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An injection mold manufacturer producing mold inserts for automotive interior trim components was drilling 12 mm diameter × 600 mm deep cooling channel bores in H13 tool steel (48–52 HRC, vacuum heat-treated and double-tempered). Using standard gun drilling with C2 grade carbide and TiAlN coating at Vc = 50 m/min, f = 0.025 mm/rev, and coolant pressure of 60 bar, tool life was only 1.8 meters per edge. The failure mechanism was rapid flank wear accelerated by the chromium carbide and vanadium carbide particles in the H13 microstructure — these carbides have hardnesses of 1,800–2,800 HV, comparable to or harder than the tungsten carbide tool substrate (1,500–1,800 HV for C2 grade). The wear rate was approximately 0.08 mm of flank wear per meter of cutting, and the tool was considered failed when flank wear reached 0.15 mm. The scrap rate from out-of-tolerance bore diameter (caused by the progressive reduction in effective cutting diameter as flank wear advanced) was 8%. A systematic tool material evaluation compared: submicron carbide with AlCrN coating (baseline improvement), TiAlN-coated carbide with higher cobalt content, CBN-tipped (60% CBN, ceramic binder), CBN-tipped (90% CBN, metallic binder), and PCD-tipped. The CBN-tipped gun drill with 60% CBN content and ceramic binder, operating at Vc = 35 m/min, f = 0.035 mm/rev, and coolant pressure of 120 bar, achieved tool life of 14.5 meters per edge — an 8× improvement over carbide. The bore surface finish improved from Ra 2.5 µm to Ra 0.9 µm, and the scrap rate dropped from 8% to 0.5%. The cycle time increased by 22% due to the lower cutting speed, but the total cost per bore decreased by 52% because tool changes were reduced from one every 72 mm of drilling to one every 580 mm.
Metallurgical Behavior of Hardened Steels During Drilling
Hardened steels in the 40–60 HRC range have a tempered martensite microstructure with dispersed carbide particles. The machining characteristics are determined by three microstructural features: the martensite matrix hardness (which determines the cutting forces), the carbide particle type, size, and distribution (which determines the abrasive wear rate), and the retained austenite content (which affects work hardening and built-up edge formation).
Cutting Mechanism
In hardened steel cutting, the chip formation mechanism transitions from continuous ductile shearing to a "segmented chip" or "saw-tooth chip" formation — also known as adiabatic shear banding. The chip forms by localized shear in narrow bands where the temperature rise is sufficient to cause thermal softening, alternating with periods of elastic deformation and fracture. This produces a chip with a characteristic saw-tooth profile and a segmented structure that is beneficial for chip evacuation (the chips are short and broken) but produces fluctuating cutting forces that can excite vibration in the tool and workpiece system.
The specific cutting force for hardened steel at 50 HRC is approximately 3,000–4,000 N/mm² — 1.5–2× that of annealed carbon steel at 200 HB (approximately 2,000 N/mm²). The cutting forces fluctuate by 20–40% during each chip segmentation cycle, and this cyclic loading can cause micro-chipping of the cutting edge, particularly if the tool substrate lacks sufficient toughness.
Abrasive Wear Mechanism
The carbide particles in hardened steels are the primary driver of tool wear. The type and hardness of the carbides vary by steel grade: cementite (Fe₃C) in carbon steels — hardness 1,200–1,400 HV; chromium carbides (Cr₇C₃, Cr₂₃C₆) in tool steels such as H13, D2, and A2 — hardness 1,800–2,200 HV; molybdenum carbides (Mo₂C) in high-speed steels and hot-work tool steels — hardness 1,800–2,200 HV; and vanadium carbides (VC) in high-vanadium tool steels such as D2 and PM grades — hardness 2,500–2,800 HV.
When the carbide particle hardness exceeds the tool material hardness (1,500–1,800 HV for conventional tungsten carbide), the workpiece carbides abrade the tool material directly. This is the "hard particle abrasion" regime, and tool life decreases exponentially with increasing workpiece carbide hardness. Vanadium carbides at 2,500–2,800 HV can abrade even the hardest carbide grades, which is why CBN (3,500–4,500 HV) or PCD (6,000–8,000 HV) tooling is required for high-vanadium tool steels and for hardened steels above 55 HRC.
