Duplex 2205 vs 904L: Which Stainless Steel to Choose

By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012

Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360

Quick Answer: Duplex 2205 vs 904L

Specify duplex 2205 (UNS S32205/S31803) where high strength and weight saving matter in a chloride-bearing service up to about 300 C, as in pressure vessels, tanks and structural components. Specify 904L (UNS N08904) where toughness, weldability and freedom from embrittlement matter in acidic or marine service, as in sulfuric acid plants, heat exchangers and flue gas desulfurisation.

Key Takeaways

• They reach a similar pitting resistance by different routes. Both grades carry a pitting resistance equivalent number of roughly 35, but 2205 gets there through chromium, molybdenum and nitrogen in a duplex structure while 904L gets there through chromium, molybdenum and a high nickel content in a fully austenitic structure.
• 2205 is roughly twice as strong in yield. Duplex 2205 typically delivers a yield strength well above 400 MPa against roughly 220 to 300 MPa for 904L, which allows thinner sections and lower weight in a strength-limited design.
• 904L is the more forgiving material. Being fully austenitic, it is tougher, easier to form, easier to weld and essentially non-magnetic, while duplex 2205 requires control of the ferrite-to-austenite balance in welding and is more sensitive to intermetallic phase formation.
• Temperature limits the duplex grades. Duplex 2205 should not be used above about 300 C in long-term service because of the risk of 475 C embrittlement and sigma-phase formation, whereas 904L has no comparable structural limitation in the same way.
• 904L is the better answer for acid service. Its high nickel and copper content gives it useful resistance in sulfuric and phosphoric acid service where duplex grades are more marginal.
• The decision is usually strength versus toughness and weldability. If the design is strength-limited, 2205 wins on weight and cost; if the design is corrosion-limited in an acid or a welded fabrication, 904L is the safer choice.

Duplex 2205 and 904L Compared

Duplex 2205 and 904L are both high-alloy stainless steels that a buyer reaches for when 316L is not enough, and they are frequently quoted as alternates because their pitting resistance is similar. The similarity is real but superficial, because the two grades achieve it through different metallurgy and therefore behave differently in every respect that matters to a fabricator and to a plant engineer: strength, toughness, weldability, magnetic behaviour, temperature limits and acid resistance.

Duplex 2205, designated UNS S32205 with the closely related UNS S31803, is a duplex stainless steel, which means its microstructure is roughly half ferrite and half austenite. Its composition is built around roughly 22 per cent chromium, 3 per cent molybdenum, 5 to 6 per cent nickel and, critically, about 0.14 to 0.20 per cent nitrogen. The nitrogen is what makes the grade work: it stabilises the austenite during welding, raises the pitting resistance without adding more molybdenum, and contributes to the high strength that duplex grades are known for. The result is a material with roughly twice the yield strength of an austenitic grade at a similar alloy cost, which is why 2205 dominates pressure vessels, storage tanks, chemical tankers, desalination plants and structural components where weight and wall thickness matter.

904L, designated UNS N08904, is a super-austenitic stainless steel, which means its microstructure is fully austenitic and it is stabilised with a high nickel content of roughly 23 to 28 per cent together with chromium, molybdenum and a copper addition. It is the classic material for sulfuric and phosphoric acid service, for pulp and paper bleach plants, for flue gas desulfurisation and for marine and offshore applications where toughness and weldability are required as well as corrosion resistance. Because it is fully austenitic, it is essentially non-magnetic, it has excellent toughness, it welds with none of the phase-balance discipline that duplex grades need, and it is not subject to the embrittlement that limits duplex grades at elevated temperature.

The practical question is therefore rarely "which is more corrosion resistant", because in chloride pitting terms the two are close. The question is which of the two sets of properties the application actually needs, and the answer usually turns on two things: whether the design is limited by strength or by corrosion, and whether the fabrication involves welding at the limits of what duplex can tolerate. Our stainless steel range covers both grades along with 254SMO, AL-6XN, 20Mo6 and 27-7MO for duties that need more than either, and our Hastelloy alloy range covers the nickel alloys for the duties beyond the stainless family altogether.

One further distinction shapes the commercial comparison, and it is worth stating early. Duplex 2205 is a high-volume commodity grade: it is produced in large tonnages, stocked in most forms and sizes, and priced competitively because its alloy content is moderate. 904L is a higher-alloy, lower-volume grade with a much higher nickel content, and it costs materially more per kilogram even though it is the weaker material. That combination, weaker but more expensive, is not a contradiction: 904L is bought for corrosion resistance and fabrication reliability, not for strength, and the buyer who compares the two on price per kilogram alone will misread the decision.

Chemical Composition Compared

The composition table shows the two routes to a similar pitting resistance number. 2205 uses nitrogen and a duplex structure; 904L uses nickel, molybdenum and copper in a fully austenitic structure. The elements that make duplex strong, nitrogen and the ferrite phase, are absent from 904L, and the elements that make 904L resistant in acid service, nickel and copper, are not present in 2205 in the same proportions.

