Cemented Carbide Grade Selection for Deep Hole Drilling: Complete Guide to WC-Co Substrates and Coatings

The cutting edge of every gun drill and BTA drill head is made from cemented carbide — a composite of tungsten carbide (WC) particles bonded with a cobalt (Co) binder. The performance of the tool depends as much on the carbide grade selection as on the geometry of the cutting edge.

Cemented carbide is not a single material. The combination of WC grain size, cobalt content, and coating type produces grades that vary in hardness from HV 1,300 to HV 2,400, in toughness from K₁c 8 to K₁c 25+ MPa·m¹⁄², and in recommended cutting speed by a factor of 5× or more between grades for the same workpiece material.

Selecting the wrong grade for a deep hole drilling operation produces results that look like incorrect parameters: rapid flank wear, chipping, cratering, or thermal cracking. The tool manufacturer’s standard grade may not be the best grade for the specific workpiece material and drilling conditions.

This guide covers the cemented carbide material system from the substrate up, with specific grade recommendations for deep hole drilling applications.

The WC-Co System

Cemented carbide consists of tungsten carbide particles (the hard phase) embedded in a cobalt binder (the ductile phase). The carbide particles provide wear resistance; the cobalt provides toughness by absorbing impact energy through plastic deformation.

WC Grain Size

The WC grain size is the primary determinant of hardness and wear resistance. Finer grains produce more grain boundaries, which resist crack propagation and increase hardness.

Ultra-fine and fine grain grades are preferred for deep hole drilling because they provide the wear resistance needed for the long cutting times per hole.

Cobalt Content

The cobalt content determines toughness. Higher cobalt content allows the material to absorb more impact energy before cracking.

For deep hole drilling, the 6–10% cobalt range is the most common, providing adequate wear resistance for production volumes while maintaining sufficient edge toughness to withstand entry impacts and chip interruption.

The Hardness-Toughness Trade-off

The fundamental trade-off in cemented carbide is: increasing hardness (by finer grain size or lower cobalt) reduces toughness, and increasing toughness (by coarser grain size or higher cobalt) reduces hardness.

For any given hardness level, there is an optimum combination of grain size and cobalt content that maximizes fracture toughness. This optimum varies by manufacturer and grade.

ISO 513 Classification System

ISO 513 classifies carbide grades by workpiece material group (letter) and hardness-toughness position (number).

Material Groups

Subgroup Number (01–50)

The number indicates the balance between hardness and toughness:

• Lower numbers (01–15): High hardness, high wear resistance, high-speed finishing
• Middle numbers (15–30): Balanced, general purpose machining
• Higher numbers (30–50): High toughness, lower speeds, roughing, interrupted cuts

For deep hole drilling, the general-purpose range (P15–P30, M15–M25, K10–K20) covers most production operations.

Coating Systems

Modern cemented carbide tools for deep hole drilling are almost always coated. The coating provides a harder surface than the substrate, a thermal barrier that reduces heat transfer to the carbide, and chemical inertness that reduces diffusion wear and built-up edge formation.

PVD (Physical Vapor Deposition)

PVD coatings are applied at 400–600°C and produce thin (1–5 µm) coatings that maintain the sharp cutting edge required for drilling.

PVD coatings are standard for gun drills and BTA drill heads because they maintain edge sharpness.

CVD (Chemical Vapor Deposition)

CVD coatings are applied at 700–1,050°C and produce thicker coatings (5–15 µm) with excellent adhesion. The higher temperature causes some edge rounding, making CVD more suitable for indexable inserts than for solid carbide drills.

Typical CVD multilayer structure (outside to inside):

• TiN — gold outer layer, provides visual wear indication
• Al₂O₃ — thermal barrier, chemical inertness, crater wear resistance
• MT-Ti(C,N) — abrasive wear resistance, bonds to substrate
• Carbide substrate — tough core

CVD coatings are used on indexable BTA inserts for steel and cast iron where the thicker coating provides longer abrasive wear life.

Grade Selection by Workpiece Material

Steel (ISO P)

For deep hole drilling of carbon and alloy steels:

Stainless Steel (ISO M)

For austenitic (304, 316), duplex (2205, 2507), and martensitic stainless:

Cast Iron (ISO K)

Cast iron is abrasive but generates lower cutting temperatures than steel, allowing grades optimized for wear resistance:

Non-Ferrous Metals (ISO N)

Aluminum, brass, bronze, copper, and plastics:

Heat-Resistant Superalloys and Titanium (ISO S)

These materials generate the highest cutting temperatures and require grades with maximum thermal stability:

Hardened Steel (ISO H)

For drilling hardened steel above 45 HRC, specialized grades are required:

Grade Selection for Guide Pads

Guide pads in BTA and gun drilling tools have different requirements than cutting edges. They must resist abrasive wear from sliding contact with the bore wall without scoring the finished surface.

Guide pads should generally be one step tougher (higher ISO number) than the cutting edge grade for the same workpiece material, because the pads experience impact loading at each rotation.

Sandvik Coromant Grade System

Sandvik Coromant grades are designated by a four-character code that encodes the application area and generation.

Common Sandvik Drilling Grades

Sandvik Turning Grades (Used for Indexable BTA Inserts)

Mitsubishi Carbide Grade System

Mitsubishi uses a different designation system, but the ISO classification is cross-referenced.

Common Mitsubishi Drilling Grades

Grade Selection Decision Flowchart

1. Identify the workpiece material group (ISO P/M/K/N/S/H)
2. Determine the operation type (finishing, general, roughing, interrupted cut)
3. Select the appropriate ISO subgroup number (lower for finishing, higher for roughing)
4. Choose the coating type (PVD for sharp edges and drilling, CVD for maximum wear resistance on indexable inserts)
5. Verify substrate properties (grain size and cobalt content appropriate for the material)
6. Select the manufacturer’s equivalent grade from their grade chart
7. Test — run a comparative test with the current grade at the same parameters

For related reading, see the Tool Coatings for Deep Hole Drilling Guide, the Deep Hole Drilling Tool Regrinding Guide, and the Material Machinability Ratings for Deep Hole Drilling.