Key takeaways
- Duplex stainless steel scrap earns its value only when sold as an identified grade to a buyer who credits chromium and molybdenum.
- Duplex is magnetic, so a magnet pulls it out of 304 and 316; an XRF reading then separates 2205, 2507 and S32760.
- Copper and tungsten at about 0.75 per cent each identify S32760; forging stamps F51, F60, F53 and F55 identify the grade directly.
- Duplex ships as stainless scrap under HS 7204.21, so the grade has to be carried on the invoice and package labels.
Duplex stainless steel scrap comes mostly out of seawater and hydrocarbon service: offshore topsides, firewater rings, desalination plant and chemical tankers. It looks like any other stainless pipe or plate, which is exactly the problem. Thrown into a general stainless pile, a length of super duplex is weighed and paid for as ordinary stainless, and the chromium and molybdenum it carries go unrecognised.
This guide is for project scrap managers, fabricators and dealers who handle duplex alongside 304 and 316. It covers what duplex is in scrap terms, why it is not simply “better 316”, how a magnet and an XRF reading sort it in minutes, how forging stamps identify the grade, and how to prepare a lot a melt shop will take as duplex.
What Counts as Duplex Stainless Steel Scrap?
Duplex stainless steel scrap is any stainless steel with a two-phase structure of austenite and ferrite grains, usually close to half and half. It sits between the familiar austenitic grades and the nickel alloys, and it is traded in three tiers.
| Tier | Grade and UNS | Forging designation | Nominal chemistry that matters |
|---|---|---|---|
| Lean duplex | 2101 (S32101), 2304 (S32304) | – | Low molybdenum, under 1% |
| Standard duplex | Duplex 2205 (S32205 / S31803) | F60 / F51 | About 22.5% Cr, 5.5% Ni, 3.1% Mo |
| Super duplex | Super duplex 2507 (S32750) | F53 | About 25% Cr, 7% Ni, 4% Mo |
| Super duplex | Super duplex S32760 | F55 | About 25% Cr, 7% Ni, 3.5% Mo, 0.75% Cu, 0.75% W |
254 SMO (S31254, F44) often travels in the same lots and the same trade category, but it is a super-austenitic stainless, not a duplex: it is non-magnetic and carries about 18% nickel and 6% molybdenum. Describe it separately.
Nominal values from the grade pages; specification limits vary by product form.
Is Duplex Stainless Steel Scrap Worth More than 316 Scrap?
Duplex scrap is usually worth more than 316 when it is sold as identified duplex to a buyer who can use its chromium and molybdenum. Sold as generic stainless, it may be worth no more, because standard duplex carries less nickel than 316, and nickel is the element most stainless scrap pricing follows.
The comparison shows the trade-off. Duplex 2205 has about half the nickel of 316 but more chromium and more molybdenum. A melt shop producing duplex or other high-molybdenum grades values exactly those elements, and it also needs to know the scrap is duplex so it can manage nitrogen and the phase balance of the new heat. A buyer pricing only on nickel sees 2205 as a low-nickel stainless. Your job as a seller is to make sure the lot reaches the first kind of buyer with the evidence they need.
Super duplex 2507 and S32760 carry more of everything: more chromium, more molybdenum and more nickel than 2205. That is why they should never share a lot with 2205, let alone with 316. For how stainless grades are priced generally, see what decides the stainless steel scrap rate.
Is Duplex Stainless Steel Magnetic?
Yes. Duplex and super duplex stainless steels are clearly magnetic because of their ferrite, while 304, 316, 254 SMO and 904L are non-magnetic when annealed. That makes a magnet the fastest first sort in a stainless yard: anything stainless that pulls is either duplex, a ferritic 400-series grade or carbon steel.
The magnet cannot do the rest. It does not separate 2205 from 2507, it does not separate duplex from ferritic stainless, and heavily cold-worked 304 or 316 can pick up a slight pull. Use the magnet to build a short pile of suspects, then read each piece. The magnet test article covers its limits across all the families.
- Non-magnetic?Yesaustenitic stainless or nickel alloy. Read it as 304, 316, 254 SMO or 904L
- Magnetic and mostly iron with little or no chromium?Yescarbon or low-alloy steel. Remove it
- Magnetic with chromium but almost no nickel?Yesferritic stainless, 400 series
- Copper and tungsten each near 0.75 per cent with 25 per cent chromium?Yessuper duplex S32760
- About 25 per cent chromium and 4 per cent molybdenum?Yessuper duplex 2507
- About 22.5 per cent chromium and 3 per cent molybdenum?Yesduplex 2205
- Molybdenum under 1 per cent?Yeslean duplex. Record the grade from markings
- Anything else: hold apart and confirm by OES
How Do You Tell 2205, 2507 and S32760 Apart?