Element (wt %)	Duplex 2205 (UNS S32205)	904L (UNS N08904)	Standard basis	Notes
Cr	approx. 22.0-23.0	approx. 19.0-23.0	ASTM A240 / ASTM B625	pitting resistance in both
Ni	approx. 4.5-6.5	approx. 23.0-28.0	ASTM A240 / ASTM B625	904L is nickel-rich
Mo	approx. 3.0-3.5	approx. 4.0-5.0	ASTM A240 / ASTM B625	higher in 904L
N	approx. 0.14-0.20	max 0.10	ASTM A240 / ASTM B625	the duplex strengthening element
Cu	not specified	approx. 1.0-2.0	ASTM B625	904L acid resistance
C	max 0.030	max 0.020	ASTM A240 / ASTM B625	low carbon in both
Mn	max 2.00	max 2.00	ASTM A240 / ASTM B625	residual
Si	max 1.00	max 1.00	ASTM A240 / ASTM B625	residual
P	max 0.030	max 0.045	ASTM A240 / ASTM B625	residual
S	max 0.020	max 0.035	ASTM A240 / ASTM B625	residual
Fe	balance	balance	ASTM A240 / ASTM B625	both iron-base

Table note: Composition ranges are the published limits of the wrought-product standards named in the Standard basis column, with ASTM A240 covering plate, sheet and strip for the duplex and pressure-vessel grades and ASTM B625 covering the nickel-rich austenitic plate, sheet and strip including UNS N08904; the bar and tubular forms are covered by the standards listed in the Standard Index. Values are shown as published ranges and should be confirmed against the edition named on the purchase order, because a revision can move a limit. The nitrogen range is the single most important entry for duplex 2205, because nitrogen controls both the pitting resistance and the phase balance of the weld, and a heat at the bottom of the nitrogen range can behave noticeably differently from one at the top.

The composition explains the temperature limit that applies to the duplex grade. A duplex microstructure is not thermodynamically stable at all temperatures: above roughly 300 C in long-term service, and much more rapidly at higher temperatures, the ferrite phase tends to transform into embrittling phases such as sigma, and the familiar result is a loss of toughness and a risk of cracking. That is why duplex grades are not specified for hot-wall refinery or high-temperature process duty, and why a grade selection that puts 2205 into a hot stream is a design error rather than a cost saving. 904L has no equivalent structural transformation, because it is fully austenitic, and it can therefore be used in the temperature range where duplex grades must be avoided. This is one of those cases where the cheaper and stronger material is the one with the narrower operating envelope, and the narrowness is a metallurgical fact rather than a limitation of any particular supplier.

Mechanical Properties: Strength, Toughness and Magnetic Behaviour

The mechanical comparison is lopsided in favour of duplex 2205 on strength and in favour of 904L on toughness, and the practical consequences of both are large. The table below sets out typical room-temperature values and the structural characteristics that follow from them, and it should be read alongside the temperature limit discussed above.

Property (typical, annealed)	Duplex 2205 (S32205)	904L (N08904)	Test method	Source type
Tensile strength	approx. 620-700 MPa	approx. 490-620 MPa	ASTM E8/E8M or A370	typical, not a standard minimum
0.2 % yield strength	approx. 450-500 MPa	approx. 220-300 MPa	ASTM E8/E8M or A370	typical, not a standard minimum
Elongation	approx. 25-35 %	approx. 35-45 %	ASTM E8/E8M or A370	typical, not a standard minimum
Impact toughness	good, sensitive to phase balance	excellent	ASTM E23	typical behaviour
Hardness	approx. 250-300 HV	approx. 150-200 HV	ASTM E18	typical, not a standard minimum
Magnetic behaviour	ferromagnetic (approx. half ferrite)	essentially non-magnetic	—	structural characteristic
Long-term service temperature limit	approx. 300 C	no comparable structural limit	—	design guidance
Useful weight saving versus 316L	high, from the higher yield	none from strength	—	design guidance

Table note: Values are typical, from published handbook and mill data, and are explicitly not the minimum values of any standard; a design that depends on a guaranteed minimum must take that minimum from the current edition of the governing standard named on the purchase order, which for plate, sheet and strip is ASTM A240 for duplex 2205 and ASTM B625 for 904L, with the bar and tubular standards named in the Standard Index. Tensile testing is performed to ASTM E8/E8M or ASTM A370, impact testing to ASTM E23, hardness to ASTM E18 and grain or phase assessment by metallographic examination. The two entries with the largest commercial consequences are the yield strength, which roughly doubles between the grades, and the temperature limit, which caps duplex 2205 at around 300 C in long-term service.