Thermal Effects
The cutting zone temperature in hardened steel drilling is 600–900 °C at typical cutting speeds of 25–50 m/min — lower than for stainless steel or superalloys because the chip segmentation process carries heat away in the chip more efficiently than continuous chip formation. However, the high compressive strength of hardened steel means that the stress on the tool edge at these temperatures is very high, and the combination of high stress and elevated temperature creates a demanding environment for the tool coating — the coating must maintain its hardness and oxidation resistance at the cutting temperature while withstanding the cyclic loading from chip segmentation.
Tool Material Selection
CBN (Cubic Boron Nitride)
CBN is the recommended tool material for production deep hole drilling of hardened steels above 45 HRC. CBN has a hardness of 3,500–4,500 HV (second only to diamond), thermal stability to 1,200 °C (superior to PCD, which degrades above 700 °C), chemical inertness to iron (unlike PCD, which reacts chemically with iron at elevated temperatures), and high abrasion resistance against carbide particles.
CBN tool materials are classified by CBN content and binder type:
Low-content CBN (40–65% CBN, ceramic binder) — The recommended grade for hardened steel deep hole drilling. The ceramic binder (typically TiCN or Al₂O₃) provides chemical stability and wear resistance, and the lower CBN content provides a tougher, more fracture-resistant edge than high-content grades. This grade is preferred for interrupted cutting conditions (peck cycles, variable material hardness) and for finish operations where surface finish is critical.
High-content CBN (80–95% CBN, metallic binder) — Higher wear resistance but lower toughness. Recommended for continuous cutting of consistent hardened steel (same hardness throughout the bore) where maximum tool life is required and edge chipping risk is low. The metallic binder (typically cobalt or nickel) provides higher thermal conductivity, which helps conduct heat away from the cutting edge.
PCD (Polycrystalline Diamond)
PCD (6,000–8,000 HV) offers the highest abrasion resistance of any tool material but is chemically reactive with iron at the cutting temperatures encountered in hardened steel drilling (above 600 °C, carbon from the diamond diffuses into the iron chip, producing rapid chemical wear). PCD is therefore not recommended for deep hole drilling of ferrous hardened steels. It can be considered for non-ferrous hardened materials (such as hardened aluminum bronzes or Stellite cobalt alloys) where abrasion resistance is required and chemical reactivity with the workpiece is not a concern.
Advanced Carbide Grades
For hardened steels in the 40–48 HRC range, advanced carbide grades — submicron or ultrafine tungsten carbide with 6–8% cobalt content and advanced coatings (AlTiN, AlCrN, or TiAlSiN) — can provide acceptable tool life for lower-volume production or where CBN tooling is not available. The carbide grade must have: submicron grain size (0.4–0.8 µm) for edge sharpness, low cobalt content (6–8%) for hot hardness, and a coating with high oxidation stability (AlTiSiN or AlCrN) for cutting temperature resistance. Expected tool life for advanced carbide in 40–48 HRC steel is 3–8 meters per edge — approximately 30–50% of CBN tool life at the same parameters.
Tool Geometry for Hardened Steel
Tool geometry requirements for hardened steel deep hole drilling differ significantly from those for annealed steel. The cutting edge requires: a negative to neutral rake angle (0 to −6° for carbide, 0 to −10° for CBN) to provide edge strength under the high compressive loads; a large edge hone (30–50 µm for carbide, 20–40 µm for CBN) to distribute the cutting forces and prevent edge chipping; a small relief angle (4–7°) to provide adequate clearance while maintaining edge support; and a wiper flat or chamfer on the secondary cutting edge (0.1–0.3 mm width) to improve surface finish.
