Tool Steel Machining Strategies: D2, H13, A2, M2 and P20

Tool steels are the materials that make other materials. They form the dies, moulds, punches, and cutting tools that shape everything from automotive body panels to medical device components. Machining tool steels presents unique challenges because these alloys are specifically engineered to resist wear, deformation, and heat, the very properties that make metal cutting possible. Each tool steel grade demands a tailored approach based on its composition, heat treatment state, and the hardness at which it will be machined.

The fundamental decision in tool steel machining is whether to cut the material in its annealed (soft) state before heat treatment or in its hardened state after heat treatment. This decision determines tooling material, machine requirements, cutting parameters, and achievable tolerances. Understanding the hardness switch point between carbide and CBN tooling is essential for economical processing.

The Hardness Switch Point

Workpiece hardness is the primary factor determining cutting tool material selection for tool steels. The industry has established clear boundaries based on decades of application data:

In the overlap zone (45-50 HRC), the decision often comes down to economics and machine capability. CBN inserts cost 5-10x more per edge than carbide but may deliver 3-5x longer tool life at these borderline hardnesses. For interrupted cuts, CBN’s brittleness becomes a liability, and tougher carbide grades with PVD coatings may prove more reliable despite shorter life.

D2 Tool Steel: High-Chrome Cold Work

D2 is a high-carbon (1.5%), high-chromium (12%) cold-work tool steel used extensively for blanking dies, forming tools, slitting knives, and wear plates. Its microstructure contains massive chromium carbides that provide extraordinary wear resistance but are extremely abrasive to cutting tools.

Annealed Condition (18-22 HRC)

In the annealed state, D2 machines reasonably well with coated carbide tooling, but the large chromium carbides cause abrasive wear far exceeding what the hardness number would suggest. Use CVD-coated carbide (Al2O3 layer for abrasion resistance) with speeds of 80-120 m/min for turning and 60-90 m/min for milling. Feed rates should be moderate (0.15-0.25 mm/rev turning) to prevent the hard carbide particles from chipping the cutting edge. Depth of cut should exceed the carbide particle size (typically 0.5mm minimum) to shear through rather than drag carbides across the flank.

Hardened Condition (58-62 HRC)

After heat treatment, D2 reaches 58-62 HRC and demands CBN tooling. High-CBN content grades (80-90% CBN) with ceramic binders are preferred for continuous cuts. Use cutting speeds of 100-150 m/min, feed rates of 0.08-0.15 mm/rev, and light depths of cut (0.1-0.3mm). The massive chromium carbides cause notch wear at the depth-of-cut line; varying DOC by 0.05mm between passes helps distribute this wear pattern.

H13 Tool Steel: Hot-Work Workhorse

H13 (5% Cr, 1.5% Mo, 1% V) is the dominant hot-work tool steel for die casting dies, extrusion tooling, and forging dies. It operates at elevated temperatures in service, so machining strategies must account for its toughness and relatively lower carbide content compared to cold-work steels.

Machining at Working Hardness (50-55 HRC)

H13 is typically machined in the hardened state (50-55 HRC) to avoid distortion during subsequent heat treatment. At this hardness level, two tooling approaches work:

SiAlON ceramics: For roughing at high speeds (300-600 m/min) with light depths (0.5-2.0mm). SiAlON’s chemical stability resists the diffusion wear mechanism active at high temperatures. Feed rates of 0.1-0.2 mm/rev. Requires rigid setup and continuous cutting; any interruption risks thermal shock fracture.

Low-CBN grades (50-65% CBN): For finishing at 120-200 m/min with fine feeds (0.05-0.12 mm/rev) and depths below 0.3mm. Low-CBN grades with TiN ceramic binders offer better toughness than high-CBN for the interrupted cuts common in die machining (cooling channels, ejector pin holes, parting line geometry).

H13’s vanadium content creates fine, well-distributed carbides that cause less localized tool wear than D2’s massive carbides. This makes H13 one of the more predictable tool steels to machine in the hardened state.

