17-7PH and 17-4PH have very similar names but belong to different classes. Both are precipitation hardening stainless steels; one is martensitic, the other semi-austenitic — and that difference changes how the material is processed from the ground up.
Copper against aluminium
According to our datasheets, 17-4PH has nickel at 3.00–5.00%, copper at 3.00–5.00% and niobium + tantalum at 0.15–0.45%. Hardening occurs through copper precipitates and the material is already martensitic in the annealed condition.
17-7PH contains no copper. Nickel rises to 6.50–7.80% and hardening is provided by 0.70–1.50% aluminium. The high nickel keeps the material austenitic in the annealed condition — that is, soft, ductile and very easy to form.
That gives 17-7PH a distinctive manufacturing advantage: the part is deep drawn, bent or coiled in the soft austenitic condition; then a conditioning heat treatment (for example TH1050 or RH950) transforms the structure to martensite and ages it. The result is a complex-shaped part with very high strength.
That is not possible with 17-4PH; since the material is hard from the outset, shaping is done by machining.
Comparison in numbers
| PROPERTY | 17-7PH | 17-4PH |
|---|---|---|
| Structure (annealed) | Semi-austenitic | Martensitic |
| Carbon (C) | max 0.080% | max 0.070% |
| Chromium (Cr) | 16.00 – 18.00% | 15.00 – 17.50% |
| Nickel (Ni) | 6.50 – 7.80% | 3.00 – 5.00% |
| Aluminium (Al) | 0.70 – 1.50% | — |
| Copper (Cu) | — | 3.00 – 5.00% |
| Niobium + Tantalum | — | 0.15 – 0.45% |
| Manganese (Mn) | max 1.00% | max 1.00% |
| Silicon (Si) | max 0.70% | max 1.00% |
| Sulphur (S) | max 0.015% | max 0.030% |
| Tensile strength Rm | 1170 MPa | — |
| Density | 7.80 g/cm³ | — |
| Modulus of elasticity | 204 GPa | — |
| UNS | S17700 | S17400 |
| W.Nr (DIN/EN) | 1.4568 | 1.4542 |
| AMS | 5528 · 5529 · 5568 · 5644 · 5678 · 5824 | 5604 · 5622 · 5642 · 5643 |
| ASTM | A564 · A579 · A693 · A705 | A484 · A564 · A693 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
17-7PH is preferred — springs and spring elements, bellows and diaphragm parts, clips and clamps, thin sheet structures, high-strength parts produced by cold forming. Anywhere forming has to be done before hardening. If the specification calls out AMS 5528 or AMS 5529, this is the grade being asked for.
17-4PH is preferred — pump and valve bodies, shafts, fasteners, tooling parts, high-strength stainless components produced by machining. It is far easier to source and its heat treatment regime is simpler.
The other members of the family
For the reduced-delta-ferrite version, see our 17-4PH – 15-5PH comparison; for higher toughness, our 17-4PH – 13-8PH article.
Technical datasheets
17-7PH — chemical composition, mechanical values, standard equivalents
17-4PH — chemical composition, mechanical values, standard equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.