Parameter Guidelines by Hardness Range
| Hardness Range | Tool Material | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure (bar) | Edge Hone (µm) | Expected Tool Life (m) | Typical Ra (µm) |
|---|---|---|---|---|---|---|---|
| 40–45 HRC | Submicron WC, 6–8% Co | 30–50 | 0.025–0.050 | 60–120 | 20–35 | 5–12 | 1.0–2.0 |
| 40–45 HRC | CBN (60%, ceramic) | 40–60 | 0.030–0.060 | 60–120 | 20–30 | 15–30 | 0.5–1.2 |
| 45–50 HRC | Submicron WC, 6% Co | 20–35 | 0.020–0.040 | 80–150 | 30–45 | 3–8 | 1.2–2.5 |
| 45–50 HRC | CBN (60–90%, ceramic/metallic) | 30–50 | 0.025–0.050 | 80–150 | 25–35 | 12–25 | 0.6–1.5 |
| 50–55 HRC | CBN (60%, ceramic) | 25–40 | 0.020–0.040 | 100–180 | 30–40 | 8–20 | 0.8–1.8 |
| 50–55 HRC | CBN (90%, metallic) | 25–40 | 0.025–0.045 | 100–180 | 20–30 | 10–22 | 0.7–1.6 |
| 55–60 HRC | CBN (60–90%, ceramic) | 15–30 | 0.015–0.030 | 120–200 | 35–50 | 5–15 | 1.0–2.2 |
Application-Specific Strategies
Mold Cooling Channels (H13, 420SS, P20 at 45–52 HRC)
Mold cooling channels are among the most challenging hardened steel deep hole drilling applications because: the bores are typically long (500–2,000 mm) with small diameters (6–16 mm), the material is at full hardness, the bores must be smooth for water flow (cooling efficiency), and the mold geometry often requires complex cross-drilled patterns with intersecting bores. The recommended strategy for mold cooling channel gun drilling is: CBN-tipped gun drills (60% CBN, ceramic binder) for tool life and surface finish; cutting speed 28–35 m/min for H13 at 48–52 HRC; feed 0.025–0.040 mm/rev; coolant pressure 100–150 bar; and a steady rest at maximum 500 mm intervals to support the long, slender workpiece and prevent bore straightness deviation. For intersecting cooling channels, the cross-hole intersection should be deburred using a flexible hone or TEM process after drilling.
Bearing Races and Ring Components (52100, M50, at 58–62 HRC)
Through-hardened bearing steels such as AISI 52100 (1.0% C, 1.5% Cr) and M50 tool steel (4% Cr, 4% Mo, 1% V) at 58–62 HRC are among the hardest materials drilled in production deep hole drilling. These applications typically involve drilling small-diameter bores (6–25 mm) in rings and races for bearings and power transmission components. The recommended strategy is: CBN-tipped tools (60% CBN with ceramic binder) with a negative rake angle of −5 to −10° and edge hone of 40–50 µm; cutting speed of 12–22 m/min; feed of 0.015–0.030 mm/rev; and coolant pressure of 150–200 bar. The extreme hardness requires exceptionally rigid machine tools and fixturing — any vibration or deflection will cause edge chipping and rapid tool failure.
Die Components (D2, A2, PM Steels at 58–62 HRC)
Cold-work tool steels such as D2 (12% Cr, 1.5% C) and powder metal (PM) tool steels contain very high volume fractions of hard chromium and vanadium carbides that produce extreme abrasive wear. For these materials, only CBN tools provide acceptable tool life. The recommended approach is: high-content CBN (85–95% with metallic binder) for maximum abrasion resistance; cutting speed of 10–18 m/min; feed of 0.012–0.025 mm/rev; and coolant pressure of 150–200 bar. The cutting speed is limited by the cutting temperature — D2 and PM steels generate higher cutting temperatures than H13 at equivalent speeds due to their higher carbide content, and the tool edge temperature must be kept below 800–900 °C to prevent CBN binder softening.
FAQ
What tool material is best for deep hole drilling 50+ HRC steel?
