- Type: Semi-austenitic precipitation hardening (PH) stainless steel
- Designations: UNS S17700 / AISI 631 / EN 1.4568 / SUS 631
- Composition: 17% Cr, 7% Ni, ~1% Al, balance Fe
- Max UTS: 1,655 MPa (H900 condition)
- Standard UTS: 1,310 MPa (TH1050 — most specified)
- Conditions: A, H900, H950, H1050, TH1050, RH950
- Key standard: ASTM A705 Grade 631 (forgings)
- PREN: ~17.0 (similar to 304 stainless)
- Max service temp: ~315°C (600°F)
- Primary use: Aerospace springs, fatigue-critical components
Origin & Classification
17-7PH is one of the most important alloys in the precipitation hardening stainless steel family — engineered to solve a problem that neither austenitic nor martensitic grades could address alone: combining very high fatigue strength with stainless corrosion resistance in a single formable alloy.
17-7PH stainless steel was first developed by Armco Steel Corporation (now part of Cleveland-Cliffs) in the early 1950s. Engineers of that era faced a fundamental material trade-off: austenitic stainless steels (such as 304 and 316) offered excellent corrosion resistance and formability but could only achieve moderate strength levels. Martensitic stainless steels could be hardened to high strength, but were brittle, difficult to form, and more prone to corrosion in service. 17-7PH was engineered to bridge that gap — and 70 years later, it remains the alloy of choice wherever high fatigue strength and stainless corrosion resistance must coexist.
The designation encodes the nominal composition: approximately 17% chromium and 7% nickel, while "PH" denotes precipitation hardening — the thermomechanical strengthening mechanism. A critical addition of approximately 1% aluminium differentiates 17-7PH from its close relative 17-4PH. This aluminium enables the formation of coherent Ni3Al intermetallic precipitates within the martensitic matrix, which is the source of 17-7PH's exceptional fatigue resistance and the reason it dominates spring-component applications in aerospace worldwide.
Classification: Semi-Austenitic Precipitation Hardening Stainless Steel
Unlike martensitic PH grades such as 17-4PH, 17-7PH is delivered in a soft, fully formable austenitic state (Condition A) and only transforms to hardened martensite through a controlled two-stage heat treatment sequence. This is why it is classified as semi-austenitic: it gives engineers the ability to perform complex cold-forming, stamping, and roll-forming operations on an essentially soft stainless steel, then harden the finished component in a batch furnace without significant dimensional distortion.
The alloy is recognised under a consistent set of international specifications: ASTM A564/A705 Grade 631, AMS 5528/5529/5568, ASME SA-564, and the European standard EN 10088-3 as 1.4568 / X7CrNiAl17-7. In Japan it is designated SUS 631 per JIS G4303/G4304. Understanding these equivalent designations is critical when procuring globally, as engineering drawings may reference different regional standards.
Chemical Composition
The composition of 17-7PH is tightly controlled across all major standards. Every element plays a specific and critical metallurgical role.
| Element | ASTM A693 (wt %) | EN 1.4568 (wt %) | Typical Value | Metallurgical Role |
|---|---|---|---|---|
| Chromium (Cr) | 16.00 – 18.00 | 16.00 – 18.00 | ~17.0 % | Passivity and corrosion resistance; forms Cr2O3 passive layer |
| Nickel (Ni) | 6.50 – 7.75 | 6.50 – 7.75 | ~7.1 % | Austenite stabiliser in Condition A; participates in Ni3Al precipitate |
| Aluminium (Al) | 0.75 – 1.50 | 0.70 – 1.50 | ~1.1 % | ★ Key hardening agent — forms coherent Ni3Al intermetallic precipitates during aging |
| Carbon (C) | ≤ 0.09 | ≤ 0.09 | ~0.06 % | Kept low to preserve ductility, toughness, and weldability |
| Manganese (Mn) | ≤ 1.00 | ≤ 1.00 | ~0.60 % | Deoxidiser during melting; mild austenite stabiliser |
| Silicon (Si) | ≤ 1.00 | ≤ 1.00 | ~0.40 % | Deoxidiser during steelmaking |
| Phosphorus (P) | ≤ 0.040 | ≤ 0.040 | — | Impurity — minimised to preserve toughness and ductility |
| Sulphur (S) | ≤ 0.030 | ≤ 0.015 | — | Impurity — minimised for hot forgeability and transverse toughness |
| Iron (Fe) | Balance | Balance | ~73 % | BCC/FCC matrix base metal |
Why Aluminium Is the Defining Element
The 0.75–1.5% aluminium content is what separates 17-7PH from 17-4PH, which uses copper (3.0–5.0%) and niobium (0.15–0.45%) as hardening agents. Aluminium forms tightly coherent Ni3Al precipitates within the martensitic matrix, creating a dispersion-hardened microstructure with superior fatigue resistance under cyclic loading. This is why 17-7PH dominates spring, clip, and diaphragm applications where 17-4PH is not the correct choice.
