Introduction: When Toughness Outweighs Strength
17-4PH (UNS S17400) is the most widely specified precipitation-hardening stainless steel in the world. Since its development by Armco Steel in the 1950s, it has become the default answer whenever an engineer needs a stainless steel that can deliver yield strengths above 1000 MPa while maintaining reasonable corrosion resistance. It is the first PH stainless steel most engineers learn, and for many, it is the only one they ever specify.
15-5PH (UNS S15500) exists for one reason: 17-4PH has a toughness problem.
The issue is not the alloy’s longitudinal properties — those are excellent. The problem is transverse toughness. 17-4PH contains a network of delta-ferrite stringers — elongated bands of retained ferrite that run parallel to the forging direction. In the longitudinal direction, these stringers are harmless. In the transverse direction, they act as crack initiation sites under impact loading, stress corrosion, or cyclic fatigue. The result is a material whose longitudinal Charpy impact energy is 40–60 J, but whose transverse value can drop to 8–15 J — a factor of 3–5× reduction that makes 17-4PH unacceptable for critical aerospace, nuclear, and high-pressure valve applications where loading is multi-directional.
15-5PH was developed specifically to eliminate this problem. By adjusting the Cr/Ni ratio and reducing the Cr content from 15.0–17.5% to 14.0–15.5% while increasing Ni from 3.0–5.0% to 3.5–5.5%, the alloy composition moves outside the δ-ferrite phase field on the Schaeffler diagram. The result is a ferrite-free martensitic matrix that delivers uniform toughness in all directions — transverse Charpy values of 35–50 J instead of 8–15 J.
This is not a “better or worse” decision. It is a loading-direction decision. If your component is loaded primarily in the forging direction (shafts, bolts, longitudinal pressure boundaries), 17-4PH is the cost-effective choice. If your component experiences multi-directional loading, impact, or stress corrosion in the transverse direction (aircraft landing gear, valve bodies, nuclear pump casings), 15-5PH is the correct specification — and in some industries, it is the only PH stainless steel that is permitted.
For background on 17-4PH’s standalone properties, see our 17-4PH stainless steel guide. For the broader stainless steel selection context, our 304L vs 316L vs 316Ti comparison covers the non-PH stainless alternatives.
1. The Delta-Ferrite Problem: Why 17-4PH Fails in Transverse Loading
What Is Delta-Ferrite?
When 17-4PH solidifies from the melt, the first phase to form is delta-ferrite (δ) — a body-centered cubic (BCC) iron structure with high Cr and Mo content. As cooling proceeds through 1200–800°C, most of this ferrite transforms to austenite (γ) via a diffusion-controlled solid-state reaction. However, in 17-4PH’s composition range (15.0–17.5% Cr, 3.0–5.0% Ni), the transformation is not complete. A fraction of the original δ-ferrite — typically 5–15% by volume — remains as elongated stringers aligned with the hot-working direction.
These stringers are not random defects. They are a thermodynamic consequence of the alloy’s composition sitting near the boundary between the single-phase austenite region and the two-phase austenite-plus-ferrite region on the Schaeffler diagram. At 16% Cr and 4% Ni, 17-4PH sits on the borderline. Any heat-to-heat variation in Cr or Ni content shifts the alloy across this boundary, producing different amounts of retained ferrite.
Why Delta-Ferrite Destroys Transverse Toughness
The mechanical consequence of δ-ferrite stringers is directional:
- Crack initiation: Ferrite is softer than the surrounding martensitic matrix (ferrite hardness ~200 HV vs martensite ~400 HV after H900 aging). Under transverse tensile or impact loading, plastic strain concentrates in the ferrite bands, which deform while the martensite remains elastic. This strain mismatch creates micro-cracks at the ferrite/martensite interface.
- Crack propagation: Once initiated, cracks propagate along the ferrite stringers because the interface provides a low-energy path. The result is delamination — the ferrite bands separate from the martensite matrix, creating a layered fracture surface that resembles wood splitting along the grain.
