Appearance
Maraging steel is not machined — it is carved with patience. At 52 HRC in the aged condition, it demands tool materials and geometries that would seem excessive for most deep hole drilling applications. Yet the solution-annealed condition machines more like a tough low-alloy steel than a hardened tool steel. The key to successful deep hole drilling of maraging steel lies in understanding this duality: choose the right heat treatment condition for the drilling operation, not the final application.
Understanding Maraging Steel
Metallurgy and Heat Treatment
Maraging steels are low-carbon (typically < 0.03% C) alloys hardened by precipitation of intermetallic compounds rather than by carbon content. The name derives from "martensite age-hardening."
Key alloying elements:
- Nickel (18%): Provides the martensitic matrix
- Cobalt (7–12%): Promotes precipitation hardening
- Molybdenum (3–5%): Forms the primary hardening precipitates (Ni₃Mo)
- Titanium (0.2–0.7%): Additional precipitation hardening
| Grade | UNS | Ni | Co | Mo | Ti | Yield (MPa) | Hardness |
|---|---|---|---|---|---|---|---|
| C250 (Maraging 250) | K92890 | 18 | 7.5 | 4.8 | 0.4 | 1,700 | 48–50 HRC |
| C300 (Maraging 300) | K93120 | 18 | 9.0 | 4.8 | 0.6 | 2,000 | 50–52 HRC |
| C350 (Maraging 350) | K93240 | 18 | 12.0 | 4.8 | 0.7 | 2,300 | 52–54 HRC |
| C400 | — | 18 | 12.5 | 4.2 | 1.6 | 2,600 | 54–56 HRC |
Heat Treatment Sequence
| Condition | Heat Treatment | Hardness | Machinability |
|---|---|---|---|
| Annealed (solution-treated) | 820°C, air cool | 28–35 HRC | Good — similar to 4140 at 32 HRC |
| Aged (hardened) | 480°C for 3–6 hours, air cool | 48–54 HRC | Poor — similar to D2 tool steel at 54 HRC |
| Overaged | 590°C for 4 hours | 40–45 HRC | Fair — intermediate condition for rough machining |
Aerospace Tooling Applications
| Application | Typical Grade | Why Maraging |
|---|---|---|
| Composite mold dies (autoclave) | C300 | Excellent dimensional stability during cure cycles |
| Injection molding tools | C350 | High hardness, no heat check cracking |
| Rocket motor casings | C250 | High strength-to-weight ratio, weldable |
| Landing gear components | C300 | Toughness at high strength |
| Extrusion dies | C300 | Wear resistance, thermal stability |
The Critical Decision: When to Drill
Maraging steel is unique in that the deep hole drilling operation can be performed at three different points in the heat treatment cycle, each with different trade-offs.
| Condition | When to Drill | Advantages | Disadvantages |
|---|---|---|---|
| Solution-annealed | Before aging | Lower cutting forces, longer tool life, fewer tool changes | Must account for 0.05–0.08% shrinkage during aging |
| Aged (hardened) | After age hardening | No shrinkage to account for; final size stable | 50%+ reduction in tool life; very high cutting forces |
| Rough drill in annealed, finish after aging | Two operations | Best of both: rough in easy condition, final size stable | Two setups, additional cost |
Recommendation: For deep hole drilling in production, drill in the solution-annealed condition and account for the predictable shrinkage during aging. The 0.05–0.08% dimensional change (approximately 0.005–0.008 mm per 10 mm of diameter) is consistent and can be compensated in tool diameter selection.
