XM-25 (Custom 450) and 17-4 PH are the two most commonly specified martensitic PH stainless grades for shafts, valves, gears and aerospace hardware. They look interchangeable on a spec sheet but are not. The short version: 17-4 PH is the right default for the majority of pump, valve and fastener work; XM-25 earns its premium when you also need the corrosion resistance of a 316 and you do not want to compromise on strength.
Both grades reach roughly 1,100–1,200 MPa yield strength after a single low-temperature ageing treatment. The difference is in the corrosion envelope and the machinability of the annealed (solution-treated) condition.
| Parameter (H900 / H1025) | 17-4 PH (S17400) | XM-25 / Custom 450 (S45000) |
|---|---|---|
| Cr / Ni / Cu / Mo | 15.5 / 4.5 / 3.5 / — | 15 / 6 / 1.5 / 0.75 |
| Yield (H900, typical) | ≈ 1,170 MPa | ≈ 1,170 MPa |
| Corrosion (PREN) | ~ 17 | ~ 20 |
| Better than 316? | Comparable to 304 in many environments | Yes — close to 316 in most chloride media |
| Machinability (annealed) | Good | Very good — easier than 17-4 PH |
| Weldability | Good (17-4 PH filler recommended) | Very good (Custom 450 filler) |
For the majority of pump, valve and fastener duty in clean (low-chloride) service, 17-4 PH is the right call — it is more readily available, less expensive, and the standard filler (17-4 PH weld wire) is on every stockist's shelf.
Default to 17-4 PH. Specify XM-25 only when (a) the corrosion envelope is past 17-4 PH but you still need PH-level strength, or (b) the machinability advantage of the annealed condition is going to save you serious cost on a complex part.
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