Last verified 19 August 2026.
The comparison
| Property | 17-4 PH | 17-7 PH | Evidence |
|---|---|---|---|
| Proof strength Rp0.2, typical | 850 MPa | 210 MPa | E3 E3 |
| Tensile strength Rm, typical | 1100 MPa | 700 MPa | E3 E3 |
| Elongation A, typical | 22 % | 50 % | E3 E3 |
| Density | 7.8 g/cm³ | 7.8 g/cm³ | E3 E3 |
| Modulus of elasticity | 200 GPa | 200 GPa | E3 E3 |
| Thermal conductivity | 16 W/(m·K) | 16 W/(m·K) | E3 E3 |
| Proof strength Rp0.2, typical (bar) | 600 MPa | not established here | E3 / — |
| Proof strength Rp0.2, typical (heat treated) | not established here | 330 MPa | — / E3 |
All shared rows compare like for like.
Where each side’s numbers come from
See 17-4 PH and 17-7 PH — each page lists every source, every demotion and every gap. Nothing on this page is derived from anything not on those two, and our coverage limits are stated separately.
Where a PREN appears above, read it as a ranking number rather than a qualification: what PREN can and cannot tell you.
When to choose which
17-4 PH. Parts that need high strength, moderate corrosion resistance and dimensional stability through hardening. Pump and valve shafts, aerospace fittings, injection moulds, firearm and turbine components. It is machined soft and aged afterwards at a low enough temperature that the part barely moves, which is the property that distinguishes it from a quenched martensitic grade.
17-7 PH. Formed springs, clips, diaphragms, bellows and thin strip parts that must be shaped first and strong afterwards. Aerospace and instrument work uses it for exactly that sequence. Where a part could be made from hardened martensitic stock but cannot be formed in that state, this is the alloy that solves the manufacturing problem rather than the service problem.
Against 17-4 PH: Chlorides, temperature and toughness at the top of its range. Its corrosion resistance sits between 410 and 304, nowhere near 316L, and the shortest ageing treatments trade notch toughness and stress corrosion resistance for strength. Above roughly 300 °C the precipitates that give it strength begin to over-age, and no source consulted for this page covers that.
Against 17-7 PH: Process control and corrosion. The transformation sequence needs accurate temperature control through several steps, and getting it wrong leaves retained austenite and unpredictable properties; that is a real risk in a small shop. Its corrosion resistance is roughly that of 410 — below 304, well below 316L — so it is a manufacturing choice, not an environmental one.
What this comparison does not cover
- The heat treatment condition, which for this alloy is the difference between two grades’ worth of strength.
- No specified minima. Everything here is a mill typical value and cannot be used as a design allowable.
- No hardness values.
- No corrosion data, and the alloy is often chosen where 316L would be safer.
- Behaviour above roughly 300 °C, where the ageing that gives it strength starts to reverse. No source consulted here covers it.
- The condition behind every number, which for a precipitation hardening alloy is the specification.
- No properties for the standard aged conditions, which are the ones a drawing calls out.
- No corrosion data.
Generated from data/materials/17-4ph.yaml and 17-7ph.yaml.