Chromium and molybdenum separate 2205 from the super duplex grades, and copper and tungsten separate S32760 from 2507. On a clean spot, a handheld XRF reads about 22.5% chromium and 3% molybdenum for 2205, against 25% chromium and around 4% molybdenum for 2507. S32760 reads like 2507 but shows copper and tungsten at about 0.75% each, which 2507 does not contain.
Two points catch people out. First, nitrogen, which is an important alloying element in every duplex grade, cannot be measured by handheld XRF. That is also why S31803 and S32205, which differ mainly in their limits on chromium, molybdenum and nitrogen, are traded as one grade. Second, the small copper and tungsten peaks in S32760 need a reasonable count time; a two-second reading on a painted pipe may miss them. Clean the spot and read for longer. The guide to handheld XRF explains surface preparation.
For the full element-by-element view, see 2205 vs 2507 and 2507 vs S32760, or put any two grades side by side in the alloy comparison tool.
What PREN Tells You, and What It Does Not
Engineers rank these grades by their pitting resistance equivalent number, PREN = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N. Calculated from nominal compositions, 2205 comes out around 35, 2507 around 43 and S32760 around 42, against roughly 24 for 316. It explains why super duplex is specified for seawater, and why a buyer is interested in super duplex scrap. It is not a scrap grading tool: you cannot calculate it from a handheld reading, because nitrogen is missing.
Where Does Duplex Scrap Come from?
Duplex scrap comes mostly from replacement and decommissioning of seawater and process systems, plus offcuts from the fabricators who build them.
- Offshore platforms and FPSO topsides. Super duplex seawater injection, firewater and cooling piping, valves and manifolds. See oil and gas scrap.
- Desalination plant. Super duplex high-pressure piping and 254 SMO intake systems, released during rebuilds. See desalination plant scrap.
- Chemical tankers and storage. 2205 plate from tank and cargo system work.
- Pulp and paper, and chemical plant. 2205 and lean duplex vessels and piping.
- Fabrication shops. Plate crops, pipe offcuts and burning skeletons, often the cleanest duplex scrap there is, because the grade is known from the cutting list.
How Do Forging Stamps and Markings Identify Duplex Scrap?
A forging stamp showing ASTM A182 with an F-number identifies the duplex grade directly: F51 or F60 for 2205, F53 for 2507 and F55 for S32760, with F44 marking 254 SMO. Flanges, fittings and valve bodies usually carry these stamps even after years in service.
A182forging specificationF53grade designation for S32750S32750UNS number, often stamped as wellHeat no.links the flange to its certificatePlate is made to ASTM A240 and pipe to A790, usually stencilled with the UNS number. A summary of the ASTM A specifications for duplex explains each one. Markings plus a reading on the same piece is the evidence a buyer wants; see positive material identification for scrap lots for how readings are recorded.
How Should a Duplex Scrap Lot Be Prepared?
Prepare duplex so each lot is one grade, free of carbon steel and other stainless grades, and free of coatings and insulation that hide what it is. Process piping is rarely one material: a super duplex spool may carry 316 instrument tubing, carbon steel supports and flanges in a different grade.
- Pipe and spools. Cut off supports and small-bore 316 connections, and read every flange separately rather than assuming it matches the pipe. See Duplex 2205 pipe scrap and S32760 pipe scrap.
- Plate. Keep fabrication crops by grade and thickness; remove primer where it hides stencils. See 2507 plate scrap.
- Valves and pumps. Strip trim, seats and bolting, which are often other alloys.
- Turnings. One grade per bin, dry and free of carbon steel chips.
- Offshore material. Remove paint systems, passive fire protection and marine growth where practical, and declare what remains.
Send the grade, form, approximate quantity and location.
- Grade split2205, 2507, S32760 and 254 SMO each in their own lot
- No austenitic stainless304 and 316 removed, including small-bore tubing
- No carbon steelsupports, clamps and bolting cut off
- Flanges read separatelyF-numbers recorded and photographed
- Readings with element valueschromium, molybdenum, copper and tungsten shown
- Coatings declaredpaint, fire protection and insulation removed or noted
Which Mistakes Cost Duplex Sellers the Most?