The strength advantage is why duplex 2205 is often the cheaper answer even though the two grades cost similar amounts per kilogram in some forms. A pressure vessel or a tank designed in 904L is designed on a yield strength of roughly 220 to 300 MPa, whereas the same vessel designed in 2205 is designed on roughly 450 to 500 MPa, and because the required wall thickness is inversely related to the allowable stress, the duplex vessel can be substantially thinner. The thinner wall saves material, saves weight, saves welding time and reduces the cost of supports and foundations, and in a large tank or vessel those downstream savings can exceed the difference in material price. The advantage disappears where the design is governed not by pressure but by corrosion allowance or by stiffness, because in those cases the thickness is set by something other than strength and the duplex advantage in strength buys nothing.

The toughness advantage of 904L matters in a different set of circumstances. A fully austenitic material retains excellent toughness at low temperature, tolerates impact and thermal shock, and accommodates the strain of forming and welding without the phase-balance problems that duplex grades present. That is why 904L is the more comfortable choice for a welded fabrication that must pass impact testing, for a component that will see thermal cycling, and for a part whose failure mode must be leak-before-break rather than fast fracture. The magnetic difference is a smaller but occasionally decisive point: 2205 is ferromagnetic because roughly half its structure is ferrite, and 904L is essentially non-magnetic, so an application that forbids magnetic material, such as certain instrument or sensor components, cannot use duplex 2205 at all.

Heat Treatment, Forming and Welding

The fabrication behaviour is where the two grades diverge most sharply in day-to-day shop practice, and it is the area where a design decision made on corrosion data alone can turn into a production problem. Duplex 2205 is formable and weldable, but it demands discipline; 904L is one of the more forgiving high-alloy stainless steels to fabricate.

Process	Duplex 2205 (S32205)	904L (N08904)	Standard basis	Notes
Solution anneal	approx. 1020-1100 C, rapid quench	approx. 1050-1150 C, rapid quench	ASTM A240 / ASTM B625	controls phase balance
Avoid temperature band	long exposure near 300-950 C	no comparable structural band	—	sigma and 475 C embrittlement
Cold forming	possible; higher springback	easy; low springback	mill practice	duplex needs more force
Hot forming	requires close control and re-anneal	more tolerant	mill practice	phase balance in duplex
Welding	control heat input and interpass temperature	straightforward, matching filler	ASME Section IX	nitrogen in shielding gas for duplex
Filler metal	matching duplex or over-alloyed	matching 904L or higher alloy	ASME Section IX	preserve corrosion resistance
Post-weld heat treatment	usually none; solution anneal if required	usually none	mill practice	re-anneal if duplex phase balance lost
Machining	work hardens; positive feeds	gummy; austenitic behaviour	mill practice	both differ from carbon steel

Table note: The process bands are normal shop practices given as planning guidance rather than standardised values, and the controlling requirements are those of the governing product standard and of the fabrication code, normally ASME Section IX for procedure and operator qualification. The duplex entries reflect the fact that the properties of duplex 2205 depend on the balance between ferrite and austenite, which welding disturbs and which must be restored by the right filler, the right heat input and, where necessary, a solution anneal. The 904L entries reflect the behaviour of a fully austenitic material, which is easier to weld and to form but more prone to distortion and to galling, so the shop controls shift from phase balance to restraint and to surface protection.

Welding is the clearest illustration. For duplex 2205 the welding procedure must preserve the phase balance in the weld and the heat-affected zone, which usually means a matching or over-alloyed filler, a nitrogen-bearing shielding gas, controlled heat input and a controlled interpass temperature, together with limits on how long the material may remain in the embrittling temperature band. A procedure that ignores those points can pass visual and radiographic inspection and still produce a weld with poor toughness and reduced corrosion resistance, because the problem is metallurgical rather than geometric. For 904L the welding problem is the ordinary one of an austenitic steel: manage distortion through restraint and sequencing, use a matching or higher-alloy filler, protect the surface from contamination, and avoid the sensitisation that a high-carbon austenitic grade would suffer, which is straightforward in a low-carbon grade such as 904L. The practical consequence is that a fabricator who is not experienced with duplex will usually find 904L easier to deliver, and the price difference between the grades is partly the price of that fabrication risk. Our knowledge base sets out the welding and phase-balance controls that a duplex fabrication should document.

Corrosion Performance: Chloride, Acid and Seawater Duty

In chloride pitting terms the two grades are close, which is exactly why the decision usually falls to other properties. In acid service, however, they are not close, and the difference is the reason 904L retains its position despite being weaker and more expensive. The table below summarises the comparison by environment and is written as engineering guidance rather than as a guarantee.

