ISO P/M/K/N/S/H Carbide Turning Grades: Complete Cutting Parameter Reference Guide

Introduction: Why Grade Selection Matters More Than You Think

Selecting the right carbide turning grade is arguably the single most impactful decision in any turning operation. The grade determines your cutting speed, tool life, surface finish, and ultimately your cost per part. Yet many machinists rely on habit or supplier recommendations without fully understanding the underlying grade chemistry, coating architecture, and application boundaries.

This reference guide provides a comprehensive comparison of carbide turning grades across all six ISO workpiece material classes — P, M, K, N, S, and H — from three leading manufacturers: Walter, Seco, and Sumitomo. Whether you’re running high-volume production of steel components or tackling a one-off Inconel job, having the right grade parameter data at your fingertips directly translates to higher productivity and lower tooling costs.

We’ll cover grade designations, coating types, recommended cutting parameters (Vc, f, ap ranges), and application guidance for each material class. All values represent typical starting parameters for continuous turning in stable conditions — always adjust based on your specific machine rigidity, coolant availability, and part setup.

Understanding the ISO Classification System

The ISO 513 standard categorizes workpiece materials into six main groups, each identified by a letter and color code. Carbide insert grades are similarly classified to indicate which material groups they’re optimized for.

Most modern grades are designed to handle multiple ISO classes but have a primary application where they excel. The grade designation suffix (e.g., P10, P25, M30) indicates both the material class and the toughness/wear resistance balance — lower numbers mean higher wear resistance for finishing, higher numbers mean higher toughness for roughing.

ISO P — Steel Turning Grades

Steel is the most commonly machined material group, and CVD-coated grades dominate this segment. The key challenge in steel turning is managing crater wear on the rake face caused by high cutting temperatures (800–1100°C) and diffusion wear mechanisms.

Coating Architecture for Steel Turning

Modern steel-turning grades typically use multi-layer CVD coatings with a total thickness of 8–20 μm. The standard stack includes:

• TiCN base layer (3–8 μm) — provides abrasion resistance and adhesion to the carbide substrate
• Al₂O₃ middle layer (3–10 μm) — acts as a thermal barrier and chemical barrier against crater wear
• TiN top layer (1–2 μm) — reduces friction and provides wear detection (gold color wears away to reveal black Al₂O₃)

Steel Turning Grade Comparison

Steel Turning Recommendations

For general-purpose steel turning, Walter WPP20, Seco TP200, and Sumitomo AC820P are all solid choices. Seco’s TP200 with Duratomic® technology tends to excel at higher speeds (250–320 m/min) in alloy steels, while Sumitomo’s AC820P offers exceptional crater wear resistance at the upper end of the speed range. Walter’s WPP20 is known for reliability across varying conditions — a good choice when material consistency varies.

For high-speed production finishing, push Sumitomo AC8025P at 350–420 m/min with feeds around 0.15–0.2 mm/rev and depths of cut of 0.5–1.5 mm. This grade’s Super FF Coat technology provides a very smooth coating surface that reduces friction and built-up edge at high cutting speeds.

ISO M — Stainless Steel Turning Grades

Stainless steel machining presents unique challenges: austenitic grades like 304 and 316 work-harden rapidly, generate high heat, and tend to form built-up edge (BUE) on the cutting edge. Martensitic and precipitation-hardening grades add abrasion resistance to the mix. PVD-coated grades are generally preferred for stainless steel due to their thinner, smoother coating surfaces and sharper cutting edges.

Stainless Steel Turning Grade Comparison

Stainless Steel Turning Recommendations

The most critical factor in stainless steel turning is avoiding work hardening. Maintain a constant feed rate and depth of cut, never dwell, and ensure the cutting edge is sharp. Built-up edge is the enemy of both surface finish and tool life — grades with smooth PVD top coats like Sumitomo AC820M with its TiSiN-TiAlN nanolaminate coating excel at BUE prevention.