- Stress concentration: The elongated morphology of stringers (aspect ratios of 5:1 to 20:1 after forging) creates stress concentration factors of 2–3× at the stringer tips under transverse loading. This reduces the effective fracture toughness by the same factor.
The net effect is dramatic. In the H900 condition (aging at 480°C / 900°F):
| Property | Longitudinal | Transverse | Ratio |
| Charpy impact (17-4PH) | 40–60 J | 8–15 J | 3–5× drop |
| Reduction of area | 40–50% | 5–15% | 4–8× drop |
| Fracture toughness (K_IC) | 80–120 MPa√m | 20–40 MPa√m | 2–3× drop |
These numbers are not theoretical — they come from decades of aerospace qualification data and are the reason that AMS 5650 (17-4PH) includes a transverse toughness requirement only for certain grades, while AMS 5659 (15-5PH) mandates uniform toughness in all directions.
How 15-5PH Eliminates Delta-Ferrite
15-5PH solves this problem by shifting the alloy composition away from the δ-ferrite phase boundary on the Schaeffler diagram. The key changes:
| Element | 17-4PH | 15-5PH | Effect |
| Cr | 15.0–17.5% | 14.0–15.5% | Reduces ferrite-stabilizing tendency |
| Ni | 3.0–5.0% | 3.5–5.5% | Increases austenite-stabilizing tendency |
| Cu | 3.0–5.0% | 2.5–4.5% | Similar Cu for ε-Cu precipitation |
| Nb | 0.15–0.45% | 0.15–0.45% | Same Nb for carbide control |
| C | ≤0.07% | ≤0.07% | Same carbon level |
The Cr reduction from a maximum of 17.5% to 15.5% and the Ni increase from a minimum of 3.0% to 3.5% moves the alloy’s Schaeffler equivalent from approximately 15 (borderline) to approximately 10 (clearly austenitic during high-temperature exposure). This means that during hot working and cooling, the δ-ferrite transforms completely to austenite — no retained stringers, no directional toughness problem.
The trade-off is predictable: less Cr means slightly lower pitting corrosion resistance (PREN drops from approximately 17 to approximately 15), and the absence of ferrite stringers means slightly lower peak yield strength (the ferrite/martensite interface contributes modestly to Hall-Petch strengthening). We will quantify these trade-offs in the sections below.
2. Heat Treatment & Precipitation Hardening Mechanics
Both alloys use the same hardening mechanism: copper precipitation. During aging at 480–620°C (900–1150°F), the supersaturated Cu in the martensitic matrix precipitates as fine ε-Cu particles (face-centered cubic copper, 5–50 nm diameter) that impede dislocation motion via Orowan bowing.
Heat Treatment Comparison
| Condition | 17-4PH Aging Temp | 15-5PH Aging Temp | Typical Application |
| H900 | 480°C / 4 hr | 480°C / 4 hr | Maximum strength |
| H925 | 495°C / 4 hr | 495°C / 4 hr | Aerospace structural |
| H1025 | 550°C / 4 hr | 550°C / 4 hr | Valve stems, shafts |
| H1075 | 580°C / 4 hr | 580°C / 4 hr | Corrosion-priority |
| H1150 | 620°C / 4 hr | 620°C / 4 hr | Maximum toughness |
| H1150-D | 620°C + 760°C dual | 620°C + 760°C dual | Over-aged for weldability |
The heat treatment procedures are identical for both alloys — same temperatures, same hold times, same quench protocols. This is because the precipitation kinetics of ε-Cu are governed primarily by the Cu concentration and the martensitic matrix carbon content, which are nearly the same in both alloys.
Subtle Processing Differences
There is one important processing difference: solution annealing temperature. 17-4PH is typically solution-annealed at 1040°C for 1 hour, while 15-5PH is annealed at 1040–1065°C. The slightly higher annealing temperature for 15-5PH ensures that any trace ferrite (which may persist in heats near the upper Cr limit of 15.5%) is completely dissolved. If 15-5PH is annealed at the lower temperatures sometimes used for 17-4PH, residual ferrite may persist in borderline heats — defeating the purpose of specifying 15-5PH in the first place.
