Both X8CrNiMoAl15-7-2 (EN 1.4532 / 1.4574, AISI 632, commercially known as 15-7PH) and X7CrNiAl17-7 (EN 1.4568, AISI 631, known as 17-7PH) are semi-austenitic precipitation hardening stainless steels that can be formed soft and hardened to very high strengths through controlled heat treatment. They share a clear family relationship — yet in demanding forged component applications, the differences between them often determine whether a part succeeds or fails in service. This guide gives engineers and procurement teams the factual comparison needed to make the right selection.
Origin and Family Relationship
X8CrNiMoAl15-7-2 (1.4532) is not an independent alloy invention — it is a deliberate, precisely engineered modification of 17-7PH. The development logic is exact: 2% Chromium is substituted by 2% Molybdenum. That single compositional change, maintained within the same semi-austenitic PH framework, drives measurable improvements in elevated-temperature strength, stress corrosion cracking resistance, and resistance to pitting and reducing acid environments.
Understanding this parent–derivative relationship matters because both grades share the same semi-austenitic conditioning route, the same Condition A / C / H heat treatment sequence, and broadly comparable fabricability. What changes is the performance ceiling — particularly in applications where high temperature, chloride exposure, and mechanical stress act simultaneously.
- 17-7PH (EN 1.4568 / AISI 631 / UNS S17700): 17% Cr · 7% Ni · Al · No Molybdenum. The original semi-austenitic PH grade. Widely stocked, well-characterised in qualification databases, cost-effective for moderate-environment applications.
- 15-7PH (EN 1.4532 / 1.4574 / AISI 632 / UNS S15700): 15% Cr · 7% Ni · 2% Mo · Al. Molybdenum-modified derivative. Narrower global availability, higher unit cost, and substantially superior performance in chloride, elevated-temperature, and reducing-acid environments.
Chemical Composition Comparison
Nominal composition ranges per EN 10088-3:2014 and the relevant AMS specifications. The critical Cr vs. Mo exchange is highlighted.
| Element | 17-7PH — EN 1.4568 | X8CrNiMoAl15-7-2 — EN 1.4532 | Engineering Significance |
|---|---|---|---|
| Carbon (C) | ≤ 0.09% | ≤ 0.09% | Identical — governs carbide precipitation tendency and weld HAZ sensitisation risk |
| Chromium (Cr) | 16.0–18.0% | 14.0–16.0% | Higher Cr in 1.4568 aids passive film stability in purely oxidising media; lower Cr in 1.4532 is more than compensated by Mo |
| Nickel (Ni) | 6.5–7.8% | 6.5–7.8% | Identical — controls austenite stability and precipitation hardening response |
| Molybdenum (Mo) | — | 2.0–3.0% | Key differentiator: blocks active pit initiation sites in chloride media and provides solid-solution strengthening at elevated temperature |
| Aluminium (Al) | 0.75–1.5% | 0.75–1.5% | Identical — forms NiAl intermetallic precipitates during aging; primary strengthening mechanism in both grades |
| Manganese (Mn) | ≤ 1.0% | ≤ 1.0% | Austenite former; kept low in both grades |
| Silicon (Si) | ≤ 1.0% | ≤ 1.0% | Oxidation resistance; equivalent in both grades |
| Sources: EN 10088-3:2014 · AMS 5520 (17-7PH strip/sheet) · AMS 5528 (15-7PH strip/sheet, CH900 condition) · AMS 5529 (17-7PH bar) · ASTM A693 | |||
The 2% Mo substitution for 2% Cr might appear minor. In practice, molybdenum contributes 3.3 PREN points per 1% addition to pitting resistance — compared to chromium's 1:1 contribution — dramatically improving corrosion performance despite the lower chromium content.
Pitting Resistance — The PREN Calculation
The Pitting Resistance Equivalent Number (PREN) is the standard formula used by engineers and material specifications to rank stainless steels against chloride-induced pitting attack:
PREN = %Cr + (3.3 × %Mo) + (16 × %N)
Standard PREN formula per ISO and NACE corrosion engineering practice
| Grade | %Cr (nominal) | %Mo (nominal) | Mo Contribution to PREN | PREN (approx.) |
|---|---|---|---|---|
| 17-4PH / EN 1.4542 | 16.5 | 0 | 0 | ~16.5 |
| 17-7PH / EN 1.4568 | 17.0 | 0 | 0 | ~17.0 |
| X8CrNiMoAl15-7-2 / EN 1.4532 | 15.0 | 2.5 | +8.25 | ~23.3 |
Despite having less chromium, 1.4532 achieves a PREN of ~23.3 versus 17-7PH's ~17.0 — a 37% improvement in theoretical pitting resistance. In practical terms, 1.4532 forged components tolerate significantly higher chloride concentrations before pitting initiates, making them the preferred choice for oilfield brines, process cooling water, and marine environments.
