Alloy 255 is a 25 per cent chromium duplex ferritic-austenitic stainless steel designated UNS S32550, ordered as forgings to ASTM A182 identification symbol F61 and listed in Europe as EN 1.4507, X2CrNiMoCuN25-6-3. Copper between 1.50 and 2.50 per cent and nitrogen between 0.10 and 0.25 per cent are specified ranges rather than residuals, and they are what distinguish the grade from 22 per cent chromium duplex.

What are the minimum mechanical properties of Alloy 255 forgings?

To ASTM A182 F61: tensile strength 109 ksi or 750 MPa minimum, yield strength at 0.2 per cent offset 80 ksi or 550 MPa minimum, elongation 25 per cent minimum and reduction of area 50 per cent minimum.

What is the PREN of Alloy 255?

Using PREN equal to per cent chromium plus 3.3 times per cent molybdenum plus 16 times per cent nitrogen, a mid-range S32550 analysis of 25.5 chromium, 3.5 molybdenum and 0.22 nitrogen gives roughly 40. The figure moves between about 38 and 42 across the permitted composition range, so it depends on the actual heat analysis. PREN ranks alloys against each other and does not predict service life in a given environment.

How is Alloy 255 heat treated after forging?

Forging is carried out hot, with European practice for 1.4507 quoting 900 to 1200 C, followed by solution annealing and a water quench. EN data gives supersaturation at 1020 to 1100 C and mill practice for S32550 quotes about 1950 F. The quench has to be fast: holding anywhere between roughly 1000 and 1800 F forms M23C6 carbides, M2N nitrides and sigma phase, and alpha prime forms approaching 900 F. All of these cut toughness and corrosion resistance.

What is the service temperature limit for Alloy 255?

Duplex grades are limited at both ends. Suppliers commonly restrict continuous service to about minus 50 F to plus 600 F because of embrittlement and loss of toughness outside that band, and the maximum continuous operating temperature cited from ASME Boiler and Pressure Vessel Code Section VIII is 500 F. Confirm the limit in the code governing your equipment.

Alloy 255 or 2507, which suits an acid duty better?

Alloy 255, ordered as A182 F61, is the grade normally preferred in acidic environments over F53 which is 2507, and its copper content is the usual reason given. It performs well in sulphuric, phosphoric and nitric acid and resists organic acids such as acetic and formic acid. Highly reducing media should be avoided.

What size Alloy 255 forgings can Jiangyin Jiangnan Metal produce?

Seamless rings roll to 6,000 mm outside diameter on the 6 m radial-axial mill, with 3 m and 1 m mills for smaller sizes and an upset-and-punch route from 50 mm outside diameter. The heaviest single forging is 30 tonnes, worked on 6,300 t, 4,000 t and 2,000 t presses. Minimum order is 10 kg with no minimum piece count, and lead time runs 20 to 60 days.

Do you melt Alloy 255 in house or buy billet?

It is melted on site. Duplex grades run through the electric arc furnace, are refined in a ladle furnace and are decarburised under vacuum in the 30 t or 60 t VOD units, which is also where nitrogen is held inside the 0.10 to 0.25 per cent band the grade requires. Maximum heat size is 60 tonnes, so the certificate traces back to a melt number rather than to a purchased billet.

Super duplex stainless steel forgings

Alloy 255 UNS S32550 · ASTM A182 F61 · EN 1.4507 · Rings, flanges, shafts, discs and tube sheets forged to drawing

Q: Is Alloy 255 the same as ASTM A182 F61?

Yes. ASTM A182 lists identification symbol 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, which carries nitrogen from 0.20 to 0.35 per cent. The two are not interchangeable without the purchaser's approval.

Q: Can you supply finish-machined Alloy 255 parts?

Parts are supplied as-forged with allowance, proof-machined ready for ultrasonic testing, or finished to drawing. Heat treatment, machining, non-destructive testing and mechanical testing are all carried out on site before shipment.

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 the reason the grade is picked for sulphuric and phosphoric acid duty in preference to 2507, and the nitrogen is why melting matters: it has to be held inside a narrow band at the furnace, not corrected afterwards.