Practical rule: Always specify solution annealing at 1040–1065°C for 15-5PH. Do not assume that the 17-4PH annealing protocol (1040°C) is sufficient for 15-5PH — it is not, particularly for heats at the upper end of the Cr specification range.
3. Mechanical Properties: Strength vs Toughness Trade-off
Room-Temperature Properties (H900 Condition)
| Property | 17-4PH (H900) | 15-5PH (H900) | Difference |
| UTS | 1310 MPa | 1240 MPa | –5% |
| Yield Strength (0.2%) | 1170 MPa | 1070 MPa | –9% |
| Elongation | 10–14% (L) | 12–16% | Slightly higher |
| Reduction of Area (L) | 40–50% | 45–55% | Slightly higher |
| Reduction of Area (T) | 5–15% | 40–50% | 3–5× better |
| Hardness | 388–444 HB | 363–415 HB | Slightly lower |
| Charpy Impact (L) | 40–60 J | 45–55 J | Similar |
| Charpy Impact (T) | 8–15 J | 35–50 J | 3–5× better |
The strength trade-off is modest — approximately 5–9% lower in 15-5PH. The toughness improvement in the transverse direction is massive — 3–5× better. This is the core value proposition of 15-5PH: you sacrifice a small amount of peak strength to gain uniform, direction-independent toughness.
Condition-Dependent Strength Curve
| Condition | 17-4PH YS | 15-5PH YS | 17-4PH UTS | 15-5PH UTS |
| H900 | 1170 MPa | 1070 MPa | 1310 MPa | 1240 MPa |
| H925 | 1070 MPa | 1000 MPa | 1170 MPa | 1100 MPa |
| H1025 | 860 MPa | 790 MPa | 965 MPa | 930 MPa |
| H1075 | 690 MPa | 650 MPa | 795 MPa | 760 MPa |
| H1150 | 485 MPa | 450 MPa | 590 MPa | 565 MPa |
At all conditions, 15-5PH strength is approximately 5–9% lower than 17-4PH. The gap is consistent and predictable — it does not widen or narrow at different aging temperatures. This means that the toughness-vs-strength trade-off holds across the entire aging range, and the selection decision is the same regardless of which condition you specify.
Fracture Toughness Comparison (H900, Room Temperature)
| Direction | 17-4PH K_IC | 15-5PH K_IC |
| Longitudinal (L-T) | 80–120 MPa√m | 75–110 MPa√m |
| Transverse (T-L) | 20–40 MPa√m | 70–100 MPa√m |
The transverse fracture toughness of 15-5PH is approximately 2–3× higher than 17-4PH. For fracture-mechanics-based design (e.g., aerospace damage tolerance, pressure vessel flaw assessment), this difference can be the deciding factor between acceptance and rejection. Many aerospace prime specifications (Boeing, Airbus, Lockheed Martin) explicitly require minimum transverse K_IC values that 17-4PH cannot meet but 15-5PH can.
4. Corrosion Resistance: The PREN Penalty
Pitting Resistance Equivalent Number
PREN = %Cr + 3.3 × %Mo + 16 × %N
| Alloy | Cr (mid) | Mo (mid) | N | PREN |
| 17-4PH | 16.25% | 0.25% | — | ~17 |
| 15-5PH | 14.75% | 0.25% | — | ~15 |
Both alloys have relatively low PREN values — they are not designed for aggressive chloride environments. They compete with 304L (PREN ~19) and 316L (PREN ~24) not with duplex (PREN 35+) or super austenitic (PREN 46+) alloys. The corrosion advantage of PH stainless steels is not pitting resistance — it is the combination of moderate corrosion resistance with very high strength.
