Start with the ISO 513 group (P for steel, M for stainless, K for cast iron, N for aluminum, S for superalloys, H for hardened steel), then select the sub-number by operation severity: finishing (01–10), general (15–25), roughing (30–50). A P25 grade with CVD TiCN/Al₂O₃/TiN multi-layer covers approximately 70% of general steel turning; an M20 grade with PVD TiAlN handles most stainless work. Correct grade matching can extend carbide insert life from under 10 minutes to 60 minutes on the same geometry in steel turning under comparable conditions.
For a complete overview of cutting tool types, geometries, and coatings, see the cutting tools complete guide.
ISO Application Groups
The ISO 513 standard classifies cutting tool materials into six application groups, each designated by a letter and color code. This is the starting point for every grade selection decision.
| ISO Group | Color | Target Materials | Key Requirement |
|---|---|---|---|
| P (Steel) | Blue | Carbon steel, alloy steel, ferritic stainless | Crater wear resistance |
| M (Stainless) | Yellow | Austenitic stainless, duplex, cast steel | Toughness + heat resistance |
| K (Cast Iron) | Red | Gray iron, ductile iron, malleable iron | Abrasion resistance |
| N (Non-Ferrous) | Green | Aluminum, copper, brass, plastics | Sharp edge, low friction |
| S (Superalloys) | Brown | Titanium, Inconel, cobalt alloys | Heat resistance, edge strength |
| H (Hardened) | Gray | Hardened steel >45 HRC, chilled cast iron | Hot hardness, wear resistance |
The two-digit number indicates the balance between hardness and toughness. Lower numbers (P01) are harder but more brittle. Higher numbers (P40) are tougher but wear faster. Most general machining falls in the P20-P30 range.
If you're unsure which grade to start with for steel, P25 with a CVD multi-layer coating is the safe default across most turning applications. For milling and interrupted work on the same material, shift to PVD-coated P20-P30 grades.
ISO 1832 defines the alphanumeric designation system for indexable inserts — encoding insert shape, clearance angle, tolerance class, and fixing hole geometry into a standardized code that appears on every insert package (e.g., CNMG 120408). The ISO 513 application group and ISO 1832 insert designation together form the two-standard system that links a specific insert geometry and grade to a machining application.
Substrate Composition
Carbide inserts are sintered composites of tungsten carbide (WC) grains held together by a cobalt (Co) binder. The grain size and binder percentage determine the grade's fundamental properties.
Grain Size Effects:
- Submicron (<0.5 um): Maximum hardness and edge sharpness. Used for finishing and hard materials.
- Fine grain (0.5-1.0 um): Good balance of hardness and toughness. General-purpose grades.
- Medium grain (1.0-3.0 um): Higher toughness at the expense of wear resistance. For interrupted cuts.
Binder Content Effects:
- 6% Co: Very hard, brittle. Finishing grades (P01-P10).
- 10% Co: Balanced. General-purpose grades (P20-P30).
- 12-15% Co: Very tough. Heavy roughing grades (P40-P50).
Grain Size and Speed Relationship
Finer grain sizes maintain edge integrity at higher cutting temperatures, enabling higher cutting speeds. Manufacturer data suggests submicron grades typically run 20-30% faster than medium-grain grades in equivalent conditions before reaching equivalent wear levels.
Coating and Grade Interaction
The substrate grade and coating work as a system. Choosing them independently leads to suboptimal performance.
✦ CVD-Coated Grades (P15-P35 typical)
- Thick Al2O3 layer provides thermal barrier
- Best for continuous turning of steel and cast iron
- Handles high cutting speeds (200-400 m/min)
- Lower cost per edge in production runs
✦ PVD-Coated Grades (P10-P25 typical)
- Thin coating preserves sharp edge geometry
- Best for milling, grooving, threading, profiling
- Preferred for small inserts and positive geometries where CVD rounding reduces edge sharpness
- Superior in interrupted cuts and variable engagement
A common mistake is pairing a tough, high-cobalt substrate (P35-P40) with a CVD coating designed for high-speed continuous cutting. The substrate cannot support the speeds the coating enables. Similarly, pairing a hard P10 substrate with a thick CVD coating wastes the substrate's potential edge sharpness because CVD rounds the cutting edge.
In most steel turning applications, CVD coatings favor high cutting speed while PVD coatings favor edge sharpness in interrupted operations. Match the coating technology to the dominant operating condition — continuous cutting at high speed, or interrupted cutting that demands edge strength — not to the substrate alone. TiCN (titanium carbonitride) forms the wear-resistant base layer in most CVD multi-layer stacks for steel, and TiN (titanium nitride) is the gold-colored top layer that signals edge wear visibly as it is worn away during use. Aluminum oxide (Al₂O₃) is used as the intermediate thermal barrier layer in CVD multi-layer coatings for steel and cast iron turning because its low thermal conductivity (~30 W/m·K, versus ~60 W/m·K for TiCN) limits heat transfer into the substrate at cutting speeds above 200 m/min.