How Precipitation Hardening Works in 17-7PH
17-7PH achieves its strength through three distinct, sequential metallurgical transformations. Understanding each phase is essential for specifying, heat-treating, and troubleshooting the alloy correctly.
Phase 1: Condition A — The Soft, Formable Austenitic Starting State
17-7PH is produced and delivered in Condition A (solution annealed), where the microstructure is predominantly metastable austenite. In this state the yield strength is approximately 380 MPa — comparable to a standard 304 austenitic stainless steel. This softness is fully intentional: it enables cold-forming, deep drawing, roll-forming, stamping, or machining of complex near-net shapes before any hardening sequence is applied. The aluminium and nickel atoms are fully dissolved in the austenitic solid solution at this stage, held in a supersaturated state that cannot persist at lower temperatures.
Phase 2: Martensitic Transformation — Converting the Crystal Structure
To begin hardening, the metastable austenite must first transform to martensite. Because 17-7PH is semi-austenitic (the martensite start temperature Ms is close to room temperature), this transformation does not occur spontaneously on cooling from solution anneal. It must be driven by one of two controlled routes: a conditioning treatment (heating to ~954°C / 1,750°F for 10 minutes, then air-cooling below 15°C) which depletes the austenite of carbon and solute-stabilising elements to raise the Ms above room temperature; or a refrigeration treatment (cooling to −73°C / −100°F for a minimum of 8 hours), which mechanically depresses the alloy below the Mf (martensite finish temperature). The resulting fresh martensite still contains aluminium and nickel atoms trapped in supersaturated solid solution, ready for the final precipitation step.
Phase 3: Precipitation Aging — The Actual Strengthening Mechanism
Aging at a controlled temperature between 482°C (H900) and 566°C (TH1050/H1050) causes the dissolved aluminium and nickel atoms to nucleate and grow as coherent Ni3Al precipitates — ordered intermetallic particles with a crystal structure closely matched to the surrounding BCC martensitic matrix. These nanometre-scale particles create coherency strain fields that dramatically increase resistance to dislocation glide — the mechanism by which metals plastically deform. The result is a dramatic strength increase: tensile strength rises from ~1,030 MPa (Condition A) to as high as 1,655 MPa (H900), with the exact value determined by the aging temperature and duration chosen.
"Unlike 17-4PH, where hardening is driven by Cu-rich epsilon-copper precipitates, 17-7PH uses aluminium-nickel intermetallics. This mechanism produces superior fatigue crack initiation resistance under cyclic loading — precisely why 17-7PH remains the alloy of choice for aerospace springs 70 years after its invention."
