Appearance
Nitronic 60 has a special property: it is designed to resist galling. The same metallurgy that prevents two Nitronic 60 surfaces from welding together under pressure also makes the material resist chip formation during cutting. When deep hole drilling these alloys, the first rule is never to stop feeding — a tool that dwells for even a fraction of a second will encounter a work-hardened surface that may destroy the cutting edge on re-engagement.
Understanding Nitronic and High-Strength Stainless Alloys
Nitronic Alloy Family
| Grade | UNS | Key Alloying | Tensile Strength | Hardness | Machinability Rating |
|---|---|---|---|---|---|
| Nitronic 30 | S20400 | 4% Mn, 0.3% N | ~690 MPa | ~28 HRC | ~50% |
| Nitronic 40 | S21904 | 9% Mn, 0.4% N | ~760 MPa | ~30 HRC | ~40% |
| Nitronic 50 | S20910 | 5% Mn, 10% Mo, 0.3% N | ~860 MPa | ~32 HRC | ~35% |
| Nitronic 60 | S21800 | 8% Mn, 4% Si, 0.2% N | ~830 MPa | ~32 HRC | ~23% |
The machinability rating is relative to B-1112 free-cutting steel (100%). For comparison, 304 stainless is approximately 45–50% and 316 is approximately 35–40%.
The nitrogen strengthening mechanism is key: nitrogen dissolved in the austenitic matrix provides solid-solution strengthening without the ductility loss that carbon would cause. This gives Nitronic grades their high strength-to-weight ratio but also drives the rapid work-hardening behavior.
What Makes Nitronic 60 Unique
Nitronic 60's galling resistance is exceptional:
| Self-Mated Pair | Threshold Galling Stress | Comparison |
|---|---|---|
| Nitronic 60 vs. Nitronic 60 | > 345 MPa (50 ksi) — did not gall | Best-in-class |
| 304 vs. 304 | 96 MPa (14 ksi) | Typical austenitic |
| 316 vs. 316 | ~100 MPa | Moderate |
This galling resistance comes from the high silicon content (3.5–4.5%) combined with manganese and nitrogen. The silicon forms a protective oxide layer that prevents metal-to-metal adhesion — but this same layer makes chip formation more difficult and increases cutting forces.
Precipitation-Hardening Stainless Steels
| Grade | UNS | Strengthening | Aged Hardness | Common Applications |
|---|---|---|---|---|
| 17-4 PH | S17400 | Cu precipitation | 40–44 HRC | Valve stems, shafts, aerospace |
| 15-5 PH | S15500 | Cu precipitation | 38–45 HRC | Aircraft components, fittings |
| A286 | S66286 | Gamma-prime | 32–36 HRC | Turbine components, fasteners |
These alloys are typically machined in the solution-annealed or overaged condition (28–34 HRC) and then aged to final hardness after machining.
The Work-Hardening Challenge
Work-Hardening Rate Comparison
| Material | Work-Hardening Rate | Depth of Work-Hardened Layer |
|---|---|---|
| 304 stainless | Moderate | 0.02–0.05 mm |
| Nitronic 50 | High | 0.05–0.10 mm |
| Nitronic 60 | Very high | 0.08–0.15 mm |
| 17-4 PH (solution-treated) | Low-Moderate | 0.02–0.04 mm |
| 17-4 PH (aged) | Low | < 0.02 mm |
The work-hardened layer in Nitronic 60 can reach 0.15 mm depth under aggressive conditions — thick enough to cause immediate edge chipping when a reamer or secondary tool attempts to follow the gun drill.
Preventing Work-Hardening During Deep Hole Drilling
| Principle | Practical Application |
|---|---|
| Never stop feeding | Continuous feed from entry to breakthrough; avoid dwell at any depth |
| Use adequate feed | Minimum 0.008 mm/rev for small diameters; 0.015–0.030 for larger |
| Maintain sharp edge | Regrind at shorter intervals — dull edges accelerate work hardening |
| Keep coolant flowing | Interrupted coolant flow allows heat buildup and surface hardening |
| Avoid re-cutting chips | Ensure chip evacuation keeps chips from rubbing against the bore wall |
TIP
The simplest diagnostic test for work-hardening during drilling: run a fine file across the bore surface immediately after drilling. If the file skates without cutting, the surface has work-hardened. This surface must be removed by the next operation — it will not break down during subsequent cutting.
