Aerospace structural
Landing gear components, hinges, fittings, fasteners. The aluminum-replacement at high stress.
Precipitation-hardening stainless · UNS S17400
17-4PH Stainless Steel
Martensitic precipitation-hardening stainless. Combines the corrosion resistance of stainless with the strength of alloy steel — the answer when 316L isn’t strong enough and 4140 will rust. Standard for aerospace landing gear, valves, shafts, and high-stress fittings.
UTS (H900)
1310 MPa
Density
7.75 g/cc
Modulus
196 GPa
Mach.
45 /100
When to use
Pick 17-4PH when you need stainless that’s actually strong.
The "PH" stands for precipitation hardening. Solution-treated and aged 17-4PH reaches yield strengths of 1170 MPa (170 ksi) — about 4× the yield of 316L stainless. The corrosion resistance is good (better than 4140, not as good as 316L), and unlike most martensitic stainless it’s weldable.
17-4PH ships in several aging conditions, named for the aging temperature. H900 (aged at 900 °F / 482 °C) gives the highest strength. H1025, H1075, and H1150 trade strength for ductility, toughness, and corrosion resistance. A common route is machining in the solution-annealed (Condition A) state, then having the part aged to the specified H-condition by a heat-treat partner — but it can be machined fully aged when geometry is simple.
Strengths
Why aerospace specs it.
- Strength + corrosion combinedYield 1170 MPa AND moderate corrosion resistance — uniquely useful in aircraft hydraulic, fuel, and engine accessory parts.
- Weldable martensiticMost martensitic stainless is hard to weld. 17-4PH welds with matching filler and a post-weld solution + age.
- Multiple aging conditionsH900 to H1150 spans 1310–930 MPa minimum UTS (862 MPa for H1150-D) — pick the right balance for your application.
- Holds tolerance through HTDimensional change during precipitation aging is a small, predictable contraction (~0.0004–0.0006″/inch). Good for tight final tolerances.
- High operating temperatureUseful up to ~315 °C (600 °F). Higher than 316L; below Inconel.
Trade-offs
What you give up.
- Less corrosion than 316LPitting in chloride environments. For seawater or sustained chloride exposure, 316L still wins.
- Stress-corrosion susceptibility (H900)H900 is most prone to stress-corrosion cracking. H1025 or H1150 is the safer choice in aggressive environments.
- Cost~1.4× the cost of 316L bar. Justified by the strength gain — but not always required.
- Hydrogen embrittlement riskAged 17-4PH at high strength can pick up hydrogen during plating or pickling. Bake-out cycles required after acid exposure.
- Slower to machine than 316LIn hardened condition, machinability drops. Most parts are rough-machined soft, then aged, then finish-ground or bored.
Specs
Mechanical, physical & chemical data.
Mechanical values shown for the H900 condition (highest strength); UTS, yield and elongation are ASTM A564 minimums. The aging condition matrix follows below.
Mechanical
Properties (H900)
| Ultimate tensile strength | 1310 MPa (190 ksi) |
|---|---|
| Yield strength (0.2% offset) | 1170 MPa (170 ksi) |
| Elongation at break | 10% |
| Reduction of area | 40% |
| Modulus of elasticity | 196 GPa |
| Shear modulus | 76 GPa |
| Poisson's ratio | 0.27 |
| Hardness | 40–44 HRC |
| Charpy V-notch impactat 21 °C | 20 J |
Source: AK Steel datasheet; Carpenter datasheet. H1025: UTS 1070 MPa, YS 1000 MPa. H1150: UTS 930 MPa, YS 724 MPa (A564 minimums).
Physical & thermal
Properties at room temp
| Density | 7.75 g/cc (0.280 lb/in³) |
|---|---|
| Melting range | 1404 – 1440 °C |
| Specific heat | 0.460 J/g·°C |
| Thermal conductivity | 17.8 W/m·K |
| CTE (20–100 °C) | 10.8 µm/m·°C |
| Electrical resistivity | 0.80 µΩ·m |
| Magnetic permeability | Ferromagnetic |
Magnetic — distinguishing feature vs austenitic stainless. CTE lower than austenitic stainless (helps with assemblies).