Mechanical Properties by Heat Treatment Condition
Both grades share the same conditioning sequence. Properties below apply to bar and forging product forms per EN 10088-3 and AMS specifications.
| Condition | Property | 17-7PH — 1.4568 | X8CrNiMoAl15-7-2 — 1.4532 |
|---|---|---|---|
| Condition A Solution Annealed (soft) | UTS minimum | ~896 MPa | ~896 MPa |
| 0.2% YS minimum | ~450 MPa | ~450 MPa | |
| Elongation minimum | ~25% | ~25% | |
| Condition TH1050 566°C / 90 min age | UTS minimum | ~1,170 MPa | ~1,240 MPa |
| 0.2% YS minimum | ~1,070 MPa | ~1,140 MPa | |
| Elongation minimum | ~6% | ~6% | |
| Condition CH900 Peak hardness — 482°C / 1 h | UTS minimum | ~1,310 MPa | ~1,380 MPa |
| 0.2% YS minimum | ~1,170 MPa | ~1,240 MPa | |
| Hardness (HRC) | ~40 HRC | ~41 HRC | |
| Red = higher performer. Actual results depend on section size and forging process parameters. Data: EN 10088-3:2014, AMS 5520, AMS 5528, AMS 5529 and manufacturer test reports. Forged bar properties may exceed stated minima. | |||
At room temperature the strength advantage of 1.4532 over 1.4568 is measurable but modest (~5–7% higher UTS and yield strength). The gap widens significantly at elevated temperatures, where Mo solid-solution strengthening provides a 10–15% UTS retention advantage above 300°C.
High-Temperature Strength — Where 1.4532 Pulls Ahead
Molybdenum stabilises the martensitic matrix at elevated temperature by inhibiting dislocation recovery and providing solid-solution strengthening. 1.4532 retains a higher proportion of its room-temperature UTS at operating temperatures above 200°C — a critical advantage for power generation and oil & gas applications.
| Temperature | Young's Modulus — 1.4568 | Young's Modulus — 1.4532 | Thermal Expansion |
|---|---|---|---|
| 20°C (RT) | ~200 GPa | ~200 GPa | — |
| 100°C | ~195 GPa | ~195 GPa | ~10.5 × 10⁻⁶ /K |
| 200°C | ~185 GPa | ~185 GPa | ~11.0 × 10⁻⁶ /K |
| 300°C | ~175 GPa | ~175 GPa | ~11.5 × 10⁻⁶ /K |
| 400°C | ~168 GPa | ~170 GPa | ~12.0 × 10⁻⁶ /K |
| Note: Tensile strength retention at temperature is the more critical differentiator. 1.4532 retains approximately 10–15% more UTS than 1.4568 at 300–400°C — data from manufacturer test reports and AMS material data. Physical properties (density, specific heat, conductivity) are very similar between the two grades. | |||
For components operating continuously above 250°C — gas turbine guide rings, steam turbine seal rings, nuclear coolant pump impellers, downhole drilling motor shafts — X8CrNiMoAl15-7-2 (1.4532) is the correct selection. Molybdenum solid-solution strengthening of the martensitic matrix provides a performance level that 17-7PH cannot achieve.
For ambient to 200°C service with benign corrosion exposure, 17-7PH (1.4568) remains technically adequate and is the more economical choice per unit of material.
Corrosion Resistance in Detail
Pitting and Crevice Corrosion
Molybdenum is the most effective alloying element for suppressing anodic dissolution at active pit initiation sites. In neutral-to-acidic chloride environments — seawater, oilfield brines, process cooling water — 1.4532 shows substantially longer time-to-pitting initiation and higher critical pitting temperatures than 17-7PH. Both grades have limited resistance to severe crevice corrosion in stagnant chloride media — a structural limitation of the PH stainless family, not a differentiator between the two grades specifically.
Stress Corrosion Cracking (SCC)
SCC in PH stainless steels is driven by hydrogen embrittlement under tensile stress in chloride-containing environments. The Mo addition in 1.4532 improves SCC resistance through multiple mechanisms: raising the critical stress intensity factor KISCC, strengthening the passive film against chloride-induced breakdown, and reducing hydrogen diffusivity in the martensitic lattice. In sour gas (H₂S-containing) service environments, 1.4532 outperforms 17-7PH when hardness is maintained below HRC 33 per NACE MR0175 / ISO 15156.