The 2-point PREN difference between 17-4PH and 15-5PH translates to a measurable but modest difference in critical pitting temperature (CPT) in chloride solutions:
- 17-4PH (H900): CPT approximately 15–20°C in 3.5% NaCl
- 15-5PH (H900): CPT approximately 10–15°C in 3.5% NaCl
In practice, this difference matters only in marginal chloride exposures. In atmospheric exposure, mild chemical service, or fresh-water applications, both alloys perform similarly. In seawater or concentrated chloride service, neither alloy is appropriate — you should specify 316L, duplex 2205, or a higher-PREN alloy instead.
Stress Corrosion Cracking
This is where the δ-ferrite issue reappears in the corrosion domain. Ferrite stringers are not only mechanical weak points — they are also electrochemical weak points. The ferrite/martensite interface has a galvanic potential difference of approximately 50–100 mV in chloride solutions, creating preferential anodic dissolution along the stringers. This means that 17-4PH’s transverse SCC resistance is significantly worse than its longitudinal SCC resistance.
| Condition | 17-4PH SCC Threshold (L) | 17-4PH SCC Threshold (T) | 15-5PH SCC Threshold |
| H900 | 35–50% YS | 10–20% YS | 30–45% YS (uniform) |
| H1025 | 55–70% YS | 20–30% YS | 50–65% YS (uniform) |
| H1150 | >80% YS | 50–60% YS | >80% YS (uniform) |
15-5PH’s SCC threshold stress is direction-independent because there are no ferrite stringers to serve as preferential crack paths. At the H900 condition, 15-5PH’s uniform SCC threshold (30–45% YS) is comparable to 17-4PH’s longitudinal threshold — but dramatically higher than 17-4PH’s transverse threshold (10–20% YS).
Design implication: If your component is exposed to chloride stress corrosion in any direction other than the forging direction, 17-4PH at H900 is potentially unsafe. 15-5PH or an over-aged condition (H1150) should be specified.
5. Welding & Fabrication
Weldability Comparison
Both alloys are weldable using standard arc processes (GTAW, GGTAW, PAW, SMAW) with appropriate filler metals. The key welding considerations:
| Aspect | 17-4PH | 15-5PH |
| Pre-weld condition | Solution-annealed or over-aged | Solution-annealed or over-aged |
| Filler metal (matching) | 17-4PH filler (AMS 5825) | 15-5PH filler (AMS 5826) |
| Filler metal (dissimilar) | ER308L / ER309L for non-PH joints | ER308L / ER309L for non-PH joints |
| Post-weld heat treatment | Full age per condition spec | Full age per condition spec |
| HAZ ferrite risk | Yes — δ-ferrite re-forms in HAZ | No — composition eliminates HAZ ferrite |
| Passivation requirement | Yes — post-weld passivation per ASTM A967 | Yes — same requirement |
The critical welding difference is HAZ ferrite. In 17-4PH, the thermal cycle of welding temporarily re-enters the δ-ferrite phase field in the heat-affected zone. This produces localized ferrite stringers in the HAZ that are not aligned with the forging direction — they are aligned with the weld thermal gradient. These stringers create the same transverse-toughness degradation as the base metal stringers, but in an unpredictable orientation relative to the component loading direction.
15-5PH does not form HAZ ferrite because its composition is outside the δ-ferrite phase field. The HAZ remains fully martensitic with the same toughness characteristics as the base metal.
Practical rule for weld repairs: If you are weld-repairing a 17-4PH component in a transverse-loaded application, the repair zone is the weakest point in the structure — regardless of filler metal selection. 15-5PH weld repairs on 15-5PH base metal maintain uniform toughness. This is why many aerospace repair procedures specify 15-5PH filler metal (AMS 5826) even for 17-4PH base metal welds — it provides a ferrite-free weld deposit that mitigates (but does not eliminate) the base metal HAZ ferrite problem.
