# Alloy 255 Forgings — UNS S32550 / ASTM A182 F61

**Alloy 255 is a 25 per cent chromium duplex ferritic-austenitic stainless steel, designated UNS S32550 and ordered as forgings to ASTM A182 identification symbol F61.** What separates it from ordinary 22 per cent chromium duplex is deliberate copper, 1.50 to 2.50 per cent, together with nitrogen from 0.10 to 0.25 per cent. The copper is why the grade is chosen for sulphuric and phosphoric acid duty in preference to 2507. The nitrogen is why melting matters: it has to be held inside a narrow band at the furnace rather than corrected later.

Jiangyin Jiangnan Metal Co., Ltd. melts this grade on site. Duplex heats run through the electric arc furnace, a ladle furnace and vacuum oxygen decarburisation, then go to the presses and the ring mill without leaving the works, so the certificate traces a finished ring back to its melt number.

Canonical page: https://www.steelforgepieces.com/Stainless-Steel/ALLOY-255.html

## Grade data

| Field | Detail |
|---|---|
| UNS number | S32550 |
| ASTM A182 symbol | F61 |
| EN number and name | 1.4507, X2CrNiMoCuN25-6-3 |
| JIS equivalent | SUS329J4L |
| Structure | Duplex, ferrite plus austenite |
| Tensile strength, min | 750 MPa (109 ksi) |
| Yield strength 0.2 % offset, min | 550 MPa (80 ksi) |
| Elongation, min | 25 % |
| Reduction of area, min | 50 % |
| Melting route | EF + LF + VOD on site, 60 t maximum heat |
| Rolled ring diameter | 50 mm to 6,000 mm OD |
| Single piece weight | 10 kg to 30,000 kg |
| Certification | EN 10204 3.1, or 3.2 witnessed |
| Minimum order | 10 kg, no minimum piece count |
| Lead time | 20 to 60 days |

Prices are quoted against a drawing. There is no list price for this grade, because every part is made to order.

## 01 Designations

ASTM A182 lists identification symbol **F61** against **UNS S32550**, described in the specification as 26 chromium, 6 nickel, 3.5 molybdenum with nitrogen and copper, 255. **F59 is a separate grade**, UNS S32520, with nitrogen from 0.20 to 0.35 per cent. Where a drawing says only "F255", confirm which of the two is meant before the heat is scheduled, because the nitrogen aim differs and cannot be adjusted after annealing.

| System | Designation | Note |
|---|---|---|
| UNS | S32550 | Primary North American number |
| ASTM / ASME | A182 F61 | Forged flanges, fittings, valves and pressure parts |
| EN 10088-1 | 1.4507, X2CrNiMoCuN25-6-3 | Ranges differ slightly from A182 |
| JIS | SUS329J4L | Close equivalent, no copper requirement |
| Also seen as | Alloy 255, Super Duplex 255, Grade 255, UR52N | Commercial and historic names |
| Not the same | S32520 = A182 F59 | Higher nitrogen, not a substitute |

Ferralium 255 is a registered trademark of Meighs Ltd, Langley Alloys Division, noted here only so buyers searching that name can find the equivalent UNS number. Material is supplied to UNS S32550 and ASTM A182 F61.

## 02 Chemical composition

ASTM A182 F61 / UNS S32550, per cent by weight. Copper and nitrogen carry minimum limits; an analysis without them is not S32550.

| Limit | C | Mn | P | S | Si | Ni | Cr | Mo | Cu | N | Fe |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Min | – | – | – | – | – | 4.5 | 24.00 | 2.9 | 1.50 | 0.10 | Balance |
| Max | 0.040 | 1.50 | 0.040 | 0.030 | 1.00 | 6.5 | 27.00 | 3.9 | 2.50 | 0.25 | Balance |

### Where EN 1.4507 differs

EN 1.4507 runs carbon to 0.03 max, silicon to 0.7 max, chromium 24.0 to 26.0, nickel 6.0 to 8.0, molybdenum 3.0 to 4.0, copper 1.0 to 2.5 and nitrogen 0.20 to 0.30. Nickel and nitrogen sit higher than A182 F61 and only partly overlap it.