Cutting Parameters for Deep Hole Drilling
Gun Drilling Parameters
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure |
|---|---|---|---|
| Nitronic 50 (S20910) | 8–15 | 0.008–0.020 | 150–200 bar |
| Nitronic 60 (S21800) | 6–12 | 0.008–0.018 | 150–250 bar |
| 17-4 PH (solution-treated) | 15–25 | 0.010–0.025 | 100–150 bar |
| 15-5 PH (solution-treated) | 15–25 | 0.010–0.025 | 100–150 bar |
| A286 (S66286) | 10–18 | 0.008–0.020 | 120–180 bar |
BTA Drilling Parameters
| Material | Cutting Speed (m/min) | Feed (mm/rev) | Coolant Pressure |
|---|---|---|---|
| Nitronic 50 | 10–18 | 0.020–0.050 | 50–100 bar |
| Nitronic 60 | 8–15 | 0.015–0.040 | 60–120 bar |
| 17-4 PH (solution-treated) | 18–28 | 0.030–0.060 | 40–80 bar |
| 17-4 PH (aged H-900) | 10–15 | 0.015–0.030 | 60–100 bar |
Parameter Effects on Tool Life in Nitronic 60
| Change | Effect | Comment |
|---|---|---|
| Speed reduced 30% (12 → 8 m/min) | Tool life +80–120% | Significant improvement |
| Feed increased 50% (0.012 → 0.018 mm/rev) | Tool life –20–30% | But chip control improves |
| Coolant pressure 150 → 200 bar | Tool life +30–50% | Better chip evacuation |
| Edge hone 0.015 → 0.030 mm | Tool life +50% on entry | Less edge chipping |
The strong effect of speed reduction on tool life reflects the heat-sensitive nature of Nitronic machining. Unlike 304 or 316, where speed can be pushed higher, Nitronic grades punish high speeds with rapid edge failure.
Tool Geometry
Gun Drill Geometry for Nitronic Grades
| Geometry Feature | Standard (316L) | Nitronic 50 | Nitronic 60 |
|---|---|---|---|
| Outer point angle (ϕ) | 30–35° | 25–30° | 22–28° |
| Inner point angle (ψ) | 20–25° | 18–22° | 18–20° |
| Outer relief angle | 10–15° | 8–10° | 6–8° |
| Inner relief angle | 15–20° | 12–15° | 10–12° |
| Edge hone | 0.01–0.02 mm | 0.02–0.03 mm | 0.03–0.05 mm |
| Back taper | 0.02× d₀/100 mm | 0.025× d₀/100 mm | 0.03× d₀/100 mm |
The trend is clear: as the material becomes more difficult (Nitronic 50 → 60), the tool geometry must become more robust — smaller angles, smaller relief, larger edge hone.
Coating Selection
| Material | Recommended Coating | Reason |
|---|---|---|
| Nitronic 50 | TiAlN | Good heat resistance; standard choice |
| Nitronic 60 | AlCrN or TiSiN | Superior notch wear resistance; handles the abrasive silicon-rich matrix |
| 17-4 PH (solution-treated) | TiAlN | Standard for martensitic stainless |
| 17-4 PH (aged) | AlCrN | Higher temperature capability |
| A286 | TiAlN or AlCrN | Both suitable; AlCrN preferred for tough conditions |
Chip Control
Chip Characteristics
| Material | Chip Form | Breaking Strategy |
|---|---|---|
| Nitronic 50 | Tough, continuous, stringy | Aggressive chip breaker, feed > 0.012 mm/rev |
| Nitronic 60 | Ribbon-like, abrasive | Short, broken chips difficult to achieve; aim for manageable lengths |
| 17-4 PH (solution-treated) | Moderate curl, breaks well | Standard chip breaker sufficient |
| 17-4 PH (aged) | Short, segmented | Easy chip breaking |
Nitronic 60 produces some of the most difficult chips in deep hole drilling. The combination of high strength, work-hardening tendency, and the abrasive silicon-rich matrix means chips resist breaking and are hard on both the tool and the chip evacuation path.
Feed Rate and Chip Breaking in Nitronic Grades
| Feed (mm/rev) | Chip Length | Chip Form | Assessment |
|---|---|---|---|
| < 0.008 | Very long (> 100 mm) | Continuous string | Unacceptable — high packing risk |
| 0.010–0.015 | 20–50 mm | Moderate curls | Acceptable |
| 0.015–0.020 | 8–20 mm | Short curls | Good |
| > 0.020 | 3–8 mm | Segmented | Excellent — but may overload edge |
For Nitronic 60, the minimum feed to achieve acceptable chip breaking is approximately 0.010 mm/rev for diameters above 6 mm, and 0.008 mm/rev for smaller diameters.
