivalent — verify with catalogue".

Reading a grade record on InsertBase

Every grade record on InsertBase carries the same fields, pulled from the manufacturer's official catalogue with a source link and an access date:

Manufacturer and grade designation — for example, Sandvik 4325.
Catalogue description — the full descriptive string, for example 4325 / HC / CVD TiCN+Al2O3+TiN, which encodes substrate, coating family and coating composition.
ISO 513 workpiece groups — the groups the manufacturer files the grade under. Multi-group grades list more than one letter.
Coating family — CVD or PVD, the most important single field for predicting cutting behaviour.
Substrate — typically HC (hard cemented carbide) or HM (micro-grain).
Source — a link to the manufacturer's official catalogue record and the date it was accessed.

Below the identification block, the record lists every catalogued insert the grade is offered on, with the ordering code, chipbreaker, ISO material group, operation and source link for each. A separate cross-brand equivalents section lists any verified equivalents — or honestly states that none are published yet, in which case the inquiry box on any model page is the route to a verified mapping returned with sources.

Where to go next

Read the ISO 1832 designation walkthrough first if you have not — geometry is the other half of the grade decision, and the two guides are written to be read together.
Open the ISO 513 material groups hub and the per-group pages (P, M, K, N, S, H) for the catalogued grades filed under each group.
Browse the carbide grade index for the full list of grade records on InsertBase, each linking back to every model it is offered on.
Use the ISO 1832 decoder for any code of your own, and the cross-reference finder to see what catalogued models match.
Read the editorial and correction policy for how grade and source data is gathered, cited and corrected on InsertBase.

If a specific grade mapping you need is not catalogued yet, the inquiry box on any model page reaches the editorial team directly — verified mappings are returned with their sources, not guessed.