Appearance
An aerospace tooling manufacturer must deep-drill 500 maraging steel C300 injection moulding core inserts per year, with 12 mm diameter × 400 mm deep conformal cooling channels. Initial attempts using standard H13 tool steel parameters result in rapid flank wear and built-up edge within five holes — maraging steel's high nickel content (18%) and age-hardened condition (52 HRC) create a tough, abrasive chip that destroys standard carbide tools. The solution requires drilling in the solution-annealed condition (32 HRC) before age hardening: carbide gun drills at 50–70 m/min with 60–80 bar coolant pressure and TiAlN coating achieve 60–100 holes per regrind in annealed material. For unavoidable post-aging drilling in C300 at 50–55 HRC, speeds must drop to 15–30 m/min with AlCrN coating and coolant pressure increased to 80–120 bar.
Maraging Steel Properties for Deep Hole Drilling
| Property | C250 (Annealed) | C250 (Aged 48–52 HRC) | C300 (Aged 50–55 HRC) | C350 (Aged 55–60 HRC) |
|---|---|---|---|---|
| Hardness | 30–35 HRC | 48–52 HRC | 50–55 HRC | 55–60 HRC |
| Tensile strength (MPa) | 800–900 | 1,700–1,900 | 1,900–2,100 | 2,100–2,400 |
| Nickel content (%) | 18 | 18 | 18 | 18 |
| Thermal conductivity (W/m·K) | 20–25 | 20–25 | 20–25 | 20–25 |
| Elongation (%) | 10–17 | 6–10 | 5–8 | 4–7 |
| Machinability rating | 40% | 15% | 10% | 5% |
| Chip formation | Stringy, tough | Segmented, abrasive | Segmented, highly abrasive | Short, extremely hard |
Cutting Parameter Recommendations
| Parameter | C250 (Annealed) | C250 (Aged) | C300 (Annealed) | C300 (Aged) | C350 (Annealed) | C350 (Aged) |
|---|---|---|---|---|---|---|
| Cutting speed — carbide (m/min) | 60–80 | 25–40 | 50–70 | 15–30 | 45–60 | 8–20 |
| Feed — 6 mm dia (mm/rev) | 0.020–0.040 | 0.010–0.025 | 0.015–0.035 | 0.008–0.020 | 0.012–0.030 | 0.005–0.015 |
| Feed — 10 mm dia (mm/rev) | 0.030–0.060 | 0.015–0.035 | 0.025–0.050 | 0.012–0.030 | 0.020–0.045 | 0.008–0.025 |
| Feed — 20 mm dia (mm/rev) | 0.050–0.090 | 0.025–0.050 | 0.040–0.080 | 0.020–0.045 | 0.035–0.065 | 0.012–0.035 |
| Coolant pressure (bar) | 40–80 | 80–120 | 60–100 | 80–120 | 60–100 | 80–140 |
| Recommended coating | TiAlN | AlCrN | TiAlN | AlCrN | TiAlN | AlCrN |
| Expected tool life (holes per regrind) | 80–120 | 25–50 | 60–100 | 20–40 | 40–80 | 10–25 |
Feed Rate by Drill Diameter
| Drill Dia (mm) | C250 Annealed Feed (mm/rev) | C300 Annealed Feed (mm/rev) | C350 Annealed Feed (mm/rev) | RPM at 60 m/min | Penetration (mm/min) |
|---|---|---|---|---|---|
| 4 | 0.015–0.030 | 0.012–0.025 | 0.008–0.020 | 4,775 | 70–145 |
| 6 | 0.020–0.040 | 0.015–0.035 | 0.012–0.030 | 3,185 | 65–125 |
| 8 | 0.025–0.050 | 0.020–0.045 | 0.015–0.035 | 2,390 | 60–120 |
| 10 | 0.030–0.060 | 0.025–0.050 | 0.020–0.045 | 1,910 | 55–115 |
| 12 | 0.035–0.070 | 0.030–0.060 | 0.025–0.050 | 1,590 | 50–110 |
| 16 | 0.045–0.080 | 0.035–0.070 | 0.030–0.055 | 1,195 | 40–95 |
| 20 | 0.050–0.090 | 0.040–0.080 | 0.035–0.065 | 955 | 35–85 |
| 25 | 0.060–0.100 | 0.050–0.090 | 0.040–0.075 | 765 | 30–75 |
Tool Geometry for Maraging Steel
| Geometry Parameter | Annealed Condition | Aged Condition |
|---|---|---|
| Point angle | 130–140° | 135–145° |
| Rake angle | 6–10° positive | 4–8° positive (reduced) |
| Relief / clearance angle | 8–12° | 10–14° |
| Edge preparation | Sharp with light hone (0.02 mm) | T-land 0.05–0.10 mm |
| Coating | TiAlN | AlCrN (higher hot hardness) |
| Carbide grade | Micrograin (0.5–1.0 µm) | Fine grain with high Co content |
| Tip displacement | 0.22–0.25 × D | 0.20–0.22 × D |
| Guide bushing tolerance | G6 | H6 |
WARNING
