Ferrite / Weldability Calculator (WRC-1992, ASME BPE)

Computes the WRC-1992 chromium and nickel equivalents, the BN2 low-ferrite discriminant Cr_eq − 0.91·Ni_eq against its qualifying limit, and the autogenous-weld sulfur weldability window of ASME BPE MM-5.2.1.1(a), with a PASS/FAIL verdict on the sulfur check.

Method last updated 2026-07-04 (calculation changelog) · fixture-verified on every build — most recently 2026-09-03.

Built and fixture-verified by Matthew Norris, P.E. — active P.E. licensure in Arizona, California, Kansas, Missouri, North Carolina, Texas.

Two different questions decide whether a hygienic tube heat welds well, and this calculator answers both from one chemistry. The first is how much delta ferrite the weld will solidify with — governed by the WRC-1992 chromium and nickel equivalents, which are the coordinates of the constitution diagram, plus the BN2 linear discriminant that qualifies a heat as low-ferrite. The second is whether the heat will fuse predictably without filler at all, which for autogenous orbital GTAW is set by sulfur: too little and the Marangoni convection that drives a deep, narrow weld pool reverses, giving wide shallow beads and erratic penetration; too much and hot cracking and reduced corrosion resistance follow. ASME BPE MM-5.2.1.1(a) bounds sulfur on both sides for that reason. Reach for this at heat qualification, when a supplier offers a substitute heat mid-project, and when an orbital weld procedure that was stable on the last delivery starts wandering on this one — that symptom is a chemistry problem far more often than it is a machine problem.

Method

Cr_eq = %Cr + %Mo + 0.7·%Nb

Ni_eq = %Ni + 35·%C + 20·%N + 0.25·%Cu

BN2 qualifier: Cr_eq − 0.91·Ni_eq ≤ limit

The calculator evaluates the two WRC-1992 equivalents from the certified chemistry, then two independent screens on them. The equivalents themselves are the constitution-diagram coordinates: chromium equivalent gathers the ferrite-forming elements, nickel equivalent the austenite-formers, with carbon and nitrogen weighted at 35 and 20 because interstitials are overwhelmingly the strongest austenite stabilisers per unit mass — 0.02 % carbon carries the same weight in the nickel equivalent as 0.7 % nickel does. The BN2 discriminant is the linear low-ferrite qualifier: subtract 0.91 times the nickel equivalent from the chromium equivalent and compare against the qualifying limit, which arrives as a defaulted input. Separately, and only when a sulfur content is supplied, the autogenous-weld window is applied — a lower and an upper bound, both defaulted inputs, with the verdict reported as in-range, low or high and a PASS only for in-range. A heat below the window draws the erratic-penetration warning; above it, the hot-cracking and corrosion-resistance warning. The Ferrite Number itself is deliberately not returned: the FN is read from the WRC-1992 diagram, which is copyrighted plate content and is not embedded here. What this card gives you are the two coordinates you carry onto your own licensed copy of that diagram, plus the linear qualifiers that do not require it.

Limitations — what this calculator is not

• The Ferrite Number is not returned. Cr_eq and Ni_eq are the coordinates you plot on your own licensed WRC-1992 diagram; the copyrighted plate is not embedded and no FN is interpolated from it here.
• WRC-1992 predicts as-solidified weld metal ferrite. It does not describe the parent tube's wrought structure, and it does not survive post-weld thermal exposure — sigma-phase formation and ferrite transformation during any subsequent heat treatment are outside it entirely.
• The diagram was developed over a bounded composition range for austenitic and duplex stainless weld metal. Heats far outside that range — high-manganese grades, nickel-base alloys, heavily stabilised compositions — fall outside the correlation, and the equivalents remain computable while the prediction stops meaning anything.
• Manganese does not appear in the WRC-1992 nickel equivalent, unlike the older Schaeffler and DeLong forms. That is deliberate in the 1992 revision, not an omission here — a chemistry evaluated against the wrong diagram generation will not agree with this one.
• The sulfur window applies to autogenous welding, where pool convection sets penetration. A filler-added joint is governed by the filler and the procedure, and the window does not transfer to it.
• Sulfur is a range, not a target: a heat inside the window still welds differently at the bottom of it than at the top, which is why weld procedure qualification is done on the heat, or at minimum on a heat of the same narrow sulfur band, rather than on the grade.
• All limits — the BN2 qualifier and both window bounds — are defaulted inputs so they can be aligned with the edition and the specification you are contracted to. Confirm them against your own copy rather than assuming the defaults are your acceptance criteria.
• Chemistry is necessary and not sufficient. Purge quality, joint fit-up, tube ovality and wall-thickness variation within the certified tolerance will defeat a perfectly qualified chemistry, and none of them is an input here. A heat that passes this screen and still welds badly is telling you to look at the machine and the joint, in that order.

