Appearance
A deep hole drilling operation producing 25 mm × 1,500 mm bores in Inconel 718 experiences rapid insert wear — standard K20 grade carbide fails after only 12 m of drilling due to crater wear and thermal cracking. Replacing the inserts with ISCAR IC806 (hard submicron substrate with TiAlN/AlTiN PVD coating and SUMOTEC surface treatment) increases tool life to 55 m — a 4.6× improvement. The IC806 grade's fine grain size (0.8 µm), optimised cobalt content (10%), and AlTiN coating provide the hot hardness and diffusion resistance required for superalloy machining.
WC-Co Composition and Microstructure
Tungsten carbide-cobalt (WC-Co) cemented carbide consists of tungsten carbide particles (the hard phase) embedded in a cobalt binder (the ductile phase). The properties of the grade are determined by three primary variables: cobalt content, WC grain size, and the addition of cubic carbides.
| Component | Function | Typical Range |
|---|---|---|
| Tungsten carbide (WC) | Hard phase; provides wear resistance | 70–97% by weight |
| Cobalt (Co) | Binder phase; provides toughness | 3–30% by weight |
| Titanium carbide (TiC) | Additive for steel-cutting grades | 0–15% |
| Tantalum/niobium carbide (TaC/NbC) | Additive for hot hardness | 0–5% |
| Chromium carbide (Cr₃C₂) | Grain growth inhibitor | 0.2–0.5% |
WC Grain Size Classification
| Grain Class | Grain Size (µm) | Hardness Range | Toughness | Application |
|---|---|---|---|---|
| Ultra-fine / Nano | < 0.5 | 93–95 HRA | Low | High-speed finishing; aluminium; composites |
| Submicron | 0.5–1.3 | 91–93 HRA | Moderate to high | Gun drills; BTA inserts; general deep hole drilling |
| Fine | 1.3–2.5 | 89–91 HRA | Good | End mills; drills; indexable inserts |
| Medium | 2.5–4.0 | 87–89 HRA | High | Wear parts; cold heading dies |
| Coarse | 4.0–6.0 | 85–87 HRA | Very high | Rock drilling; mining tools |
| Extra-coarse | > 6.0 | 83–85 HRA | Maximum | Heavy percussion; downhole drilling |
For deep hole drilling tools, submicron grain carbide (0.5–1.3 µm) is the standard choice. It provides the edge sharpness needed for low cutting forces while maintaining sufficient toughness for the torsional and bending loads of deep hole drilling.
Cobalt Content and Toughness Trade-Off
Cobalt content is the primary control over the hardness-toughness balance. Each 1% increase in cobalt reduces hardness by approximately 0.5 HRA while increasing transverse rupture strength (TRS) by 50–100 MPa.
