Appearance
An aerospace actuator manufacturer must deep-drill 600 components per year from 17-4PH stainless steel in the H900 condition (44 HRC), with 10 mm diameter × 350 mm deep hydraulic passages. Initial attempts using 304 stainless steel parameters cause rapid edge chipping and tool failure within five holes — the precipitation-hardened condition at 44 HRC is significantly more abrasive than austenitic grades. Drilling in the solution-annealed Condition A (33 HRC) produces gummy chip formation and built-up edge. The process team determines that the H1150 condition (28–32 HRC) offers the best machinability for deep hole drilling, with carbide gun drills at 30–46 m/min cutting speed, 0.025–0.050 mm/rev feed, and 70–100 bar coolant pressure achieving 60–100 holes per regrind.
PH Stainless Steel Properties for Deep Hole Drilling
| Property | Condition A (Solution Annealed) | H900 (Aged 900°F) | H1025 (Aged 1,025°F) | H1150 (Aged 1,150°F) |
|---|---|---|---|---|
| Hardness (HRC) | 30–33 | 40–44 | 35–38 | 28–32 |
| Tensile strength (MPa) | 800–900 | 1,310–1,380 | 1,070–1,170 | 930–1,000 |
| Elongation (%) | 10–15 | 10–15 | 15–20 | 18–22 |
| Thermal cond. (W/m·K) | 18 | 18 | 18 | 18 |
| Machinability rating | 38% | 31% | 37% | 45% |
| Work hardening rate | High (gummy) | Moderate | Moderate | Moderate |
| Chip formation | Stringy, tough | Segmented, nice break | Segmented | Segmented |
| Suitability for deep hole drilling | Poor | Fair | Good | Excellent |
Cutting Parameter Recommendations
| Parameter | 17-4PH Condition A | 17-4PH H900 | 17-4PH H1025 | 17-4PH H1150 | 15-5PH H1025 |
|---|---|---|---|---|---|
| Cutting speed — carbide (m/min) | 20–35 | 18–30 | 25–40 | 30–46 | 25–40 |
| Cutting speed — carbide (SFM) | 65–115 | 60–100 | 82–130 | 100–150 | 82–130 |
| Feed — 6 mm dia (mm/rev) | 0.015–0.030 | 0.010–0.025 | 0.015–0.030 | 0.020–0.040 | 0.015–0.030 |
| Feed — 10 mm dia (mm/rev) | 0.025–0.050 | 0.018–0.035 | 0.025–0.050 | 0.030–0.060 | 0.025–0.050 |
| Feed — 20 mm dia (mm/rev) | 0.040–0.080 | 0.030–0.060 | 0.040–0.080 | 0.050–0.100 | 0.040–0.080 |
| Coolant pressure (bar) | 70–120 | 80–120 | 70–100 | 60–100 | 70–100 |
| Recommended coating | TiAlN | AlCrN | TiAlN | TiAlN | TiAlN |
| Expected tool life (holes per regrind) | 20–40 | 25–50 | 40–70 | 60–100 | 40–70 |
Feed Rate by Drill Diameter
| Drill Diameter (mm) | H1150 Feed (mm/rev) | H900 Feed (mm/rev) | RPM at 38 m/min | Penetration (mm/min) |
|---|---|---|---|---|
| 4 | 0.012–0.025 | 0.008–0.020 | 3,025 | 35–75 |
| 6 | 0.020–0.040 | 0.010–0.025 | 2,015 | 40–80 |
| 8 | 0.025–0.050 | 0.015–0.030 | 1,510 | 35–75 |
| 10 | 0.030–0.060 | 0.018–0.035 | 1,210 | 35–70 |
| 12 | 0.035–0.070 | 0.020–0.040 | 1,010 | 35–70 |
| 16 | 0.045–0.085 | 0.025–0.050 | 755 | 35–65 |
| 20 | 0.050–0.100 | 0.030–0.060 | 605 | 30–60 |
| 25 | 0.060–0.110 | 0.035–0.070 | 485 | 30–55 |
TIP
The choice of heat treatment condition is the single most important factor for successful deep hole drilling of 17-4PH and 15-5PH stainless steels. While Condition A (solution annealed, 33 HRC) is the softest condition, it is paradoxically the most difficult to deep hole drill — the fully martensitic structure is tough and gummy, producing long stringy chips that clog evacuation passages and cause built-up edge. The H900 condition (44 HRC) produces excellent chip breakage but is too hard for economical tool life in deep hole drilling. The H1150 condition (28–32 HRC) offers the best balance: it is soft enough for good tool life but precipitation-hardened enough to produce short, segmented chips. Whenever possible, specify H1150 for deep hole drilling operations. If the final application requires H900 or H1025 strength, consider drilling in H1150 and re-aging to the final condition — the hardness change from H1150 to H900 actually reduces hole diameter by only 0.02–0.05%, which is manageable.