Worked example — fixture-verified

A 316L hygienic tube heat offered for autogenous orbital welding. Certificate: 17.0 % Cr, 12.0 % Ni, 0.020 % C, 0.050 % N, 2.10 % Mo, 0.010 % S.

Step by step

1. Cr_eq = 17 + 2.1 + 0.7 × 0 = 19.1.
2. Ni_eq = 12 + 35 × 0.02 + 20 × 0.05 + 0.25 × 0 = 12 + 0.7 + 1.0 = 13.7.
3. BN2 = 19.1 − 0.91 × 13.7 = 19.1 − 12.467 = 6.633 → within the qualifying limit.
4. Sulfur 0.010 % sits inside the autogenous window → verdict in-range: PASS.

A heat worth buying for orbital work: it qualifies on the low-ferrite discriminant and sits comfortably inside the sulfur window. Worked example 2 takes the identical chemistry and moves only the sulfur, which is the single change most likely to arrive unannounced in the next delivery.

Worked example 2 — same heat chemistry, sulfur below the window

The same 316L composition, but a low-sulfur heat at 0.002 %. Nothing else changes — this is the case that arrives when a supplier substitutes a cleaner heat believing lower sulfur is unambiguously better.

Step by step

1. Cr_eq and Ni_eq are unchanged; BN2 = 6.633, still within the qualifying limit.
2. Sulfur 0.002 % is below the window lower bound → verdict low: FAIL.
3. Warning raised: risk of erratic penetration and lack of fusion in autogenous welds.

The ferrite screen is untouched and the heat still fails, which is the whole point of running both checks. Below roughly the window's lower bound the surface-tension gradient in the weld pool reverses sign: the pool convects outward instead of downward, and the same machine settings that gave full penetration on the last heat now give a wide, shallow, intermittently unfused bead. The fixes are a heat inside the window, or a requalified procedure on this heat — not more current.

Fixture case “316L, S below window -> fail” (tolerance 0.001) — locked in the same release gate as the example above.

Sources & citations

• WRC-1992 (Kotecki–Siewert) constitution diagram — Cr/Ni equivalent forms; the diagram plate itself is not embedded.
• ASME BPE — App. G (ferrite determination) and MM-5.2.1.1(a), autogenous-weld sulfur window (bounds are defaulted inputs).

Per the source & citation policy, allowable-stress and factor table values are user-supplied. Where a page does reproduce specific ASME data (the B16.5 ratings, the quick-reference tables), it states the source table and conditions inline.

FAQ

Why does the tool not just give me the Ferrite Number?

Because the Ferrite Number is read off the WRC-1992 diagram, and that diagram is copyrighted plate content. Embedding a digitised version of it — or interpolating an FN from a reconstruction of it — would be reproducing licensed material, which this site does not do for any standard. What the calculator returns instead are the two things that are genuinely public: the closed-form equivalents that give you the coordinates, and the BN2 linear discriminant that answers the low-ferrite question without the plate at all. Plot the coordinates on your own licensed copy and read the FN there. For the common case where the actual question is "does this heat qualify as low-ferrite", the BN2 value answers it directly and the diagram is not needed.

What is the BN2 discriminant actually doing?