Service condition	Recommendation	Reason	Test / basis
Seawater, ambient, flowing	either; 2205 for strength	both resist chloride pitting	ASTM G48
Seawater, chlorinated, elevated temperature	2205 preferred up to its limit	higher strength permits thinner section	ASTM G48
Sulfuric acid, dilute to moderate	904L preferred	nickel and copper content gives acid resistance	ASTM G28 / G48
Phosphoric acid service	904L preferred	proven in acid plant duty	ASTM G28
Flue gas desulfurisation, mixed duty	904L often preferred	toughness and welding reliability	ASTM G48
Pulp and paper bleach plant	904L preferred	acid and chloride combination	ASTM G48
Pressure vessel, chloride service, under 300 C	2205 preferred	strength allows thinner wall	ASTM A240 design basis
Hot process stream above 300 C	904L required	duplex embrittlement limit	design guidance
Structural component, weight-critical	2205 preferred	high yield strength	ASTM A240 design basis
Strongly reducing acid	904L or a nickel alloy	stainless limits reached	ASTM G28

Table note: The recommendation column is engineering guidance based on the metallurgy of the two grades and on the corrosion test standards named in the final column, and it is not a substitute for a corrosion assessment of the specific stream. ASTM G48 covers pitting and crevice corrosion testing and ASTM G28 covers intergranular corrosion testing, and where a service is governed by a proprietary or industry specification, such as a refinery or offshore material selection document, that document governs. Where a stream is beyond the reach of both grades, the correct move is to a nickel alloy such as Hastelloy C-276, Inconel 625 or Monel 400 rather than to a higher condition of either stainless steel.

Two conclusions follow. The first is that the chloride comparison between these grades is not decisive, because both sit at a pitting resistance equivalent number of roughly 35 and a plant that is choosing between them is usually choosing on strength, toughness, temperature or weldability rather than on a narrow pitting margin. The second is that the acid comparison is decisive, because in sulfuric and phosphoric acid service the nickel-rich, copper-bearing austenitic grade is the proven material and the duplex grades are more marginal, which is why 904L remains the specification of record in acid plants even where the walls could be thinner in duplex. A third point is worth adding for plants that are standardising: 2205 is the better choice for the structural and pressure-retaining parts of a chloride-bearing process, and 904L is the better choice for the acid-wetted and welded parts, and a plant that uses both deliberately will usually spend less than one that uses either across the board.

Standards and Specifications

The two grades are bought under different standard families, and the difference reflects their metallurgy: duplex 2205 is covered by the general stainless-steel standards, while 904L is covered by the nickel-alloy standards because of its high nickel content. The table below maps the standards for each grade and form.

Area	Duplex 2205 (S32205)	904L (N08904)	Covers	Form
Plate, sheet and strip	ASTM A240	ASTM B625	composition + mechanical	plate, sheet, strip
Bar and rod	ASTM A276 / ASTM A479	ASTM B649	composition + mechanical	bar, rod
Seamless and welded tube	ASTM A789	ASTM B677	composition + mechanical	tube
Pipe	ASTM A790	ASTM B673	composition + mechanical	pipe
Forgings	ASTM A182	ASTM B564	composition + mechanical	forging
Fittings	ASTM A815	ASTM B366	composition + mechanical	fittings
ASME equivalent	ASME SA240 / SA789 / SA790	ASME SB625 / SB649 / SB677	composition + mechanical	all forms
European	EN 10088 / EN 10216-5	EN 10088 where listed	composition + properties	plate, tube
Corrosion testing	ASTM G48 / ASTM G28	ASTM G48 / ASTM G28	test method	all forms
Toughness testing	ASTM E23	ASTM E23	test method	all forms
Inspection documents	EN 10204	EN 10204	documents	all forms

Table note: The table names standards by number and scope, and where an edition year is not quoted the controlling document is the one named on the purchase order. The most important structural point is that 904L is governed by the nickel-alloy standards (ASTM B625 for plate, B649 for bar, B677 for tube, B673 for pipe and B366 for fittings) rather than by the stainless-steel series, because of its nickel content, so a specification that names a stainless-steel standard for 904L is incomplete. Where the ASME code governs a pressure-retaining fabrication, the S-prefixed equivalent is called up and the code's own design and testing requirements are added, and where a European project applies EN 10088 or EN 10216-5 the grade designations are numeric and equivalence must be confirmed rather than assumed.

Two procurement consequences follow. First, because the grades sit in different standard families, a bill of materials that mixes them must name the correct document for each line rather than applying one house standard across the package, and a supplier who quotes both from a single standard is signalling that the enquiry was not read carefully. Second, the acceptance tests should be matched to the requirement: hardness and phase balance for duplex 2205 where the phase balance is a design assumption, impact toughness to ASTM E23 where toughness matters, and corrosion testing to ASTM G48 or G28 where the corrosion resistance is the reason for the grade. Naming the test alongside the standard is what converts a material specification into something that can be verified on delivery.

Selection Guide: Vessels, Heat Exchangers and Acid Service

The selection logic is easiest to apply by asking two questions in order: is the design strength-limited or corrosion-limited, and does the service temperature exceed about 300 C. The table below turns that logic into a component-level guide.