Machinability
Both alloys are machinable in the solution-annealed condition (hardness ~30 HRC) and difficult to machine in the aged condition (hardness ~38–44 HRC for H900). Machinability is nearly identical because the matrix structure is the same (martensite + ε-Cu precipitates) and the hardness values are close.
| Condition | 17-4PH Machinability Rating | 15-5PH Machinability Rating |
| Solution-annealed | 55–60% (relative to 304L = 100%) | 55–60% |
| H900 aged | 30–35% | 30–35% |
| H1150 aged | 45–50% | 45–50% |
Recommended machining parameters for both alloys in the H900 condition:
- Cutting speed: 15–25 m/min (carbide tools)
- Feed rate: 0.05–0.15 mm/rev
- Depth of cut: 0.5–2.0 mm
- Coolant: Sulfur-based cutting oil (not water-based — chloride coolants risk SCC initiation on freshly machined surfaces)
6. Specifications & Industry Standards
ASTM / AMS Standards
| Standard | 17-4PH | 15-5PH | Coverage |
| ASTM A564 | S17400 | S15500 | Bar, rod, wire, forging |
| ASTM A693 | S17400 | S15500 | Plate, sheet, strip |
| AMS 5650 | Yes | — | 17-4PH bar and forging (aerospace) |
| AMS 5659 | — | Yes | 15-5PH bar and forging (aerospace) |
| AMS 5825 | Yes | — | 17-4PH welding wire |
| AMS 5826 | — | Yes | 15-5PH welding wire |
| ASME SA-564 | S17400 | S15500 | Pressure vessel bar |
| ASME SA-693 | S17400 | S15500 | Pressure vessel plate |
Aerospace Qualification Status
The aerospace industry is where the 17-4PH vs 15-5PH decision matters most, and where the standards are most explicit about which alloy is permitted for which application:
| Specification | 17-4PH | 15-5PH | Notes |
| Boeing BMS 7-173 | Permitted with transverse testing | Permitted without transverse testing | 15-5PH preferred for critical structures |
| Airbus AIMS 04-02 | Restricted to longitudinal-loaded parts | Permitted for all loading directions | 15-5PH mandatory for multi-directional |
| Lockheed Martin ES-217 | Permitted for non-critical | Permitted for critical flight structures | Damage tolerance analysis required for 17-4PH |
| MIL-S-25017 | Both alloys listed | Both alloys listed | Military spec permits both; Navy prefers 15-5PH for submarine valves |
The pattern is clear: aerospace primes that have experienced transverse toughness failures in 17-4PH components (landing gear side braces, wing attachment fittings, hydraulic actuator bodies) have migrated their critical structural specifications to 15-5PH over the past 20–30 years. 17-4PH remains permitted for bolt-loaded, shear-loaded, and longitudinally-loaded components.
7. Applications & Selection Decision Guide
Application Matrix
| Application | Loading Direction | Corrosion Severity | Recommended Alloy | Reason |
| Aircraft landing gear brace | Multi-directional | Moderate (atmospheric) | 15-5PH | Transverse impact loading mandates ferrite-free structure |
| Valve stem (globe valve) | Longitudinal | Moderate (process fluid) | 17-4PH | Longitudinal shear; strength advantage matters |
| Valve body (high-pressure) | Multi-directional | Moderate to severe | 15-5PH | Transverse hoop stress + potential SCC |
| Pump shaft (rotating) | Longitudinal + torsional | Moderate | 17-4PH | Longitudinal and torsional loading; no transverse concern |
| Hydraulic actuator body | Multi-directional | Moderate (hydraulic fluid) | 15-5PH | Transverse fatigue + burst pressure loading |
| Fasteners (bolts, studs) | Longitudinal | Moderate | 17-4PH | Longitudinal tensile loading; strength priority |
| Nuclear pump casing | Multi-directional | Low (controlled water) | 15-5PH | Transverse fracture toughness mandate per ASME III |
| Surgical instrument handle | Multi-directional | Low (body fluid) | 15-5PH | Impact resistance for drop/impact scenarios |
| Paper mill roll | Longitudinal | Moderate (chloride wash) | 17-4PH | Longitudinal bending; H1025 condition for SCC margin |
| Offshore valve (subsea) | Multi-directional | Severe (seawater) | Neither — specify 625 or C-276 | Both PH steels lack seawater pitting resistance |
Selection Decision Flowchart
- Is the loading primarily longitudinal? (shafts, bolts, pins) → Yes: 17-4PH is the cost-effective choice. Use H1025 or H1075 for moderate SCC margin. → No: Continue to step 2.
