17-7PH – UNS S17700 (Type 631)
Semi-austenitic precipitation-hardening stainless steel (Cr-Ni-Al)
Semi-austenitic stainless: austenitic and formable in condition A, transformed to martensite by thermal conditioning, sub-zero treatment or cold rolling and then hardened by precipitation of Ni-Al compounds. Supplied mainly as thin sheet, strip and wire for springs and aircraft parts, with strengths of 1,240 to 1,650 MPa.
Designations and equivalents
| System | Designation |
|---|---|
| UNS | S17700 |
| W.Nr./EN | 1.4568 (X7CrNiAl17-7) |
| DIN | X7CrNiAl17-7 |
| JIS | SUS631 |
| Trade names | 17-7PH (Cleveland-Cliffs trademark), Type 631 |
| Others | AMS 5528, 5529, 5644, 5678 |
Standards by product form
| Form | Standard |
|---|---|
| Sheet/strip | ASTM A693 Type 631; AMS 5528 (A), AMS 5529 (C) |
| Bar/forging | ASTM A564 Type 631; AMS 5644 |
| Spring wire | ASTM A313 Type 631; AMS 5678 |
| Forging | ASTM A705 Type 631 |
Tolerances and HS codes
- Forged bars (round and square): machining allowance from 2 mm (Ø 6–16 mm) to 24 mm (Ø 400–500 mm) on the diameter. E.g. Ø 100–125 mm finished: +7 mm, tolerance +6 / −0 (length up to 3,500 mm).
- Forged discs: allowance of 4 to 9 mm on the diameter and 3 to 11 mm on the height, plus-only tolerance.
- Cutting and machining: band saw −0 / +5 to +15 mm on length; waterjet +10 % of thickness on the diameter (min. 2 mm); machined parts −0 / +2 to +5 mm on the diameter. See the full tables (INF 002 and INF 003, the same for every alloy).
| Form | NCM (Brazil, 8 digits) | HS (international, 6 digits) | HTS (USA, 10 digits)* | CN (European Union, 8 digits)* |
|---|---|---|---|---|
| Forged bar | 7222.30.00 | 7222.30 | 7222.30.0024 | 7222 30 51 |
| Hot-rolled bar | 7222.11.00 | 7222.11 | 7222.11.0059 | 7222 11 11 |
| Plate (> 10 mm, width ≥ 600 mm) | 7219.21.00 | 7219.21 | 7219.21.0020 | 7219 21 10 |
| Flange and fitting | 7307.21.00 | 7307.21 | 7307.21.5000 | 7307 21 00 |
Tariff class: stainless steel (by chemical composition, under the Chapter 72 Notes and Section XV Note 5).
*: US and EU codes for reference. NCM per current legislation (General Rules of Interpretation, Notes to Chapters 72, 74 and 75 and Section XV Note 5). Confirm the final code with your customs broker according to the item size and finish. HS = first 6 digits of the NCM.
Chemical composition (%)
ASTM A693 Type 631
| Element | Range (%) |
|---|---|
| C | ≤ 0.09 |
| Mn | ≤ 1.00 |
| Si | ≤ 1.00 |
| P | ≤ 0.040 |
| S | ≤ 0.030 |
| Cr | 16.0 – 18.0 |
| Ni | 6.50 – 7.75 |
| Al | 0.75 – 1.50 |
| Fe | balance |
Mechanical properties
ASTM A693 sheet – minimums by condition (minimum elongation depends on thickness)
| Property | Value |
|---|---|
| Condition A | Rm ≤ 1,035 MPa; Rp ≤ 380 MPa; ≤ 92 HRB |
| TH1050 | Rp ≥ 1,035 / Rm ≥ 1,240 MPa; ≥ 38 HRC |
| RH950 | Rp ≥ 1,310 / Rm ≥ 1,450 MPa; ≥ 44 HRC |
| CH900 | Rp ≥ 1,585 / Rm ≥ 1,655 MPa; ≥ 46 HRC |
| Typical elongation | A 35%; TH1050 8%; RH950 7%; CH900 2% |
Typical physical properties
| Property | Value |
|---|---|
| Density | 7.81 g/cm³ (A) / 7.65 g/cm³ (TH1050, RH950) |
| Melting range | ≈ 1,400 – 1,440 °C (typical) |
| Modulus of elasticity | 200 GPa (TH1050/RH950) |
| Thermal conductivity (150 °C) | 16.9 W/m·K (TH1050) |
| Thermal expansion (20–93 °C) | 10.1 µm/m·K (TH1050, 21–93 °C) / 15.3 (A) |
Aging conditions
| Condition | Cycle | Rp0.2 | Rm | Hardness | Elongation (typ.) |
|---|---|---|---|---|---|
| A (solution annealed) | 1,066 ± 14 °C, air | ≤ 380 MPa | ≤ 1,035 MPa | ≤ 92 HRB | 35% |
| TH1050 | A → 760 °C/90 min, cool < 15 °C (T) → 566 °C/90 min | ≥ 1,035 MPa | ≥ 1,240 MPa | ≥ 38 HRC | 8% |
| RH950 | A → 954 °C/10 min, air → −73 °C/8 h (R100) → 510 °C/60 min | ≥ 1,310 MPa | ≥ 1,450 MPa | ≥ 44 HRC | 7% |
| C (cold rolled) | from the mill | ≥ 1,205 MPa | ≥ 1,380 MPa | ≥ 41 HRC | 5% |