Reducing Acids and Process Water
1.4532 demonstrates measurably better resistance to dilute reducing acids — including dilute H₂SO₄ and organic acids — and to the mixed chloride-bearing water common in river water intakes and cooling tower circuits. 17-7PH is adequate in purely atmospheric or controlled humidity conditions but should not be the preferred choice where reducing acids are present in the process stream.
Marine Atmosphere
Both grades can show surface rust initiation in severe marine splash-zone environments. 1.4532's PREN of ~23 provides a meaningful advantage over 17-7PH's ~17 for marine atmospheric exposure — particularly relevant for propeller shafts, marine pump components, and offshore fasteners. Neither grade is suitable for long-term submerged seawater service without cathodic protection or protective coatings.
Heat Treatment — Shared Logic, Nearly Identical Temperatures
A key practical advantage of 1.4532 over higher-alloyed alternatives is that its heat treatment protocol is almost identical to 17-7PH. Facilities already qualified for 17-7PH processing can handle 1.4532 without significant furnace recertification or new procedure qualifications — reducing changeover cost when upgrading to the more capable grade.
| Stage | 17-7PH — EN 1.4568 | X8CrNiMoAl15-7-2 — EN 1.4532 |
|---|---|---|
| Solution Anneal (Condition A) | 1038–1066°C, water or air cool | 1038–1066°C, water or air cool |
| Austenite Conditioning (Condition B) | 955°C / 10 min, air cool to ≤15°C | 955°C / 10 min, air cool to ≤15°C |
| Martensite Transform (Condition C) | −73°C / 8 h (cryogenic treatment) | −73°C / 8 h (cryogenic treatment) |
| Precipitation Age — CH900 | 482°C / 1 h, air cool | 482°C / 1 h, air cool |
| Precipitation Age — TH1050 | 566°C / 90 min, air cool | 566°C / 90 min, air cool |
| Dimensional Distortion | Minimal — key advantage of the PH route | Minimal — same advantage fully retained |
| Note: The low-temperature aging step (CH900 at 482°C) produces minimal distortion compared to conventional quench-and-temper processes — a major advantage for close-tolerance forged components in both grades. | ||
Weldability and Fabricability
Both grades are classified as weldable, but with important caveats for any engineer or fabricator considering welded assemblies in either material.
| Aspect | 17-7PH — EN 1.4568 | X8CrNiMoAl15-7-2 — EN 1.4532 |
|---|---|---|
| Weldability rating | Good (for PH stainless) | Moderate — slightly reduced vs. 17-7PH |
| Primary limitation | Al oxide inclusions possible in weld pool | Same Al limitation plus Mo raises delta-ferrite risk in heat-affected zone |
| Recommended filler metal | ER630 (17-4PH type) commonly used | ER630 or matching 15-7PH filler; confirm with welding engineer before proceeding |
| Preheat | Not typically required for sections <10 mm | 93–120°C preheat recommended for sections >10 mm |
| Post-weld treatment | Full re-solution anneal + age for full properties | Full re-solution anneal + age is mandatory; partial treatment causes severe property loss in the HAZ |
| Design guidance | More forgiving for repair welding in Condition A | Prefer net-shape forged designs to eliminate or minimise welds; welded joints demand full procedural control |
- Where design permits, specify net-shape forged components in 1.4532 to eliminate welds entirely — this is the manufacturing approach Jiangsu Liangyi optimises for in all 1.4532 / 1.4574 component programs. The full range of available shapes — bars, seamless rolled rings, shafts, discs and valve components — is listed on the X8CrNiMoAl15-7-2 (1.4532 / 1.4574) forged parts product page.
- If welding is unavoidable, plan a full post-weld re-anneal and re-age cycle across the entire assembly. Treating only the weld zone will not restore the corrosion resistance or mechanical properties of the heat-affected zone in either grade.
- For 17-7PH in Condition A (pre-hardening, fully soft), repair welding is more forgiving — a practical advantage in maintenance and field repair contexts where a full re-heat-treatment cycle is not feasible.