Most value lost on duplex stainless steel scrap comes from treating it as ordinary stainless at some point between the plant and the buyer.
- Letting it reach the general stainless pile. Once super duplex offcuts are mixed into baled 300-series, nobody will pay to find them again. Separate at the point of generation.
- Mixing 2205 with super duplex. The two look identical and often meet in the same project, with 2205 in utilities and super duplex in seawater lines. Mixed, the lot is valued at the lower grade.
- Assuming the flange matches the pipe. Spools are regularly fitted with flanges in a different duplex grade, or in 316. Each stamp takes seconds to photograph.
- Reading through paint. Offshore coatings can mask molybdenum and hide the copper and tungsten that identify S32760. Grind a small spot first.
- Calling 254 SMO “super duplex”. It is non-magnetic and chemically different; describing it correctly keeps the buyer’s confidence in the rest of your list.
How Is Duplex Scrap Classified and Shipped?
Duplex is an iron-base stainless steel, so its scrap is normally classified under HS 7204.21, waste and scrap of stainless steel, not under the nickel scrap heading 7503.00 used for Inconel or Monel. National tariff lines add further digits, so confirm with your customs broker. The packing and document set are the same as for other stainless, covered in exporting stainless steel scrap.
What changes is description. Because duplex shares a tariff heading with ordinary stainless, the invoice, packing list and package labels are where its grade is recorded. Label each bundle or box with grade and weight, and keep duplex packages physically apart from 304 and 316 in the container so they can be checked on arrival.
What Should You Send to Get an Offer?
Send the grade or best identification, the form, approximate weight and location, photographs of the lot and of stamps or stencils, and XRF readings on several pieces that show chromium, molybdenum and, for S32760, copper and tungsten. Say where the material came from, because “offshore firewater replacement” tells a buyer a lot about likely grades and coatings. The duplex and 254 SMO scrap section has a page for every grade and form.
A line such as “Super duplex S32760 pipe, 4 to 10 inch, cut free of supports, flanges read and stamped F55, about 4 tonnes” gets a firmer first offer than “duplex scrap, mixed”. Send the details and we’ll review your enquiry.
Submit a requirement with the grade, form, quantity and delivery location.
Source a GradeFAQ
Is duplex stainless steel scrap classed as nickel scrap?
No. Duplex is an iron-base stainless steel with about 5.5 to 7 per cent nickel, so it is traded and shipped as stainless steel scrap under HS 7204.21. Nickel alloys such as Inconel and Monel, which are nickel-base, ship under HS 7503.00. Describing duplex as nickel scrap would mislead a buyer and could cause customs problems.
Can S31803 and S32205 scrap be sold as one lot?
Yes. S31803 is the original UNS number for duplex 2205 and S32205 is a later version with tighter limits on chromium, molybdenum and nitrogen. Most mills certify material to both, and handheld XRF cannot measure nitrogen, so the two are traded together as 2205. Record whichever number appears on the markings.
Is lean duplex scrap valued like 2205?
Usually not. Lean duplex grades such as 2101 and 2304 carry well under 1 per cent molybdenum, so they lack much of the molybdenum that gives 2205 its extra value. They are still magnetic, so they end up in the same suspect pile. Read molybdenum on every magnetic piece and keep lean duplex as its own lot.
Why does duplex scrap sometimes get a lower offer than 316?
Because standard duplex carries less nickel than 316, and a buyer who prices stainless purely on nickel will see it as a low-nickel grade. The chromium and molybdenum in duplex are only credited by buyers who can use them, and only when the grade is proved. Selling identified duplex as its own lot is how you reach those buyers.
Does welding or heat damage change how duplex scrap is valued?
Not much for remelting. Welds, heat-affected zones and service damage can change the phase balance and properties of duplex, which matters for reuse but not for a melt shop, which only needs the chemistry. Keep welded spools clean of carbon steel and filler from other alloys, and the lot is valued on its grade.
Can I use the duplex grade from the piping drawings?
Use drawings as a guide to where to look, not as proof. Repairs and substitutions over a plant’s life often put a different grade into a line than the drawing shows, especially in flanges and valves. Read each piece, record F-numbers from forgings and send those readings; buyers trust measured values over design documents.