| Cobalt Content (%) | Hardness (HRA) | TRS (MPa) | Toughness | Typical Deep Hole Application |
|---|---|---|---|---|
| 3–6 | 92–94 | 1,500–2,200 | Low | Brazed tips for abrasive materials; cast iron |
| 6–8 | 91–92 | 2,200–2,800 | Moderate | General gun drilling; K20 grade; guide bushings |
| 8–10 | 90–91 | 2,800–3,400 | Moderate to high | BTA indexable inserts; general purpose |
| 10–12 | 89–90 | 3,400–3,800 | High | Interrupted cuts; stainless steel; superalloys |
| 12–15 | 88–89 | 3,800–4,200 | Very high | Heavy roughing; high feeds |
TIP
The ideal cobalt content for deep hole drilling is 6–10%, depending on the specific operation:
- Gun drills (solid carbide): 8–12% Co — the higher cobalt content provides the toughness needed for the slender tool body to resist torsional fracture
- BTA brazed tips: 6–8% Co — the carbide is supported by a steel body, so higher hardness can be prioritised
- BTA indexable inserts: 8–10% Co — balance of wear resistance and edge toughness for multiple cutting edges
- Guide bushings: 6–8% Co — maximum wear resistance for the bushing bore
ISO Classification for Deep Hole Drilling
The ISO classification system (ISO 513) categorises carbide grades by workpiece material group. The primary classes relevant to deep hole drilling are:
| ISO Class | Material Group | Typical Application | Recommended Co% | Grain Size |
|---|---|---|---|---|
| P | Steel (carbon, alloy, tool steel) | General BTA drilling; gun drilling | 8–12% | Submicron to fine |
| M | Stainless steel (austenitic, duplex) | Stainless steel deep hole drilling | 8–12% | Submicron |
| K | Cast iron, non-ferrous | Cast iron BTA; aluminium gun drilling | 6–8% | Fine to medium |
| N | Aluminium, non-ferrous, plastics | Aluminium deep hole drilling | 6–10% | Ultra-fine to submicron |
| S | Superalloys (Inconel, titanium) | Aerospace deep hole drilling | 8–12% | Submicron |
| H | Hardened steel (> 400 HB) | Hard machining; die steel drilling | 6–10% | Ultra-fine to submicron |
ISO P Grades for Steel Drilling
| Grade | Co% | Hardness | Coating | Best For |
|---|---|---|---|---|
| IC907 (ISCAR) | ~8% | 92 HRA | TiAlN PVD | Steel, alloy steel; moderate speeds |
| IC508 (ISCAR) | ~10% | 90 HRA | TiN/TiCN PVD | General steel; interrupted cuts |
| IC808 (ISCAR) | ~9% | 91 HRA | TiN/TiAlN + SUMOTEC | Wide range; chipping resistance |
| KCU05A (Kennametal) | ~9% | 91.5 HRA | Multilayer PVD | Steel, stainless; high production |
IC907 is the most widely used PVD grade for steel BTA drilling, offering excellent wear resistance in carbon and alloy steels at cutting speeds of 60–140 m/min.
ISO M Grades for Stainless Steel
Stainless steel deep hole drilling requires grades with enhanced thermal stability and resistance to built-up edge formation. The work-hardening characteristic of austenitic stainless steel demands a sharp cutting edge combined with a tough substrate.
| Grade | Co% | Grain Size | Coating | Characteristic |
|---|---|---|---|---|
| IC520 (ISCAR) | ~10% | Submicron | TiN PVD | Austenitic stainless; notch wear resistance |
| IC806 (ISCAR) | ~10% | Submicron | TiAlN/AlTiN + SUMOTEC | Highest temperature resistance |
| IC830 (ISCAR) | ~11% | Submicron | TiN/TiAlN + SUMOTEC | High toughness for interrupted cuts |
ISO K Grades for Cast Iron and Non-Ferrous
| Grade | Co% | Hardness | Application |
|---|---|---|---|
| K20 (standard) | 6% | 92 HRA | General cast iron drilling; bushings |
| HB-406 (HB Carbide) | 6% | 92.3 HRA | Dedicated gun drilling; bimodal grain |
| K30 | 8–10% | 89 HRA | Higher toughness; interrupted cuts |
HB-406 is a specialised gun drilling grade with a bimodal grain structure (mixed grain sizes averaging 4 µm) that provides outstanding wear resistance combined with tool stability for deep hole drilling.
ISO S Grades for Superalloys
Superalloy drilling demands the highest hot hardness and diffusion resistance. ISCAR's IC806 grade was specifically developed for deep hole drilling in heat-resistant superalloys and titanium.
| Grade | Co% | Substrate | Coating | Tool Life vs K20 |
|---|---|---|---|---|
| IC806 | 10% | Hard submicron | TiAlN/AlTiN + SUMOTEC | 4.6× in Inconel 718 |
| IC903 | 9% | Ultra-fine grain | AlTiN PVD | 3× in titanium alloys |
| IC520 | 10% | Submicron | TiN PVD | 2× in austenitic stainless |
Substrate-Costing Interaction
The carbide substrate and coating must be selected as a system. The substrate provides bulk properties (toughness, edge strength), while the coating provides surface properties (wear resistance, thermal barrier, friction reduction).