- Is there significant transverse or multi-directional loading? (valve bodies, actuator housings, fittings) → Yes: 15-5PH is required for uniform toughness. Use H925 or H1025 condition. → No: 17-4PH may be acceptable, but verify with fracture mechanics analysis.
- Is the service environment chloride-exposed? (seawater, brine, HCl-containing process) → Yes: Neither alloy is appropriate for direct seawater immersion. Consider 316L (moderate), duplex 2205 (good), or nickel alloys (excellent) instead. For atmospheric or splash-zone exposure, 15-5PH at H1150 offers the best SCC resistance. → No: Both alloys are acceptable for atmospheric, fresh-water, and mild chemical service.
- Is the application aerospace critical structure? → Yes: 15-5PH is required by most prime specifications for multi-directional loading. 17-4PH is restricted to longitudinal-loaded and non-critical parts. → No: Both alloys are available; select based on the loading-direction analysis above.
- Is cost a primary driver? → 17-4PH bar stock is typically 10–15% cheaper than 15-5PH bar stock due to wider availability and larger production volumes. If the transverse toughness requirement does not apply, 17-4PH is the economical choice.
8. Cost, Availability & Total Cost of Ownership
Raw Material Cost Comparison
| Product Form | 17-4PH (per kg) | 15-5PH (per kg) | Premium |
| Bar (25–100 mm) | $18–22 | $20–25 | +10–15% |
| Plate (6–25 mm) | $20–24 | $22–28 | +10–15% |
| Forging stock | $22–26 | $24–30 | +10–15% |
| Welding wire | $35–45 | $38–50 | +8–12% |
The cost premium for 15-5PH is modest — 10–15% for most product forms. This is because the alloy’s composition changes (lower Cr, higher Ni) are small and do not significantly affect melting or processing costs. The premium is primarily driven by lower production volumes and narrower supply chain availability.
Availability Comparison
| Product Form | 17-4PH Lead Time | 15-5PH Lead Time |
| Standard bar sizes | 2–4 weeks | 4–8 weeks |
| Non-standard sizes | 6–10 weeks | 8–12 weeks |
| Plate > 12 mm | 4–6 weeks | 6–10 weeks |
| Certified aerospace (AMS 5650/5659) | 8–12 weeks | 10–14 weeks |
17-4PH has significantly better availability because it is produced in larger volumes by more mills worldwide. 15-5PH availability is adequate for planned procurement but may be problematic for emergency replacement or short-lead-time projects. The 2–4 week additional lead time for 15-5PH should be factored into project planning.
Total Cost of Ownership Case Study
Application: High-pressure valve body for a chemical processing plant (multi-directional loading, moderate chloride exposure, 10-year design life).
| Factor | 17-4PH Option | 15-5PH Option |
| Material cost (valve body forging) | $2,800 | $3,100 (+11%) |
| Heat treatment (H1025) | $400 | $400 (same) |
| NDE (transverse Charpy + K_IC testing) | $1,200 (extensive testing needed) | $400 (standard testing) |
| Risk of rejection (transverse test failure) | 15–20% rejection rate | <2% rejection rate |
| Expected replacement cost (if rejected) | $4,400 (re-forging + re-test) | Negligible |
| Warranty/insurance cost (SCC risk) | $600/year (higher SCC risk) | $300/year |
| 10-year TCO | $13,800 | $10,000 |
Despite the 11% material cost premium, 15-5PH’s TCO is approximately 28% lower because it eliminates the transverse testing burden, the rejection risk, and the higher SCC insurance costs. This TCO advantage is typical for multi-directionally loaded components and is the reason that 15-5PH has displaced 17-4PH in many valve, actuator, and structural applications over the past two decades.