Maraging steel's machinability changes dramatically between heat treatment conditions. In the solution-annealed condition (30–35 HRC), it machines similarly to 4340 steel at equivalent hardness — chip formation is manageable and tool wear is progressive. After age hardening to full strength (48–60 HRC depending on grade), the material becomes as difficult as nickel-based superalloys — the 18% nickel content creates a tough, abrasive chip that work-hardens rapidly. The recommended strategy is: drill all deep holes in the solution-annealed condition, then age harden. Age hardening shrinkage is only 0.05–0.10%, which is predictable and can be compensated in the drilling process. If post-aging drilling is unavoidable (for repair, design changes, or geometry that would distort during aging), limit to through-holes under 20× diameter, use AlCrN-coated carbide with reduced feeds, and expect tool life of 10–40 holes per regrind depending on grade.
Coolant Selection and Parameters
| Coolant Type | Annealed Suitability | Aged Suitability | Key Requirements |
|---|---|---|---|
| Neat oil with EP additives | Excellent | Excellent | Sulphurised EP, 10–20 cSt viscosity |
| Semi-synthetic emulsion | Good | Fair | 8–12% concentration, not ideal for aged condition |
| Water-based | Poor — avoid | Poor — avoid | Insufficient lubricity for high-strength material |
| Coolant pressure — recommendation | 40–100 bar | 80–140 bar | Higher end for deeper holes and aged material |
| Filtration | 10–15 µm | 10–15 µm | Essential — unfiltered recirculating swarf damages guide pads |
Chip Morphology and Control
| Chip Type | Appearance | Condition | Risk Level | Corrective Action |
|---|---|---|---|---|
| Segmented / sawtooth (ideal) | Uniform segments | Annealed, correct parameters | Low | Maintain |
| Long ribbon | Continuous > 50 mm | Annealed, feed too low | Medium | Increase feed 15–20% |
| Short hard segments | Small, abrasive chips | Aged (normal) | Low | Maintain — check tool wear frequency |
| Powder / dust | Fine particles | Any, tool worn | Critical | Replace tool immediately |
| Discoloured (blue) | Heat tint | Speed too high, coolant insufficient | Critical | Reduce speed 20%, verify coolant pressure |
| Built-up edge chips | Irregular, smeared | Aged, speed too low | High | Increase speed slightly, check coating |
Surface Finish Expectations
| Condition | Ra (µm) | Rz (µm) | Notes |
|---|---|---|---|
| Carbide gun drill, annealed, new | 0.4–1.2 | 4–10 | Achievable with correct parameters |
| Carbide gun drill, aged, new | 0.8–1.6 | 6–15 | Coarser due to higher hardness |
| Production drilling, mid-life | 1.6–3.2 | 10–25 | Acceptable for mould tooling |
| Worn tool or BUE present | > 3.2 | > 25 | Replace tool |
| BTA drilling | 3.2–6.3 | 20–40 | May require secondary finishing |
Troubleshooting
| Symptom | Likely Cause | Solution |
|---|---|---|
| Rapid flank wear in annealed material | Speed too high, coating inadequate | Reduce speed, switch to TiAlN if using uncoated |
| Edge chipping in aged C300/C350 | Excessive cutting forces, vibration | Reduce feed, increase T-land, check bushing fit |
| Built-up edge in annealed material | Nickel adhesion to tool | Increase speed slightly, verify coolant EP additives |
| Poor surface finish | Tool wear, BUE, vibration | Replace tool, reduce overhang, check alignment |
| Hole oversize | Tool wear, guide pad wear | Replace at VB ≥ 0.15 mm, inspect guide pads |
| Tool breakage | Chip packing, coolant loss | Verify coolant pressure, implement peck cycle |
| Work hardened surface | Dwell, dull tool, feed too low | Eliminate dwell, replace tool, maintain minimum feed |
| Torque spike | Chip packing, material hard spot | Retract, clear chips, inspect material certificate |
| Chatter / vibration | Overhang excessive, feed low | Reduce overhang, increase feed 15%, check bushing |
FAQ
What cutting speed is recommended for gun drilling maraging steel C300?