Tool Geometry for PH Stainless Steel
| Geometry Parameter | H1150 / H1025 (Recommended) | H900 (If Unavoidable) |
|---|---|---|
| Point angle | 130–140° | 135–145° |
| Rake angle | 6–10° positive | 4–8° positive |
| Relief / clearance angle | 8–12° | 10–14° |
| Edge preparation | Sharp with light hone | T-land 0.03–0.08 mm |
| Coating | TiAlN | AlCrN |
| Carbide grade | Micrograin (0.5–1.0 µm) | Fine grain with high Co |
| Tip displacement | 0.22–0.25 × D | 0.20–0.22 × D |
| Guide bushing tolerance | G6 | H6 |
Coolant Selection and Parameters
| Coolant Type | Suitability | Pressure Required | Key Requirements |
|---|---|---|---|
| Neat oil with EP additives | Excellent | 60–120 bar | Sulphurised EP, 10–20 cSt viscosity |
| Semi-synthetic emulsion | Good | 60–120 bar | > 10% concentration, anti-corrosion |
| Oil-based coolant | Recommended for gun drilling | 70–100 bar | Better lubricity than water-based |
| Coolant filtration | Essential | — | 10–15 µm for PH grades |
Chip Morphology and Control
| Chip Type | Appearance | Condition | Risk Level | Corrective Action |
|---|---|---|---|---|
| Segmented / sawtooth (ideal) | Uniform short segments | H1150, H1025 | Low | Maintain parameters |
| Long stringy ribbon | Continuous > 50 mm | Condition A (common) | High | Switch to H1150 condition if possible |
| Short hard segments | Small, abrasive chips | H900 (normal) | Low | Maintain — check tool wear frequency |
| Burned / discoloured | Heat tint | Any, speed too high | Critical | Reduce speed, verify coolant |
| Built-up edge chips | Irregular, smeared | Condition A | High | Increase speed, switch to H1150 |
| Powder / dust | Fine particles | Any, tool worn | Critical | Replace tool immediately |
Surface Finish Expectations
| Condition | Ra (µm) | Rz (µm) | Notes |
|---|---|---|---|
| H1150, optimised carbide gun drill, new | 0.4–1.2 | 4–10 | Best condition for finish |
| H1025, production drilling | 0.8–1.6 | 8–18 | Acceptable for hydraulic components |
| H900, new tool | 0.8–1.6 | 8–18 | Shorter tool life, consistent finish |
| Condition A | 0.8–2.0 | 10–25 | BUE degrades finish |
| BTA drilling (any condition) | 3.2–6.3 | 20–40 | May require secondary finishing |
Troubleshooting
| Symptom | Likely Cause | Solution |
|---|---|---|
| Built-up edge in Condition A | Gummy material, speed too low | Switch to H1150 or increase speed, use TiAlN coating |
| Edge chipping in H900 | Material too hard, feed excessive | Reduce feed, increase T-land, switch to AlCrN |
| Rapid flank wear in any condition | Speed too high, coolant insufficient | Reduce speed, increase coolant pressure |
| Poor surface finish | Tool wear, BUE, vibration | Replace tool, check alignment, verify material condition |
| Stringy chip clogging | Condition A, feed too low | Switch to H1150 condition, increase feed |
| Tool breakage | Chip packing, coolant loss | Verify coolant, implement peck cycle |
| Hole oversize | Tool wear, guide pad wear | Replace at VB ≥ 0.15 mm, inspect guide pads |
| Exit burr excessive | Dull tool, material too ductile | Replace tool, reduce feed at exit 50% for last 3 mm |
BTA Drilling Parameters
| Parameter | 17-4PH H1150 | 17-4PH H900 | 15-5PH H1025 |
|---|---|---|---|
| Cutting speed — BTA (m/min) | 30–50 | 20–35 | 25–45 |
| Feed — BTA 20 mm (mm/rev) | 0.05–0.12 | 0.04–0.08 | 0.05–0.10 |
| Feed — BTA 40 mm (mm/rev) | 0.08–0.18 | 0.06–0.12 | 0.08–0.15 |
| Coolant pressure — BTA (bar) | 40–80 | 60–100 | 50–80 |
| Insert grade | TiAlN-coated | AlCrN-coated | TiAlN-coated |
| Surface finish Ra (µm) | 3.2–6.3 | 3.2–6.3 | 3.2–6.3 |
FAQ
What cutting speed is recommended for gun drilling 17-4PH stainless steel?
