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

Computes the WRC-1992 chromium and nickel equivalents that place a heat on the constitution diagram, the ASME BPE Appendix G BN2 low-ferrite criterion on its own chromium and nickel equivalents, and the autogenous-weld sulfur weldability window of MM-5.2.1.1(a), with a PASS/FAIL verdict on the sulfur check.

Method last updated 2026-10-02 (calculation changelog) · fixture-verified on every build — most recently 2026-10-12.

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 criterion — a heat-selection limit that ASME BPE Appendix G cites and evaluates on its own, different chromium and nickel equivalents. 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

WRC-1992: Cr_eq = %Cr + %Mo + 0.7·%Nb · Ni_eq = %Ni + 35·%C + 20·%N + 0.25·%Cu

BN2: Cr(eq) = %Cr + 1.5·%Si + %Mo + 2·%Ti · Ni(eq) = %Ni + 0.5·%Mn + 30·%C + 30·(%N − 0.02)

BN2 criterion: Cr(eq) − 0.91·Ni(eq) ≤ limit

ASME BPE Appendix G uses two different pairs of equivalents for two different jobs, and the calculator keeps them apart. The WRC-1992 pair (G-1) places a heat on the constitution diagram: 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 criterion (G-3(d) and its footnote, citing Basler Norm BN2) is a heat-selection limit for low-ferrite material such as grade 1.4435, and it is written on its own equivalents: silicon and titanium join the chromium term, manganese joins the nickel term, carbon counts 30 and nitrogen counts 30 above a 0.02 % offset. The 0.91 coefficient and the limit belong to that pair only — applied to the WRC-1992 pair they give a different number, which is why BN2 needs the certified silicon and manganese and is not evaluated without them. Separately, and only when a sulfur content is supplied, the autogenous-weld window is applied — a lower and an upper bound, both taken from MM-5.2.1.1(a) unless your specification sets its own, 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: it is read from the WRC-1992 diagram, which is published plate content and is not embedded here. What this card gives you are the two coordinates you carry onto your own copy of that diagram, plus the criteria 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 copy of the WRC-1992 diagram; the plate itself is not reproduced here and no FN is interpolated from it.
• 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 — deliberate in the 1992 revision. It does appear in the BN2 nickel equivalent, which belongs to a separate criterion: the two pairs are not interchangeable, and a BN2 value computed on the WRC-1992 pair is a different number.
• BN2 is evaluated only when silicon and manganese are entered. Titanium, when omitted, is taken as zero — right for unstabilised grades such as 316L, wrong for a titanium-stabilised heat, where it understates the BN2 chromium equivalent and flatters the result.
• 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 limit (App G, G-3(d) footnote 3) and both window bounds (MM-5.2.1.1(a)) — are inputs that open at the BPE-2026 values, 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, 0.50 % Si, 1.50 % Mn, 0.010 % S.

Step by step

1. WRC-1992: Cr_eq = 17 + 2.1 + 0.7 × 0 = 19.1; Ni_eq = 12 + 35 × 0.02 + 20 × 0.05 + 0.25 × 0 = 12 + 0.7 + 1.0 = 13.7.
2. BN2 equivalents: Cr(eq) = 17 + 1.5 × 0.5 + 2.1 + 2 × 0 = 19.85; Ni(eq) = 12 + 0.5 × 1.5 + 30 × 0.02 + 30 × (0.05 − 0.02) = 12 + 0.75 + 0.6 + 0.9 = 14.25.
3. BN2 = 19.85 − 0.91 × 14.25 = 19.85 − 12.9675 = 6.883 → 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 meets the BN2 low-ferrite criterion with margin 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. Both pairs of equivalents are unchanged; BN2 = 6.883, 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.

Worked example 3 — a higher-silicon heat the BN2 criterion rejects

A 316L heat on the rich side of its ranges: 17.2 % Cr, 12.2 % Ni, 0.020 % C, 0.040 % N, 2.30 % Mo, 0.70 % Si, 0.90 % Mn, 0.010 % S.

Step by step

1. WRC-1992: Cr_eq = 17.2 + 2.3 = 19.5; Ni_eq = 12.2 + 35 × 0.02 + 20 × 0.04 = 12.2 + 0.7 + 0.8 = 13.7.
2. BN2 equivalents: Cr(eq) = 17.2 + 1.5 × 0.7 + 2.3 = 20.55; Ni(eq) = 12.2 + 0.5 × 0.9 + 30 × 0.02 + 30 × (0.04 − 0.02) = 12.2 + 0.45 + 0.6 + 0.6 = 13.85.
3. BN2 = 20.55 − 0.91 × 13.85 = 20.55 − 12.6035 = 7.947 → above the qualifying limit: the heat does not meet BN2.
4. Sulfur 0.010 % is in range → the sulfur verdict is still PASS.

