17-4 PH vs 15-5 PH Stainless
15-5 PH is 17-4 PH with the chemistry trimmed and the melt cleaned up. Both are martensitic precipitation-hardening stainless steels strengthened by copper, both use the same Condition A and H900 to H1150 heat treatments, and ASTM A564 gives them the same minimum strengths in each condition. The difference is delta ferrite: 17-4 carries some, which leaves large bar and forgings with poor ductility and toughness across the grain. 15-5 has a little less chromium and a little more nickel, is ferrite-free, and is usually consumable-electrode remelted, so its transverse properties hold up in large sections.
For most industrial parts 17-4 PH is the right answer: it is cheaper, stocked everywhere, and also comes as sheet, plate, castings, MIM feedstock and printing powder. Choose 15-5 PH when the drawing calls out AMS 5659, when the part is a large forging or thick bar loaded across the grain, or when transverse toughness is tested on the cert.
| Property | 17-4 PH | 15-5 PH |
|---|---|---|
| UNS / ASTM A564 type | S17400 / Type 630 | S15500 / Type XM-12 |
| Chromium / nickel | 15.0-17.5% / 3.0-5.0% | 14.0-15.5% / 3.5-5.5% |
| Copper / niobium | 3.0-5.0% / 0.15-0.45% | 2.5-4.5% / 0.15-0.45% |
| Delta ferrite | Some; stringers in large sections | Essentially none |
| Usual melt | Air melt (AOD); remelted grades available | Consumable-electrode remelted |
| Tensile / yield, H900 min | 1310 / 1170 MPa (190 / 170 ksi) | 1310 / 1170 MPa (190 / 170 ksi) |
| Tensile / yield, H1025 min | 1070 / 1000 MPa (155 / 145 ksi) | 1070 / 1000 MPa (155 / 145 ksi) |
| Hardness, H900 | 40-47 HRC | 40-47 HRC |
| Transverse ductility and toughness | Lower, especially in large bar and forgings | Higher; close to longitudinal values |
| Density | 7.8 g/cm³ | 7.8 g/cm³ |
| Typical uses | Pump shafts, valve stems, fittings, molds, fasteners, printed parts | Aerospace fittings, actuator and landing-gear parts, large forgings |
Typical values for the stated conditions. Confirm against the mill certificate and your spec.
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Decision matrix
A check marks the side with the edge on that dimension.
| Dimension | 17-4 PH | 15-5 PH |
|---|---|---|
| Machining | Good in Condition A or H1150; fair at H900 | Essentially the same as 17-4 PH |
| Welding | Good; weld in Condition A with ER630 filler, then re-age | Good; same procedure and filler |
| Forming | Poor; limited bending in Condition A | Poor; limited bending in Condition A |
| Strength | Same minimums; transverse ductility and impact drop in large sections | ✓ Same minimums; transverse properties hold in large sections |
| Corrosion resistance | Near 304 in atmosphere when overaged; below 316 in chlorides | About the same as 17-4 PH |
| Finishing | Passivate after machining; left machined or bead blasted | Same as 17-4 PH |
| Material cost | ✓ About 1.5-2x 304 bar per pound | About 1.2-1.5x 17-4 PH per pound |
| Availability | ✓ Bar, plate, sheet, castings, MIM and printing powder | Mainly bar and forgings from aerospace distributors |
Choose 17-4 PH when
- Industrial shafts, valve stems, pump parts, fittings and fasteners in small to medium sections
- Sheet, strip, plate, castings (CB7Cu-1), metal injection molded or printed parts
- Cost and short lead time matter more than transverse toughness
- Drawings calling out AMS 5643 or ASTM A564 Type 630
Choose 15-5 PH when
- Aerospace parts specified to AMS 5659
- Large forgings and thick bar that carry load across the grain
- Parts where transverse ductility or impact toughness is tested on the cert
- Fatigue- and impact-critical fittings, actuator rods and landing-gear parts in H1025 to H1150
Frequently asked questions
- What is the difference between 17-4 and 15-5 stainless?
- Mainly cleanliness and structure. 15-5 PH has less chromium, a little more nickel, no delta ferrite, and is usually remelted, which gives it better ductility and toughness across the grain. Heat treatments, minimum strengths, corrosion resistance and machinability are essentially the same.
- Is 15-5 PH stronger than 17-4 PH?
- Not in the longitudinal direction: both have the same ASTM A564 minimums, 1170 MPa (170 ksi) yield in H900. 15-5 PH is better across the grain and in large sections, where 17-4 PH can fall short on transverse elongation and impact energy.
- Can I substitute 17-4 PH for 15-5 PH?
- Not without engineering approval. Going from 15-5 to 17-4 gives up transverse toughness the designer may be relying on, and aerospace drawings name the spec. Going the other way, 15-5 in place of 17-4, is usually acceptable technically, but the customer still has to agree.
- Is 15-5 PH stainless magnetic?
- Yes. Like 17-4 PH it is martensitic and magnetic in every condition, solution annealed or aged.
