Ferralium 255 vs Zeron 100
Which super duplex for your forging?

Ask three metallurgists whether Ferralium 255 or Zeron 100 is the "better" super duplex and you'll get three answers — usually shaped by whoever sold them the last heat. The honest answer is that neither is universally better. They are two branches of the same 25% chromium family that diverged over a single deliberate alloying choice, and once you understand that choice the selection becomes straightforward.

This guide is written from the perspective of a company that forges both grades. We have no brand to defend, so we'll lay out the metallurgy plainly, put the numbers side by side, and finish with a decision map you can actually use on a purchase order. Think of it as a selection guide rather than a datasheet: once you have settled on S32550, the full forging specifications, sizes and ordering details live on our dedicated product page, linked below.

Two super duplex grades, one shared backbone

Both alloys sit in the super duplex tier — the class of duplex stainless steels with a Pitting Resistance Equivalent Number (PREN) of 40 or above and a roughly 50/50 mix of austenite and ferrite. Both are built on the same core: about 25% chromium, 3.5% molybdenum, nitrogen for strength and pitting resistance, and enough nickel to balance the microstructure. Both were engineered for the same job — surviving where 316L and even standard duplex 2205 corrode away.

The history is worth one sentence, because it explains the rivalry. Ferralium 255 was patented by Langley Alloys in 1967 and is the original super duplex stainless steel; Zeron 100 arrived in the 1980s from Mather & Platt (its trademark now sits with Rolled Alloys). Two makers, two patents, two philosophies — and that's exactly where they part ways.

Copper vs tungsten: the element that decides everything

Strip away the marketing and the difference between these grades comes down to two elements you won't find in ordinary stainless steel.

Ferralium 255 leans on copper. It carries a deliberate 1.5–2.0% Cu addition. Copper does something the standard PREN formula never captures: it dramatically improves resistance in reducing acids such as sulphuric and phosphoric acid, and it contributes a mild self-healing effect at incipient pitting sites. This is why S32550 has spent five decades in fertiliser plants, phosphoric-acid trains and mixed chemical process equipment.

Zeron 100 leans on tungsten. It swaps most of that copper for roughly 0.7% W (with a smaller ~0.7% Cu retained). Tungsten behaves chemically like molybdenum, reinforcing resistance to chloride pitting and crevice corrosion. Crucially, it lets the grade guarantee a minimum PREN of 40 — the tungsten is even written into its PREN formula.

Ferralium 255 vs Zeron 100 at a glance

Typical values below; always confirm against the actual heat certificate for your order.

Where each grade actually wins

Corrosion performance is not a single scoreboard — it depends entirely on what the metal is touching.

Chloride pitting & seawater

This is Zeron 100's home turf. The tungsten addition and slightly higher nitrogen give it a firm, guaranteed PREN and a critical pitting temperature comfortably above 50 °C, and it has an extensive North Sea and subsea pedigree in seawater, firewater and cooling systems. Ferralium 255 is also excellent here — both grades vastly outclass 6-moly and austenitic steels — but for a pure seawater specification, Zeron's documented track record is often the deciding factor for the client's approval engineer.

Sulphuric, phosphoric & reducing acids

This is where Ferralium 255 pulls ahead, and it's not close. The 2% copper suppresses corrosion in reducing-acid environments that will slowly attack tungsten-bearing grades. In sulphuric-acid handling, phosphoric-acid production and fertiliser process equipment, S32550 is frequently specified precisely because of that copper. If your process stream is acidic rather than chloride-driven, the copper grade is usually the right forging.

Mechanicals and phase balance

On strength, Ferralium 255 holds a small but genuine edge: it's the only super duplex grade that guarantees a minimum yield of 85 ksi (586 MPa) as solid bar, against 80 ksi (552 MPa) for Zeron 100. In heavy forged sections both are typically qualified to a 500 MPa minimum, so the gap narrows for large rings and blocks — but for highly stressed shafts and fasteners the S32550 headroom can matter.

Zeron 100 answers with its higher nickel (~7% vs ~5.5%). More nickel stabilises austenite, which gives a slightly more forgiving phase balance through welding and heat treatment. For heavily welded fabrications, some fabricators find the tungsten grade a touch more tolerant — though with correct procedures both weld successfully using over-alloyed 25-9-4 fillers and nitrogen-bearing shielding gas.

Standards: F61 vs F55

For forged flanges, fittings and valve bodies the two grades appear under different ASTM A182 grade numbers — F61 for S32550 and F55 for S32760 — and different EN numbers (1.4507 vs 1.4501). Beyond that, their qualification landscape is nearly identical: both are covered for sour service under NACE MR0175 / ISO 15156, both are routinely supplied to NORSOK M-650 requirements for oil-and-gas forgings, and both appear in API 6A material classes for wellhead and valve components. In practice, the governing project specification will name one UNS number, so your job is usually to forge and certify to that exact grade rather than to substitute.

A decision map you can use

Match your dominant service condition to a column. When conditions conflict, weight the one that actually threatens the part.

What changes on the shop floor

Here's the reassuring part: from a forging and heat-treatment standpoint, S32550 and S32760 are handled almost identically. Both demand solution annealing in the ~1,025–1,120 °C window followed by a rapid water quench to dissolve sigma and chi phases and lock in a 40–60% ferrite balance. Both are ruined by slow cooling through the 700–950 °C sigma range, and both require careful ferrite mapping for HISC-critical subsea work. The nuances are small — Zeron's higher nickel gives marginally more phase-balance latitude, while Ferralium's copper content should be verified by optical emission spectrometry on every heat so it's not confused with a cheaper duplex.

If you already know which grade your drawing calls for, the practical questions are capacity, ferrite control and certification — which is where our Ferralium 255 (UNS S32550) super duplex forgings capability comes in. We forge open-die shapes, seamless rolled rings and custom geometries in both S32550 and S32760, with controlled solution annealing, nine-point ferrite mapping on request, and EN 10204 3.1 material certificates (3.2 available through customer-nominated third-party inspection).

FAQ

Governing standards & sources

The material data and qualification requirements in this guide are drawn from the following recognised standards and technical sources. Always specify against the current published edition.