Heat Treatment Conditions: H900 through RH950
17-7PH is available in five standard heat treatment conditions, each trading strength against ductility and toughness. Choosing the correct condition is the single most important engineering decision when specifying a 17-7PH forging or component.
| Condition | Full Process Route | UTS Min (MPa) | 0.2% YS Min (MPa) | Elong. Min (%) | Hardness |
|---|---|---|---|---|---|
| A | Solution anneal only — as-supplied | ≥ 1,030 | ≥ 380 | ≥ 20 | ≤ 92 HRB |
| H900 | Condition A → Age 482°C (900°F) / 1 hr, air cool | ≥ 1,655 | ≥ 1,520 | ≥ 6 | 46–50 HRC |
| H950 | Condition A → Age 510°C (950°F) / 1 hr, air cool | ≥ 1,520 | ≥ 1,380 | ≥ 6 | 43–47 HRC |
| TH1050 ★ | 954°C/10 min → air cool <15°C → −73°C/8 hr → Age 566°C/1.5 hr | ≥ 1,310 | ≥ 1,170 | ≥ 10 | 40–44 HRC |
| RH950 | 1,750°F/10 min → −73°C/8 hr → Age 510°C (950°F) / 1 hr | ≥ 1,448 | ≥ 1,310 | ≥ 9 | 42–46 HRC |
| H1050 | Condition C → Age 566°C (1,050°F) / 1 hr, air cool | ≥ 1,170 | ≥ 1,000 | ≥ 10 | 37–42 HRC |
Condition Selection Guide
H900 — Maximum strength; use only for static, non-chloride environments
At ≥1,655 MPa UTS and ≥1,520 MPa 0.2% yield strength, H900 is the strongest available condition. However, minimum elongation drops to 6% and toughness is substantially reduced. More critically, H900 is notably susceptible to stress corrosion cracking (SCC) in chloride-containing environments. Reserve H900 for static structural aerospace components in controlled environments where mass minimisation is paramount.
TH1050 — Best all-round balance; the most-specified forging condition
TH1050 is the most widely specified condition for 17-7PH forgings. The three-stage conditioning-refrigeration-aging sequence produces a microstructure with excellent toughness, predictable fatigue performance, and good SCC resistance at a UTS exceeding 1,310 MPa. It is the preferred condition for turbine disc forgings, structural airframe forgings, valve bodies, and any application involving dynamic, impact, or fatigue loading. Jiangsu Liangyi supplies 17-7PH forging parts in all five standard conditions — specify your required condition on the enquiry form and we will confirm the processing route and lead time.
RH950 — High strength with better toughness than H900
RH950 uses refrigeration (rather than elevated-temperature conditioning) to drive the martensitic transformation, then ages at 510°C. This produces a finer martensitic substructure than H900, yielding better toughness at comparable strength (≥1,448 MPa UTS). RH950 is preferred for forgings that require H900-range strength but where greater ductility (minimum 9% elongation) is a design requirement.
H1050 — Maximum ductility among hardened conditions
H1050 provides the highest elongation (minimum 10%) and best SCC resistance of all hardened conditions. It is the correct choice when moderate strength is acceptable, when post-heat-treatment machining is required, or when the component will be exposed to mildly corrosive environments.
Specification Best Practice
Always specify the full condition code on both the engineering drawing and the purchase order — for example, "17-7PH per ASTM A705 Grade 631 Condition TH1050." A forging delivered in Condition A and one delivered in TH1050 have radically different mechanical properties and must never be treated as interchangeable in service.
Mechanical & Physical Properties
Typical properties for 17-7PH in TH1050 condition — the most commonly specified forged state. All production forgings from Jiangsu Liangyi are individually tested and certified with a Mill Test Certificate (MTC).
High-Temperature Performance
17-7PH retains useful mechanical properties up to approximately 315°C (600°F). Above this temperature, the Ni3Al precipitates begin to coarsen through Ostwald ripening, reducing their strengthening efficiency. For applications requiring sustained service above 315°C, consider PH15-7Mo (EN 1.4532), which incorporates 2% molybdenum for significantly improved thermal stability, or Grade A-286 for service up to 650°C.
Fatigue Strength
The endurance limit of 17-7PH in TH1050 condition (fully reversed bending, stress ratio R = −1, run-out at 107 cycles) is approximately 690–760 MPa — among the highest of any stainless steel grade and superior to 17-4PH in equivalent conditions. This exceptional fatigue resistance, combined with inherent stainless corrosion protection, is the primary reason 17-7PH dominates aircraft spring clips, belleville washers, and flat retention springs requiring millions of load cycles in service.