Composition
Chemistry, weight %
| Chromium (Cr) | 15.0 – 17.5 |
|---|---|
| Nickel (Ni) | 3.0 – 5.0 |
| Copper (Cu) | 3.0 – 5.0 |
| Niobium + Tantalum | 0.15 – 0.45 |
| Manganese (Mn) | 1.0 max |
| Silicon (Si) | 1.0 max |
| Carbon (C) | 0.07 max |
| Phosphorus (P) | 0.04 max |
| Sulfur (S) | 0.03 max |
| Iron (Fe) | balance |
Copper is the precipitation-strengthening element; Nb stabilizes against carbide formation during aging.
Aging conditions
Pick the right H-number.
Aging temperature determines the strength/ductility trade. The number after "H" is the aging temperature in degrees Fahrenheit. UTS and yield for the aged conditions are ASTM A564 minimums; hardness ranges and the Condition A strengths are typical (A564 sets only a maximum hardness for Condition A).
| Condition | Aging temp | UTS (min) | Yield (min) | Hardness | Best for |
|---|---|---|---|---|---|
| A (annealed) | — | ~1030 MPa (typ.) | ~760 MPa (typ.) | ≤38 HRC | Stock condition before aging; for machining |
| H900 | 482 °C / 900 °F | 1310 MPa | 1170 MPa | 40–44 HRC | Highest strength — fasteners, shafts, valves |
| H925 | 496 °C / 925 °F | 1170 MPa | 1070 MPa | 39–43 HRC | Slightly more ductility than H900 |
| H1025 | 552 °C / 1025 °F | 1070 MPa | 1000 MPa | 35–40 HRC | Balanced — most common production condition |
| H1075 | 580 °C / 1075 °F | 1000 MPa | 860 MPa | 32–37 HRC | Toughness gain, used in shock-loaded parts |
| H1150 | 621 °C / 1150 °F | 930 MPa | 724 MPa | 28–34 HRC | Maximum toughness; weld zones; cryogenic |
| H1150-D (double age) | 621 °C / 1150 °F ×2 | 862 MPa | 724 MPa | 24–33 HRC | Lowest hardness; best stress-corrosion resistance |
Specifications
Common purchase specs.
AMS 5643
Bar, wire, forgings — Condition A
AMS 5604
Sheet, strip, plate
AMS 5622
Bar, wire, forgings — vacuum-melted, solution treated
AMS 5825
Welding wire
ASTM A564
Bar — heat-treated PH stainless
ASTM A693
Sheet, strip — PH stainless
AISI 630
Equivalent designation
Machining
Machining considerations.
17-4PH machines best in Condition A (solution annealed). Fully aged H900 (40–44 HRC) is harder than 4140 prehardened (28–32 HRC) but still machinable — tougher and slower than mild steels.
Condition A first
Rough and finish in annealed state when possible. Age after — dimensional change is small and predictable.
Cutting speed
150–250 SFM with carbide in Condition A. Drop to 80–120 SFM for fully aged H900.
Feed rate
Aggressive constant feed. Like 316L, work-hardens if you let the tool rub.
Tooling
Coated carbide grades (TiAlN, AlTiN). HSS only for very low volume.
Coolant
Flood. Through-spindle high-pressure for production.
Climb mill
Standard. Conventional milling generates heat that work-hardens the surface.
Allow for HT shrinkage
Aging from Condition A to H900 contracts the part ~0.0004–0.0006″/inch in all directions — leave finish stock for post-aging cleanup on tight features.
Bake out before plating
Aged 17-4PH absorbs hydrogen from acid pickling. Required bake-out (375 °F, 8–24 hrs) for plating ops.
Applications
Where 17-4PH earns its place.
Hydraulics & valves
Valve bodies, pump shafts, hydraulic actuator pistons. High-pressure fluid environments where 4140 would corrode.
Defense & marine
Submarine fittings, missile components, weapon platforms. Tough, strong, and stainless in salt.
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