Tool Materials: Classification, Properties, Heat Treatment, Coatings, and Applications

This page is a comprehensive, reference-grade overview of tool materials used in machining, forming, and cutting operations. It covers classification, compositions and microstructures, key properties, wear mechanisms, heat treatment, surface engineering, selection methodology, cutting data envelopes, and failure troubleshooting. It is written to support curriculum delivery and practical decision-making in manufacturing engineering.

Contents

1. Classification of tool materials
2. Representative compositions and microstructures
3. Key properties and performance metrics
4. Wear mechanisms and failure modes
5. Heat treatment of tool steels
6. Coatings and surface engineering
7. Material selection for applications
8. Typical cutting data envelopes
9. Standards and designations
10. Troubleshooting and optimization
11. Glossary

1. Classification of tool materials

• Plain carbon tool steels (W-group): 0.6–1.5% C; low cost; water/oil quenched; suitable for low-speed cutting and simple forming tools.
• Alloy tool steels: Cr, V, Mo, W additions for hardenability and wear resistance; used for dies, punches, and cold work tools.
• High-speed steels (HSS): T-series (tungsten-based) and M-series (molybdenum-based); retain hardness at elevated temperature (red hardness); general-purpose cutting tools.
• Cast cobalt-base alloys (e.g., Stellite): Co–Cr–W; hot hardness and wear resistance; used for hot-working tools and hardfacing.
• Cemented carbides: WC–Co (and variants with TiC/TaC/NbC); powder metallurgy; primary choice for most modern machining inserts.
• Cermets: Ti(C,N)-based with Ni/Co binder; high wear resistance and good surface finish in finishing/semi-finishing of steels.
• Ceramics: Alumina-based (Al₂O₃), SiAlON, mixed ceramics; high hot hardness; used for high-speed finishing of cast irons and superalloys.
• Polycrystalline cubic boron nitride (PCBN): For hardened steels and cast irons; excellent abrasion resistance at high temperatures.
• Polycrystalline diamond (PCD): For non-ferrous metals, composites, and abrasive non-metals; unmatched wear resistance but not suitable for ferrous alloys at high temps.
• Ultra-hard thin films: DLC variants, diamond coatings; niche for Al–Si alloys, graphite, and gummy non-ferrous materials.

2. Representative compositions and microstructures

2.1 Tool steels (carbon, alloy, HSS)

• Carbon tool steel: 0.9–1.2%C; Fe–C pearlite/spheroidized carbides in annealed state; martensite + carbides after heat treatment.
• Cold-work alloy steels (e.g., D2): High C (~1.5–2.0%) and Cr (~11–12%); abundant primary carbides (M₇C₃, M₂₃C₆); wear-resistant.
• HSS M2 (typical): ~0.85%C, 6%W, 5%Mo, 4%Cr, 2%V; tempered martensite with fine carbides (MC, M₆C, M₂₃C₆).
• HSS M42 (cobalt-bearing): ~1.1%C, 9.5%Mo, 8%Co, 3.8%Cr, 1.5%W, 1.2%V; improved red hardness and crater wear resistance.

2.2 Carbides and cermets

• WC–Co carbides: WC grains in a Co binder; grain size ranges from submicron to coarse; finer grains increase hardness and wear resistance.
• Complex carbides (WC–TiC–TaC/NbC): Improve resistance against crater wear in steel cutting; typically lower toughness than straight WC–Co.
• Cermets (Ti(C,N)–Ni/Co): Ceramic hard phase in metallic binder; excels in finishing steels with stable wear and excellent surface finish.

2.3 Ceramics, PCBN, PCD

• Alumina ceramics: High-purity Al₂O₃ or Al₂O₃ with TiC/TiN whiskers; excellent chemical stability and hot hardness.
• SiAlON: Silicon–aluminum–oxygen–nitrogen ceramics; improved thermal shock resistance over pure alumina; suited for Ni-based superalloys and cast irons.
• PCBN: Binders may be TiC/TiN/Al; varying CBN content for balance of toughness and wear resistance; ideal for hardened steels (≥ 45 HRC).
• PCD: Sintered diamond grains with metallic binder; best for Al, Cu, Mg, MMCs, fiber-reinforced polymers, and abrasive non-metals.

3. Key properties and performance metrics

3.1 Red hardness and hot strength

Red hardness is the ability to retain hardness at elevated temperature. HSS achieves this via alloy carbides and secondary hardening; carbides, ceramics, PCBN, and PCD rely on intrinsically stable hard phases that resist softening and diffusion wear.