We melt this grade here. Duplex heats run through the electric arc furnace, a ladle furnace and vacuum oxygen decarburisation, then go straight to the presses and the ring mill without leaving the site, so the certificate traces a finished ring back to its melt number.

Request a price What to send with an enquiry

Designations, and the one that gets confused

Enquiries for this grade arrive under at least six names, and two of them are routinely mixed up. 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. If a drawing says only "F255", tell us which of the two is meant before the heat is scheduled, because the nitrogen aim differs and it cannot be adjusted after the forging is annealed.

Alloy 255 cross-reference
System	Designation	Note
UNS	S32550	Primary North American number
ASTM / ASME	A182 F61	Forged flanges, fittings, valves and parts for pressure service
EN 10088-1	1.4507 · X2CrNiMoCuN25-6-3	Ranges differ slightly from A182, see section 02
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. Do not substitute without written approval

Ferralium 255 is a registered trademark of Meighs Ltd, Langley Alloys Division. It appears here only so buyers searching that name can identify the equivalent UNS number. We supply to UNS S32550 and ASTM A182 F61, not under any trademarked designation.

Chemical composition

The table below is the ASTM A182 grade F61 requirement. Copper and nitrogen carry minimum limits, which is worth checking on any certificate offered to you for this grade: an analysis without them is not S32550.

ASTM A182 F61 / UNS S32550, per cent by weight
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

If your order is written to the European number rather than the ASTM one, the aim changes. 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. The nickel and nitrogen windows sit higher than A182 F61 and only partly overlap it. Dual certification to both documents is possible, but the heat has to be aimed at the overlap from the start, so say so at enquiry stage rather than after the ingot is cast.

A182 F61 against EN 1.4507, selected elements
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

Pitting resistance equivalent

For duplex grades PREN is taken as chromium plus 3.3 times molybdenum plus 16 times nitrogen. A mid-range S32550 analysis, say 25.5 chromium with 3.5 molybdenum and 0.22 nitrogen, works out near 40; across the full permitted range the figure moves between roughly 38 and 42. That is a calculation from the composition limits, not a measured property, and it says nothing about how the alloy will behave in a specific liquid at a specific temperature. Where pitting resistance is a contract requirement, ask for a G48 test rather than relying on the arithmetic.

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	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. State the figure you want on the order

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 numbers are lower than the A182 forging minimums because the product form and test basis are different. We certify against whichever document your order names, and we will not quote EN figures on an ASTM certificate.

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, rising to 14.0 at 300 C
Electrical resistivity	0.8 micro-ohm·m

Temperature limits

This is the constraint that catches people out, because 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, which is why the grade is not offered for duty outside that band. For pressure equipment the governing figure is tighter again: the maximum continuous operating temperature cited from ASME Boiler and Pressure Vessel Code Section VIII is 500 F. Check the limit in the code your vessel is built to before you specify.

Corrosion behaviour

Against the common austenitic grades, Alloy 255 offers better resistance in most media than 316, and it generally outperforms 22 per cent chromium duplex 2205. It resists chloride stress corrosion cracking, crevice attack and pitting better than lower-alloyed stainless steel, which is the practical reason it ends up in seawater and sour-service hardware.

Mineral acids. Performs well in sulphuric, phosphoric and nitric acid. The copper addition is the usual explanation given for its edge over 2507 in acidic 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 the duty, F61 is normally the better candidate. We forge all three and can price them side by side on one enquiry.

One warning about the microstructure, because it bears directly on corrosion resistance rather than just on strength: anything that puts intermetallic phase into the ferrite will cost you both toughness and pitting resistance at the same time. That is a heat-treatment and quench-rate question, covered next.

Forging, heat treatment, machining and welding

Hot work is carried out in the range European practice gives for 1.4507, 900 to 1200 C, then the part is solution annealed and water quenched. EN data puts supersaturation at 1020 to 1100 C; mill practice for S32550 is commonly quoted around 1950 F. The quench rate is not a detail to be relaxed for a heavy section.

Why the quench matters

Hold this alloy anywhere between roughly 1000 and 1800 F and it will precipitate M₂₃C₆ carbides, M₂N nitrides and sigma phase. Approaching 900 F, alpha prime forms instead. Each of these lowers toughness and corrosion resistance, and none of them can be machined out. On thick rings and tube sheets the risk sits at the mid-wall, where the cooling rate is slowest, so section thickness has to be part of the heat-treatment plan from the quotation onward. Our furnace charts are issued with the certificate for exactly this reason.