Magnetic Properties
17-7PH in Condition A is essentially non-magnetic (austenitic). After conditioning and aging treatments (TH1050, H900, RH950), the martensitic microstructure renders the alloy weakly to moderately ferromagnetic. This is an important consideration for applications near magnetic-sensitive instruments, and for selecting appropriate non-destructive testing methods (magnetic particle testing is not reliable for TH1050 condition material — use liquid penetrant testing instead).
Corrosion Resistance
17-7PH provides corrosion resistance broadly equivalent to 304 austenitic stainless steel — excellent in atmospheric and freshwater environments, with well-defined limitations in chloride-containing or acidic media.
| Environment | Rating | Recommended Condition | Notes |
|---|---|---|---|
| Atmospheric (industrial / coastal) | ✓ Good | Any | Passive Cr2O3 film stable; suitable for most outdoor applications |
| Freshwater / potable water | ✓ Good | Any | Negligible corrosion rate at ambient temperature |
| Steam (low-pressure, clean) | ✓ Good | TH1050 / H1050 | Below 315°C only; above this, mechanical properties degrade |
| Mild organic acids (dilute) | ⚠ Limited | H1050 / TH1050 | Testing required; higher-aged conditions provide better resistance |
| Chloride solutions / seawater | ✗ Not Suitable | — | Risk of pitting and crevice corrosion; consider duplex 2205 or PH15-7Mo |
| Stress corrosion cracking (SCC) | ⚠ High risk (H900) | Avoid H900 in Cl− | H900 is SCC-susceptible in chloride environments; TH1050/H1050 significantly better |
| Strong oxidising acids (HNO3) | ⚠ Limited | H1050 | Passive in dilute HNO3; may corrode in concentrated solutions |
Critical Warning: SCC Susceptibility in H900 Condition
17-7PH in H900 condition is notably susceptible to stress corrosion cracking in chloride-bearing environments. There are documented service failures of aerospace structural components in H900 condition exposed to de-icing fluids (which contain chlorides) and salt-fog. If any chloride exposure is possible in service, specify TH1050 or H1050 and document this decision on the engineering drawing. Consult your materials engineer before finalising the condition code for any part that may contact chloride-containing fluids, lubricants, or cleaning chemicals.
The PREN (Pitting Resistance Equivalent Number) of 17-7PH is approximately 17.0, compared to approximately 18.0 for 316L, ~34 for duplex 2205, and ~23 for PH15-7Mo (1.4532). PREN = %Cr + 3.3×%Mo + 16×%N.
After any machining, grinding, welding, or heat treatment operation, passivation per ASTM A967 Method C or AMS 2700 Method 1 is strongly recommended to restore and maximise the protective chromium oxide passive layer. This step is especially important for parts intended for use in humid or mildly corrosive environments.
Forging 17-7PH Stainless Steel
Open die forging and seamless ring rolling produce 17-7PH components with a directional, refined grain structure that delivers fatigue resistance 40% or more above an equivalent cast or bar-stock component of the same alloy.
Forging Temperature Range
17-7PH must be forged in the temperature range of 1,065–1,175°C (1,950–2,150°F). Forging below 1,010°C results in excessive forging pressures, risk of cracking from insufficient plasticity, and poor grain refinement. Forging above 1,200°C leads to abnormal grain growth (grain coarsening), which permanently degrades mechanical properties and fatigue performance and cannot be recovered by heat treatment alone. For large-section or heavy forgings, multiple reheats are required to maintain temperature uniformity across the entire cross-section throughout the forging sequence.
