Three alloys, one family — and very different answers
PH15-7Mo (UNS S15700 / AISI 632), 17-4PH (UNS S17400 / AISI 630), and 15-5PH (UNS S15500) belong to the same precipitation-hardening stainless family — but they are not interchangeable. The right choice depends on the strength level you need, the environment your part will see, the dimensional tolerance you can afford to lose during heat treatment, and whether you need to form or weld before hardening.
Highest strength
Best cost/availability
Best toughness (VAR)
The fundamental difference: how each alloy hardens
All three alloys achieve high strength through precipitation hardening — but the mechanism and precipitate chemistry differ significantly, and those differences drive most of the practical property gaps in the data below.
17-4PH and 15-5PH: copper-rich precipitates, spontaneous transformation
Both 17-4PH and 15-5PH are martensitic PH steels. When solution-annealed and cooled to room temperature, they transform spontaneously to martensite. Final aging at 900–1150°F then precipitates copper-rich ε-phase particles (10–50 nm) within the martensite matrix, blocking dislocation motion and producing the strength increase.
The practical difference between these two: 15-5PH is produced by vacuum melting (AOD or VAR), eliminating the delta-ferrite stringers present in conventionally-melted 17-4PH. Delta-ferrite stringers reduce transverse toughness and fatigue life. In through-thickness loading (flanges, heavy rings, impellers), 15-5PH consistently shows better Charpy impact values and higher fracture toughness (KIC) at the same heat treatment condition.
PH15-7Mo: NiAl intermetallic precipitates, controlled transformation
PH15-7Mo is a semi-austenitic PH steel — the only one in this comparison. Its annealed structure is predominantly austenitic (FCC), not martensitic. Transformation requires a deliberate intermediate step — austenite conditioning — which raises the martensite-start (Ms) temperature above room temperature by precipitating Cr₂₃C₆ carbides at grain boundaries. Subsequent cooling or sub-zero treatment (−73°C for 8 h in the R-condition) then drives the transformation. Final aging precipitates NiAl (B2-ordered) intermetallic precipitates just 2–10 nm in diameter — far finer than copper-rich precipitates — generating peak strengths up to 1828 MPa (CH900).
| Metallurgical Feature | PH15-7Mo (S15700) | 17-4PH (S17400) | 15-5PH (S15500) |
|---|---|---|---|
| PH Family | Semi-austenitic | Martensitic | Martensitic |
| Annealed Structure | Austenite + δ-ferrite | Martensite + δ-ferrite | Martensite + δ-ferrite |
| Hardening Precipitate | NiAl — B2 ordered | Cu-rich ε-phase | Cu-rich ε-phase |
| Precipitate Size | 2–10 nm (very fine) | 10–50 nm | 10–50 nm |
| Delta-ferrite Stringers | Minimal | Present (air melt) | Eliminated (VAR) |
| Pre-hardening Formability | Excellent (austenitic) | Limited | Limited |
| Pre-hardening Weldability | Excellent — weld in Cond. A | Good | Good |
Maximum tensile strength by heat treatment condition
The chart below shows peak tensile strength in each alloy's strongest standard condition. UNS S15700 open die forgings in the CH900 condition are in a class of their own among precipitation-hardening stainless steels — 32% stronger than the best 15-5PH condition, and 40% stronger than the best 17-4PH.
| Condition | PH15-7Mo — UTS / YS (MPa) | 17-4PH — UTS / YS (MPa) | 15-5PH — UTS / YS (MPa) |
|---|---|---|---|
| Highest strength | 1828 / 1724 (CH900) | 1310 / 1170 (H900) | 1380 / 1310 (H900) |
| Mid condition | 1655 / 1552 (RH950) | 1170 / 1070 (H950) | 1310 / 1170 (H950) |
| Ductile condition | 1448 / 1310 (TH1050) | 860 / 795 (H1150) | 930 / 862 (H1150) |
| Hardness range (HRC) | 42–54 | 28–44 | 28–44 |
| Elongation — high strength cond. | 1–6% (CH900) | 5–10% (H900) | 5–10% (H900) |
| Max service temperature | 315°C / 600°F | 260°C / 500°F | 260°C / 500°F |
| Strength retained at 315°C | ~85% of RT value | ~70% | ~70% |
| Fracture toughness KIC (mid cond.) | 55–75 MPa√m (RH950) | 55–70 MPa√m (H950) | 70–95 MPa√m (H950) |
PH15-7Mo in CH900 achieves its extreme strength at the cost of ductility (<2% elongation) and fracture toughness (KIC ≈ 40–55 MPa√m). For fracture-critical aerospace structural members, the TH1050 condition (1448 MPa, KIC ≈ 80–110 MPa√m, ~10% elongation) is often the better engineering choice, even though it sacrifices peak strength.
PREN: why PH15-7Mo dominates in chloride environments
The Pitting Resistance Equivalent Number (PREN) is the standard industry formula for predicting resistance to pitting corrosion in chloride-bearing media. Molybdenum is weighted at 3.3× chromium because it is far more effective per weight-percent at maintaining the protective passive film.
PH15-7Mo's 2.0–3.0% molybdenum addition is the decisive factor. Neither 17-4PH nor 15-5PH contains molybdenum — their PREN is chromium only. Despite having 15% Cr vs 17% Cr, PH15-7Mo's Mo addition at 3.3× weighting produces a 45% higher PREN than 17-4PH.
A PREN above 20 is the commonly accepted threshold for reliable service in ambient seawater. PH15-7Mo clears this threshold with significant margin; 17-4PH and 15-5PH do not. In practice:
- Marine hardware, offshore valves, subsea actuators: PH15-7Mo is the natural choice. 17-4PH and 15-5PH will experience pitting in long-term seawater immersion.