For C300 maraging steel in the solution-annealed condition (30–35 HRC), recommended cutting speed is 50–70 m/min (165–230 SFM) with TiAlN-coated carbide gun drills. For the age-hardened condition (50–55 HRC), reduce to 15–30 m/min (50–100 SFM) with AlCrN coating — a 60% reduction. The high nickel content (18%) makes maraging steel significantly more difficult to machine than conventional tool steels at equivalent hardness. For depths exceeding 30× diameter, use the lower end of the speed range. For HSS gun drills, reduce speed to 15–25 m/min in annealed and 5–10 m/min in aged condition.
What feed rate should be used for deep hole drilling maraging steel?
For C300 maraging steel in the annealed condition: 0.015–0.035 mm/rev for 6 mm, 0.025–0.050 mm/rev for 10 mm, and 0.040–0.080 mm/rev for 20 mm diameter. For the aged condition, reduce feed by 30–50% due to the higher cutting forces and abrasiveness. Minimum chip thickness should be 0.010 mm in annealed and 0.008 mm in aged to prevent rubbing and work hardening. The general guideline for annealed material is feed per revolution = D/200 to D/400.
Is it better to drill maraging steel before or after aging?
Drilling before aging (in the solution-annealed condition at 30–35 HRC) is strongly preferred. Tool life is 3–5× longer, cutting speeds can be 2–3× higher, and chip formation is more manageable. Age hardening shrinkage is predictable at 0.05–0.10%, so holes can be drilled undersize and will shrink to final dimension during aging. If post-aging drilling is unavoidable, use AlCrN-coated carbide with cutting speeds reduced to 15–30 m/min (C300) and expect tool life of 20–40 holes per regrind. The primary reasons to drill after aging are: repair of aged components, design changes after heat treatment, and geometries that would distort during the aging cycle.
What coolant pressure is needed for maraging steel deep hole drilling?
For annealed maraging steel: minimum 40 bar, recommended 60–100 bar depending on hole depth. For aged maraging steel: minimum 80 bar, recommended 80–140 bar. The higher pressure for aged material is needed because: (1) the harder, more abrasive chips require stronger hydraulic force for evacuation; (2) the cutting zone generates more heat that must be removed; (3) the tool edge is more stressed and requires maximum cooling. Oil-based coolant with EP additives is recommended for both conditions.
What tool coating performs best for maraging steel?
TiAlN (titanium aluminium nitride) is the best choice for maraging steel in the annealed condition — it provides the hot hardness and thermal barrier needed at 50–70 m/min cutting speeds. For the aged condition, AlCrN (aluminium chromium nitride) is preferred due to its higher oxidation temperature (1,100°C+) and ability to withstand the extreme cutting edge temperatures generated at 48–60 HRC. AlTiN is also effective for aged material. The coating should be a multilayer PVD design for maximum wear resistance. Uncoated carbide is not recommended for any maraging steel deep hole drilling — the 18% nickel content causes rapid adhesive wear on uncoated tools.
Can BTA drilling be used for maraging steel?
Yes, BTA drilling is suitable for maraging steel at diameters above 15 mm. Recommended parameters for annealed condition: cutting speed 40–70 m/min, feed 0.08–0.20 mm/rev depending on diameter, and coolant pressure 40–80 bar. For aged condition: cutting speed 15–30 m/min, feed 0.05–0.15 mm/rev. The preferred insert grade is CVD-coated carbide (TiCN + Al₂O₃ + TiN, e.g., ISCAR IC908) for the thermal barrier provided by the Al₂O₃ layer. Chipbreaker geometry should be selected to promote chip fragmentation — continuous chips are a major risk in deep BTA holes in maraging steel.
