ASTM A182 F60 vs F51: why nitrogen decides your PREN

Buyers ask us this almost every week: "Our spec says F51/F60 — are they the same steel?" The honest answer is no. F60 and F51 are close cousins built on the same 22Cr duplex recipe, but F60 (UNS S32205) tightens the chemistry floor so its pitting resistance is guaranteed, while F51 (UNS S31803) can legally arrive at a lower number. If your project imposes a minimum PREN, that single distinction decides whether the forging you receive is compliant or not.

01 / ORIGINSTwo grades, one family — and why there are two at all

Both grades are "2205" duplex stainless steel: a roughly 50/50 mix of ferrite and austenite that combines the strength and chloride-cracking resistance of ferritic steel with the toughness and formability of austenitic steel. When 2205 was first standardized in the 1980s it was assigned UNS S31803 — the alloy behind ASTM A182 Grade F51.

Over the following years, foundries and end users learned that S31803's composition window was wide enough to allow inconsistent behavior. Heats near the low end of the range — lower chromium, lower molybdenum, and especially lower nitrogen — sometimes fell short on pitting resistance and produced weld heat-affected zones with too much ferrite. To close that gap, a tighter version was introduced and registered as UNS S32205, which ASTM A182 designates as Grade F60. Same alloy family, narrower and higher floors, more predictable performance.

So F60 is not a "different steel" so much as a disciplined redraw of the same one. Every F60 heat also satisfies the S31803 limits, which is why suppliers can dual-certify a compliant heat as "S31803 / S32205." The reverse is not true: an S31803 heat does not automatically meet S32205.

02 / THE MATHThe PREN formula and the 16× rule

Pitting Resistance Equivalent Number (PREN) is the industry's single-figure index for how well a stainless steel resists localized chloride pitting — the failure mode that eats through 316L in seawater. For duplex grades it is calculated from just three alloying elements:

That multiplier is the whole story. Nitrogen is the lightest and cheapest of the three contributors, yet the formula rewards it more than four times as heavily as molybdenum and sixteen times as heavily as chromium. A modest bump of 0.06% nitrogen — a fraction of a percent, at almost no alloy cost — adds nearly a full PREN point.

Now put the two grades' minimum floors into the equation:

• F51 (S31803) at its floors — Cr 21.0%, Mo 2.5%, N 0.08% → PREN = 21.0 + (3.3 × 2.5) + (16 × 0.08) = 30.5
• F60 (S32205) at its floors — Cr 22.0%, Mo 3.0%, N 0.14% → PREN = 22.0 + (3.3 × 3.0) + (16 × 0.14) = 34.1

All three floors rise from F51 to F60, but nitrogen is the most efficient lever and the emblem of the change: raising the nitrogen minimum from 0.08% to 0.14% alone contributes almost a full point of PREN, without spending anything extra on molybdenum or chromium. That is why the industry calls S32205 "the nitrogen grade."

03 / TRY ITWatch nitrogen move the number across 34

Drag the sliders — or load a preset — and see how each element feeds the total. Nitrogen's segment grows fastest for the smallest move, and the badge flips the instant PREN crosses the 34 line that most critical specifications require.

04 / THE NUMBERSComposition side by side

Every element they share is identical in intent. The rows that decide PREN — chromium, molybdenum and nitrogen — are where F60 pulls the floor upward. The nitrogen row (highlighted) is the defining one.

Values per the S31803 / S32205 registrations as adopted in ASTM A182. Both grades carry the same minimum mechanical properties in A182 (≈450 MPa / 65 ksi yield). Always confirm against the current edition of the standard for your order.

05 / CONSEQUENCESWhat the difference actually means in the field

PREN 34 is not an arbitrary line. It is the acceptance floor written into the standards that govern the most demanding uses of duplex forgings, and it is roughly the point below which 22Cr duplex loses its safety margin against pitting in warm, high-chloride water.

• NORSOK M-122 (North Sea): qualification of 22Cr duplex for offshore service is built around guaranteed pitting resistance and verified ferrite content. An F51 heat that dips under 34 does not clear the bar.
• NACE MR0175 / ISO 15156 (sour service): S32205 is listed for H₂S environments under specific hardness and heat-treatment conditions. Specifying the guaranteed-PREN grade removes any ambiguity for the corrosion engineer signing off the material.
• Seawater-cooled and produced-water systems: the gap between PREN 30 and PREN 34 is the gap between early pitting along the weakest zones and reliable service life.

In practice this means a forging can pass its chemical certificate as "duplex 2205" and still be the wrong material for the job — if it was bought as F51 and landed near the low-nitrogen floor.

06 / BONUS EFFECTSNitrogen isn't only about pitting

The PREN multiplier gets the attention, but nitrogen earns its keep in three more ways that reinforce the case for F60:

• Austenite balance: nitrogen is a strong austenite former. Higher nitrogen helps austenite re-form quickly as a weld cools, keeping the heat-affected zone from becoming too ferritic — the exact weakness that pushed the industry from S31803 to S32205.
• Strength: nitrogen dissolves into the lattice and raises yield strength through solid-solution strengthening, letting designers thin down pressure-retaining walls without losing margin.
• Cleaner intermetallics: nitrogen mildly delays the precipitation of sigma and chi phases, giving a little more processing latitude during the critical cooling from solution-anneal temperature.

07 / PROCUREMENTThe "F51/F60" dual-marking trap

A large share of duplex purchase orders still read "A182 F51/F60" or simply "duplex 2205." That phrasing is a historical habit, and it quietly shifts risk onto the buyer. Dual marking is legitimate when the heat genuinely meets S32205 — but a purchase order that only names both grades does not compel the supplier to hit PREN 34. It leaves the door open to an S31803-floor heat.

If your application has a PREN requirement, write the order so it cannot be misread:

• Specify "ASTM A182 Grade F60, UNS S32205" — not "F51/F60," not "2205" alone.
• Require the calculated PREN, using %Cr + 3.3×%Mo + 16×%N, reported on the MTR, with a stated minimum of 34.
• Require the full nine-element analysis including nitrogen, plus ferrite content per ASTM E562 or a calibrated Feritscope.
• State the heat-treatment condition explicitly: solution annealed and water quenched.

08 / BALANCEWhen F51 is still a perfectly good choice

None of this makes F51 a bad steel. Where the governing standard permits S31803 and does not impose a PREN floor — general structural duplex parts, moderately corrosive service, applications chosen for strength and cracking resistance rather than marginal pitting performance — the two grades behave alike, and F51 may already be qualified in a client's system. The point is not that F51 is inferior; it is that F60 removes a variable. When the environment is aggressive enough that PREN 34 is a design requirement, paying the negligible premium for a guaranteed floor is cheap insurance against an expensive failure.

09 / QUESTIONSFrequently asked