International Standards and Grade Equivalents
| Standard / Region | 17-7PH — EN 1.4568 | X8CrNiMoAl15-7-2 — EN 1.4532 / 1.4574 |
|---|---|---|
| EN (Europe) | EN 1.4568 · X7CrNiAl17-7 | EN 1.4532 · EN 1.4574 · X8CrNiMoAl15-7-2 |
| AISI / UNS (USA) | AISI 631 · UNS S17700 | AISI 632 · UNS S15700 |
| AMS (Aerospace) | AMS 5520 (sheet/strip) · AMS 5529 (bar) | AMS 5520 (sheet/strip) · AMS 5528 (CH900 condition) |
| ASTM | ASTM A693 Type 631 | ASTM A693 Type 632 |
| Common trade name | 17-7PH stainless steel | 15-7PH stainless steel · PH 15-7Mo |
| Global stock availability | Very wide — distributed globally | Moderate — typically manufactured to order |
| Note: 1.4574 is listed as a variant of 1.4532 in EN 10088-3 with a slightly tighter composition range. Both designations refer to the same alloy family and are frequently used interchangeably by specifications and forging manufacturers. | ||
Grade Selection Decision Matrix
Use this matrix as a first-pass selection tool. Your specific operating conditions, section size, heat treatment infrastructure, and long-term risk tolerance should all factor into the final engineering decision.
- Operating temperature stays continuously below 200°C
- Chloride exposure is low or atmospheric only
- Budget is constrained; material premium cannot be justified
- Short lead time is critical; off-the-shelf stock is acceptable
- Welded assemblies are required and post-weld re-heat-treatment is limited
- An established 17-7PH qualification history exists for the application
- Service is in benign, non-corrosive environment
- Operating temperature exceeds 200–250°C under sustained load
- Chloride-bearing environments — brines, seawater, oilfield fluids
- SCC resistance is a stated specification requirement
- Reducing acid exposure is expected in the process stream
- Maximum achievable UTS (≥1,380 MPa Condition CH900) is required
- Application is in power generation, oil & gas, nuclear, aerospace, or marine
- Net-shape forged design eliminates or minimises welds
Once you have confirmed that X8CrNiMoAl15-7-2 (1.4532) meets your application requirements, the next step is to specify your geometry and heat treatment condition. Jiangsu Liangyi can manufacture custom 1.4532 / 1.4574 forged bars, rings, shafts and valve components from 30 kg to 30,000 kg per piece, with lead times of 4–10 weeks.
Typical Applications by Grade
Applications Where 17-7PH (EN 1.4568) Is Commonly Specified
- Springs, flat springs and spring elements in moderate temperature and benign environments
- Diaphragms, bellows and thin-section formed parts at ambient temperature
- Structural brackets and clips in aerospace cabin environments (non-engine, non-fuel zone)
- Medical device components requiring moderate strength and good formability in Condition A
- Food processing equipment where chloride exposure is controlled and temperatures are low
- Precision instrument housings, frames and retaining components
Applications Where X8CrNiMoAl15-7-2 (EN 1.4532) Is the Correct Forged Grade
- Power generation: Gas turbine seal rings, guide rings, labyrinth rings, turbine discs and blisks operating above 250°C; rotor end rings and casing rings; nuclear reactor coolant pump impellers and casing shells
- Oil & gas: High-pressure valve bodies, valve seat rings, valve stems, valve balls; downhole mud motor drive shafts; electric submersible pump (ESP) motor shafts; high-pressure vessel components
- Nuclear power: Reactor coolant pump impellers, pump casing shells, containment seal chambers requiring long-term reliability under irradiation
- Aerospace: High-strength valve components, fuel system parts, actuator housings, structural parts requiring maximum strength-to-weight ratio at moderate elevated temperatures
- Marine: Propeller shafts, marine engine valve assemblies, shipboard high-pressure piping and pressure vessel components
- Turbomachinery: Centrifugal compressor impellers, pump barrels, shrouded impellers, pump shafts and wear rings for process industry applications
This comparison covers material selection. For the complete dimensional capability, available stock conditions, inspection standards, quality documentation and quotation process, refer to the dedicated X8CrNiMoAl15-7-2 (1.4532 / 1.4574) forged parts product page — which covers all product forms from round bars up to 2,000 mm diameter and seamless rolled rings up to 6 metres, together with the full mechanical and physical property data tables, manufacturing process flow, and quality assurance details that are outside the scope of this grade comparison guide.
Cost, Lead Time and Availability
X8CrNiMoAl15-7-2 carries a material premium of approximately 15–30% over 17-7PH in equivalent mill forms, driven primarily by the molybdenum alloying addition and narrower global production volumes. For forged components, where total part cost is dominated by machining time, heat treatment cycles, inspection, and logistics rather than raw material price, this premium is frequently recovered through longer service life and reduced replacement frequency.
