PH 15-5 vs 17-4 Stainless: Tool Selection by Aging Condition
Precipitation-hardening (PH) stainless steels 15-5 PH and 17-4 PH are the workhorses of aerospace structural components, valve bodies, and high-strength fittings. Both alloys achieve their properties through copper-rich precipitate formation during aging, but their different metallurgical architectures create distinct machining behaviors. This guide maps insert grades and cutting parameters to specific aging conditions for both alloys.
Alloy Profiles
17-4 PH (UNS S17400, AISI 630):
- Composition: 17% Cr, 4% Ni, 4% Cu, 0.3% Nb
- Most widely available PH stainless; stocked in all conditions
15-5 PH (UNS S15500):
- Composition: 15% Cr, 5% Ni, 3.5% Cu, 0.3% Nb
- Improved transverse ductility and toughness vs 17-4; preferred for thick-section forgings
Hardness by Aging Condition
| Condition | Aging Temp / Time | 17-4 PH Hardness | 15-5 PH Hardness | 17-4 UTS (MPa) |
|---|---|---|---|---|
| H1150 (overaged) | 620 deg C / 4hr | 26-30 HRC | 26-30 HRC | 930-1000 |
| H1100 | 593 deg C / 4hr | 30-34 HRC | 30-34 HRC | 1000-1100 |
| H1075 | 580 deg C / 4hr | 33-37 HRC | 33-37 HRC | 1100-1200 |
| H1025 | 550 deg C / 4hr | 36-40 HRC | 36-40 HRC | 1200-1310 |
| H900 (peak aged) | 480 deg C / 1hr | 42-46 HRC | 42-46 HRC | 1310-1400 |
| Condition A (solution) | Not aged | 28-32 HRC | 28-32 HRC | 1000-1100 |
Insert Grade Selection by Condition
H1150 and H1100 (26-34 HRC) – Both Alloys:
This is the most machinable aged condition. Standard P-Group (steel) inserts perform well.
- Roughing: Sandvik GC4325 (CVD TiCN/Al2O3/TiN) or Kennametal KCP25B. CNMG 120412 with -MR chipbreaker for good chip control.
- Semi-finishing: Sandvik GC4315 or Mitsubishi MC6025. CNMG 120408 with wiper flat.
- Finishing: Sandvik GC4305 or Tungaloy T9115. DNMG 150404 for fine surface finish.
H1075 and H1025 (33-40 HRC) – Both Alloys:
Increasing copper precipitate density raises abrasiveness. Harder P-grades or M-grades recommended.
- Roughing: Sandvik GC4330 (tough grade for higher hardness) or Kennametal KCP30B. CNMG 120412 with reinforced edge.
- Finishing: Sandvik GC4315 or Mitsubishi VP15TF (M-group PVD). Use slightly reduced feed rates versus H1150.
H900 (42-46 HRC) – Both Alloys:
Peak-aged condition is significantly more abrasive. Shift from P-group carbide to harder substrates or PCBN.
- Roughing: Sandvik GC4340 (hardest carbide grade available for steel) or PCBN Sandvik CB7015. CNMX 120408 with T-land edge prep.
- Finishing: PCBN preferred: Kennametal KD050 or Sumitomo BN250. DNMX 150404.
- Budget alternative: Fine-grain PVD carbide (Mitsubishi VP15TF or Kyocera PR1535) at reduced speeds. Accept 5-10 minute tool life per edge.
Cutting Parameters by Condition
17-4 PH Turning:
| Condition | Vc (m/min) | fn (mm/rev) | ap (mm) | Tool Life (min/edge) |
|---|---|---|---|---|
| H1150 (rough) | 150-220 | 0.20-0.35 | 2.0-5.0 | 40-80 |
| H1150 (finish) | 200-300 | 0.10-0.20 | 0.3-1.0 | 60-120 |
| H1025 (rough) | 100-170 | 0.18-0.30 | 2.0-4.0 | 25-50 |
| H1025 (finish) | 140-220 | 0.08-0.18 | 0.3-0.8 | 30-60 |
| H900 (rough, PCBN) | 80-140 | 0.12-0.22 | 1.0-3.0 | 20-40 |
| H900 (finish, PCBN) | 120-200 | 0.06-0.15 | 0.2-0.8 | 30-60 |
| H900 (rough, carbide) | 50-80 | 0.12-0.20 | 1.0-2.5 | 8-15 |
15-5 PH Turning:
Reduce all speeds by 10-15% compared to 17-4 PH at equivalent conditions. 15-5 PH’s slightly higher nickel content increases toughness and work-hardening tendency.
Key Differences Between 15-5 and 17-4 in Machining
- Chip formation: 17-4 PH produces shorter, more manageable chips due to its slightly higher chromium content forming harder carbides that act as chip breakers. 15-5 PH generates longer, tougher chips requiring more aggressive chipbreaker geometries.
- Built-up edge: 15-5 PH is more prone to BUE formation at lower cutting speeds (below 80 m/min) due to its higher nickel content increasing chemical affinity with the insert coating. Maintain Vc above 100 m/min in H1150 condition to avoid BUE.
- Surface finish: 17-4 PH generally achieves better turned surface finish (Ra 0.4-0.8 micrometers) compared to 15-5 PH (Ra 0.6-1.2 micrometers) at equivalent parameters due to its more uniform precipitate distribution.
- Interrupted cuts: 15-5 PH’s superior toughness makes it more forgiving of interrupted cuts on keyed or splined components. Use tougher insert grades (GC4330, KC5525) for 15-5 PH interrupted applications.
Milling Parameters
Face and Profile Milling – H1150 to H1075:
- 4-5 flute solid carbide end mills, AlTiN coating, 12-25mm diameter
- Vc: 120-200 m/min
- fz: 0.06-0.12 mm/tooth
- Radial engagement: 50-70% of diameter
- Axial depth: 0.8-1.5 x diameter
Face and Profile Milling – H1025 to H900:
- Same tooling but reduce Vc to 60-120 m/min
- fz: 0.04-0.08 mm/tooth
- Radial engagement: 30-50% of diameter
- Use trochoidal toolpaths to reduce radial forces
Drilling
- Solid carbide, 135 deg point, TiAlN coating, through-tool coolant
- H1150: Vc 80-120 m/min, fn 0.08-0.15 mm/rev (8mm drill)
- H1025: Vc 50-80 m/min, fn 0.06-0.12 mm/rev
- H900: Vc 30-50 m/min, fn 0.04-0.08 mm/rev. Consider peck depth of 1.0-1.5 x diameter.
Process Planning Recommendations
- Specify H1150 or H1100 condition for machining whenever the design allows the lower strength. Many non-critical structural components can function at H1150 strength levels with significant machining cost savings.
- If H900 strength is required, perform all roughing and semi-finishing in Condition A (solution annealed, 28-32 HRC), then age to H900 and finish only critical dimensions with PCBN tooling.
- Account for dimensional change during aging. 17-4 PH shrinks approximately 0.05-0.08% during H900 aging. 15-5 PH shrinks 0.04-0.07%. Program offsets accordingly.
- Both alloys are magnetic in all conditions. Magnetic chuck clamping is viable for surface grinding and EDM operations.
- Passivation per AMS2700 or ASTM A967 is required after machining to restore corrosion resistance. Budget this as a post-machining operation in your process flow.
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