Recommended cutting speed depends on heat treatment condition. For H1150 (28–32 HRC) — the best condition for deep hole drilling — use 30–46 m/min (100–150 SFM) with TiAlN-coated carbide gun drills. For H1025 (35–38 HRC), use 25–40 m/min. For H900 (40–44 HRC), reduce to 18–30 m/min with AlCrN coating. For Condition A (solution annealed, 33 HRC), use 20–35 m/min — despite being the softest condition, the gummy chip formation requires reduced speeds to manage BUE. Speeds above 50 m/min in any condition cause rapid thermal tool failure. For HSS gun drills, reduce speeds by 60%.
Which heat treatment condition is best for deep hole drilling 17-4PH?
H1150 (aged at 1,150°F, 28–32 HRC) provides the best machinability for deep hole drilling. At this condition, the material is precipitation-hardened enough to produce short, segmented chips that evacuate cleanly, but soft enough for economical tool life (60–100 holes per regrind). The machinability rating of 45% is the highest of any 17-4PH condition. H1150D (double-aged) provides similar machinability. Avoid drilling 17-4PH in Condition A (solution annealed) — it produces long stringy chips and severe BUE. If final hardness must be H900 or H1025, the recommended sequence is: rough machine in H1150 → deep hole drill → re-age to final condition.
What feed rate should be used for gun drilling 17-4PH H1150?
For 17-4PH in H1150 condition, recommended feed rate depends on hole diameter: 0.020–0.040 mm/rev for 6 mm, 0.030–0.060 mm/rev for 10 mm, and 0.050–0.100 mm/rev for 20 mm diameter. Minimum feed should be 0.015 mm/rev to prevent rubbing and work hardening. The general guideline is feed per revolution = D/200 to D/400. Feed should be consistent and uninterrupted — PH stainless steels work harden rapidly if the tool dwells or rubs.
What coolant pressure is required for 17-4PH deep hole drilling?
For 17-4PH in H1150 or H1025 condition: minimum 60 bar, recommended 70–100 bar. For H900 condition: 80–120 bar. For Condition A: 70–120 bar. Oil-based coolant with EP additives is recommended for all conditions. For BTA drilling of 15-5PH, documented production operations use 69 bar (1,000 PSI) with 80 GPM flow rate for 40 mm diameter holes. Coolant filtration to 10–15 µm is essential for PH stainless steels to prevent recirculating carbide particles in the swarf from damaging guide pads.
What tool coating works best for 17-4PH deep hole drilling?
TiAlN (titanium aluminium nitride) is recommended for H1150 and H1025 conditions. For H900 (40+ HRC), AlCrN (aluminium chromium nitride) provides better hot hardness and oxidation resistance. For Condition A, TiAlN is effective but cannot fully eliminate BUE — switching to H1150 is a better solution. The coating functions as a thermal barrier that protects the carbide substrate from the heat generated at the cutting edge. Multilayer PVD coatings provide the best balance of wear resistance and edge toughness. Uncoated carbide is not recommended for any PH stainless steel deep hole drilling.
Can BTA drilling be used for 15-5PH stainless steel?