For austenitic stainless (304/316) at moderate production rates, start with Seco TM2000 at 120–180 m/min, 0.2–0.3 mm/rev feed, and 2–3 mm depth of cut. Use high-pressure coolant (70+ bar) if available — it dramatically improves tool life by penetrating the vapor barrier.

For duplex and super-duplex stainless, reduce speeds by 20–30% and lean toward tougher grades like Walter WMP30 or Sumitomo AC830M. Duplex materials’ high strength and work-hardening tendency demand more robust edge preparation.

ISO K — Cast Iron Turning Grades

Cast iron machining is dominated by abrasive wear from free graphite and silicon carbide particles in the workpiece. Gray cast iron (GCI) is relatively easy to machine but generates abrasive chips. Ductile iron (GJS/Ni-Resist) is tougher and more abrasive. Compacted graphite iron (CGI) sits between the two in machinability but has a strong work-hardening tendency.

Cast Iron Turning Grade Comparison

Cast Iron Turning Recommendations

Cast iron can run at surprisingly high cutting speeds — gray cast iron at 500+ m/min is achievable with the right grade and machine. The key is thermal cracking resistance: cast iron’s intermittent nature (from the casting skin, sand inclusions, or interrupted cuts) combined with high temperatures causes thermal fatigue cracks.

For high-volume gray iron production (e.g., brake discs, engine blocks), Seco TK1001 at 400–550 m/min delivers exceptional productivity. The Duratomic® Al₂O₃ coating provides excellent thermal barrier properties.

For ductile iron (GGG40/GGG60), reduce speeds to 250–350 m/min and move to a tougher grade like Sumitomo AC510U or Walter WKP20. Ductile iron’s higher strength and toughness cause more mechanical load on the cutting edge.

For CGI (compacted graphite iron), expect speeds 30–40% lower than gray iron. CGI has significantly higher tensile strength and produces continuous chips that transfer more heat to the tool. Stick to moderate speeds (150–250 m/min) and ensure adequate coolant delivery.

ISO N — Non-Ferrous Turning Grades

Non-ferrous materials (aluminum, copper, brass, bronze) are generally considered “easy” to machine, but achieving high surface quality and dimensional accuracy at high production rates requires the right tooling. The primary challenges are built-up edge, especially with pure or soft alloys, and maintaining a mirror-like surface finish.

For aluminum and copper alloys, uncoated carbide, polished PCD (polycrystalline diamond), or Diamond-Like Carbon (DLC) coated inserts are used. Standard TiN/TiCN/TiAlN coatings actually increase friction with non-ferrous materials and promote BUE.

Non-Ferrous Turning Grade Comparison

Non-Ferrous Turning Recommendations

For general aluminum turning (6061, 7075, cast A356), an uncoated fine-grain carbide grade like Walter WK1 or Sumitomo ND2025 running at 500–700 m/min is the most cost-effective choice. Use positive rake inserts with highly polished rake faces to minimize BUE.

For high-volume production or high-silicon aluminum (12–18% Si), PCD-tipped inserts are the clear winner despite higher initial cost. Tool life increases by 10–50× compared to carbide, and surface finish is consistently excellent. Seco CD10 and Walter WBK20 are both excellent choices at 1500–2500 m/min.

Pro tip: When machining aluminum, ensure your coolant has good lubricity (semi-synthetic or soluble oil with at least 8% concentration) to prevent BUE. Dry machining aluminum with carbide inserts is almost never a good idea for production.

ISO S — Superalloy and Titanium Turning Grades

Superalloys (Inconel, Hastelloy, Waspaloy, René) and titanium alloys (Ti-6Al-4V, Ti-5553) are among the most difficult-to-machine materials. Their high temperature strength, low thermal conductivity, and chemical reactivity with carbide at high temperatures make tool life short and cutting speeds low. PVD-coated grades with high-aluminum-content coatings (TiAlN, AlTiN) are the standard choice.

Superalloy and Titanium Turning Grade Comparison

Superalloy and Titanium Turning Recommendations

Cutting speeds for superalloys are an order of magnitude lower than for steel — typically 20–70 m/min for most operations. The key is maintaining a consistent chip load and managing heat. Because these materials conduct heat poorly, 80%+ of the cutting heat goes into the tool rather than the chip.