Required Minimum Forging Ratio
A minimum forging reduction ratio of 4:1 is required for standard commercial forgings. This ratio is necessary to break down the cast ingot dendritic structure, close internal porosity and shrinkage defects, and establish a uniform, fine-grained wrought microstructure. Critical aerospace, nuclear, and pressure vessel applications frequently specify ratios of 6:1 or above, with enhanced ultrasonic testing acceptance criteria (UT Class 2 or higher per EN 10228-3) applied as a consequence.
Complete Heat Treatment Sequence for TH1050 Forgings
| Step | Operation | Temperature | Hold Time | Cooling Method | Purpose |
|---|---|---|---|---|---|
| 1 | Solution Anneal (Condition A) | 1,065°C ± 14°C (1,950°F) | 30 min + 1 min/mm section | Water quench or rapid air cool | Re-dissolve precipitates; restore austenitic microstructure |
| 2 | Conditioning Treatment | 954°C ± 14°C (1,750°F) | 10 min ± 1 min | Air cool to below 15°C (59°F) | Deplete austenite of stabilising solutes; raise Ms above room temperature |
| 3 | Refrigeration Treatment | −73°C ± 6°C (−100°F) | 8 hours minimum | Warm to room temperature in still air | Complete martensitic transformation below Mf |
| 4 | Precipitation Aging | 566°C ± 6°C (1,050°F) | 90 min ± 5 min | Air cool to room temperature | Nucleate and grow Ni3Al precipitates; achieve target mechanical properties |
Non-Destructive Testing (NDE) Requirements
Standard NDE requirements for 17-7PH forgings include 100% volumetric ultrasonic testing (UT) per ASTM A388 or EN 10228-3, and liquid penetrant testing (PT) per ASTM E165 or EN 10228-2 for surface inspection. Magnetic particle testing (MT) is generally not applicable to TH1050, H1050, and Condition A material because these conditions do not provide sufficient ferromagnetic response for reliable defect detection.
Dimensional Distortion During Heat Treatment
One of the key advantages of 17-7PH compared to conventional through-hardening steels is that the precipitation aging step (the final hardening step) occurs at relatively low temperature (482–566°C) and involves no quenching. This means that dimensional distortion during final hardening is minimal — typically less than 0.05% linear in the aging direction for precision forgings held in restraining fixtures. This is a major manufacturing advantage for precision aerospace components where tight tolerances must be maintained through the full heat treatment sequence.
Custom 17-7PH Forgings — Jiangsu Liangyi Co., Limited
We manufacture 17-7PH (UNS S17700 / AISI 631 / EN 1.4568) open die forgings and seamless rolled rings from 30 kg to 30,000 kg per piece, fully compliant with ASTM A705 Grade 631, available in all five conditions (H900, H950, H1050, TH1050, RH950). Mill Test Certificate (MTC) provided with every order. Third-party inspection available on request (customer's nominated inspection agency accepted). 25+ years, 120,000 tons annual capacity, exported to 50+ countries.
→ View 17-7PH open die forging and seamless rolled ring specifications
Applicable Standards
17-7PH is covered by a comprehensive and consistent set of international standards. The table below enables procurement engineers and quality managers to confirm cross-regional equivalency when sourcing globally.
| Standard Body | Document Number | Grade / Designation | Scope & Notes |
|---|---|---|---|
| ASTM | A564/A564M | Grade 631 | Hot-rolled and cold-finished age-hardening stainless steel bars and shapes |
| ASTM | A705/A705M | Grade 631 | Age-hardened stainless steel forgings — the primary forging procurement standard |
| ASTM | A693 | Grade 631 | Precipitation hardening stainless steel strip, sheet, and plate |
| AMS (SAE) | AMS 5528F | 17-7PH | Aerospace sheet, strip, and plate in Condition A |
| AMS (SAE) | AMS 5529D | 17-7PH | Aerospace sheet, strip, and plate in Conditions C and CH |
| AMS (SAE) | AMS 5568 | 17-7PH | Welding wire (bare) for aerospace applications |
| ASME | SA-564 | Grade 631 | Pressure vessel bars — BPVC Section II, Part A |
| EN | EN 10088-3:2014 | 1.4568 / X7CrNiAl17-7 | European standard — bars, rods, wire, sections, bright products |
| EN | EN 10250-4:2000 | 1.4568 | Open die steel forgings for general engineering — stainless steels |
| JIS | JIS G4303 / G4304 | SUS 631 | Japanese Industrial Standard equivalent for bars and plates |
| GB (China) | GB/T 1220 | 0Cr17Ni7Al | Chinese national standard equivalent designation |
Industry Applications
17-7PH occupies a well-defined and largely unique application space: wherever extremely high fatigue strength must coexist with stainless corrosion resistance, and where the component must be formed into a complex shape before final hardening.