| Coating Type | Hardness (HV) | Oxidation Temp | Best Substrate | Application |
|---|---|---|---|---|
| TiN (PVD) | 2,600 | 600°C | Tough submicron (8–12% Co) | Low-speed; general purpose |
| TiCN (PVD) | 3,000 | 400°C | Tough submicron | Steel; wear resistance |
| TiAlN (PVD) | 3,000 | 900°C | Hard submicron (6–10% Co) | Steel, stainless; moderate-high speeds |
| AlTiN (PVD) | 3,300 | 1,000°C | Hard submicron (6–10% Co) | Superalloys; hardened steel |
| TiAlN/AlTiN multilayer | 3,500 | 950°C | Hard submicron | Production deep hole drilling |
| CVD TiCN + Al₂O₃ | 2,500 + 2,100 | 1,000+°C | Tough substrate (8–12% Co) | High-speed steel drilling |
| DLC | 2,000–3,000 | 350°C | Ultra-fine grain | Aluminium; non-ferrous |
Substrate Requirements by Coating Type
PVD coatings require a substrate with sufficient hardness to support the coating under load without plastic deformation. A substrate hardness below 90 HRA risks coating collapse under the high point pressures of deep hole drilling. CVD coatings, applied at higher temperatures (900–1,000°C), require a substrate with higher cobalt content (10–12%) to maintain toughness after the thermal cycle.
For deep hole drilling, PVD-coated submicron grades (IC806, IC907, IC903) are the standard choice because the coating process temperature (450–550°C) does not degrade the substrate properties.
Commercial Grade Recommendations by Application
Gun Drilling (Solid Carbide)
| Workpiece Material | Recommended Grade | Grain | Co% | Coating | Tool Life Expectancy |
|---|---|---|---|---|---|
| Carbon steel (1018, 1045) | Submicron, 10% Co | 0.8–1.0 µm | 10 | TiAlN | 80–120 m |
| Alloy steel (4140, 4340) | Submicron, 10% Co | 0.8–1.0 µm | 10 | TiAlN or AlTiN | 60–100 m |
| Stainless steel (304, 316) | Submicron, 10–12% Co | 0.8–1.0 µm | 10–12 | TiAlN | 40–70 m |
| Inconel 718 | Submicron, 10% Co | 0.5–0.8 µm | 10 | AlTiN | 15–35 m |
| Titanium Ti-6Al-4V | Submicron, 10% Co | 0.5–0.8 µm | 10 | AlTiN | 30–55 m |
| Aluminium 6061 | Ultra-fine grain | < 0.5 µm | 6–8 | Uncoated or DLC | 100–200 m |
| Cast iron | Fine grain, 6% Co | 1.0–2.0 µm | 6 | Uncoated | 80–150 m |
BTA Drilling (Indexable Inserts)
| Workpiece Material | ISCAR | Kennametal | Sandvik | Specialist Grade |
|---|---|---|---|---|
| Carbon steel | IC907 | KCU05A | GC4334 | — |
| Alloy steel | IC808 | KCU05A | GC4334 | — |
| Stainless steel (304) | IC806 | KCU10 | GC3330 | IC520 (austenitic) |
| Stainless steel (duplex) | IC806 | KCU10 | S205 | — |
| Cast iron (grey) | IC508 | KCK20 | GC3210 | — |
| Cast iron (ductile) | IC907 | KCK15 | GC4334 | — |
| Inconel 718 | IC806 | KCU10 | S205 | — |
| Titanium Ti-6Al-4V | IC806 | KCU10 | GC3330 | IC903 |
| Hardened steel (40–55 HRC) | IC903 | KCK15 | GC4334 | — |
| Aluminium | IC508 | KCK20 | GC3210 | Uncoated micro-grain |
BTA Brazed Tips and Guide Bushings
| Component | Recommended Grade | Co% | Hardness | Key Requirement |
|---|---|---|---|---|
| Brazed cutting tip (steel) | K20–K25 | 6–8% | 91–92 HRA | Wear resistance |
| Brazed cutting tip (stainless) | K25–K30 | 8–10% | 89–91 HRA | Edge toughness |
| Guide pad (general) | K20 | 6% | 92 HRA | Abrasion resistance |