Lead times for 1.4532 forged parts from a specialist manufacturer such as Jiangsu Liangyi are typically 4–10 weeks depending on part geometry and volume. Standard 1.4568 bar stock is more widely available from distribution, but custom forgings in either grade must be manufactured to order regardless of the alloy.
In critical infrastructure — gas turbines, subsea valves, nuclear coolant circuits — a single component failure typically costs far more in downtime, remediation, and liability than the material premium between 1.4568 and 1.4532. When risk-adjusted total life cycle cost is the evaluation metric, 1.4532 is frequently the more economical choice, even though its per-kilogram material price is higher.
Summary — Engineer's Quick Reference
| Criterion | Better Grade | Notes |
|---|---|---|
| Room-temperature UTS (Cond. CH900) | 1.4532 | ~5–7% higher than 1.4568 |
| High-temperature strength (>250°C) | 1.4532 | Mo solid-solution strengthening; 10–15% UTS retention advantage |
| Pitting resistance (PREN) | 1.4532 | ~23.3 vs ~17.0 — 37% improvement in chloride pitting resistance |
| Stress corrosion cracking resistance | 1.4532 | Measurably better in chloride / H₂S environments |
| Reducing acid resistance | 1.4532 | Mo addition improves resistance to dilute H₂SO₄ and organic acids |
| Weldability | 1.4568 | Slightly easier; 1.4532 requires more procedural rigour |
| Global stock availability | 1.4568 | Wider distribution stock; faster off-the-shelf delivery |
| Unit material cost | 1.4568 | Approximately 15–30% lower unit cost |
| Heat treatment process | Equivalent | Virtually identical temperatures, sequences and equipment requirements |
| Density | Equivalent | Both ~7.78 g/cm³ |
| Hardening mechanism | Equivalent | NiAl intermetallic precipitate strengthening in both grades |
| Dimensional stability during hardening | Equivalent | Minimal distortion in both grades — key PH advantage vs. conventional H&T |
Frequently Asked Questions
X8CrNiMoAl15-7-2 (EN 1.4532) is a molybdenum-modified version of 17-7PH (EN 1.4568) in which 2% Chromium is replaced by 2% Molybdenum. This results in higher tensile strength at elevated temperatures, superior pitting resistance (PREN ~23 vs ~17), better stress corrosion cracking resistance in chloride environments, and improved resistance to reducing acids. Heat treatment sequences are nearly identical for both grades.
X8CrNiMoAl15-7-2 (1.4532) achieves a PREN of approximately 23.3, compared to approximately 17.0 for 17-7PH (1.4568). Despite having less chromium, the 2–3% molybdenum content in 1.4532 adds approximately 8.25 PREN points (3.3 × %Mo), making it 37% more resistant to chloride-induced pitting by this metric.
Yes. Both grades use virtually identical heat treatment sequences: solution anneal at 1038–1066°C (Condition A), austenite conditioning at 955°C, cryogenic martensite transformation at −73°C (Condition C), and precipitation aging at 482°C/CH900 or 566°C/TH1050. Heat treatment shops qualified for 17-7PH can process 1.4532 without significant process changes or equipment recertification.
Yes, 1.4532 is slightly more difficult to weld than 17-7PH due to its molybdenum content raising the risk of delta-ferrite formation in the heat-affected zone. A preheat of 93–120°C is recommended for sections over 10 mm, and a full post-weld re-solution anneal plus complete aging cycle is mandatory to restore full mechanical properties and corrosion resistance across the weld and HAZ.
X8CrNiMoAl15-7-2 (1.4532/1.4574) forged parts are used in power generation (turbine seal rings, guide rings, discs), oil and gas (high-pressure valve bodies, valve seat rings, downhole drive shafts, ESP pump shafts), nuclear power (coolant pump impellers, casing shells), aerospace (high-strength valve and fuel system components), marine (propeller shafts), and turbomachinery (compressor impellers, pump barrels).
X8CrNiMoAl15-7-2 is designated EN 1.4532 and EN 1.4574 in European standards, AISI 632 and UNS S15700 in US designations, AMS 5528 (sheet/strip CH900 condition) in aerospace specifications, and ASTM A693 Type 632. It is commercially known as 15-7PH or PH 15-7Mo. The equivalent 17-7PH is EN 1.4568, AISI 631, UNS S17700, AMS 5520/5529, and ASTM A693 Type 631.
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