| CH900 | C → 482 °C/60 min | ≥ 1,585 MPa | ≥ 1,655 MPa | ≥ 46 HRC | 2% |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | Up to ≈ 315–340 °C for springs and structural parts; above that it loses strength by overaging. |
| Heat treatment | Condition A: 1,066 ± 14 °C, air. TH1050: condition at 760 °C/90 min, cool below 15 °C within 1 h (T), age 566 °C/90 min. RH950: condition at 954 °C/10 min, air, sub-zero −73 °C/8 h (R100), age 510 °C/60 min. CH900: from C (cold rolled), 482 °C/60 min. Minimums: TH1050 Rp 1,035/Rm 1,240 MPa, 38 HRC; RH950 1,310/1,450 MPa, 44 HRC; CH900 1,585/1,655 MPa, 46 HRC. |
| Welding | Weldable by TIG/MIG/resistance, but worse than 17-4PH (Al hurts penetration and produces slag). After welding, conditioning and aging are required. Matching consumable AMS 5824 (ER631 / UNS S17780). |
| Machining | In condition A it is ductile and work hardens like an austenitic (304-type machining); in TH/RH, machine with carbide tooling. |
Typical applications
- Flat and coil springs
- Washers and retaining rings
- Diaphragms and bellows
- Aircraft blades and components
- High-strength stamped parts
- Surgical instruments
Alternatives: PH 15-7Mo for higher hot strength; 17-4PH for bar and forgings; cold-worked (full hard) 301 for lower-cost springs; Inconel X-750 for springs above 350 °C.
17-7PH: complete guide
What 17-7PH stainless steel is
17-7PH belongs to the family of special stainless steels, in the semi-austenitic precipitation-hardening group. In condition A the structure is metastable austenite, ductile and only slightly magnetic; a thermal or sub-zero conditioning treatment transforms it into martensite, and aging precipitates nickel-aluminum compounds that raise the strength.
Forming, machining and welding
The rule for 17-7PH is to form in condition A, when it behaves like an austenitic, and treat afterwards. Machining is also easier in condition A. Welding is done in condition A, by TIG, MIG or resistance, with 17-7PH consumable, and the welded part then receives the full treatment. Dustre performs the TH and RH cycles in its own furnaces as part of its heat treatment service.
Corrosion and limits of use
Corrosion resistance is similar to 304 in condition A and slightly lower in the treated conditions, with a PREN of 16 to 18; combined chlorides and stress should be avoided in the harder conditions. Under load, use goes up to about 315 °C, above what cold-worked 301 springs withstand, and that is one of the reasons for choosing it.
Applications
Compression and leaf springs, instrument diaphragms and bellows, blades and clips, sheet-formed aircraft components, valve and actuator parts, and components that require high strength after being formed into complex shapes.
Similar grades
PH 15-7 Mo adds molybdenum for better strength at temperature; 15-5PH and 17-4PH are the martensitic PH grades for bar and forgings; cold-worked 301 is the economical alternative for less demanding springs.
Supply forms and Dustre services
Round bars and blocks forged in open die, cut to size on a band saw, with rough or finish machining, heat treatment (quenching and tempering, annealing, solution annealing and aging) in our own furnaces and hardness, tensile and impact testing. All material is supplied with a 3.1 inspection certificate and heat traceability.
Frequently asked questions
What is the chemical composition of 17-7PH?
Per ASTM A693 Type 631: C ≤ 0.09; Mn ≤ 1.00; Si ≤ 1.00; P ≤ 0.040; S ≤ 0.030; Cr 16.0 – 18.0.
Which standards cover 17-7PH?
Sheet/strip: ASTM A693 Type 631; AMS 5528 (A), AMS 5529 (C). Bar/forging: ASTM A564 Type 631; AMS 5644. Spring wire: ASTM A313 Type 631; AMS 5678. Forging: ASTM A705 Type 631.
Up to what temperature can 17-7PH be used?
Up to ≈ 315–340 °C for springs and structural parts; above that it loses strength by overaging.
Which materials can replace 17-7PH?
PH 15-7Mo for higher hot strength; 17-4PH for bar and forgings; cold-worked (full hard) 301 for lower-cost springs; Inconel X-750 for springs above 350 °C.
