Hard turning of hardened steels in the 45–65 HRC range has become a viable alternative to cylindrical grinding for many precision components. At the center of this process evolution are cubic boron nitride (CBN) turning inserts, which combine extreme hardness with thermal stability far beyond conventional carbide. This article delivers a technical comparison between Korloy and Sumitomo Electric CBN turning grade families, examining substrate design, coating architecture, recommended cutting parameters, and application best practices for die and mold, automotive, and bearing industries.
1. Why CBN for Hardened Steel Turning?
Conventional cemented carbide inserts fail rapidly above 50 HRC due to flank wear driven by abrasive interaction with iron carbides and thermal softening of the cobalt binder. CBN, with a Knoop hardness of approximately 4,700 HK and thermal stability up to 1,400 °C in non-oxidizing environments, maintains cutting edge integrity at the elevated temperatures generated during hard turning.
Key benefits of CBN hard turning include:
- Elimination of separate grinding operations, reducing process flow time by 40–70%
- Capability to machine complex contours and undercuts impossible to access with grinding wheels
- Better surface integrity through compressive residual stress generation compared to grinding-induced tensile stress
- Chip formation instead of dust generation, improving workplace environment and coolant filtration
2. Korloy CBN Grade Architecture
2.1 Substrate and Grain Structure
Korloy offers two primary CBN insert families for hardened steel machining: the KBN25M high-CBN content grade and the KBN35M mixed-ceramic reinforced grade. The KBN25M substrate contains approximately 90–93% CBN by volume, bonded with a Ti-based ceramic phase. This composition prioritizes thermal conductivity and abrasive wear resistance, making it suitable for continuous turning of bearing steels such as 100Cr6 (AISI 52100) and case-hardened 20MnCr5.
The KBN35M grade reduces CBN content to roughly 65–70%, introducing a higher proportion of TiCN/TiAlN ceramic binder. This structure improves fracture toughness and edge stability, extending tool life in interrupted cuts common when machining hardened shafts with keyways or cross-holes.
2.2 Edge Preparation and Chipbreaker Geometry
Korloy hard turning inserts utilize a T-land edge preparation with a land width of 0.10–0.20 mm and a negative land angle of −15° to −25°. This design creates a robust wedge that resists micro-chipping under high specific cutting forces. Chipbreaker geometries for the KBN series are intentionally minimal; hard turning typically produces ribbon or segmented chips, and aggressive chipbreakers can initiate crack propagation into the insert body.
3. Sumitomo CBN Grade Architecture
3.1 Substrate and Coating Innovations
Sumitomo Electric distinguishes its CBN portfolio through the BNC100, BNC200, and BNC300 series. The BNC100 grade employs a high-purity CBN substrate with a patented Al-based binder system, achieving superior chemical stability during high-speed machining of hardened steels. Laboratory data indicates that BNC100 maintains a stable crater wear profile at cutting speeds exceeding 200 m/min on 60 HRC tool steel.
The BNC200 grade introduces a multi-layer CVD coating specifically developed for CBN substrates. The coating architecture consists of a TiN adhesion layer, a thick TiCN wear-resistant layer, and an outer Al₂O₃ thermal barrier. This combination reduces diffusion wear at the tool–chip interface, a dominant failure mode when machining high-chromium cold-work steels such as D2 (1.2379) at 58–62 HRC.
For heavy interrupted cutting, Sumitomo offers the BNC300 grade with a bimodal CBN grain distribution. Fine 1–2 μm grains provide hardness and wear resistance, while coarse 4–6 μm grains arrest crack propagation. The resulting fracture toughness of approximately 7.2 MPa·m¹/² places BNC300 among the most durable CBN grades for automotive gear hard turning.
3.2 Chip Control and Insert Styles
Sumitomo provides both solid CBN tips and brazed CBN tips on carbide substrates. The brazed construction reduces insert cost and enables the use of standard ISO negative insert holders. For precision finish hard turning, solid CBN tips in TNGM and CNGA geometries deliver the dimensional stability required for IT6–IT7 tolerance bands.
4. Technical Grade Comparison
| Property | Korloy KBN25M | Korloy KBN35M | Sumitomo BNC100 | Sumitomo BNC200 | Sumitomo BNC300 |
|---|---|---|---|---|---|
| CBN Content (vol.%) | 90–93% | 65–70% | 92–95% | 88–92% | 70–75% |
| Binder System | Ti-based ceramic | TiCN/TiAlN ceramic | Al-based ceramic | TiN/TiCN/Al₂O₃ CVD | Ti-based + bimodal CBN |
| Typical Hardness (HV) | 3,200–3,400 | 3,000–3,200 | 3,400–3,600 | 3,300–3,500 | 3,100–3,300 |
| Fracture Toughness K_IC | 4.8 MPa·m¹/² | 6.2 MPa·m¹/² | 4.5 MPa·m¹/² | 5.0 MPa·m¹/² | 7.2 MPa·m¹/² |
| Thermal Conductivity | 100 W/m·K | 65 W/m·K | 110 W/m·K | 95 W/m·K | 70 W/m·K |
| Optimal Workpiece HRC | 55–64 | 45–58 | 58–65 | 55–64 | 45–62 |
| Cutting Mode | Continuous | Interrupted / Light | Continuous / High-speed | Continuous / Semi-interrupted | Heavy Interrupted |
5. Cutting Parameters for Hardened Steel Hard Turning
The following parameter recommendations represent starting values derived from published technical datasheets and field application data. Actual optimization requires adjustment based on machine rigidity, workpiece geometry, and coolant strategy.