Aerospace & Defence
Aerospace is the primary and historically most important application sector for 17-7PH. AMS-recognised properties, combined with the ability to form complex geometries in soft Condition A before batch-oven aging at modest temperature, make it irreplaceable for flat springs, precision retention clips, and structural brackets that must survive tens of millions of fatigue cycles. Structural airframe forgings are almost universally specified in TH1050 or RH950 condition; sheet-metal spring components are typically H900 or RH950.
Oil & Gas
In oil and gas, 17-7PH is used primarily in instrument, control, and measurement components rather than structural pressure-containing forgings (where duplex stainless or martensitic grades are typically preferred). Its exceptional spring-back, fatigue life, and corrosion resistance make it the material of choice for instrument diaphragms, valve springs, and precision measurement housings in surface wellhead equipment and production facilities.
Medical & Food Processing
The stainless composition allows 17-7PH to be used in hygienic and biocompatible environments. Surgical instrument springs and clamps maintain precise, consistent clamping force through thousands of autoclave sterilisation cycles. In food processing equipment, it provides spring material that resists the aggressive caustic and acidic CIP (clean-in-place) chemicals used in production line cleaning cycles.
Electronics & Precision Manufacturing
17-7PH sheet and strip in Condition A is widely used for precision stamped and drawn components in electronics assemblies, connector contacts, and chassis retention hardware. The ability to stamp and form in the soft annealed state, then age-harden in a batch oven without significant dimensional distortion, is a key manufacturing advantage over high-carbon spring steels that must be processed in the hard temper.
17-7PH vs. Similar Precipitation Hardening Grades
When evaluating PH stainless steels, 17-7PH is most frequently compared to 17-4PH and PH15-7Mo. Selecting the wrong grade is a common and costly specification error.
- 17-7PH vs. 17-4PH: 17-7PH achieves significantly higher maximum tensile strength (1,655 MPa vs. 1,310 MPa at H900 condition) and superior fatigue life due to the Ni3Al precipitation mechanism. 17-4PH is simpler to process (single aging step; no refrigeration), offers better weldability, and is easier to qualify for oil & gas sour service applications (API 6A 105K). For structural forgings and oil & gas: specify 17-4PH. For springs and fatigue-critical components: specify 17-7PH.
- 17-7PH vs. PH15-7Mo (1.4532): PH15-7Mo replaces 2% chromium in 17-7PH with molybdenum, boosting PREN from ~17 to ~23 and significantly improving resistance to stress corrosion cracking, pitting, and elevated-temperature performance (up to ~400°C). If service involves temperatures above 315°C or any sustained exposure to chloride solutions, PH15-7Mo is the correct upgrade path from 17-7PH.
- 17-7PH vs. 316L austenitic: 316L stainless offers better corrosion resistance (PREN ~23), excellent weldability, and superior low-temperature toughness. However, its yield strength (~220 MPa annealed) is approximately one-fifth that of 17-7PH in TH1050 condition. Where both corrosion resistance and high cyclic strength are design requirements, 17-7PH in TH1050 or H1050 condition is the engineeringly sound choice.
- 17-7PH vs. 301 austenitic (cold-worked): AISI 301 in full-hard condition can reach ~1,300 MPa UTS through cold work alone and is a direct competitor for spring applications. However, 17-7PH in TH1050 condition exceeds 301 full-hard in both tensile strength and fatigue endurance limit, while also providing better corrosion resistance. For demanding aerospace spring applications, 17-7PH is the preferred choice.