Yes, BTA drilling is well-documented for 15-5PH. A production case study (TechniDrill Systems) demonstrates 40 mm diameter × 500 mm deep holes in 15-5PH at 36 HRC with a penetration rate of 198 mm/min (7.8 IPM), coolant pressure of 69 bar, and coolant flow of 80 GPM. Another case study (Allied Machine T-A Pro) showed 44.45 mm diameter × 508 mm deep holes with cutting speed 76 m/min, feed 0.20 mm/rev, achieving 60 holes per insert edge — double the competitor's tool life. BTA is 3–5× faster than gun drilling for larger diameters in these materials.
What is the primary challenge when drilling 17-4PH in Condition A?
The primary challenge is built-up edge caused by the tough, gummy nature of the solution-annealed martensitic structure. Condition A (33 HRC) has high elongation (10–15%) and produces long, stringy chips that adhere to the cutting edge. The BUE degrades surface finish, increases cutting forces, and can cause catastrophic tool failure when pieces of built-up material break off, taking carbide particles with them. Additionally, the stringy chips clog evacuation passages. The solution is to avoid drilling in Condition A — specify H1150 instead. If drilling in Condition A is unavoidable, use TiAlN-coated carbide at 20–35 m/min with coolant pressure above 100 bar and polished rake face tools.
What surface finish can be expected when gun drilling 17-4PH H1150?
With an optimised TiAlN-coated carbide gun drill in H1150 condition, surface finish of Ra 0.4–1.2 µm is achievable. Production runs typically achieve Ra 0.8–1.6 µm through the tool life. This is comparable to or better than austenitic stainless steel gun drilling. H900 condition achieves slightly coarser finish (Ra 0.8–1.6 µm with new tool) due to higher cutting forces and tool wear. Condition A produces the poorest finish (Ra 0.8–2.0 µm) due to BUE. When Ra exceeds 2.0 µm, inspect and replace the tool.
How do 17-4PH and 15-5PH compare for deep hole drilling?
17-4PH and 15-5PH are very similar in deep hole drilling behavior — both are precipitation-hardening martensitic stainless steels with nearly identical chemistries (15-5PH has slightly lower chromium and 5% nickel versus 4% in 17-4PH). Machining parameters are virtually interchangeable between the two grades. 15-5PH has marginally improved toughness and transverse properties but presents the same drilling challenges and requires the same parameter adjustments by heat treatment condition. The same guidance applies: drill in H1150 condition for best results, avoid Condition A, and use TiAlN-coated carbide with 60–100 bar coolant pressure.
What is the most common mistake in deep hole drilling 17-4PH and 15-5PH?
The most common mistake is attempting to drill in the wrong heat treatment condition. Operators often assume that the softest condition (Condition A, 33 HRC) will be easiest to drill, but it is actually the most difficult due to gummy chip formation and built-up edge. The H900 condition (44 HRC) is too hard for economical deep hole drilling. The H1150 condition (28–32 HRC) — which many shops overlook — is the optimal condition. The second most common mistake is using 304/316 stainless steel parameters on PH grades without adjusting for the higher strength and abrasiveness. PH stainless steel at H1150 requires 30–40% lower cutting speeds than 304 stainless steel. The third mistake is inadequate coolant pressure — PH grades generate higher cutting forces than austenitic grades and require 70–100 bar minimum.
Summary
Deep hole drilling of 17-4PH and 15-5PH precipitation-hardening stainless steels requires careful selection of heat treatment condition above all other parameters. H1150 (28–32 HRC) is the optimal condition, offering the best balance of chip formation and tool life with TiAlN-coated carbide gun drills at 30–46 m/min, feed rates of 0.015–0.100 mm/rev depending on diameter, and coolant pressure of 60–100 bar. Condition A (solution annealed, 33 HRC) is the most difficult to drill despite being the softest — gummy chip formation and BUE make it unsuitable for production deep hole drilling. H900 (44 HRC) is drillable but requires AlCrN-coated tools at reduced speeds (18–30 m/min) with shorter tool life. The recommended workflow is: deep hole drill in H1150 condition, then re-age to the final required condition. BTA drilling is highly productive for larger diameters, with documented penetration rates of 198 mm/min in 15-5PH. Tool life in the optimal H1150 condition reaches 60–100 holes per regrind, with surface finish of Ra 0.4–1.2 µm. The most important process decision is not the cutting speed or feed rate — it is specifying the correct heat treatment condition before drilling begins.