3.2 Toughness vs. wear resistance trade-off

Increasing hardness and volume fraction of hard phases improves abrasion resistance but reduces fracture toughness. Tool choice balances interrupted cuts (need toughness) against continuous finishing (favor wear resistance).

4. Wear mechanisms and failure modes

• Abrasion (flank wear): Hard inclusions or work-hardened layers scratch tool; controlled by hardness and microstructure.
• Adhesion (built-up edge): Material transfer at tool–chip interface; mitigated by sharp edges, coatings, and lubricity.
• Diffusion (crater wear): High-temperature interdiffusion, especially in steel cutting; mitigated by TiC/TaC additions and stable coatings.
• Oxidation wear: At moderate–high temperatures; reduced in inert or coated systems.
• Thermal fatigue (cracking): Cycling in interrupted cuts; requires tougher substrates and proper coolant strategy.
• Chipping/notching: Entry/exit impacts, scale, hard skin; needs edge prep, tougher grade, or chamfered hone.

5. Heat treatment of tool steels

5.1 General sequence

1. Preheat: One or two steps to reduce thermal gradients.
2. Austenitize: Heat to dissolve carbides to desired extent; control time to avoid grain growth.
3. Quench: Oil, air, salt, or gas quench depending on steel and section size.
4. Temper (multiple): 2–3 tempers to relieve stresses and precipitate secondary carbides; avoid secondary hardening valleys.
5. Cryogenic treatment (optional): Convert retained austenite, refine residual carbides in some grades.

5.2 Typical ranges (indicative)

• Carbon tool steel: Austenitize ~760–820°C; oil/water quench; temper 150–250°C; target HRC 58–62.
• D2 (high Cr): Austenitize ~1010–1040°C; air/quench plate; temper 3× at 500–540°C; HRC ~58–62.
• HSS M2: Austenitize ~1180–1230°C; salt bath or vacuum/gas; temper 3× at 540–560°C; HRC ~64–66.
• HSS M42: Austenitize ~1180–1210°C; temper 3× at 540–570°C; HRC ~66–68.

5.3 Microstructural control

• Carbide population: Primary carbides for wear, secondary carbides for red hardness.
• Grain size: Finer austenite grains improve toughness; controlled via preheat and hold times.
• Retained austenite: Manage via quench severity, cryo, and tempering schedule.

6. Coatings and surface engineering

6.1 PVD coatings (typical)

• TiN: General-purpose; reduces adhesion; golden color; good for HSS drills and taps.
• TiCN: Higher hardness than TiN; better for abrasive steels; lower friction.
• TiAlN/AlTiN: Excellent hot hardness via Al₂O₃ scale; ideal for dry/high-speed cutting.
• CrN: Good for sticky non-ferrous alloys; corrosion-resistant; lower chemical affinity.
• nACo/Multilayers: Nanolaminates balancing toughness and hardness; used in milling.

6.2 CVD coatings (carbide inserts)

• TiC/TiN/Al₂O₃ stacks: Thick coatings for crater and flank wear resistance at elevated temperatures.
• Al₂O₃ outer layers: Thermal barrier; suited for continuous turning of steels.

6.3 Surface treatments

• Nitriding/nitrocarburizing: Hard case on tool steels for wear and fatigue resistance in forming dies.
• Honing/chamfering: Edge preparation improves chipping resistance and reduces BUE.
• Polishing/lapping: Reduces friction and adhesion for aluminum and gummy alloys.

7. Material selection for applications

7.1 Quick selection map (qualitative)

7.2 Decision factors

• Cut type: Continuous vs. interrupted; interrupted cuts need toughness (tough carbide, HSS, PM HSS).
• Temperature: High heat favors TiAlN-coated carbide, ceramics, PCBN.
• Chemistry: Avoid chemically reactive pairs (PCD with Fe at high T).
• Surface finish: Cermets and sharp polished edges for fine finishes.
• Cost and regrind: HSS tools can be reground; inserts reduce downtime.

8. Typical cutting data envelopes (indicative ranges)

Actual values depend on grade, geometry, rigidity, coolant, and machine capability. Start mid-range and tune based on wear pattern.