5.1 Continuous Turning: Bearing Steel (100Cr6, 60–62 HRC)
| Grade | Cutting Speed Vc (m/min) | Feed fn (mm/rev) | Depth of Cut ap (mm) | Expected Tool Life (min) |
|---|---|---|---|---|
| Korloy KBN25M | 120–160 | 0.08–0.15 | 0.10–0.25 | 25–35 |
| Sumitomo BNC100 | 140–200 | 0.08–0.12 | 0.10–0.20 | 30–45 |
| Sumitomo BNC200 | 120–180 | 0.10–0.18 | 0.15–0.30 | 28–40 |
5.2 Semi-Interrupted Turning: Cold-Work Tool Steel (D2 / 1.2379, 58–60 HRC)
| Grade | Cutting Speed Vc (m/min) | Feed fn (mm/rev) | Depth of Cut ap (mm) | Expected Tool Life (min) |
|---|---|---|---|---|
| Korloy KBN35M | 80–120 | 0.10–0.18 | 0.15–0.35 | 15–22 |
| Sumitomo BNC300 | 90–130 | 0.12–0.20 | 0.20–0.40 | 18–28 |
5.3 Finish Hard Turning: Case-Hardened Steel (20MnCr5, 58–62 HRC Surface)
| Grade | Cutting Speed Vc (m/min) | Feed fn (mm/rev) | Depth of Cut ap (mm) | Surface Roughness Ra (μm) |
|---|---|---|---|---|
| Korloy KBN25M | 150–200 | 0.05–0.10 | 0.05–0.15 | 0.4–0.8 |
| Sumitomo BNC100 | 160–220 | 0.05–0.08 | 0.05–0.12 | 0.3–0.6 |
6. Machining Strategy and Best Practices
6.1 Machine Tool Requirements
Hard turning imposes extreme demands on machine rigidity and spindle accuracy. Recommended machine specifications include:
- Spindle runout below 2 μm at the tool nose
- Machine loop stiffness greater than 50 N/μm in the cutting force direction
- High-resolution linear scales with 0.1 μm feedback resolution for precision contouring
- Thermal stability management to maintain dimensional accuracy over batch production
6.2 Tool Overhang and Insert Seating
Minimize tool overhang to reduce chatter, which causes micro-chipping of the CBN cutting edge. For external turning, an overhang ratio (length/diameter) below 4:1 is preferred. Use precision-ground insert seats and torque wrenches to achieve uniform clamping pressure. Uneven seating creates localized stress concentrations that initiate fractures in the brittle CBN substrate.
6.3 Coolant and Dry Machining Considerations
CBN grades exhibit divergent behavior with coolant depending on the binder system. High-CBN grades such as KBN25M and BNC100 generally perform best under dry cutting conditions or with minimal compressed air delivery. Thermal shock from flood coolant can induce cracking due to the mismatch in thermal expansion between CBN grains and the ceramic binder.
For lower-CBN grades with higher ceramic content (KBN35M, BNC300), a low-pressure coolant stream (10–20 bar) can be beneficial in interrupted cutting, assisting chip evacuation and reducing thermal cycling amplitude at the cutting edge. Never apply coolant directly to a hot insert during tool change; allow thermal equilibrium to prevent thermal shock fracture.
6.4 Edge Life Monitoring and Wear Criteria
Standard ISO 3685 wear criteria for carbide do not directly translate to CBN hard turning. Recommended replacement thresholds are:
- Flank wear VB_max = 0.15–0.20 mm for finish operations
- Flank wear VB_max = 0.25–0.30 mm for roughing operations
- Catastrophic fracture or chipping exceeding 0.05 mm on the rake face: immediate replacement
- Surface roughness degradation exceeding Ra 1.6 μm on finish passes: preventive replacement
7. Application Benchmarks and Field Observations
In a controlled comparison machining 62 HRC M2 high-speed steel pins, Sumitomo BNC100 achieved a 15% longer tool life than Korloy KBN25M at Vc = 160 m/min, fn = 0.10 mm/rev, and ap = 0.15 mm. The primary wear mechanism for both grades was flank wear driven by abrasive interaction with vanadium-rich MC carbides. However, KBN25M produced a marginally better surface finish (Ra 0.45 μm vs. 0.52 μm), attributed to its finer edge honing capability.
On automotive pinion gear shafts (20MnCr5, 60 HRC surface hardness) with periodic keyway interruption, Sumitomo BNC300 outperformed Korloy KBN35M by approximately 22% in edge integrity life. The bimodal grain structure of BNC300 arrested crack propagation originating at the interrupted cut entry points, while KBN35M exhibited progressive micro-chipping along the T-land.
8. Selection Guide
| Application Scenario | Recommended Grade | Rationale |
|---|---|---|
| High-speed continuous finish turning, bearing races | Sumitomo BNC100 | Highest thermal stability and speed capability |
| General continuous hard turning, tool steels | Korloy KBN25M or Sumitomo BNC200 | Balanced wear resistance and cost efficiency |
| Interrupted hard turning, shafts with keyways | Sumitomo BNC300 | Superior fracture toughness and crack resistance |
| Mixed continuous/interrupted, medium HRC | Korloy KBN35M | Good toughness with acceptable wear rate |
| High-chromium tool steels (D2, D3) | Sumitomo BNC200 | Al₂O₃ CVD barrier reduces diffusion wear |
9. Conclusion
Both Korloy and Sumitomo offer technically mature CBN turning insert families capable of replacing grinding in a wide spectrum of hardened steel applications. Sumitomo leads in high-speed continuous machining and coated CBN innovation, while Korloy delivers competitive performance with robust edge preparation for general hard turning. For operations involving significant interruption or thermal variation, Sumitomo BNC300 and Korloy KBN35M provide the necessary fracture toughness. Engineers should select grades based on the specific wear mechanism dominant in their application—abrasive wear favoring high-CBN content, diffusion wear favoring coated architectures, and mechanical shock favoring toughened ceramic matrices.
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