9. Summary: Quick Reference Decision Table
| Decision Factor | Choose 17-4PH | Choose 15-5PH |
| Loading is purely longitudinal | ✅ | ✅ (acceptable but over-specified) |
| Multi-directional or transverse loading | ❌ | ✅ (mandatory) |
| Aerospace critical structure | ❌ (restricted) | ✅ (required by most primes) |
| Maximum strength priority (H900) | ✅ (1170 MPa YS) | ✅ (1070 MPa — 9% lower) |
| Fracture toughness priority | ❌ (transverse K_IC 20–40) | ✅ (uniform K_IC 70–100) |
| Chloride SCC exposure (transverse) | ❌ (threshold 10–20% YS) | ✅ (threshold 30–45% YS) |
| Weld repair in transverse-loaded part | ❌ (HAZ ferrite risk) | ✅ (ferrite-free HAZ) |
| Cost priority (no transverse concern) | ✅ (10–15% cheaper) | ✅ (acceptable but over-specified) |
| Seawater immersion | ❌ Neither | ❌ Neither — specify 625 or C-276 |
| Short lead-time procurement | ✅ (2–4 weeks) | ❌ (4–8 weeks longer) |
FAQ
Q1: Can I substitute 15-5PH for 17-4PH in any application?
In most longitudinally-loaded applications, yes — 15-5PH is an acceptable substitute, though you sacrifice 5–9% peak strength and pay a 10–15% material cost premium. The reverse substitution (17-4PH for 15-5PH) is not acceptable in transverse-loaded or multi-directionally-loaded applications because 17-4PH cannot meet the transverse toughness requirements that drove the original 15-5PH specification. Always verify with the design authority before substituting.
Q2: Why does 15-5PH have slightly lower pitting resistance than 17-4PH?
15-5PH reduces Cr from 15.0–17.5% to 14.0–15.5% in order to shift the alloy outside the δ-ferrite phase field on the Schaeffler diagram. This Cr reduction lowers the PREN from approximately 17 to approximately 15 — a modest difference that is only significant in marginal chloride exposures. In atmospheric, fresh-water, or mild chemical service, the pitting resistance of both alloys is practically equivalent.
Q3: Is 17-4PH acceptable for aerospace use?
Yes, but with restrictions. Most aerospace prime specifications permit 17-4PH for longitudinally-loaded, bolt-loaded, and non-critical structural components. They require 15-5PH for multi-directionally-loaded critical structures (landing gear braces, wing fittings, actuator bodies) where transverse fracture toughness must meet minimum K_IC thresholds that 17-4PH cannot achieve. Check the specific prime specification (Boeing BMS, Airbus AIMS, Lockheed Martin ES) before specifying 17-4PH for any flight structure.
Q4: Can I weld 17-4PH and 15-5PH together?
Yes, using ER308L or ER309L filler metal for the dissimilar joint. The joint will have the corrosion resistance of the weaker (lower Cr) alloy — 15-5PH. The HAZ on the 17-4PH side will contain δ-ferrite stringers oriented along the weld thermal gradient, which may create localized toughness degradation. If the weld is in a transverse-loaded region, specify 15-5PH filler (AMS 5826) on both sides to ensure a ferrite-free weld deposit, even though the 17-4PH HAZ ferrite remains a concern.
Q5: Which aging condition gives the best balance of strength and SCC resistance?
For 17-4PH: H1025 provides yield strength of approximately 860 MPa with SCC threshold stress of approximately 55–70% YS (longitudinal) — the best strength/SCC balance for longitudinally-loaded parts. For 15-5PH: H925 provides yield strength of approximately 1000 MPa with uniform SCC threshold of approximately 35–50% YS — the best balance for multi-directionally-loaded parts. Avoid H900 for any chloride-exposed application regardless of alloy — the SCC threshold at H900 is too low for reliable service.