| Guide pad (stainless) | Submicron, 8% Co | 8% | 90 HRA | Galling resistance |
| Guide bushing | K20, HB-406 | 6% | 92.3 HRA | Maximum wear life |
Troubleshooting Grade-Related Failures
| Failure Mode | Visual Characteristic | Root Cause | Corrective Action |
|---|---|---|---|
| Flank wear | Uniform wear on relief face | Grade too soft for abrasive material | Switch to higher hardness grade (lower Co%, finer grain) |
| Crater wear | Cavity on rake face | Diffusion wear at high temperature | Upgrade to AlTiN-coated grade; add TaC/NbC |
| Edge chipping | Small fractures on cutting edge | Grade too brittle for interrupted cut | Increase Co% by 2%; switch to tougher grade |
| Thermal cracking | Crazing perpendicular to edge | Thermal shock from coolant | Use grade with higher thermal conductivity (higher Co%) |
| Built-up edge | Material welded to cutting edge | Adhesion; grade too soft | Switch to AlTiN or DLC coating; increase Co% |
| Notch wear | Groove at DOC line | Work hardening at surface | Use grade with better notch resistance (IC520, IC808) |
| Plastic deformation | Edge collapse or rounding | Cutting temperature too high for substrate | Upgrade to ultra-fine grain with AlTiN coating |
| Bulk fracture | Complete tip breakage | Excessive mechanical load | Increase Co% significantly (10–15%); reduce feed |
| Coating delamination | Coating peeling from substrate | Insufficient substrate hardness | Use harder substrate (> 91 HRA); verify coating adhesion |
| Galling on guide pad | Material transfer to pad | Adhesive wear with stainless steel | Switch to submicron grade with higher Co%; PCD coating |
| Comb cracks | Fine cracks at cutting edge | Thermo-mechanical fatigue | Reduce speed; use grade with better hot hardness (IC806) |
| Chip cratering | Localised crater behind edge | Chemical diffusion at high speed | Reduce cutting speed; use Al₂O₃ CVD coating |
FAQ
What is the best carbide grade for BTA drilling steel?
IC907 (ISCAR) or KCU05A (Kennametal) — TiAlN PVD-coated submicron grades with 8–10% cobalt — are the standard choice for carbon and alloy steel BTA drilling. These grades provide the best balance of wear resistance and edge toughness for the 60–140 m/min cutting speed range. For higher speeds or more abrasive steel grades, IC808 with SUMOTEC surface treatment offers extended tool life.
What cobalt content is recommended for gun drills?
Solid carbide gun drills typically use 8–12% cobalt with submicron grain size (0.5–1.3 µm). The higher cobalt content (10–12%) is needed because the slender gun drill body must resist torsional fracture and bending stresses during drilling. Brazed carbide-tipped gun drills can use lower cobalt (6–8%) in the tip because the steel shank provides the bulk toughness.
How does grain size affect deep hole drilling performance?
Submicron grain size (0.5–1.3 µm) is optimal for deep hole drilling. It provides the edge sharpness needed for low cutting forces (critical for hole straightness) while maintaining sufficient toughness for the cyclic loading of deep hole drilling. Ultra-fine grain (< 0.5 µm) offers higher wear resistance but risks chipping. Coarse grain (> 2.5 µm) cannot maintain a sharp enough edge for consistent hole quality.