8.1 Turning speeds (vc) and feeds (f)

• HSS in steel: vc ~ 20–40 m/min; f ~ 0.05–0.25 mm/rev; DOC 0.5–3 mm.
• Carbide in steel: vc ~ 120–280 m/min (coated higher); f ~ 0.1–0.35 mm/rev; DOC 0.5–4 mm.
• Cermet finishing: vc ~ 200–400 m/min; f ~ 0.05–0.2 mm/rev; DOC 0.1–1 mm.
• Ceramic in cast iron: vc ~ 400–900 m/min; f ~ 0.1–0.3 mm/rev; DOC 0.2–2 mm (continuous).
• PCBN hard turning: vc ~ 120–250 m/min; f ~ 0.05–0.2 mm/rev; DOC 0.05–0.5 mm.
• PCD in Al–Si: vc ~ 400–1200 m/min; f ~ 0.05–0.4 mm/rev; DOC 0.2–2 mm.

8.2 Milling (vc) and feeds per tooth (fz)

• HSS end mills in steel: vc ~ 15–35 m/min; fz ~ 0.02–0.08 mm/tooth.
• Carbide end mills in steel: vc ~ 80–220 m/min; fz ~ 0.04–0.18 mm/tooth.
• Carbide in stainless: vc ~ 60–160 m/min; fz ~ 0.03–0.14 mm/tooth.
• PCD in aluminum: vc ~ 400–1500 m/min; fz ~ 0.05–0.3 mm/tooth.

8.3 Drilling (vc) and feed (f)

• HSS in steel: vc ~ 15–30 m/min; f ~ 0.05–0.25 mm/rev (per diameter-dependent).
• Carbide in steel: vc ~ 80–180 m/min; f ~ 0.08–0.35 mm/rev.
• Carbide in aluminum: vc ~ 200–400 m/min; f ~ 0.1–0.4 mm/rev.

9. Standards and designations

9.1 HSS and tool steels

• HSS grades: M1, M2, M7, M35 (Co), M42 (8% Co), T1, T15.
• Cold-work: O1 (oil hardening), A2 (air), D2/D3 (high Cr), PM grades (e.g., CPM series).
• Hot-work: H11, H13, H21–H26 (tungsten hot-work).

9.2 Carbide ISO grade families

• P (blue): Steels.
• M (yellow): Stainless steels.
• K (red): Cast irons.
• N (green): Non-ferrous.
• S (orange): Heat-resistant superalloys.
• H (grey): Hardened materials.

Within each family, sub-grades balance toughness vs. wear resistance and often indicate coating type and substrate grain size.

10. Troubleshooting and optimization

10.1 Symptom → likely cause → corrective action

• Rapid flank wear: Excessive speed, abrasive work, too soft grade → Reduce vc, choose harder grade/coating, improve filtration/coolant.
• Crater wear: High interface temperature, diffusion in steels → Use Al₂O₃/TiAlN coatings, reduce vc, increase f slightly to move heat into chip.
• Built-up edge (BUE): Low speed, gummy material, dull edge → Increase vc, use sharp polished edge and TiB₂/CrN/DLC-like coatings, apply coolant or MQL.
• Notching at DOC line: Work hardening/scale, thermal shock → Use tougher grade, apply chamfered edge, pre-remove scale, alter DOC.
• Edge chipping: Interruptions/vibration, too brittle grade → Switch to tougher substrate, increase hone, reduce overhang, optimize fixturing.
• Thermal cracks (milling): Coolant intermittency → Cut dry or ensure consistent coolant; reduce cyclic thermal loads.
• Poor surface finish: BUE, chatter, too large nose radius for feed → Increase vc, reduce f or use appropriate r_ε, select cermet/PCD for finishing.

10.2 Edge preparation

• Sharp edge: Best for non-ferrous and finishing but fragile; risk of chipping.
• Honed edge (K-factor): Improves strength; select minimal hone for stainless to limit work hardening.
• Chamfered/landed edge: Resists chipping in interrupted cuts; increases cutting forces.

11. Glossary

• Red hardness: Retention of hardness at elevated temperature during cutting.
• Secondary hardening: Increase in hardness during tempering due to fine alloy carbide precipitation (HSS).
• CVD/PVD: Chemical/physical vapor deposition coating processes for inserts and tools.
• CBN/PCBN: Cubic boron nitride; polycrystalline form used as superhard insert material.
• PCD: Polycrystalline diamond; ultra-hard, non-ferrous machining.
• Cermet: Composite of ceramic hard phase with metallic binder for finishing steels.
• SiAlON: Ceramic based on silicon, aluminum, oxygen, nitrogen; toughened alumina alternative.
• BUE: Built-up edge; adhered workpiece material on cutting edge.
• DOC: Depth of cut; radial/axial engagement in turning/milling.

For production tuning, validate initial parameters with short tool-life trials, capture wear images at fixed time-on-cut, and iterate one variable at a time (speed, then feed, then geometry/coating) to converge on a stable, economical process window.