Processing summary
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, followed by 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 is not required. Post-weld heat treatment is not mandatory but is preferred on heavy sections to protect corrosion resistance
Pickling	Where required, 15 % nitric plus 2 % hydrofluoric acid by volume, minimum 55 F

What we forge in Alloy 255

Everything here is made to the customer's drawing. There is no stock list for this grade, because duplex is melted to order so that the nitrogen can be aimed at your specification.

Rings and rolled rings

Ring-rolled from a punched preform, rectangular or contoured section, to 6,000 mm outside diameter. Below the mill range, 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, with 15 m well furnaces for vertical quenching.

Tube sheets

Solid forged tube sheets and covers for exchangers and vessels, supplied proof-machined so ultrasonic testing can be done properly.

Discs and blanks

Forged discs and blanks for valve and closure components, machined to drawing or left with allowance.

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 blanks, to 1,600 mm diameter by 10,000 mm.

Where these parts end up

Offshore and subsea production systems, downhole equipment, seawater and firewater pipework with its 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, and mining and metallurgical equipment. The grade also turns up in fasteners and in food and grain processing machinery.

Melting, testing and certification

Because the steel is melted here, the chemistry is verified at the furnace rather than copied from a billet supplier's certificate. For a nitrogen-alloyed duplex grade that distinction is the whole argument: the 30 t and 60 t vacuum oxygen decarburisation units are where the nitrogen band is actually held.

Route for an Alloy 255 forging
Stage	Detail
Melting	Electric arc furnace, ladle furnace, then VOD at 30 t or 60 t. Maximum heat 60 t. Electroslag or vacuum arc remelting 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 from 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 with the certificate
Chemistry	Optical emission spectrometry to ASTM E415, infrared carbon and sulphur to ASTM E1019, handheld PMI for verification
Mechanical	Tensile to ASTM A370 and ISO 6892 on 600 kN and 300 kN machines; Charpy V-notch with a low-temperature bath to minus 60 C
Ultrasonic	ASTM A388 and EN 10228-3, acceptance class to order. SEP 1921 on request
Surface and other NDT	Magnetic particle to ASTM E709, liquid penetrant, bore endoscope, dimensional report, grain size to ASTM E112
Placed outside	ASTM A923 phase balance and ferrite count, ASTM G48 pitting, ASTM A262 and G28 intergranular corrosion, bench Brinell, impact below minus 60 C. Run by accredited laboratories, reported alongside the mill certificate
Certification	EN 10204 3.1 as standard. EN 10204 3.2 witnessed by BV, DNV, LR, SGS, TUV or your own inspector on request. Works approvals: CCS, BV, DNV, LR, NK

For super duplex forgings, the clauses that cause argument at inspection are phase balance, ferrite count and impact toughness, not tensile strength. ASTM A923 and G48 are placed with an outside laboratory and need scheduling, so put them in the enquiry and we will confirm them in the order acknowledgement instead of discovering them at final inspection.

Requesting a price

Send a drawing, or just the dimensions. Six items get a firm price back inside two working days. Minimum order is 10 kg with no minimum piece count, and normal lead time is 20 to 60 days; duplex sits at the lower end unless a remelting step or outside corrosion testing is called for.

What to include
#	Item	Example
1	Grade and governing document	Alloy 255, UNS S32550, ASTM A182 F61
2	Dimensions, or a drawing in PDF, DWG, DXF or 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

sales@steelforgepieces.com +86 189 2135 9659

Questions we get about this grade

What is Alloy 255?

A 25 per cent chromium duplex ferritic-austenitic stainless steel, designated UNS S32550, ordered as forgings to ASTM A182 F61 and listed in Europe as EN 1.4507, X2CrNiMoCuN25-6-3. Copper from 1.50 to 2.50 per cent and nitrogen from 0.10 to 0.25 per cent are specified ranges, not residuals, and they are what separate it from 22 per cent chromium duplex.

Is Alloy 255 the same thing 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, carrying nitrogen from 0.20 to 0.35 per cent, and the two should not be swapped without the purchaser agreeing in writing.