Cutting Parameters for Deep Hole Drilling
Recommended Parameters
| Parameter | Solution-Annealed (30–35 HRC) | Aged (48–54 HRC) |
|---|---|---|
| Cutting speed (gun drilling) | 20–35 m/min | 8–15 m/min |
| Cutting speed (BTA drilling) | 25–40 m/min | 10–18 m/min |
| Feed rate (gun drill, < 10 mm) | 0.008–0.020 mm/rev | 0.005–0.012 mm/rev |
| Feed rate (gun drill, 10–25 mm) | 0.015–0.030 mm/rev | 0.008–0.018 mm/rev |
| Feed rate (BTA) | 0.020–0.050 mm/rev | 0.010–0.025 mm/rev |
| Coolant pressure | 80–150 bar | 120–200 bar |
| Expected tool life | Baseline | 30–50% of annealed |
Parameters by Diameter (Solution-Annealed Condition)
| Gun Drill Diameter | Speed (m/min) | Feed (mm/rev) | Coolant Pressure |
|---|---|---|---|
| 1–3 mm | 20–30 | 0.005–0.012 | 150–250 bar |
| 3–10 mm | 25–35 | 0.008–0.020 | 100–180 bar |
| 10–25 mm | 20–30 | 0.015–0.030 | 80–150 bar |
| 25+ mm | 18–25 | 0.020–0.040 | 60–120 bar |
Parameters by Diameter (Aged Condition)
| Gun Drill Diameter | Speed (m/min) | Feed (mm/rev) | Coolant Pressure |
|---|---|---|---|
| 1–3 mm | 8–12 | 0.004–0.008 | 180–250 bar |
| 3–10 mm | 10–15 | 0.005–0.012 | 150–200 bar |
| 10–25 mm | 8–12 | 0.008–0.018 | 120–180 bar |
| 25+ mm | 8–10 | 0.010–0.020 | 100–150 bar |
Tool Geometry and Carbide Grade
Carbide Grade Selection
| Condition | Recommended Grade | Grain Size | Co Content | Properties |
|---|---|---|---|---|
| Solution-annealed | Micro-grain (K20–K30) | 0.5–1.0 μm | 8–10% | Toughness + wear resistance |
| Aged (48–54 HRC) | Sub-micro grain (K10–K20) | 0.2–0.5 μm | 6–8% | Wear resistance, moderate toughness |
| Interrupted cuts | Toughness grade (K30–K40) | 1.0–2.0 μm | 10–12% | Maximum edge strength |
Key principle for aged maraging steel: The carbide grade must prioritize wear resistance over toughness. The aged material is abrasive, and the cutting edge experiences high localized pressure. A sub-micro grain carbide with 6–8% cobalt provides the best balance.
Tool Geometry Recommendations
| Geometry Feature | Solution-Annealed | Aged | Reason for Change |
|---|---|---|---|
| Outer point angle (ϕ) | 30–35° | 25–30° | Reduced angle strengthens edge for higher forces |
| Inner point angle (ψ) | 20–25° | 18–22° | Balanced cutting forces |
| Outer relief angle | 10–12° | 6–8° | More edge support for high hardness |
| Inner relief angle | 15–18° | 10–12° | Prevent notch wear |
| Edge hone | 0.015–0.025 mm | 0.030–0.050 mm | Strengthen edge against chipping |
| Back taper | 0.02× d₀/100 mm | 0.03× d₀/100 mm | Reduce friction in high-force drilling |
Coating Selection
| Coating | Application | Performance |
|---|---|---|
| TiAlN | Solution-annealed and aged | Good — standard choice for high-strength steel |
| AlCrN | Aged condition | Excellent — best notch wear resistance |
| TiSiN | Aged, high-speed | Excellent — superior oxidation resistance at edge |
| DLC | Not recommended for maraging | Insufficient temperature capability |
The coating must provide thermal barrier properties because the high cutting forces generate significant heat at the cutting edge. TiAlN and AlCrN both form a stable aluminum oxide layer at elevated temperatures that protects the carbide substrate.
Chip Control
Chip Characteristics in Maraging Steel
| Condition | Chip Type | Breaking Difficulty |
|---|---|---|
| Solution-annealed | Tough, continuous chips | Moderate — similar to 4140 at 32 HRC |
| Aged | Short, segmented chips | Low — brittleness aids chip breaking |
| Aged with interrupted cut | Small, fragmented chips | Low |
In the solution-annealed condition, chips can be long and tough, requiring attention to chip breaker geometry and feed rate. In the aged condition, chips break readily but the tool must be strong enough to handle the interrupted cutting action.
Chip Breaker Recommendations
| Condition | Feed Strategy | Chip Breaker |
|---|---|---|
| Solution-annealed | Minimum 0.010 mm/rev to break chips | Standard chip breaker, moderate step |
| Aged | Lower feed acceptable (0.005 mm/rev minimum) | Standard chip breaker, sharp edge not needed |
| Both conditions | Avoid feed below 0.004 mm/rev | Risk of rubbing and work hardening |
Coolant Strategy
Coolant Types and Performance
| Coolant Type | Suitability | Recommendation |
|---|---|---|
| Oil-based cutting oil | Excellent | First choice — essential for aged condition |
| Water-miscible emulsion (> 12%) | Acceptable for solution-annealed | Not recommended for aged |
| Standard emulsion (5–8%) | Poor | Insufficient lubricity for maraging |
Oil-based coolant is strongly recommended, particularly for drilling in the aged condition. The extreme pressure at the cutting edge and guide pad interface requires the highest available lubricity to prevent galling and edge chipping.
Coolant Pressure Requirements
| Condition | Minimum Pressure | Recommended Pressure |
|---|---|---|
| Solution-annealed | 80 bar | 100–150 bar |
| Aged | 120 bar | 150–200 bar |
Higher coolant pressure serves two critical functions in maraging steel drilling: it improves chip evacuation (chips are heavy and dense) and provides better cooling at the cutting edge (heat concentration is a primary failure mode).
