17-4PH and 15-5PH are the two most common precipitation hardening stainless steels. Both are hardened with copper and niobium, and both are brought to the required strength by a single-stage ageing treatment. 15-5PH is in fact an improved derivative of 17-4PH.
Chromium down, nickel up
The compositions on our datasheets show the difference clearly: chromium is 15.00–17.50% in 17-4PH and falls to 14.00–15.50% in 15-5PH. Nickel, in exchange, rises from 3.00–5.00% to 3.50–5.50%. Copper falls from 3.00–5.00% to 2.50–4.40%.
The purpose of that shift is to reduce the amount of delta ferrite in the structure. Delta ferrite is a soft, directional phase left inside the austenitic–martensitic structure; it elongates in the rolling direction and makes the material anisotropic. As a result, the toughness of 17-4PH in the transverse direction stays noticeably lower than in the longitudinal direction.
In 15-5PH the chromium–nickel balance is adjusted to suppress delta ferrite, so the material behaves more consistently in every direction. That is the only change made chemically; the strength class and the ageing regimes (H900, H1025, H1150 and so on) are the same.
Comparison in numbers
| PROPERTY | 17-4PH | 15-5PH |
|---|---|---|
| Carbon (C) | max 0.070% | max 0.070% |
| Chromium (Cr) | 15.00 – 17.50% | 14.00 – 15.50% |
| Nickel (Ni) | 3.00 – 5.00% | 3.50 – 5.50% |
| Copper (Cu) | 3.00 – 5.00% | 2.50 – 4.40% |
| Niobium + Tantalum (Nb+Ta) | 0.15 – 0.45% | 0.15 – 0.45% |
| Manganese (Mn) | max 1.00% | max 1.00% |
| Silicon (Si) | max 1.00% | max 1.00% |
| UNS | S17400 | S15500 |
| W.Nr (DIN/EN) | 1.4542 | 1.4545 |
| AMS | 5604 · 5622 · 5642 · 5643 | 5622 · 5643 · 5659 |
| ASTM | A484 · A564 · A693 | A564 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
17-4PH is sufficient — pump and valve bodies, shafts, fasteners, tooling components, general aerospace and marine parts. It is the most widely available, most economical precipitation hardening stainless steel, and the one whose heat treatment behaviour is best understood. If the specification calls out AMS 5604, this is the grade being asked for.
15-5PH is required — thick-section forgings, aircraft structural members where high toughness is wanted in the transverse direction as well, critical actuator bodies. If the specification calls out AMS 5659, 17-4PH is not an acceptable substitute.
The other members of the family
For higher toughness and a cleaner structure there is 13-8PH; we covered the difference in this article. For applications where spring properties matter, 17-7PH is an alternative, and for higher strength, Custom 455.
Technical datasheets
17-4PH — chemical composition, mechanical values, standard equivalents
15-5PH — 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.