| Element | A182 F61 / S32550 | EN 1.4507 | Effect on the order |
|---|---|---|---|
| Ni | 4.5 to 6.5 | 6.0 to 8.0 | Overlap is 6.0 to 6.5 only |
| N | 0.10 to 0.25 | 0.20 to 0.30 | Overlap is 0.20 to 0.25, set at VOD |
| Mo | 2.9 to 3.9 | 3.0 to 4.0 | Broad overlap, no constraint |
| C | 0.040 max | 0.03 max | Use the EN limit for dual certification |

Dual certification to both documents is possible, but the heat must be aimed at the overlap before the ingot is cast.

### Pitting resistance equivalent

PREN is taken as chromium plus 3.3 times molybdenum plus 16 times nitrogen. A mid-range analysis of 25.5 chromium, 3.5 molybdenum and 0.22 nitrogen gives about 40; across the permitted composition the figure moves between roughly 38 and 42. It is a calculation from the limits, not a measured property, and says nothing about behaviour in a specific liquid at a specific temperature. Where pitting resistance is contractual, specify an ASTM G48 test.

## 03 Mechanical and physical properties

ASTM A182 F61 forging requirements:

| Property | Imperial | Metric |
|---|---|---|
| Tensile strength, min | 109 ksi | 750 MPa |
| Yield strength, 0.2 % offset, min | 80 ksi | 550 MPa |
| Elongation in 2 in. or 4D, min | 25 % | 25 % |
| Reduction of area, min | 50 % | 50 % |

Hardness is not fixed by A182 for this grade. Where a ceiling is needed it comes from the purchase specification, often in the 270 to 300 HBW region.

EN-based data sheets for 1.4507 bar and plate quote a tensile band near 700 to 900 MPa with yield above 500 MPa. Those figures are lower than the A182 forging minimums because the product form and test basis differ. Certification follows whichever document the order names.

Indicative physical properties, EN 1.4507 basis:

| Property | Value |
|---|---|
| Modulus of elasticity | 200 GPa at 20 C, 194 GPa at 100 C, 180 GPa at 300 C |
| Thermal conductivity | 15 W/m·K |
| Specific heat | 500 J/kg·K |
| Linear expansion | 13.0 x 10⁻⁶/K at 20 C, 14.0 at 300 C |
| Electrical resistivity | 0.8 micro-ohm·m |

### Temperature limits

Duplex is restricted at both ends. Prolonged service below about minus 50 F or above about plus 600 F brings embrittlement and loss of toughness. For pressure equipment the figure is tighter: the maximum continuous operating temperature cited from ASME Boiler and Pressure Vessel Code Section VIII is 500 F.

## 04 Corrosion behaviour

Alloy 255 offers better resistance in most media than 316 and generally outperforms 22 per cent chromium duplex 2205. It resists chloride stress corrosion cracking, crevice attack and pitting better than lower-alloyed stainless steel.

- **Mineral acids.** Performs well in sulphuric, phosphoric and nitric acid. The copper addition is the usual explanation for its edge over 2507 in acid duty.
- **Organic acids.** Resists acetic and formic acid.
- **Avoid.** Highly reducing media.
- **Against F53 and F55.** F53, which is 2507, is the more widely stocked general-purpose super duplex. Where sulphuric or phosphoric acid dominates, F61 is normally the better candidate.

Intermetallic phase in the ferrite costs toughness and pitting resistance together, which makes quench rate a corrosion question as much as a strength one.

## 05 Forging, heat treatment, machining and welding

| Operation | Practice |
|---|---|
| Hot forging | 900 to 1200 C per EN practice for 1.4507 |
| Solution anneal | 1020 to 1100 C (EN), about 1950 F in mill practice, then a water quench |
| Range to avoid | About 1000 to 1800 F for carbides, nitrides and sigma; near 900 F for alpha prime |
| Cold work | Re-anneal after appreciable deformation; heavy cold work needs an intermediate treatment |
| Machining | As for conventional stainless steel, carbide tooling preferred. Allow for strength well above 316 |
| Welding | Weld in the annealed condition on clean surfaces. Preheat not required. Post-weld heat treatment not mandatory but preferred on heavy sections |
| Pickling | Where required, 15 % nitric plus 2 % hydrofluoric acid by volume, minimum 55 F |