For Inconel 718 turning — the most common aerospace superalloy — start with Seco TS2000 or Sumitomo AC5010S at 30–45 m/min, feed of 0.2 mm/rev, and 2–3 mm depth of cut. Use high-pressure coolant (70–100 bar) directed at the cutting edge; this can extend tool life by 50–100% compared to flood coolant.

For titanium alloys (Ti-6Al-4V), speeds are similar (30–60 m/min) but the failure mode differs. Titanium chemically reacts with carbide at cutting temperatures, causing cratering and diffusion wear. PVD AlTiN grades like Walter WSM20 are good general choices. For higher productivity, consider cermet or SiAlON ceramic grades at higher speeds (80–150 m/min), though they require more stable setups.

Important: Never use uncoated carbide for titanium — the chemical affinity will cause severe cratering and possible catastrophic tool failure within seconds.

ISO H — Hard Part Turning Grades

Hard part turning — machining hardened steel (45–65 HRC) and chilled cast iron — has revolutionized the manufacturing industry by replacing grinding operations in many applications. CBN (cubic boron nitride) and PCBN (polycrystalline CBN) inserts are the tools of choice, though ceramic grades are used for some lower-hardness applications.

Hard Part Turning Grade Comparison

Hard Part Turning Recommendations

Hard turning with PCBN can achieve surface finishes of Ra 0.2–0.8 μm and dimensional tolerances of ±5 μm — competitive with grinding in many cases. The key requirements are a rigid machine (at least 15 kW spindle power, linear guideways or heavy box ways), tooling with minimal overhang, and proper edge preparation on the insert.

For continuous finish turning of hardened steel (58–62 HRC) like bearings, gears, and shafts, Sumitomo BNC100 or Seco CBN100 at 150–200 m/min with 0.1–0.15 mm/rev feed and 0.2–0.5 mm depth of cut delivers excellent results. Use a wiper insert geometry for best surface finish.

For interrupted hard turning (e.g., shafts with keyways, gears with teeth), use a tougher PCBN grade like Walter WBN20 or Sumitomo BNC200 with a honed edge (0.02–0.05 mm × 20–30° hone) to prevent chipping. Reduce speed by 20–30% compared to continuous cutting.

Quick Reference: Grade Selection Summary

Factors That Affect Real-World Performance

The parameters in this guide are starting point recommendations. Your actual results will vary based on several factors:

• Machine rigidity: A solid lathe with box ways can run 10–20% faster than a light-duty machine with linear guideways.
• Coolant type and pressure: High-pressure coolant (70+ bar) can increase tool life by 30–100% in stainless steel and superalloys. Through-tool coolant is always better than flood.
• Insert geometry: Positive rake inserts run cooler but are more fragile. Negative rake inserts are stronger but generate more cutting force and heat.
• Chip breaker: The right chip breaker for your feed/depth combination dramatically affects surface finish and tool life. Always match the chip breaker to your operation (finishing, medium, roughing).
• Workpiece condition: Forged or cast skins, scale, decarburization, and inconsistent hardness all reduce tool life significantly.
• Cutting fluid concentration: Too-dilute coolant causes rust and poor lubrication. Too-concentrated coolant wastes money and can cause foaming. Follow the manufacturer’s recommendations (typically 5–10% for semi-synthetic).

Final Thoughts

Carbide grade selection is both a science and an art. The data in this reference guide gives you a solid starting point, but the best way to optimize your process is through controlled testing. When evaluating a new grade, run a direct comparison against your current grade at the same parameters, then systematically push the speed until you find the limit.

Remember that the cheapest insert isn’t always the lowest-cost option. A premium grade that costs 30% more but runs 50% faster with 2× tool life typically reduces your cost per part by 25–40% when you factor in machine time, labor, and tool change overhead.

Bookmark this guide as your go-to reference for turning grade selection across all six ISO material classes. The next time you’re setting up a new job, use these tables to narrow down your grade choices, then validate with real cuts on your machine.