What is the difference between K20 and P30 carbide?
K20 is a straight WC-Co grade (no TiC/TaC additives) designed for cast iron and non-ferrous materials. It provides high abrasion resistance but has limited hot hardness. P30 contains TiC and TaC additions that improve crater wear resistance at high cutting temperatures, making it suitable for steel machining. K20 is used for guide bushings and cast iron drilling; P30 for steel drilling. For deep hole drilling, PVD-coated grades have largely replaced uncoated K20 and P30 grades.
What carbide grade is recommended for drilling Inconel 718?
ISCAR IC806 is specifically designed for deep hole drilling in heat-resistant superalloys. It features a hard submicron substrate with approximately 10% cobalt and a TiAlN/AlTiN PVD coating with SUMOTEC surface treatment. It provides 4.6× the tool life of standard K20 grade in Inconel 718. The AlTiN coating provides oxidation resistance up to 1,000°C, essential for maintaining edge integrity in superalloy machining.
Why does my carbide grade fail by thermal cracking?
Thermal cracking is caused by rapid temperature cycling at the cutting edge — the carbide expands and contracts as it enters and exits the cut, with coolant providing the thermal shock. Higher cobalt content (10–12%) improves thermal conductivity and reduces thermal gradients. Grades with higher toughness (K30–K40) resist crack propagation better than harder grades (K10–K20).
Should I use coated or uncoated carbide for deep hole drilling?
Coated carbide is recommended for all production deep hole drilling. PVD coatings (TiAlN, AlTiN, TiAlN/AlTiN multilayer) improve tool life by 2–5× compared to uncoated grades in steel and stainless steel. Uncoated carbide is only appropriate for cast iron drilling (where abrasion is the primary wear mechanism) and aluminium drilling (where sharp edge geometry is prioritised).
What is the best carbide grade for guide bushings?
K20 grade with 6% cobalt and fine to medium grain size is the standard for guide bushings. HB-406 (HB Carbide) is a specialised bimodal-grain grade designed for gun drilling bushings that provides 92.3 HRA hardness with a TRS of 334,000 PSI. For maximum wear life, the bushing grade should prioritise hardness over toughness since the bushing is not subjected to impact loading.
How do I select carbide grade for different workpiece materials?
Follow the ISO classification system: P grades for steel, M for stainless steel, K for cast iron, N for aluminium, S for superalloys, H for hardened steel. Within each class, select higher hardness (lower Co%) for continuous cutting of soft materials, and higher toughness (higher Co%) for interrupted cuts, vibration-prone operations, or work-hardening materials. The specific commercial grade (IC806, IC907, IC508) should be selected based on the manufacturer's application guide.
What causes built-up edge on carbide drills in stainless steel?
Built-up edge (BUE) in stainless steel is caused by adhesion between the workpiece material and the carbide substrate at temperatures below 600°C. The cobalt binder dissolves into the chip and re-deposits on the cutting edge. Solutions include: switching to an AlTiN or DLC coating (reduces adhesion), increasing cutting speed (raises temperature above the BUE formation range), increasing cobalt content (reduces chemical affinity), or using a grade with TaC/NbC additives.
Summary
Carbide grade selection for deep hole drilling requires matching the WC-Co composition (cobalt content 6–12%, grain size 0.5–2.5 µm) and coating (TiAlN, AlTiN, multilayer) to the workpiece material group (ISO P/M/K/N/S/H). Submicron grain carbide (0.5–1.3 µm) with 8–10% cobalt and TiAlN or AlTiN PVD coating is the standard for production gun drilling and BTA drilling in steel and stainless steel. For superalloys, IC806 with AlTiN coating provides the highest hot hardness. For cast iron, uncoated K20 grades are sufficient. For aluminium, uncoated ultra-fine grain or DLC-coated grades are preferred. The substrate and coating must be selected together — the substrate provides bulk toughness while the coating provides surface wear resistance and thermal protection.