A2 Tool Steel: The Friendliest Cold-Work Grade

A2 (1% C, 5% Cr, 1% Mo) is an air-hardening cold-work tool steel that represents the best balance between wear resistance, toughness, and machinability in the cold-work category. It achieves 58-62 HRC after heat treatment but contains fewer large carbides than D2, making it significantly more cooperative during machining.

In the annealed state (12-16 HRC), A2 machines like a medium-carbon alloy steel. Speeds of 120-180 m/min with coated carbide are routine. After hardening to 58-62 HRC, CBN tooling is required but performs well with standard parameters: 120-180 m/min speed, 0.08-0.15 mm/rev feed, 0.1-0.3mm depth. The lower chromium content compared to D2 means less abrasive wear and more predictable tool life progression.

A2 is often selected for prototype tooling and short-run dies specifically because its machinability reduces lead time. When given the choice between D2 and A2 for applications where either grade meets performance requirements, A2 will typically cost 15-25% less to machine.

M2 High-Speed Steel: The Hardest Challenge

M2 (6% W, 5% Mo, 4% Cr, 2% V) is a tungsten-molybdenum high-speed steel used for cutting tools, drills, taps, and broaches. At 62-65 HRC after triple tempering, it represents the upper limit of commonly machined hardnesses and contains a dense network of extremely hard tungsten and vanadium carbides.

Machining hardened M2 is restricted to high-CBN content grades (85-95% CBN) with metallic binders for maximum thermal conductivity. Cutting speeds are limited to 80-120 m/min because the tungsten carbide particles in M2 are harder than the CBN binder phase itself. Feed rates must be conservative (0.05-0.10 mm/rev) with depths below 0.2mm. Tool life is short by CBN standards (typically 10-15 minutes per edge for finishing), and cost per part is high.

Most shops prefer to grind hardened M2 rather than single-point machine it. However, for complex geometries that cannot be ground (internal profiles, deep pockets), CBN hard turning or milling is the only option. Korloy’s high-CBN insert grades designed for hardened steel applications provide the necessary wear resistance for these demanding operations.

P20 Tool Steel: Pre-Hardened Simplicity

P20 (0.35% C, 1.7% Cr, 0.4% Mo) is a pre-hardened mould steel supplied at 28-32 HRC, ready to machine without subsequent heat treatment. This eliminates distortion concerns and allows the mould to go directly from machining to the press. At its delivered hardness, P20 machines like a medium-alloy structural steel rather than a tool steel.

Standard coated carbide tooling handles P20 efficiently at speeds of 150-250 m/min for turning and 120-200 m/min for milling. Feed rates are generous (0.2-0.35 mm/rev turning, 0.1-0.2 mm/tooth milling) because the material lacks the hard carbide particles that force conservative parameters in other tool steels. CVD or PVD coatings both work well; the primary wear mechanism is straightforward flank wear rather than the chemical or abrasive mechanisms dominant in harder grades.

The main consideration with P20 is volume: mould cavities are large, requiring significant material removal. High-feed milling strategies with indexable cutters optimize roughing, followed by ball-nose finishing with solid carbide at high speeds for surface quality.

Common Failure Modes Across Tool Steel Grades

Crater Wear

Diffusion-driven crater wear on the rake face accelerates at high cutting temperatures. It is most severe in H13 and M2 at elevated speeds. Countermeasures: reduce speed by 15-20%, select coatings with alumina barriers (CVD Al2O3 for carbide, TiN coating on CBN).

Depth-of-Cut Line Notching

A groove forms at the point where the tool exits the workpiece surface, caused by work-hardened material at the cut boundary and oxidation-assisted wear. Particularly common in D2 and M2. Countermeasures: vary DOC between passes by 0.05-0.1mm, use round inserts or large nose radii to spread the depth-of-cut zone.

Edge Chipping

Microchipping of the cutting edge from mechanical shock, common in interrupted cuts through hardened tool steels. Most frequent in CBN tooling on H13 die cavities. Countermeasures: select tougher CBN grades (low-CBN content with TiCN binder), apply edge preparation (hone radius 15-25 micron), reduce feed rate, and ensure rigid workholding eliminates vibration.