Holding this alloy between roughly 1000 and 1800 F precipitates M23C6 carbides, M2N nitrides and sigma phase; near 900 F, alpha prime forms instead. All lower toughness and corrosion resistance and none can be machined out. On thick rings and tube sheets the risk sits at the mid-wall where cooling is slowest, so section thickness belongs in the heat-treatment plan from the quotation onward. Furnace charts are issued with the certificate.

## 06 Products forged in Alloy 255

All made to customer drawing. There is no stock list, because duplex is melted to order so the nitrogen can be aimed at the governing specification.

- **Rings and rolled rings.** Ring-rolled from a punched preform, rectangular or contoured section, to 6,000 mm OD. From 50 mm, rings are upset and punched instead.
- **Forged flanges.** Weld neck, blind, slip-on, socket weld, lap joint, orifice and long weld neck to ASME B16.5, B16.47 and API 6A, certified under ASTM A182.
- **Shafts and spindles.** Pump and mixer shafts, stepped and eccentric shafts, spindles. Turned to 14 m between centres, 15 m well furnaces for vertical quenching.
- **Tube sheets and discs.** Solid forged discs, tube sheets and covers, supplied proof-machined for ultrasonic testing.
- **Valve and wellhead parts.** Bodies, bonnets, blocks, stems, seat rings, spools and manifolds for ball, gate, globe, check and plug valves. API 6A and NACE MR0175 / ISO 15156 where required.
- **Bars and blocks.** Round, flat and square bar as feedstock for machined bodies and fittings.
- **Cylinders, sleeves and pipe.** Hollow forged cylinders, sleeves, bushings and heavy-wall pipe bored from solid, to 1,600 mm by 10,000 mm.

Typical destinations: offshore and subsea production systems, downhole equipment, seawater and firewater pipework with risers and manifolds, desalination and high-pressure reverse osmosis plant, phosphoric and sulphuric acid plant, flue gas desulphurisation and scrubbers, pulp and paper bleaching plant, waste water and demineralised water treatment, mining and metallurgical equipment, fasteners, and food and grain processing machinery.

## 07 Melting, testing and certification

| Stage | Detail |
|---|---|
| Melting | Electric arc furnace, ladle furnace, then VOD at 30 t or 60 t. Maximum heat 60 t. ESR or VAR available where cleanliness is specified |
| Forging | 6,300 t, 4,000 t and 2,000 t open-die presses; 6 m, 3 m and 1 m radial-axial ring mills; hammers 750 kg to 5 t. Heaviest single piece 30 t |
| Heat treatment | 14 furnaces including two 15 m well-type furnaces for vertical quenching. Solution anneal and water quench, charts issued |
| Chemistry | Optical emission spectrometry to ASTM E415, infrared carbon and sulphur to ASTM E1019, handheld PMI |
| Mechanical | Tensile to ASTM A370 and ISO 6892 on 600 kN and 300 kN machines; Charpy V-notch to minus 60 C |
| Ultrasonic | ASTM A388 and EN 10228-3, acceptance class to order. SEP 1921 on request |
| Other NDT | Magnetic particle to ASTM E709, liquid penetrant, bore endoscope, dimensional report, grain size to ASTM E112 |
| Placed with accredited laboratories | ASTM A923 phase balance and ferrite count, ASTM G48 pitting, ASTM A262 and G28 intergranular corrosion, bench Brinell, impact below minus 60 C |
| Certification | EN 10204 3.1 as standard; 3.2 witnessed by BV, DNV, LR, SGS, TUV or the customer's inspector. Works approvals CCS, BV, DNV, LR, NK |

Because the steel is melted here, chemistry is verified at the furnace rather than copied from a billet supplier's certificate. For a nitrogen-alloyed duplex grade the VOD units are where the nitrogen band is actually held.

The clauses that cause argument at inspection on super duplex forgings are phase balance, ferrite count and impact toughness rather than tensile strength. A923 and G48 are placed outside and need scheduling, so state them in the enquiry.

