17-4 PH and 15-5 PH are precipitation-hardened stainless steels combining high strength (close to carbon steel) with corrosion resistance (close to austenitic stainless). They're nearly identical in properties; the difference is in what happens during cryogenic service. This guide covers the specific cases where the distinction matters.
PH = Precipitation Hardening. Unlike regular stainless (where strength comes from cold work or heat treat of martensite), PH stainless develops strength from copper or other elements precipitating out of solution during aging heat treatment.
The workflow:
PH stainless is used where you need high strength AND corrosion resistance — aerospace fasteners, pump shafts, marine hardware, surgical instruments.
| Property (H900 condition) | 17-4 PH | 15-5 PH |
|---|---|---|
| Tensile strength | 1310 MPa | 1310 MPa |
| Yield strength | 1170 MPa | 1170 MPa |
| Hardness | 44 HRC | 44 HRC |
| Impact strength (Charpy) | 25 J | 35 J |
| Max service temperature | 315 °C | 315 °C |
| Cryogenic toughness | Fair (delta ferrite reduces) | Good (no delta ferrite) |
| Chemistry (Cr) | 15.0-17.5% | 14.0-15.5% |
| Chemistry (Ni) | 3.0-5.0% | 3.5-5.5% |
| Chemistry (Cu) | 3.0-5.0% | 2.5-4.5% |
| Delta ferrite content | 4-15% (variable) | < 5% (tightly controlled) |
| Cost | Baseline | +5-10% premium |
For all practical purposes in non-cryogenic service, 17-4 PH and 15-5 PH are interchangeable. The strength, hardness, and corrosion resistance are identical.
PH stainless is specified by its aging heat treatment condition. Common conditions for 17-4 PH (same for 15-5 PH):
| Condition | Aging temperature | Tensile strength | Use case |
|---|---|---|---|
| H900 | 480 °C / 1h | 1310 MPa | Maximum strength |
| H1025 | 550 °C / 4h | 1070 MPa | Balance strength and toughness |
| H1075 | 580 °C / 4h | 1000 MPa | Better SCC resistance, still strong |
| H1100 | 595 °C / 4h | 965 MPa | Good SCC resistance |
| H1150 | 620 °C / 4h | 930 MPa | Maximum SCC resistance, moderate strength |
| H1150M | 620 °C / 4h, 760 °C / 2h | 795 MPa | Double-aged, maximum toughness |
Higher aging temperatures reduce strength but improve corrosion resistance and toughness. For most mechanical applications, H1025 is a good balance. For high-stress-corrosion environments, H1150 is the safer choice.
15-5 PH earns its premium over 17-4 PH in specific situations:
Delta ferrite in 17-4 PH can cause embrittlement at cryogenic temperatures. 15-5 PH's tighter ferrite control provides more consistent low-temperature toughness. Aerospace cryogenic fuel systems, liquid-hydrogen service.
15-5 PH's tighter chemistry gives more predictable fracture mechanics, important for safety-critical aerospace parts where fatigue crack growth prediction is required.
Specific military/nuclear specifications require 15-5 PH specifically, driven by the cryogenic and toughness requirements above.
For everything else (commercial aerospace, industrial, marine, medical), 17-4 PH is the practical default. The 5-10% cost savings over 15-5 PH is real money on production runs.
Both 17-4 PH and 15-5 PH machine similarly:
Standard workflow: rough-machine in solution-annealed condition, age harden to target condition, finish-machine or grind critical features. This gives best dimensional accuracy and tool life.
Distortion during aging is typically <0.1% on dimensions, but allow for it on critical features. For tight-tolerance post-aging work, leave 0.1-0.3 mm stock per surface for finishing after age hardening.
Email [email protected] with your drawing and target condition (H900 through H1150). We'll confirm grade based on service conditions and quote the heat-treat cycle.
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