Application	Recommended grade	Why	Alternative considered
Pressure vessel, chloride service, under 300 C	duplex 2205	high yield strength permits thinner wall	316L for lower duty, 904L if hot
Storage tank, chemical, ambient	duplex 2205	strength and pitting resistance	316L for benign duty
Heat exchanger, seawater, ambient	duplex 2205	strength and pitting resistance	904L for welded or acid duty
Heat exchanger, sulfuric acid	904L	acid resistance	Hastelloy for stronger acid
Sulfuric acid plant piping and tanks	904L	proven acid service material	high-nickel alloy for stronger acid
Phosphoric acid service	904L	acid resistance	nickel alloy for stronger duty
Flue gas desulfurisation absorber	904L	toughness and welding reliability	2205 for less aggressive zones
Process stream above 300 C	904L	duplex embrittlement limit	nickel alloy for higher duty
Structural, weight-critical, chloride	duplex 2205	high strength to weight	316L for lower duty
Strongly reducing acid	904L or a nickel alloy	stainless limits reached	Hastelloy C-276

Table note: A component-level guide, not a code or a design basis; the recommended grade must be confirmed against the governing specification of the equipment and against the actual temperature, pressure and chemistry, and the alternatives listed indicate the boundary of the two grades rather than being an exhaustive list. The temperature branch is the one most often overlooked: a selection that is technically sound on corrosion grounds can still be wrong if it puts duplex 2205 into a long-term service above about 300 C, because the failure mode in that case is embrittlement rather than corrosion.

The most common selection error in this pair is to treat duplex 2205 as a universal upgrade from 316L and to apply it across a whole plant, including the hot and acid-wetted parts where it is the wrong answer. The second most common error is the reverse: to specify 904L everywhere because it is the more forgiving material to weld, which adds cost and gives up the weight and strength advantage that duplex would have delivered in the structural and pressure-retaining parts. The efficient answer for most chloride-bearing plants is a deliberate split: duplex 2205 for the pressure-retaining and structural components where strength and weight matter, and 904L or a nickel alloy for the acid-wetted, welded and hot components where toughness and acid resistance matter. Where the streams go beyond both grades, the correct move is to a nickel alloy rather than to a more expensive stainless steel, and our Hastelloy alloy range covers the grades that take over at that point.

Price, Availability and Procurement Notes for 2026

Price follows the alloy content, and the nickel content of 904L is the reason it costs materially more than duplex 2205 despite having lower strength. The figures below are reference ranges only and they are not a quotation; the contract that governs is the one signed with the supplier.

Item	Grade	Reference range, 2026, EXW Shanghai	Basis
Plate, 3-20 mm	Duplex 2205	USD 8-16/kg	2026, EXW Shanghai, reference range only
Plate, 3-20 mm	904L	USD 22-38/kg	2026, EXW Shanghai, reference range only
Round bar, 20-100 mm	Duplex 2205	USD 10-18/kg	2026, EXW Shanghai, reference range only
Round bar, 20-100 mm	904L	USD 26-45/kg	2026, EXW Shanghai, reference range only
Seamless tube, 19-38 mm OD	Duplex 2205	USD 12-22/kg	2026, EXW Shanghai, reference range only
Seamless tube, 19-38 mm OD	904L	USD 30-55/kg	2026, EXW Shanghai, reference range only
Nickel surcharge exposure	904L	high, from the nickel content	reference range only
Molybdenum surcharge exposure	both	approx. 8-14 % of base metal cost	reference range only

Table note: Reference ranges only, 2026, EXW Shanghai, in USD per kilogram, and subject to movement in the nickel, molybdenum and chromium markets; these figures are indicative and are not a quotation. Delivered prices depend on quantity, specification, form, tolerance, test scope, documentation, delivery terms and on whether the material is stock or a mill order, and the percentage figures are planning conventions rather than published rates. The price gap between the two grades is the reason the strength comparison matters: because 904L is both weaker and more expensive per kilogram, a strength-limited design in 904L is expensive twice over, once in the price and once in the extra thickness it requires.

Three procurement points complete the picture. First, compare a design rather than a price: the correct comparison between these grades is the cost of a finished vessel or component designed in each grade, not the price per kilogram, because the duplex option can be thinner and lighter and may win on total cost even where the price per kilogram is close. Second, expect the availability of the two grades to differ: duplex 2205 is a commodity grade with a deep stock position, while 904L is a lower-volume grade with a narrower stock position, particularly in heavy plate and large tubular sizes, so a mill order and its lead time should be planned. Third, require a documentary package matched to the grade: an inspection document to EN 10204 3.1, heat-number traceability, positive material identification to ASTM E1476 to confirm the alloy and to separate the duplex and austenitic families, impact testing to ASTM E23 where toughness matters, a phase-balance check for the duplex grade where it is a design assumption, and corrosion testing to ASTM G48 or G28 where the corrosion resistance is the reason for the grade. Our news and buying guides track which grades and forms are tight in a given quarter, and our contact page is the place to send a package list for a stock-versus-mill-order assessment.