## 08 Requesting a price

Minimum order 10 kg with no minimum piece count. Lead time 20 to 60 days; duplex sits at the lower end unless remelting or outside corrosion testing is called for.

| # | Item | Example |
|---|---|---|
| 1 | Grade and governing document | Alloy 255, UNS S32550, ASTM A182 F61 |
| 2 | Dimensions or drawing (PDF, DWG, DXF, STEP) | OD 1200 x ID 900 x H 260 mm |
| 3 | Quantity and likely repeat | 8 pcs, about 3 per year |
| 4 | Delivery condition | Proof-machined for UT |
| 5 | Heat treatment and testing | Solution annealed and water quenched, UT to ASTM A388, A923 Method C |
| 6 | Certificate and destination | EN 10204 3.2 witnessed by DNV, Rotterdam |

Email sales@steelforgepieces.com or call +86 189 2135 9659.

## 09 Questions

**What is Alloy 255?** A 25 per cent chromium duplex ferritic-austenitic stainless steel, UNS S32550, ordered as forgings to ASTM A182 F61 and listed in Europe as EN 1.4507. Copper 1.50 to 2.50 per cent and nitrogen 0.10 to 0.25 per cent are specified ranges, not residuals.

**Is Alloy 255 the same as A182 F61?** Yes. A182 lists F61 against UNS S32550, described as 26 chromium, 6 nickel, 3.5 molybdenum with nitrogen and copper, 255. F59 is a different grade, UNS S32520, with nitrogen 0.20 to 0.35 per cent.

**Minimum mechanical properties?** 109 ksi (750 MPa) tensile, 80 ksi (550 MPa) yield at 0.2 per cent offset, 25 per cent elongation, 50 per cent reduction of area.

**PREN?** About 40 for a mid-range analysis, between roughly 38 and 42 across the permitted composition. A calculation from the limits, not a measured property.

**Heat treatment after forging?** Forge hot at 900 to 1200 C, solution anneal at 1020 to 1100 C or about 1950 F, water quench. Quench speed keeps carbides, nitrides and sigma out of the structure.

**Service temperature?** Normally inside minus 50 F to plus 600 F. For pressure equipment, 500 F is the maximum continuous operating temperature cited from ASME Section VIII.

**Alloy 255 or 2507 for acid duty?** Alloy 255, as A182 F61, is normally preferred over F53 in acidic environments, with copper content the reason usually given. Reducing media should be avoided.

**Do you melt it or buy billet?** Melted on site. Duplex runs EF, LF then VOD at 30 t or 60 t, which is where nitrogen is held in band. Maximum heat 60 t, so certificates trace to a melt number.

**Sizes?** Rings to 6,000 mm OD, heaviest single forging 30 t, shafts turned to 14 m, discs to 5,000 mm diameter, bored cylinders to 1,600 by 10,000 mm, from 10 kg.

**Finish-machined parts?** As-forged with allowance, proof-machined for UT, or finished to drawing. Heat treatment, machining, NDT and mechanical testing all on site.

## Supplier record

| Field | Detail |
|---|---|
| Legal name | Jiangyin Jiangnan Metal Co., Ltd. |
| Business | Integrated melting works, open-die forge and ring rolling mill |
| Established | 1997 |
| Employees | 460, including 9 senior engineers, 32 intermediate engineers and 140 technicians |
| Works | No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China |
| Melting | EF, LF, VOD, VIM, VAR, ESR. 60 t maximum heat |
| Forging | 6,300 t press, 6 m ring mill, 30 t maximum single piece |
| Approvals | CCS, BV, DNV, LR, NK |
| Telephone | +86 189 2135 9659 |
| Email | sales@steelforgepieces.com |
| Website | https://www.steelforgepieces.com/ |

Composition and mechanical data follow the published requirements of ASTM A182 for F61 / UNS S32550 and EN 10088-1 for 1.4507, together with mill data for the grade. Physical properties and processing temperatures are indicative. The purchase order and the specification named on it take precedence. Page last reviewed 2 October 2026.