17-4 PH vs 17-7 PH Stainless Steel
17-4 PH and 17-7 PH are precipitation-hardening stainless steels from the same family, but with different phase structure. 17-4 is martensitic, 17-7 is semi-austenitic - they age-harden via different mechanisms.
17-4 PH
UNS S17400
Composition (wt %)
| Element | Min | Max |
|---|---|---|
| Fe | 73.0 | 79.0 |
| Cr | 15.0 | 17.5 |
| Ni | 3.0 | 5.0 |
| Cu | 3.0 | 5.0 |
| Nb | 0.15 | 0.45 |
| Mn | 0.0 | 1.0 |
| C | 0.0 | 0.07 |
17-7 PH Stainless Steel
UNS S17700
Composition (wt %)
| Element | Min | Max |
|---|---|---|
| Fe | 70.0 | 80.0 |
| Cr | 16.0 | 18.0 |
| Ni | 6.5 | 7.75 |
| Al | 0.75 | 1.5 |
| Mn | 0.0 | 1.0 |
| C | 0.0 | 0.09 |
Key differences
| Aspect | 17-4 PH | 17-7 PH Stainless Steel | Engineering impact |
|---|---|---|---|
| Structure class | Martensitic PH | Semi-austenitic PH | 17-4 is martensitic as supplied and only needs a single aging step. 17-7 requires a conditioning treatment (austenite to martensite transformation) before aging. |
| Nominal composition | 17% Cr, 4% Ni, 4% Cu, Nb | 17% Cr, 7% Ni, 1% Al | 17-4 hardens via Cu-rich precipitates. 17-7 hardens via NiAl precipitates. |
| Form supplied | Bar, forging, plate, casting | Sheet and strip (dominant) | 17-7 is the go-to PH stainless for springs and formed sheet parts. 17-4 dominates machined components. |
| Heat treatment complexity | Solution + single age (H900 etc.) | Condition A -> form -> condition T/R -> age | 17-7 enables forming in soft condition then hardening in-place - ideal for complex formed springs. |
When to choose 17-4 PH
- Pump and valve shafts
- Aerospace machined fittings and fasteners
- Nuclear and petrochemical internals
When to choose 17-7 PH Stainless Steel
- Retaining rings, springs, diaphragms
- Aerospace formed sheet components
- Clips and clamps requiring in-place hardening
Frequently asked questions
Which has higher corrosion resistance?
Comparable - both are equivalent to 304 in mild environments and inferior to 316 in chloride service.
Can I weld 17-7 and harden the assembly afterwards?
Yes - that is one of its strengths. Weld in Condition A, then apply the TH or RH treatment to the whole assembly.
Why use 17-4 H1025 over H900?
Higher aging temperature (H1025, H1150) trades peak strength for better toughness and stress-corrosion resistance. H900 is peak strength but most SCC-sensitive.