Standard Index

Standard	Title / scope	Covers	Form
ASTM A240	Chromium and chromium-nickel stainless steel plate, sheet and strip for pressure vessels	composition + mechanical	plate, sheet, strip
ASTM A276	Stainless steel bars and shapes	composition + mechanical	bar, shapes
ASTM A479	Stainless steel bars and shapes for use in boilers and pressure vessels	composition + mechanical	bar
ASTM A789	Seamless and welded ferritic/austenitic stainless steel tubing	composition + mechanical	tube
ASTM A790	Seamless and welded ferritic/austenitic stainless steel pipe	composition + mechanical	pipe
ASTM A182	Forged or rolled alloy steel pipe flanges, fittings and valves	composition + mechanical	forging, flange
ASTM A815	Wrought ferritic, ferritic/austenitic and martensitic stainless steel piping fittings	composition + mechanical	fittings
ASTM B625	Ni-Fe-Cr-Mo-Cu low-carbon alloy plate, sheet and strip (incl. UNS N08904)	composition + mechanical	plate, sheet, strip
ASTM B649	Ni-Fe-Cr-Mo-Cu low-carbon alloy rod and bar (incl. UNS N08904)	composition + mechanical	bar, rod
ASTM B677	Ni-Fe-Cr-Mo-Cu low-carbon alloy seamless tube and tube	composition + mechanical	tube
ASTM B673	Ni-Fe-Cr-Mo-Cu low-carbon alloy welded pipe	composition + mechanical	pipe
ASTM B366	Nickel alloy fittings	composition + mechanical	fittings
ASME SA240 / SA789 / SA790	ASME equivalents of the duplex product standards	composition + mechanical	all forms
ASME SB625 / SB649 / SB677	ASME equivalents of the 904L product standards	composition + mechanical	all forms
ASTM E8/E8M	Tension testing of metallic materials	test method	all forms
ASTM A370	Mechanical testing of steel products	test method	all forms
ASTM E23	Notched bar impact testing	test method	all forms
ASTM E18	Rockwell hardness testing	test method	all forms
ASTM G48	Pitting and crevice corrosion testing	test method	all forms
ASTM G28	Intergranular corrosion testing of Ni-base alloys	test method	all forms
ASTM E1476	Metals identification and PMI	test method	all forms
EN 10088	Stainless steels, technical delivery conditions	composition + properties	plate, bar
EN 10216-5	Seamless steel tubes for pressure purposes, stainless	composition + properties	tube
EN 10204	Inspection document types (2.2, 3.1, 3.2)	inspection documents	all forms
ISO 9001	Quality management systems	system certification	all forms

Table note: Standards are listed by number and scope; where an edition year is not quoted the latest edition applies and the controlling edition is the one named on the purchase order. The list is deliberately split between the stainless-steel series for duplex 2205 and the nickel-alloy series for 904L, because the high nickel content of 904L places it in the B-series rather than the A-series, and a specification that ignores that distinction is incomplete. Where a grade is quoted by trade name alone without a UNS number, a product standard and a condition, the quotation cannot be verified against a specification.

FAQ

Q1: What is the main difference between duplex 2205 and 904L?

The main difference is metallurgical: duplex 2205 is a two-phase material, roughly half ferrite and half austenite, while 904L is fully austenitic. Duplex 2205 uses nitrogen and its duplex structure to achieve strength and pitting resistance, which gives it roughly twice the yield strength of 904L but also makes it sensitive to embrittlement above about 300 C in long-term service and requires careful control of the phase balance when welding. 904L uses a high nickel content together with molybdenum and copper to achieve corrosion resistance in a fully austenitic structure, which gives it excellent toughness, easy welding and resistance in acid service but a much lower strength. In short, 2205 is the strong, weight-efficient duplex grade with a temperature ceiling, and 904L is the tough, forgiving, acid-resistant austenitic grade. They reach a similar pitting resistance number by different routes, which is why the choice usually falls to strength, toughness, temperature or weldability rather than to pitting alone.

Q2: Which grade is stronger, duplex 2205 or 904L?

Duplex 2205 is substantially stronger, typically delivering a yield strength in the region of 450 to 500 MPa against roughly 220 to 300 MPa for 904L, which is roughly double. That difference is the whole commercial case for duplex in pressure-retaining and structural applications, because the wall thickness required for a given pressure falls as the allowable stress rises, so a vessel or tank designed in 2205 can be materially thinner and lighter than the same vessel designed in 904L. The saving is not limited to the material: a thinner wall reduces weight, welding time, support steel and foundation cost, and in a large tank or vessel those downstream savings can exceed the difference in material price. The advantage disappears where the design is not strength-limited, for example where the thickness is set by a corrosion allowance or by stiffness rather than by pressure, because in that case the extra strength buys nothing.

Q3: Can duplex 2205 be used at high temperature?