Material-Specific Grade Recommendations
Carbon and Alloy Steel (P group):
- Turning: P15-P25 with CVD TiCN/Al2O3/TiN multi-layer
- Milling: P20-P30 with PVD TiAlN
- Interrupted turning: P25-P35 with PVD or thin CVD
Stainless Steel (M group):
- Use M15-M25 grades with PVD TiAlN or AlCrN coating
- Higher cobalt content resists notch wear from work-hardened surface
- Avoid uncoated grades — built-up edge degrades the finish
TiAlN (titanium aluminum nitride) coatings are preferred over TiN for stainless steel because their higher oxidation resistance (stable to ~800°C) reduces crater wear during the elevated cutting temperatures that austenitic stainless generates. AlCrN (aluminum chromium nitride) offers similar heat resistance with improved performance in high-feed interrupted conditions, making it the better choice over TiAlN when stainless steel milling involves variable engagement.
Cast Iron (K group):
- Gray iron: K10-K20 with CVD Al2O3 at high speeds
- Ductile iron: K20-K30 with thicker CVD coating for abrasion
- Use ceramic inserts (Si3N4) for roughing gray iron above 500 m/min
Aluminum (N group):
- Uncoated polished carbide or PCD (polycrystalline diamond) tipped
- DLC coating prevents built-up edge
- Avoid TiAlN — aluminum has chemical affinity with the coating, causing rapid buildup
PCD (polycrystalline diamond) inserts are used for high-volume aluminum machining because diamond's extreme hardness (~8,000 HV) resists the abrasive wear that silicon-containing aluminum alloys (2000, 6000, 7000 series) cause at cutting speeds of 500–1,500 m/min. DLC (diamond-like carbon) coatings serve the same anti-buildup function at lower cost for medium-speed aluminum operations, providing a low-friction surface that prevents aluminum from welding to the insert edge.
The best insert grade for steel is rarely a single answer — it depends on whether the operation is continuous or interrupted, and on cutting speed. P25 with CVD covers most steel turning, P30 with PVD fits milling, and P30-P40 with PVD handles interrupted cuts.
Avoid Cross-Group Application
Using a P-group grade on stainless steel or an M-group grade on cast iron will typically underperform, especially in austenitic stainless and ductile cast iron where the wear mechanisms differ most versus the correct ISO group. Grades are engineered for specific wear mechanisms. Steel creates crater wear on the rake face (P grades resist this). Cast iron causes abrasive flank wear (K grades resist this). Using the wrong group means the grade is optimized against the wrong failure mode.
Practical Selection Sequence
- Identify workpiece material and match to ISO group (P, M, K, N, S, H)
- Determine operation severity: finishing (01-10), general (15-25), or roughing (30-50)
- Select coating type: CVD for continuous, PVD for interrupted
- Start at manufacturer's recommended speed and feed
- Evaluate wear pattern after first tool change and adjust grade if needed
Following this sequence — ISO group first, operation severity second, coating type third — catches most grade selection errors before a test cut is ever made.
Quick Grade Selection by Application
| Scenario | ISO Group | Grade Range | Coating | Why |
|---|---|---|---|---|
| General steel turning | P | P20–P30 | CVD TiCN/Al₂O₃/TiN | Balanced crater + flank wear resistance |
| Stainless steel (austenitic) | M | M15–M25 | PVD TiAlN or AlCrN | Resists work-hardening and built-up edge |
| Gray cast iron (high speed) | K | K10–K20 | CVD Al₂O₃ | Abrasion resistance against hard carbides |
| Aluminum / non-ferrous | N | N10–N20 | Uncoated polished or DLC | Prevents built-up edge on soft materials |
| Interrupted cuts (steel) | P | P30–P40 | PVD TiAlN | Compressive stress resists micro-chipping |
| Hardened steel >45 HRC | H | H10–H20 | CVD multi-layer | Hot hardness + crater wear resistance |
| Superalloys (Inconel, Ti) | S | S15–S25 | PVD AlCrN | Heat resistance, oxidation resistance >800°C |
Start with the ISO group, then refine by operation severity.
Grade selection follows a clear hierarchy: ISO application group first, then hardness-toughness number based on operation type, then coating matched to continuous versus interrupted cutting. A P25 CVD grade covers 70% of general steel turning. An M20 PVD grade handles most stainless work. Master these two starting points, then refine based on actual wear patterns in your shop.
What ISO insert grade covers most general steel turning?
A P25 grade with CVD TiCN/Al₂O₃/TiN multi-layer coating covers approximately 70% of general steel turning applications. It balances crater wear resistance from the Al₂O₃ layer with toughness from the P25 substrate, making it the default starting point before refining based on actual wear patterns.
Why can't I use the same carbide grade for steel and stainless steel?
Steel causes crater wear on the rake face — P grades with CVD Al₂O₃ resist this — while stainless steel causes notch wear from work-hardened surfaces, which requires the tougher cobalt content of M15–M25 grades. Using the wrong ISO group means the grade resists the wrong failure mode, reducing tool life by 30-50% in typical comparisons.
What is the difference between CVD and PVD coated insert grades?
CVD coatings (8-20 µm thick) provide a thermal barrier for continuous high-speed turning, while PVD coatings (1-8um) preserve sharp edge geometry for milling, grooving, and interrupted cuts. Choose CVD for continuous operations and PVD for interrupted ones.
How does cobalt binder content affect insert performance?
Cobalt binder controls the hardness-toughness balance: 6% cobalt produces very hard, brittle finishing grades (P01–P10); 10% cobalt creates balanced general-purpose grades (P20–P30); 12–15% cobalt delivers very tough roughing grades (P40–P50) that resist chipping in interrupted cuts and heavy interrupted entry impacts.