Frequently Asked Questions About 17-7PH Stainless Steel
Answers to the most common questions engineers, buyers, and procurement specialists ask about 17-7PH (UNS S17700 / AISI 631).
What is 17-7PH stainless steel? ▼
17-7PH is a semi-austenitic precipitation hardening (PH) stainless steel with the UNS designation S17700, AISI designation 631, and European designation EN 1.4568 (X7CrNiAl17-7). It contains approximately 17% chromium, 7% nickel, and 1% aluminium, with the balance iron. The alloy achieves its high strength through a two-stage heat treatment: first, a martensitic transformation of the microstructure, then precipitation aging at 482–566°C to form coherent Ni3Al intermetallic particles that prevent dislocation movement. In its most-specified forged condition (TH1050), it achieves a minimum tensile strength of 1,310 MPa and a minimum yield strength of 1,170 MPa — while maintaining 10% minimum elongation.
What is the difference between 17-7PH and 17-4PH stainless steel? ▼
The main differences between 17-7PH and 17-4PH are:
1. Microstructure type: 17-4PH is a martensitic PH grade (delivered as martensite, hardened by a single aging step). 17-7PH is a semi-austenitic PH grade (delivered as austenite, requiring a two-stage conditioning + aging process).
2. Hardening mechanism: 17-4PH uses copper-niobium precipitates. 17-7PH uses Ni3Al (nickel-aluminium) precipitates, which produce higher fatigue resistance.
3. Maximum strength: 17-7PH in H900 condition achieves 1,655 MPa UTS vs. 1,310 MPa for 17-4PH H900 — a 26% difference.
4. Best application: 17-4PH is preferred for structural forgings and oil & gas components requiring good weldability. 17-7PH is preferred for springs, clips, and fatigue-critical components in aerospace and precision applications.
What is the best heat treatment condition for 17-7PH forgings? ▼
TH1050 is the most widely specified condition for 17-7PH forgings because it provides the best balance of strength, toughness, fatigue resistance, and stress corrosion cracking resistance. In TH1050, the minimum tensile strength is 1,310 MPa, minimum yield strength is 1,170 MPa, and minimum elongation is 10%.
H900 should only be specified for static components in non-chloride environments where maximum strength (≥1,655 MPa) is the primary design driver. H900 is notably susceptible to stress corrosion cracking in chloride environments and has limited ductility (6% minimum elongation).
Is 17-7PH magnetic? ▼
It depends on the heat treatment condition. 17-7PH in Condition A (annealed) is essentially non-magnetic, as the microstructure is predominantly austenitic. After conditioning and aging treatments (H900, H950, TH1050, RH950, H1050), the alloy becomes weakly to moderately ferromagnetic due to the martensitic microstructure. The degree of magnetism increases with higher martensitic volume fraction. This must be considered when specifying magnetic particle testing (not reliable for most hardened conditions) and when using the material near magnetically sensitive instruments or sensors.
What is the corrosion resistance of 17-7PH stainless steel? ▼
17-7PH provides corrosion resistance broadly equivalent to 304 austenitic stainless steel in most environments. Its PREN (Pitting Resistance Equivalent Number) is approximately 17.0. It performs well in atmospheric, freshwater, and steam environments. However, it is not recommended for immersion in chloride-containing solutions (seawater, deicing fluids) or highly acidic media, where pitting and stress corrosion cracking can occur — especially in H900 condition. For chloride-resistant applications, consider PH15-7Mo (1.4532, PREN ~23) or duplex 2205 (PREN ~34).
What is the European equivalent of 17-7PH stainless steel? ▼
The European equivalent of 17-7PH stainless steel is EN 1.4568, with the compositional designation X7CrNiAl17-7. It is covered by EN 10088-3 (for bars and sections) and EN 10250-4 (for open die forgings). The chemical composition limits and heat treatment conditions are functionally equivalent to the ASTM/AMS specifications, allowing direct substitution in most engineering applications when the correct condition (H900, TH1050, etc.) is specified. The Japanese equivalent is SUS 631 per JIS G4303/G4304, and the Chinese equivalent is 0Cr17Ni7Al per GB/T 1220.
