17-4PH and 13-8PH belong to the same family — precipitation hardening stainless steels — but their hardening mechanisms differ. That difference shows itself less in the strength reached than in the cleanliness and toughness of the material.
Aluminium instead of copper
17-4PH hardens through the copper precipitates formed in the structure during ageing; its composition contains 3.00–5.00% copper. 13-8PH contains no copper; its hardening comes from the intermetallic precipitates that 0.90–1.35% aluminium forms with nickel. In addition, 2.00–2.50% molybdenum is added to 13-8PH — there is no molybdenum in 17-4PH.
The second difference, at least as important as composition, is the production method. 13-8PH is vacuum melted and its impurities are very tightly limited: phosphorus max 0.010%, sulphur max 0.008%, silicon and manganese max 0.10–0.20%. The same values in 17-4PH are 0.040%, 0.030% and 1.00% respectively. That cleanliness raises fracture toughness and fatigue life directly.
Comparison in numbers
| PROPERTY | 17-4PH | 13-8PH |
|---|---|---|
| Carbon (C) | max 0.070% | max 0.050% |
| Chromium (Cr) | 15.00 – 17.50% | 12.25 – 13.25% |
| Nickel (Ni) | 3.00 – 5.00% | 7.50 – 8.50% |
| Copper (Cu) | 3.00 – 5.00% | — |
| Molybdenum (Mo) | — | 2.00 – 2.50% |
| Aluminium (Al) | — | 0.90 – 1.35% |
| Niobium + Tantalum | 0.15 – 0.45% | — |
| Manganese (Mn) | max 1.00% | max 0.20% |
| Silicon (Si) | max 1.00% | max 0.10% |
| Phosphorus (P) | max 0.040% | max 0.010% |
| Sulphur (S) | max 0.030% | max 0.008% |
| Tensile strength Rm | — | 1480 MPa |
| Yield strength Rp0.2 | — | 1415 MPa |
| Elongation A50 | — | 13% |
| UNS | S17400 | S13800 |
| W.Nr (DIN/EN) | 1.4542 | 1.4534 |
Values are taken from our product datasheets. In both grades the mechanical values depend on the ageing regime applied (H900, H1000, H1025, H1150 and so on); the table gives the values from our datasheet for 13-8PH.
Which one, and when?
17-4PH is sufficient — pump shafts, valve components, fasteners, tooling parts, general purpose high-strength stainless components. It is easy to source and low in cost.
13-8PH is required — aircraft landing gear components, critical actuator shafts, parts where fatigue life and fracture toughness are stated as numbers in the specification. It is more expensive and takes longer to source; but when the toughness limit of 17-4PH has been reached, there is no alternative.
The grade in between
For requirements between the two there is 15-5PH; we explained its difference from 17-4PH in this article.
Technical datasheets
17-4PH — chemical composition, mechanical values, standard equivalents
13-8PH — 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.