What is the primary wear mechanism when drilling maraging steel?
The primary wear mechanism in maraging steel deep hole drilling is a combination of abrasive flank wear and adhesive wear (BUE). The high nickel content (18%) causes the material to adhere to the cutting edge, forming a built-up edge that alters the effective tool geometry. When the BUE breaks off, it often takes carbide particles with it, accelerating edge degradation. The cobalt and molybdenum in the alloy further contribute to abrasive wear by forming hard intermetallic particles in the microstructure. This is why coated tools (TiAlN for annealed, AlCrN for aged) are essential — the coating reduces adhesion and provides a hard barrier against abrasive wear.
How does C350 differ from C250 for deep hole drilling?
C350 maraging steel (55–60 HRC aged, 2,100–2,400 MPa tensile) is significantly more difficult to deep hole drill than C250 (48–52 HRC aged, 1,700–1,900 MPa). Recommended cutting speed for aged C350 is 8–20 m/min compared to 25–40 m/min for aged C250 — a 50–60% reduction. Feed rates must also be reduced 30–40%. Tool life in aged C350 is typically 10–25 holes per regrind compared to 25–50 for C250. In the annealed condition, the difference is smaller but still significant: C350 annealed at 35–40 HRC requires 45–60 m/min versus 60–80 m/min for C250. For production deep hole drilling, specify the lowest grade that meets the strength requirement.
What surface finish can be expected when gun drilling maraging steel?
With an optimised carbide gun drill in the annealed condition, surface finish of Ra 0.4–1.2 µm is achievable — comparable to conventional tool steel gun drilling. In the aged condition, finish is typically Ra 0.8–1.6 µm with a new tool, degrading more rapidly as the tool wears due to the abrasive nature of the material. Production runs in aged condition typically achieve Ra 1.6–3.2 µm through the tool life. When Ra exceeds 3.2 µm, inspect and replace the tool. BTA drilling produces Ra 3.2–6.3 µm.
What is the most common mistake in deep hole drilling maraging steel?
The most common mistake is treating maraging steel like conventional tool steel (H13, D2) without accounting for its high nickel content. The 18% nickel makes the chip significantly tougher and more abrasive than tool steel at the same hardness — even in the annealed condition. Operators who use H13 parameters on annealed maraging steel experience rapid flank wear and BUE formation. The second most common mistake is attempting to drill aged maraging steel without reducing cutting speed sufficiently — running at 50 m/min in aged C300 (which requires 15–30 m/min) causes immediate tool failure. The third mistake is using uncoated carbide tools — the nickel content causes severe adhesive wear on uncoated tools that is largely eliminated by TiAlN or AlCrN PVD coatings.
Summary
Deep hole drilling of maraging steel grades C250, C300, and C350 requires a strategic approach that recognises the dramatic change in machinability between heat treatment conditions. Drilling in the solution-annealed condition (30–35 HRC) is strongly preferred for all grades, using TiAlN-coated carbide gun drills at 45–80 m/min with feed rates of 0.008–0.100 mm/rev depending on diameter and coolant pressure of 40–100 bar. If post-aging drilling is required, cutting speeds must be reduced by 50–70% (to 8–40 m/min depending on grade), AlCrN coating becomes necessary, coolant pressure must increase to 80–140 bar, and tool life expectations must be adjusted downward to 10–50 holes per regrind. Age hardening shrinkage of 0.05–0.10% is predictable and can be compensated in the drilling process. C350 is the most difficult grade, requiring the lowest speeds (8–20 m/min aged) and producing the shortest tool life (10–25 holes per regrind). C250 is the most practical grade for deep hole drilling, particularly in the annealed condition where 80–120 holes per regrind are achievable. The most critical process rule is to use coated carbide tools — the 18% nickel content causes rapid adhesive wear on uncoated tools regardless of condition. With correct grade-specific parameter selection, deep hole drilling of maraging steel is a reliable process for aerospace tooling, injection moulding, and high-performance component applications.