Can 17-7PH stainless steel be welded? ▼
Yes, 17-7PH can be welded, but with important limitations. Welding should be performed on material in Condition A (the soft, austenitic state) wherever possible. Welding in hardened conditions (H900, TH1050, etc.) is not recommended, as the heat-affected zone will lose its hardened properties. AMS 5568 provides the specification for 17-7PH welding wire. After welding in Condition A, the full conditioning and aging heat treatment sequence must be applied to the entire assembly to restore mechanical properties. If full re-heat-treatment is not possible after welding, 17-4PH is generally a better choice due to its simpler single-step aging process and better weldability.
What is the maximum service temperature for 17-7PH? ▼
The maximum recommended continuous service temperature for 17-7PH is approximately 315°C (600°F). Above this temperature, the Ni3Al precipitates begin to coarsen (overage), reducing hardness and strength. For elevated-temperature service, the recommended upgrades are: PH15-7Mo (EN 1.4532) for service up to ~400°C, or Grade A-286 (UNS S66286) for service up to 650°C. For temperatures above 700°C, nickel-based superalloys are typically required.
Complete Specification Summary
A consolidated specification reference for 17-7PH stainless steel across all standard conditions, for use in engineering drawings and procurement documents.
| Parameter | Value / Specification | Reference |
|---|---|---|
| Common Name | 17-7PH / 17/7 PH | Industry name |
| UNS Number | S17700 | ASTM DS-56 |
| AISI / SAE | Type 631 | AISI |
| EN Number | 1.4568 | EN 10088 |
| EN Name | X7CrNiAl17-7 | EN 10088 |
| JIS | SUS 631 | JIS G4303 |
| GB (China) | 0Cr17Ni7Al | GB/T 1220 |
| Type | Semi-austenitic precipitation hardening stainless steel | — |
| Cr content | 16.00 – 18.00 wt % | ASTM A693 |
| Ni content | 6.50 – 7.75 wt % | ASTM A693 |
| Al content | 0.75 – 1.50 wt % | ASTM A693 |
| Density | 7.78 g/cm³ | Typical |
| Elastic Modulus | 204 GPa (29.6 × 10&sup6; psi) | Typical |
| CTE (20–100°C) | 10.8 µm/(m·°C) | Typical |
| Max service temp | 315°C (600°F) | Industry consensus |
| PREN | ~17.0 | Calculated |
| Primary forging standard | ASTM A705 Grade 631 | ASTM |
| TH1050: UTS | ≥ 1,310 MPa (≥ 190 ksi) | ASTM A705 |
| TH1050: YS 0.2% | ≥ 1,170 MPa (≥ 170 ksi) | ASTM A705 |
| TH1050: Elongation | ≥ 10% | ASTM A705 |
| H900: UTS | ≥ 1,655 MPa (≥ 240 ksi) | ASTM A705 |
| H900: YS 0.2% | ≥ 1,520 MPa (≥ 220 ksi) | ASTM A705 |
| Forging reduction ratio (min) | 4:1 (standard); 6:1 (aerospace / critical) | Industry practice |
| UT standard (forgings) | ASTM A388 or EN 10228-3 | ASTM / EN |
| Surface inspection (forgings) | ASTM E165 (PT) or EN 10228-2 | ASTM / EN |
| Passivation | ASTM A967 Method C or AMS 2700 Method 1 | ASTM / AMS |
Need Custom 17-7PH Forgings?
Jiangsu Liangyi manufactures 17-7PH (UNS S17700 / AISI 631 / EN 1.4568) open die forgings and seamless rolled rings from 30 kg to 30,000 kg per piece, in all five heat treatment conditions, with Mill Test Certificate (MTC) and third-party inspection available on request. Exported to 50+ countries since 1997.