Coolant Strategy
Coolant Type
| Coolant | Nitronic 50 | Nitronic 60 | 17-4 PH |
|---|---|---|---|
| Oil-based cutting oil | Recommended | Strongly recommended | Recommended |
| High-oil emulsion (> 15%) | Acceptable | Marginal | Acceptable |
| Standard emulsion (5–8%) | Not recommended | Not recommended | Not recommended |
Oil-based coolant is strongly preferred for all Nitronic grades. The need for maximum lubricity at the cutting edge and guide pad interface is critical to prevent work hardening and galling.
Coolant Pressure Requirements by Diameter
| Drill Diameter | Nitronic 50 | Nitronic 60 |
|---|---|---|
| 1–3 mm | 180–250 bar | 200–250 bar |
| 3–10 mm | 150–200 bar | 180–250 bar |
| 10–25 mm | 120–180 bar | 150–200 bar |
| 25+ mm | 80–150 bar | 120–180 bar |
Case Studies
Case 1: Gun Drilling Nitronic 50 for Shaft Component
| Parameter | Value |
|---|---|
| Process | Gun drilling, 10 mm × 400 mm in Nitronic 50 |
| Initial condition | Tool chipping after 5–8 holes at 18 m/min, 0.015 mm/rev |
| Root cause | Cutting speed too high for the material — edge temperature caused thermal softening |
| Correction | Reduced speed to 12 m/min; increased edge hone to 0.030 mm; switched to AlCrN coating |
| Result | Tool life increased to 40+ holes; consistent chip form |
Case 2: Nitronic 60 Galling on Guide Pads
| Parameter | Value |
|---|---|
| Process | Gun drilling, 6 mm × 300 mm in Nitronic 60 |
| Failure | Guide pad galling after 3 holes — workpiece material transferred to pads |
| Root cause | Standard TiAlN-coated pads — insufficient galling resistance for Nitronic 60 |
| Correction | Changed to DLC-coated guide pads; increased coolant pressure from 150 to 200 bar |
| Result | Pad galling eliminated; tool life stabilized at 25 holes per regrind |
Case 3: 17-4 PH H-900 Deep Hole Drilling
| Parameter | Value |
|---|---|
| Process | BTA drilling, 30 mm × 500 mm in 17-4 PH (aged H-900, 42 HRC) |
| Initial attempt | Edge chipping at 12 m/min, 0.025 mm/rev |
| Root cause | Aged condition required tougher carbide grade and more robust edge preparation |
| Correction | Switched from K20 to K30 grade; increased edge hone to 0.04 mm; reduced feed to 0.018 mm/rev |
| Result | 30 holes per edge; acceptable tool life for short-run production |
FAQ
Q: What is the most difficult stainless steel for deep hole drilling? Nitronic 60 is generally considered the most difficult commonly-drilled stainless grade due to its combination of rapid work hardening, high silicon content, and galling resistance — all of which work against the cutting process.
Q: What cutting speed is recommended for gun drilling Nitronic 50? 8–15 m/min, depending on diameter and depth. Start at the lower end and increase based on tool wear observations.
Q: How does Nitronic 60's work-hardening affect deep hole drilling? It creates a hardened surface layer that can reach 0.15 mm depth. Any dwell or interruption in feed will work-harden the bore surface, causing the tool to encounter a harder surface on re-engagement.
Q: What coating is best for drilling Nitronic 60? AlCrN or TiSiN coatings provide the best performance. The high silicon content of Nitronic 60 is abrasive, and these coatings offer superior notch wear resistance.
Q: Can 17-4 PH be drilled in the aged condition? Yes, but with reduced parameters. Cutting speed should be reduced by approximately 40% compared to the solution-treated condition, and a tougher carbide grade with increased edge hone should be used.
Q: What coolant pressure is needed for Nitronic deep hole drilling? 150–250 bar for diameters under 10 mm, reducing to 80–150 bar for larger diameters. Oil-based coolant is strongly recommended.
Q: Why does Nitronic 60 resist galling and why does this matter for drilling? The high silicon content (3.5–4.5%) forms a protective oxide layer that prevents metal-to-metal adhesion. This same layer increases cutting forces and makes chip formation more difficult.
Q: What is the minimum feed rate for gun drilling Nitronic 50? Minimum 0.008 mm/rev for diameters above 6 mm. Feeds below this risk rubbing instead of cutting, which work-hardens the surface.
Q: How should tool geometry change for Nitronic vs. 316L? Reduce outer and inner point angles by 5–8°, reduce relief angles by 2–4°, increase edge hone by 0.01–0.03 mm, and increase back taper by approximately 30%.
Q: What is the most important operational rule for deep hole drilling Nitronic grades? Never stop the feed. Continuous, uninterrupted cutting is essential to prevent work hardening. A dwell of even one spindle revolution can create a hardened surface that destroys the cutting edge.