Duplex 2205 should not be used in long-term service above about 300 C, because above that temperature the ferrite phase tends to transform into embrittling phases such as sigma, and the result is a loss of toughness and a risk of cracking. The transformation is slow at the lower end of the range and faster as the temperature rises, so the limit is a matter of time as well as temperature, and short excursions are more tolerable than continuous operation. The practical rule is that a duplex grade is not the right choice for hot-wall refinery, petrochemical or high-temperature process duty, and that a design which places 2205 in such a stream is a design error rather than a cost saving. Where a high-temperature chloride-bearing service must be met with a stainless steel rather than a nickel alloy, a fully austenitic super-austenitic grade such as 904L is the more suitable choice, because it has no comparable structural transformation.

Q4: Which grade is better for sulfuric acid service?

904L is the better and the proven choice for sulfuric acid service, which is one of the applications the grade was developed for. Its high nickel content together with its molybdenum and copper additions gives it useful resistance across dilute to moderately concentrated sulfuric acid service where the duplex grades are more marginal, and it has a long service record in acid plants, in phosphoric acid duty and in related chemical service. Duplex 2205 may be acceptable in some dilute or contaminated sulfuric acid streams, but it is not the grade of record for that service and the decision should be made against a specific corrosion assessment rather than by analogy. Where the acid is stronger, hotter or more aggressive than 904L can handle, the correct move is to a nickel alloy such as Hastelloy C-276 or Alloy 20, both of which are covered in our Hastelloy alloy range, rather than to a higher condition of a stainless steel.

Q5: How do duplex 2205 and 904L differ in welding?

Welding duplex 2205 is a controlled process: the welding procedure must preserve the balance between ferrite and austenite in the weld and the heat-affected zone, which usually means a matching or over-alloyed filler, a nitrogen-bearing shielding gas, controlled heat input and a controlled interpass temperature, together with limits on the time the material spends in the embrittling temperature band. A procedure that ignores those points can pass inspection and still produce a weld with poor toughness and reduced corrosion resistance, because the issue is metallurgical rather than geometric. Welding 904L is far more straightforward, because a fully austenitic low-carbon grade presents the ordinary austenitic welding problems of distortion and contamination rather than a phase-balance problem, and the filler is a matching 904L or a higher alloy. The consequence is that a fabricator without duplex experience will usually deliver a 904L fabrication more reliably, and part of the price difference between the grades is the price of that fabrication risk.

Q6: Which grade should I use for a seawater heat exchanger?

For a seawater heat exchanger at ambient temperature, duplex 2205 is a common and economic choice because its combination of pitting resistance and high strength allows a thinner wall and a lighter tube bundle, and its strength helps where the tubes are subject to vibration. Where the exchanger is welded into a fabrication that must pass impact testing, where the seawater is aggressive and chlorinated at temperature, or where the unit is in acid or mixed service, 904L becomes the more suitable answer because of its toughness, its welding reliability and its broader corrosion envelope. Many plants use both, with duplex for the structural and pressure-retaining parts and an austenitic or nickel alloy for the wetted tubes and for the acid-wetted zones. The correct choice depends on the actual seawater chemistry, the chlorination regime, the temperature and the design pressure, and the material selection should be made against those data rather than against a single generic recommendation.

Q7: Are duplex 2205 and 904L magnetic?

Duplex 2205 is ferromagnetic when measured with a magnet, because roughly half of its microstructure is ferrite, and that is a normal and expected characteristic rather than a defect. 904L is essentially non-magnetic, because it is fully austenitic, although a heavily cold-worked surface can become slightly magnetic. The magnetic difference is decisive only in a minority of applications, such as instrument components, sensor bodies or parts that must be located or identified by magnetic means, but where it applies it excludes duplex 2205 entirely. For most process and structural applications the magnetic behaviour is irrelevant to the design and should not be used as a selection criterion; the more important physical differences are the strength, the toughness and the temperature limit, which affect the design directly.

Q8: Can I substitute duplex 2205 for 904L to reduce cost?

You can sometimes, and in a strength-limited application the substitution can reduce cost substantially, but it must be justified against the properties that govern the part. If the part was specified in 904L for acid resistance, for toughness, for welded-fabrication reliability or for a service temperature above about 300 C, then duplex 2205 is a downgrade on the governing property and the substitution may be unsafe even though 2205 is the stronger material. If the part was specified in 904L only because it was available or because the specification was written conservatively, and the duty is a chloride-bearing service at moderate temperature, then 2205 may serve well and save weight and cost. The correct procedure is to identify which property governs, confirm that 2205 satisfies it with an adequate margin, and check the fabrication route for duplex compatibility, and to treat the change as an engineering substitution rather than a purchasing decision.

Q9: What tests should I require when buying duplex 2205?