Process Recommendations
Entry and Exit
| Consideration | Recommendation |
|---|---|
| Pilot hole | Required — depth 1.5–2× diameter |
| Entry feed | 50% of normal feed for first 2–3× diameter |
| Guide bushing clearance | +0.003 to +0.005 mm (tighter than standard) |
| Exit breakthrough | Reduce feed by 50% for final 2 mm |
The high cutting forces in maraging steel make proper entry support critical. A worn guide bushing that would produce a 0.02 mm bell mouth in steel can produce a 0.05 mm bell mouth in maraging steel.
Tool Wear Monitoring
| Wear Type | Appearance | Action |
|---|---|---|
| Uniform flank wear | Even wear land on flank | Normal — continue until 0.3 mm |
| Notch wear | Groove at depth of cut line | Reduce speed, check edge hone |
| Micro-chipping | Small edge fragments missing | Increase edge hone, reduce feed |
| Edge deformation | Edge rounded or bulging | Reduce speed, check coating |
| Crater wear | Depression on rake face | Check chip breaker, reduce speed |
Case Studies
Case 1: Gun Drilling Composite Mold Tooling from C300 Maraging
| Parameter | Value |
|---|---|
| Process | Gun drilling, 8 mm × 500 mm in C300 maraging steel |
| Condition | Solution-annealed (32 HRC) |
| Cutting speed | 28 m/min |
| Feed | 0.018 mm/rev |
| Tool | Solid carbide gun drill, TiAlN coated, K20 grade |
| Coolant | Oil-based, 120 bar |
| Result | 80+ holes per regrind; bore diameter held within 0.015 mm |
| Compensation | Tool ground 0.050 mm oversize to account for aging shrinkage |
Case 2: Drilling Aged C350 for Injection Molding Tool
| Parameter | Value |
|---|---|
| Process | BTA drilling, 20 mm × 300 mm in C350 maraging (52 HRC aged) |
| Initial attempt | Tool failed after 3 holes — corner chipping |
| Root cause | Carbide grade too tough (K30) — edge wore rapidly, then chipped |
| Correction | Changed to sub-micro grain K15 grade with AlCrN coating; reduced speed from 15 to 10 m/min; increased coolant pressure from 60 to 150 bar |
| Result | 25 holes per edge; consistent bore finish |
FAQ
Q: What is maraging steel and why is it used in aerospace tooling? Maraging steel is a low-carbon, nickel-cobalt-molybdenum alloy that achieves tensile strengths exceeding 2,000 MPa through precipitation hardening. It is used for aerospace tooling because of its excellent dimensional stability during heat treatment, high toughness, and resistance to heat checking.
Q: Should maraging steel be drilled before or after aging? Drilling in the solution-annealed condition (before aging) is strongly recommended. Tool life is 2–3× longer, cutting forces are lower, and fewer special tool geometries are required. Account for 0.05–0.08% linear shrinkage during aging.
Q: What cutting speed is recommended for gun drilling maraging steel? 20–35 m/min in the solution-annealed condition (30–35 HRC). Reduce to 8–15 m/min in the aged condition (48–54 HRC).
Q: What carbide grade is best for drilling aged maraging steel? Sub-micro grain carbide (0.2–0.5 μm grain size) with 6–8% cobalt content, K10–K20 ISO grade. The priority is wear resistance over toughness.
Q: What coolant is recommended for deep hole drilling maraging steel? Oil-based cutting oil is strongly recommended, particularly for the aged condition. High lubricity is essential to prevent edge chipping and guide pad galling.
Q: Can water-miscible coolant be used for maraging steel drilling? Only for the solution-annealed condition, with concentration above 12%. Standard emulsions (5–8%) provide insufficient lubricity for maraging steel.
Q: How does maraging steel chip form compare to other high-strength steels? In the solution-annealed condition, chips are tough and continuous — similar to 4140 at 32 HRC. In the aged condition, chips are short and segmented due to the brittleness of the hardened martensitic matrix.
Q: What coating is best for drilling maraging steel? TiAlN is the standard choice for both conditions. AlCrN is recommended for aged material due to its superior notch wear resistance at high cutting temperatures.
Q: Why is guide bushing clearance critical for maraging steel drilling? The high cutting forces in maraging steel amplify the effect of bushing clearance. A clearance of +0.003 to +0.005 mm is recommended — tighter than the standard +0.003 to +0.008 mm range.
Q: How do I account for shrinkage during aging when drilling in the solution-annealed condition? The linear shrinkage of maraging steel during aging is 0.05–0.08% (0.005–0.008 mm per 10 mm of diameter). Grind the gun drill or BTA head oversize by this amount so the bore shrinks to the final dimension after aging.