For hardened steel above 50 HRC, CBN (cubic boron nitride) is the recommended tool material. CBN provides the necessary combination of hardness (3,500–4,500 HV), abrasion resistance against carbide particles in the steel, chemical stability (does not react with iron at cutting temperatures), and thermal stability (stable to 1,200 °C). For the 50–55 HRC range, low-content CBN (60% CBN with ceramic binder) is recommended for its superior toughness. For 55–60 HRC, higher-content CBN (85–90% with metallic binder) provides better wear resistance. CBN-tipped gun drills cost 3–5× more than carbide equivalents but deliver 3–8× longer tool life in hardened steel, making the cost per bore significantly lower.
Can PCD tools be used for hardened steel deep hole drilling?
PCD is not recommended for hardened steel deep hole drilling. Although PCD is the hardest known tool material (6,000–8,000 HV), it is chemically reactive with iron at the cutting temperatures encountered in hardened steel drilling (600–900 °C). The carbon in the diamond diffuses into the iron chip, producing rapid chemical wear that can destroy the cutting edge within meters of cutting. PCD also has limited thermal stability — it begins to graphitize (convert from diamond to graphite) at approximately 700 °C, losing its hardness and wear resistance. PCD is suitable for non-ferrous materials (aluminum, brass, composites) but should not be used for ferrous hardened steels.
What cutting speed should be used for gun drilling H13 at 48–52 HRC?
For gun drilling H13 tool steel at 48–52 HRC with CBN-tipped tools, the recommended cutting speed range is 28–38 m/min, with 32–35 m/min as the optimal starting range. With carbide tools (submicron grade, AlCrN or AlTiSiN coating), the recommended speed is 22–30 m/min, with 25–28 m/min as the starting range. The speed should be adjusted based on tool life data: if tool life is limited by flank wear, reduce speed by 10–15%; if limited by edge chipping or notch wear, increase speed by 10–15% (the higher temperature can increase material ductility and reduce chipping). At the recommended speeds, expected tool life is 12–25 meters per edge for CBN and 3–8 meters for advanced carbide.
How does coolant pressure affect tool life in hardened steel drilling?
Coolant pressure has a direct and significant effect on tool life in hardened steel drilling. Increasing coolant pressure from 60 bar to 120 bar typically improves tool life by 50–100% for carbide tools and 30–60% for CBN tools. The improvement comes from three mechanisms: improved heat removal from the cutting edge (critical for maintaining edge hardness and preventing thermal softening of the carbide substrate or CBN binder), improved chip evacuation (hardened steel chips, though segmented, are abrasive and can score the bore surface if not evacuated quickly), and hydraulic damping of the drill tube (higher pressure provides additional support to the drill tube, reducing vibration and edge chipping). The recommended minimum coolant pressure for hardened steel above 45 HRC is 80 bar, with 120–180 bar recommended for production reliability.
What is the expected surface finish when gun drilling hardened steel?
Gun drilling of hardened steel (45–55 HRC) with CBN-tipped tools typically produces bore surface finish of Ra 0.5–1.8 µm, depending on cutting parameters, tool geometry, and the specific steel grade. The finish tends to be better than gun drilling of annealed steel because the chip segmentation process produces a cleaner cut surface with less built-up edge. With optimized parameters — CBN tool, Vc = 30–40 m/min, f = 0.025–0.040 mm/rev, coolant pressure 120+ bar — surface finish of Ra 0.6–1.2 µm is achievable. For applications requiring finer finishes (Ra < 0.4 µm), a subsequent honing or skiving and burnishing operation should be considered.
Disclaimer: The process parameters, tool selection recommendations, and performance data presented in this article are based on published technical literature, tooling manufacturer specifications, and industry-reported experience with deep hole drilling of hardened steels. Actual results depend on specific steel grade and heat treatment condition, carbide particle type and distribution, machine tool rigidity, coolant system capability, and tooling quality. The cutting parameters provided should be used as starting recommendations and verified through process development trials for each specific application. CBN and PCD tooling require appropriate machine tool rigidity and coolant system capability to realize their full potential. No guarantee of specific tool life, bore quality, or process stability is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.