- Mildly chloride-bearing process streams: All three perform acceptably; 17-4PH is typically the cost-optimised selection.
- Crevice corrosion: Molybdenum provides even greater benefit against crevice corrosion than pitting. PH15-7Mo's advantage is larger wherever crevices are unavoidable (flanged joints, threaded fasteners, valve seats).
- Stress-corrosion cracking (SCC) in chloride: All three are susceptible under high stress in aggressive environments. Lower-strength conditions (TH1050, H1150) offer better SCC resistance than high-strength conditions (CH900, H900).
Condition naming conventions and hardening sequences
One common source of confusion is that the naming systems for these alloys are completely different, even though the underlying concept (solution anneal → condition → age to precipitate) is the same.
17-4PH and 15-5PH use the letter H followed by the aging temperature in °F (e.g. H900 = aged at 900°F / 482°C). PH15-7Mo uses a two-letter prefix: T (air-cooled from 1400°F austenite conditioning), R (refrigerated after 1750°F conditioning), or C (cold-worked), followed by the aging temperature. RH950 in PH15-7Mo is often compared to H900 in 17-4PH as the representative "high strength" condition — but their production routes are very different.
The R-condition requires holding at −73°C (−100°F) for 8 hours following the austenite-conditioning step. This sub-zero treatment drives the austenite-to-martensite transformation to >95% completion. Not all heat treatment shops have this capability. Confirm this with your supplier before specifying PH15-7Mo in the RH950 condition.
Forging distortion during hardening — a critical difference for precision parts
When a forging is heat-treated to its final hardened condition, it changes dimensions. For a pump shaft, turbine disc, or precision valve body, this distortion directly affects how much post-heat-treatment machining allowance must be built into the blank — and whether that machining is even achievable. This is where PH15-7Mo's semi-austenitic nature gives it a decisive structural advantage.
Martensite forms gradually and uniformly during austenite conditioning — not in a sudden quench. Distortion is minimal and highly repeatable across the entire forging cross-section.
Martensite transformation occurs during solution-anneal cooling. Thermal gradients in large or complex sections produce asymmetric distortion and warpage that requires heavier machining allowances.
For a 500 mm diameter forged shaft, the difference between 0.0005 in/in and 0.002 in/in distortion is the difference between a 0.25 mm and a 1.0 mm runout after hardening. Precision bearings and close-tolerance bores cannot tolerate 1.0 mm runout — meaning 17-4PH and 15-5PH require heavier finish-machining allowances and often grinding operations that PH15-7Mo avoids entirely.
For PH15-7Mo forged rings and bars supplied in TH1050, RH950, or CH900 condition, we add only 1.5–2.5 mm of finish-machining allowance per face on precision components. For equivalent 17-4PH forgings requiring similar dimensional tolerances, allowances of 3–6 mm per face are standard, and some complex geometries require post-heat-treatment straightening operations.
Which alloy does each industry actually use — and why?
| Industry / Application | PH15-7Mo | 17-4PH | 15-5PH |
|---|---|---|---|
| Aerospace — structural members | ✔ Primary choice — material meets AMS 5520/5812 specification requirements | ◑ Secondary | ◑ Vacuum melt only |
| Aerospace — springs & clips | ★ Best (ultra-high UTS + low distortion) | — Too low strength | — Too low strength |
| Oil & gas — valve bodies | ◑ Preferred in offshore/marine | ✔ Widely used — compatible with API 6A and NACE MR0175 service requirements | ✔ Common |
| Marine / offshore / subsea | ★ Best (PREN 23.25 > 20 threshold) | — PREN 16.0, insufficient | — PREN 15.0, insufficient |
| Power generation turbines | ★ Best (315°C service limit) | — Limited to 260°C | — Limited to 260°C |
| Pump shafts / impellers | ✔ Preferred — minimal distortion | ◑ Standard with extra allowance | ✔ Better toughness than 17-4PH |
| Food & beverage processing | ◑ Overkill unless aggressive CIP | ★ Best (cost + corrosion balance) | ✔ Common |
| General industrial machinery | — Cost premium unjustified | ★ Best (most stocked, lowest cost) | ◑ When 17-4PH stock not available |
| Defense / ordnance | ✔ Material meets requirements of MIL-S-25043 | ◑ Material meets specification | ◑ Material meets specification |
A practical decision framework for engineers
| Criterion | Winner | Runner-up | Key difference |
|---|---|---|---|
| Max tensile strength | PH15-7Mo CH900 — 1828 MPa | 15-5PH H900 — 1380 MPa | PH15-7Mo wins by ≥32% |
| Corrosion resistance (PREN) | PH15-7Mo — PREN 23.25 | 17-4PH — PREN 16.0 | Only PH15-7Mo clears seawater threshold |
| Dimensional stability | PH15-7Mo — <0.0005 in/in | — | 17-4PH / 15-5PH 4–6× more distortion |
| Elevated temperature performance | PH15-7Mo — 315°C limit | — | Others limited to 260°C |
| Pre-hardening formability | PH15-7Mo — austenitic Cond. A | — | Can be deeply cold-worked before aging |
| Transverse toughness (thick sections) | 15-5PH (VAR) | PH15-7Mo (VOD/VAR) | 15-5PH eliminates δ-ferrite stringers |
| Cost and availability | 17-4PH | 15-5PH | PH15-7Mo carries 15–30% material premium |
| Heat treatment simplicity | 17-4PH / 15-5PH | — | PH15-7Mo RH950 requires sub-zero step |
Questions engineers ask most often
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Jiangsu Liangyi — ISO 9001:2015 certified, 25+ years specialising in precipitation-hardening stainless steel open die forgings and seamless rolled rings. Single-piece weight 30 kg to 30,000 kg. Exported to 50+ countries.