The card's PASS is the sulfur verdict; the BN2 row is the one that decides whether this is a low-ferrite purchase, and here it is not. The same subtraction done on the WRC-1992 pair gives 19.5 − 0.91 × 13.7 = 7.033 and would have passed the heat — which is exactly why the criterion has to be evaluated on its own equivalents. Here silicon, weighted 1.5 in the BN2 chromium term and absent from the WRC-1992 pair, is the largest single difference between the two.

Fixture case “high-Si 316L heat, BN2 not met though WRC-1992 pair reads 7.033” (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 as given in ASME BPE-2026 App. G-1; the diagram plate itself is not embedded.
• ASME BPE-2026 — App. G, G-3(d) and its footnote: the BN2 low-ferrite criterion and its own Cr(eq)/Ni(eq) (Basler Norm BN2).
• ASME BPE-2026 — 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 inputs — typed from your governing edition, or autofilled on paid tiers from the embedded datasets enumerated there, always cited by standard, edition, table and line. 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 the diagram itself is not reproduced here — neither as a digitised copy nor as an FN interpolated from a reconstruction of it. What the calculator returns instead are the two things that need no plate: the closed-form equivalents that give you the coordinates, and the BN2 criterion, on its own equivalents, that answers the low-ferrite question without the plate at all. Plot the coordinates on your own copy of the diagram 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. It stops being enough the moment the question is an actual ferrite level rather than a yes/no — CF8M and CF3M castings can carry 10 to 20 vol.% ferrite, against roughly 0 to 3 vol.% in solution-annealed wrought 316L (App. G-1) — because a level like that has to be read off the plotted point, not screened by one limit.

What is the BN2 criterion actually doing?

It screens a heat for low weld ferrite before the heat is bought, using a limit ASME BPE Appendix G takes from Basler Norm BN2. G-3 lists four ways to control ferrite in austenitic stainless welds: postweld solution annealing, a higher-nickel filler, nitrogen added to the shielding gas, and choosing heats with a high nickel-to-chromium ratio, such as grade 1.4435, whose Cr(eq) to Ni(eq) ratio is restricted per BN2. That criterion is written on its own equivalents: silicon and titanium join chromium and molybdenum, manganese joins nickel, carbon counts 30 and nitrogen counts 30 above a 0.02 % offset. The 0.91 coefficient and the limit belong to that pair. They are not a line on the WRC-1992 diagram, whose coordinates use different weights and leave silicon and manganese out altogether. Worked example 3 shows the consequence: a heat that clears the limit when the subtraction is done on WRC-1992 coordinates fails it on the BN2 pair. It is a screen, not a measurement — a heat near the limit still calls for the diagram or a magnetic measurement on a real weld.

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. ASME BPE's own Nonmandatory Appendix D names the same mechanism in its rouge-formation table: welds on tubing with dissimilar sulfur concentrations are flagged as a strong-influence variable, because the resulting lack of penetration can leave a crevice, and a crevice is a rouging and corrosion-initiation site long after the weld itself passed inspection. 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. ASME BPE's own Nonmandatory Appendix G is explicit that measuring ferrite in production welds is done to AWS A4.2M (ISO 8249) — a magnetic method, not an arithmetic one. 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 AWS A4.2M measurement is the compliance evidence, and this calculator is the tool that stops you buying material that cannot meet it in the first place.

Which elements matter most if my certificate is incomplete?

Chromium, nickel and carbon are required for everything; the rest depends on which screen you are running. For the WRC-1992 coordinates, nitrogen is the one to chase: weighted at 20 in the nickel equivalent, an unreported 0.05 % is worth a full point of Ni_eq, while molybdenum and niobium sit in the chromium equivalent, so zeroing any of them moves the plotted point. For the BN2 criterion, silicon and manganese are required — the card will not evaluate BN2 without them — because silicon enters the chromium equivalent at 1.5 times its weight and a missing value would flatter the heat. Nitrogen enters BN2 as 30 × (N − 0.02); left blank it is taken as zero, which understates Ni(eq), overstates the BN2 value and makes the screen conservative rather than lenient. Titanium is taken as zero, which is right for 316L and wrong for a titanium-stabilised heat. Copper and niobium do not enter BN2 at all. If anything is missing and the result sits near a limit, get the full chemistry rather than accepting a 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
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