Require the standard mechanical package plus the tests that support the specific reason for choosing the grade. The baseline is an inspection document to EN 10204 3.1 with heat-number traceability, positive material identification to ASTM E1476 to confirm the grade and to separate the duplex family from the austenitic grades, tensile testing to ASTM E8/E8M or ASTM A370, and hardness testing to ASTM E18. Where toughness matters, require notched bar impact testing to ASTM E23 at the specified temperature, and where the corrosion resistance is the governing property, require pitting or intergranular corrosion testing to ASTM G48 or G28. Where the phase balance is a design assumption, require the ferrite content to be reported from a metallographic or magnetic measurement, because an out-of-balance microstructure is the most likely hidden defect in a duplex supply. Naming those tests on the enquiry is what makes the acceptance criteria verifiable rather than assumed.

Q10: What is the pitting resistance equivalent number and why does it matter here?

The pitting resistance equivalent number, usually abbreviated PREN, is an empirical ranking that weights the elements that resist pitting corrosion, most commonly calculated from chromium plus 3.3 times molybdenum plus 16 times nitrogen. It is a comparative tool rather than a guarantee, but it is useful because it explains why two chemically different grades can have similar pitting resistance. Duplex 2205 and 904L both land at roughly 35, reached in the duplex by a moderate molybdenum level together with a substantial nitrogen addition and in the austenitic grade by a higher molybdenum level with almost no nitrogen. The practical consequence is that pitting resistance is unlikely to be the deciding difference between these two grades, so the choice should be made on strength, toughness, temperature, weldability and acid resistance instead, which is where the two grades genuinely differ.

Q11: How does Shanghai Hangbo Alloy Group supply duplex 2205 and 904L?

Shanghai Hangbo Alloy Group Co., Ltd. supplies duplex 2205 and 904L in plate, sheet, strip, bar, seamless tube, welded pipe and fittings, together with the higher super-austenitic grades such as 254SMO, AL-6XN, 20Mo6 and 27-7MO and the nickel alloys for duties beyond the stainless family. We issue an inspection document to EN 10204 3.1 with third-party inspection by SGS, BV or TUV and positive material identification to ASTM E1476 on delivery, and we operate an ISO 9001:2015 quality system. For duplex material we report the phase balance where it is a design assumption, and for both grades we will discuss the actual stream chemistry and the service temperature and will say plainly when a nickel alloy is the better answer rather than quoting a stainless steel that will not last. Send the forms, quantities, service conditions and any fabrication requirements through our contact page and we will return a recommendation with the standards and the lead time stated.

Conclusion: Strength or Toughness, Temperature Decides

Duplex 2205 and 904L are both high-performance stainless steels that a buyer reaches for when 316L is not enough, and they are not interchangeable even though their pitting resistance is similar. Duplex 2205 is the strong, weight-efficient duplex grade: its nitrogen-bearing two-phase structure gives it roughly twice the yield strength of 904L, which allows thinner and lighter pressure-retaining and structural components, but it carries a long-term service limit of about 300 C because of embrittlement and it demands disciplined welding to preserve the phase balance. 904L is the tough, forgiving austenitic grade: its high nickel content with molybdenum and copper gives it resistance in sulfuric and phosphoric acid service, and its fully austenitic structure gives it excellent toughness, easy welding and freedom from the duplex temperature limit, at the cost of much lower strength and a higher price per kilogram.

The decision sequence is therefore short. First, check the service temperature: above about 300 C in long-term service, duplex 2205 is excluded and the choice moves to 904L or to a nickel alloy. Second, ask whether the design is strength-limited: if it is, duplex 2205 offers real savings in weight and wall thickness, and if it is not, the extra strength buys nothing. Third, ask whether the service is acid-wetted or requires high toughness or a heavily welded fabrication: if it does, 904L is the safer material and the price difference is the price of that safety. Where the stream is beyond both grades, move to a nickel alloy rather than to a higher condition of a stainless steel. Shanghai Hangbo Alloy Group Co., Ltd. supplies both grades along with the super-austenitic and nickel-alloy families with ISO 9001:2015 process control, mill test certification to EN 10204 3.1, third-party inspection by SGS, BV or TUV, and PMI verification to ASTM E1476 on delivery. Send your forms, quantities and service conditions through our contact page and we will return a grade recommendation with the standards and the lead time stated.

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Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier

Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com

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Ni: NI200-N6 | Monel: 400-502, K-500, R-405 | Inconel: 600-740H, X-750, 725 | Incoloy: 800-27-7MO | Hastelloy: C-276-HYBRID-BC1, N | Super Austenitic: 904L-AL-6XN | Chromium-Rich: Alloy 33 | High-Strength Stainless: Nitronic 50/60, PH13-8Mo | Chlorination: Alloy 45 | Nimonic: 75-263 | Haynes: 25-HR-160 | RA: 330/333 | Precision: 1J22-4J45 | High-Strength: 18Ni250-AerMet100 | Heating: Cr20Ni80/60 | Specialty: Alloy 20-602CA

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