It is a straight line drawn across the WRC-1992 constitution plane, and the check is which side of that line a heat falls on. Because the diagram's ferrite iso-lines run roughly parallel over the composition range of interest, a single linear combination of the two equivalents — the chromium equivalent minus 0.91 times the nickel equivalent — separates low-ferrite heats from the rest without reading the plate. The 0.91 coefficient is the slope that makes the line parallel to those iso-lines. It is a screen, not a measurement: a heat well below the limit is confidently low-ferrite, and one sitting within a few tenths of it is close enough that the diagram, or a magnetic ferrite measurement on an actual weld, is the honest next step rather than the arithmetic.

Is low sulfur not always better for stainless steel?

For corrosion resistance and hot-cracking, yes — sulfur forms manganese sulfide inclusions that are pit initiation sites, and it is the classic hot-shortness element. For autogenous orbital welding it is emphatically not, and this is one of the few places in stainless metallurgy where cleaner material welds worse. Sulfur is surface-active in the molten pool and controls the sign of the surface-tension temperature gradient. Above roughly 0.005 % the gradient drives Marangoni flow inward and downward, producing the deep, narrow, repeatable bead orbital welding depends on. Below it the flow reverses outward, the pool spreads, and penetration collapses at the same energy input. This is why ASME BPE bounds sulfur from below as well as above for hygienic tube, and why "we sourced a cleaner heat" is a change that requires requalification rather than a free improvement.

Can I weld two heats from opposite ends of the sulfur window together?

You can, and it is a known source of asymmetric welds. When a high-sulfur and a low-sulfur tube are butted, the pool convects asymmetrically across the joint: penetration is deeper on the high-sulfur side and the bead drifts off centre, which can present as apparent misalignment or as one-sided lack of fusion on the weld acceptance check. Both heats can be individually inside the window and the joint still behaves badly. Where heats are being mixed on one system, the practical controls are to qualify the procedure on the worst-case pairing rather than on a single heat, to keep heat traceability at the joint so the anomaly is diagnosable later, and to treat a sudden run of off-centre beads as a chemistry-mismatch symptom before rebuilding the weld head.

Does this replace a magnetic ferrite measurement on the finished weld?

No. This is a pre-weld screen on chemistry; a ferritescope or equivalent measurement is a post-weld measurement on the actual joint, and they answer different questions. The screen tells you what the heat should do given its composition and lets you reject a bad heat before it is installed. The measurement tells you what the weld actually did, including the effects of cooling rate, restraint, purge quality and any nitrogen picked up or lost at the arc — none of which the chemistry knows about. Where a specification sets an FN acceptance range on production welds, the measurement is the compliance evidence and this calculator is the tool that stops you buying material that cannot meet it.

Which elements matter most if my certificate is incomplete?

Chromium, nickel and carbon are required and carry the calculation; nitrogen matters more than its magnitude suggests. Because carbon is weighted at 35 and nitrogen at 20 in the nickel equivalent, an unreported nitrogen of 0.05 % is worth a full point of nickel equivalent — enough to move the BN2 value by nearly a point and to shift a marginal heat across the qualifying limit. Nitrogen and copper omitted from a certificate are taken as zero, which lowers the nickel equivalent and therefore raises the computed BN2 value — conservative for the low-ferrite screen, but not for a ferrite-minimum criterion, where an over-predicted BN2 flatters the heat rather than rejecting it. Molybdenum and niobium behave the opposite way: they sit in the chromium equivalent, so zeroing them lowers BN2, and a certificate missing Mo or Nb must not be run through this screen at all. If any of the four is missing and the result is anywhere near the limit, get the full chemistry rather than accepting the zero default.

Related calculators & tools

• Tube Weld Acceptance Calculator (ASME BPE MJ-8.4-1) — Chemistry screen first, then the weld's dimensions
• PREN Calculator (Pitting Resistance Equivalent, ASME BPE) — Same certificate, the corrosion-resistance question
• Ra Surface-Finish Calculator (ASME BPE SF Designations) — Weld and HAZ finish is a separate acceptance population
• ASME BPE hygienic design guide — How heat qualification feeds the fabrication sequence