What are the minimum mechanical properties?

To A182 F61: 109 ksi or 750 MPa tensile, 80 ksi or 550 MPa yield at 0.2 per cent offset, 25 per cent elongation and 50 per cent reduction of area, all minimums. EN data sheets for 1.4507 bar and plate quote lower numbers because the product form and test basis differ.

What is the PREN?

Taking 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, and the figure moves between roughly 38 and 42 across the permitted composition. It is a calculation from the limits, useful for ranking alloys and no use for predicting service life. Where it matters contractually, specify a G48 test.

How should it be heat treated after forging?

Forge hot, then solution anneal and water quench. EN practice for 1.4507 gives forging at 900 to 1200 C and supersaturation at 1020 to 1100 C; mill practice for S32550 is quoted around 1950 F. Speed through the quench is what keeps M₂₃C₆ carbides, M₂N nitrides and sigma phase out of the structure. Those form on any hold between about 1000 and 1800 F, and alpha prime forms near 900 F. On heavy sections the mid-wall cooling rate is the thing to watch.

What temperature can it run at in service?

Continuous service is normally kept inside about minus 50 F to plus 600 F, since toughness falls away outside that band. For pressure equipment the limit is lower: 500 F is the maximum continuous operating temperature cited from ASME Boiler and Pressure Vessel Code Section VIII. Work to whichever code governs your equipment.

Alloy 255 or 2507 for an acid duty?

Alloy 255, as A182 F61, is the grade normally preferred over F53 which is 2507 in acidic environments, and the copper content is the reason usually given. It handles sulphuric, phosphoric and nitric acid and resists organic acids such as acetic and formic. Reducing media should be avoided. We forge both grades, so send one enquiry and we will price them together.

Do you melt it, or buy billet?

We melt it. Duplex runs through the electric arc furnace, is refined in a ladle furnace and is decarburised under vacuum in the 30 t or 60 t VOD units, which is where the nitrogen is held inside the specified band. Maximum heat size is 60 tonnes. The practical consequence is that the certificate traces your ring back to a melt number rather than stopping at a purchased billet, which is as far as most forging shops in China can document.

What sizes and weights can you make?

Rings roll to 6,000 mm outside diameter on the 6 m mill, with 3 m and 1 m mills below that and an upset-and-punch route from 50 mm. The heaviest single forging is 30 tonnes on the 6,300 t press. Shafts turn to 14 m, discs and tube sheets machine to 5,000 mm diameter, and bored cylinders run to 1,600 mm by 10,000 mm. Minimum order is 10 kg.

Can you supply finish-machined parts?

As-forged with allowance, proof-machined ready for ultrasonic testing, or finished to drawing. Heat treatment, machining, NDT and mechanical testing are all done on site before shipment.

Supplier record

Manufacturer of the Alloy 255 forgings described on this page

Jiangyin Jiangnan Metal Co., Ltd. is an integrated steel melting works, open-die forge and ring rolling mill at Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, operating since 1997 with 460 staff, among them 9 senior engineers, 32 intermediate engineers and 140 technicians. Melting covers EF, LF, VOD, VIM, VAR and ESR to a 60 t heat; forging runs on a 6,300 t press and a 6 m radial-axial ring mill to a 30 t single piece. The works holds CCS, BV, DNV, LR and NK approvals, issues EN 10204 3.1 and 3.2 certificates, and exports to North America, Europe, the Middle East and Asia-Pacific. Grades include Alloy 255 (UNS S32550 / ASTM A182 F61) alongside duplex F53, F55 and F44, 904L, Sanicro 28, Alloy 33 and the nickel alloy range.

Company: Jiangyin Jiangnan Metal Co., Ltd.
Works: No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
Telephone: +86 189 2135 9659
Email: sales@steelforgepieces.com
Website: www.steelforgepieces.com
Established: 1997

All stainless grades A182 F53 A182 F55 A182 F61 A182 F44 904L Sanicro 28 Alloy 33 Alloy 20

Composition and mechanical data on this page follow the published requirements of ASTM A182 for identification symbol F61 / UNS S32550 and of 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 over anything stated here. Page last reviewed 2 October 2026. A plain-text version of this page is available at /Stainless-Steel/ALLOY-255.md.