Why predict ferrite in a stainless weld at all?

A few percent delta-ferrite in austenitic stainless welds suppresses solidification (hot) cracking, but too much harms ductility, toughness and corrosion resistance and can embrittle in service. The constitution diagrams let you aim composition at a target ferrite window before you weld.

Schaeffler, DeLong or WRC-1992 — which one?

Schaeffler (1949) is the original and most general but ignores nitrogen. DeLong (1973) added nitrogen and the Ferrite Number scale for common austenitic grades. WRC-1992 is the most accurate modern diagram and is the default for predicting Ferrite Number in austenitic and duplex weld metal.

What is a Ferrite Number?

Ferrite Number (FN) is a magnetically measured, reproducible scale for ferrite content in stainless weld metal. At low levels it is close to volume percent ferrite, but it is defined to be measurable consistently, which percent ferrite is not.

Welding · guide

Schaeffler, DeLong & WRC-1992 diagrams

Stainless-steel weld metal can solidify austenitic, ferritic, martensitic, or some mix — and the mix decides cracking resistance, toughness and corrosion behaviour. Constitution diagrams predict it from composition, by collapsing the alloying into two equivalents.

Two axes: chromium- and nickel-equivalent

Every constitution diagram works the same way: ferrite-promoting elements are summed into a chromium-equivalent (the x-axis) and austenite-promoting elements into a nickel-equivalent (the y-axis). The plotted point lands in a microstructure field. The original Schaeffler equivalents are:

Cr_eq = Cr + Mo + 1.5·Si + 0.5·Nb

Ni_eq = Ni + 30·C + 0.5·Mn

Carbon and nitrogen are powerful austenite stabilisers, which is why carbon carries a factor of 30.

DeLong: adding nitrogen and Ferrite Number

Schaeffler predates routine nitrogen control. DeLong added nitrogen to the nickel-equivalent and recalibrated the diagram against the magnetically measured Ferrite Number for austenitic grades:

Ni_eq (DeLong) = Ni + 30·C + 30·N + 0.5·Mn

DeLong reads out in Ferrite Number rather than percent ferrite, which is what fabrication specs (e.g. 3–10 FN to balance cracking and toughness) actually call out.

WRC-1992: the modern default

The WRC-1992 diagram refits the equivalents and boundaries against a large weld dataset and adds the solidification-mode lines (A, AF, FA, F) that govern hot-cracking. Its equivalents drop some of the older coefficients in favour of a better fit:

Cr_eq = Cr + Mo + 0.7·Nb

Ni_eq = Ni + 35·C + 20·N + 0.25·Cu

For predicting Ferrite Number in austenitic and duplex weld metal, WRC-1992 is the one to reach for.

Open the calculatorSchaeffler / DeLong / WRC-1992 calculator →Enter the weld composition (including dilution) to plot the point on each diagram and read the predicted microstructure and Ferrite Number.

Don’t forget dilution

The composition that matters is the weld metal, which is a blend of filler and melted base metal set by the dilution of the process. A filler that would give the right ferrite undiluted can land in the cracking-prone fully-austenitic field once diluted by a low-ferrite base metal. Compute the diluted composition first, then read the diagram.

Frequently asked

Why predict ferrite in a stainless weld at all?: A few percent delta-ferrite in austenitic stainless welds suppresses solidification (hot) cracking, but too much harms ductility, toughness and corrosion resistance and can embrittle in service. The constitution diagrams let you aim composition at a target ferrite window before you weld.
Schaeffler, DeLong or WRC-1992 — which one?: Schaeffler (1949) is the original and most general but ignores nitrogen. DeLong (1973) added nitrogen and the Ferrite Number scale for common austenitic grades. WRC-1992 is the most accurate modern diagram and is the default for predicting Ferrite Number in austenitic and duplex weld metal.
What is a Ferrite Number?: Ferrite Number (FN) is a magnetically measured, reproducible scale for ferrite content in stainless weld metal. At low levels it is close to volume percent ferrite, but it is defined to be measurable consistently, which percent ferrite is not.

References

A.L. Schaeffler, "Constitution diagram for stainless steel weld metal," Metal Progress 56, 1949.
W.T. DeLong, "Ferrite in austenitic stainless steel weld metal," Welding Journal 53, 1974.
D.J. Kotecki, T.A. Siewert, "WRC-1992 constitution diagram for stainless steel weld metals," Welding Journal 71, 1992.
J.C. Lippold, D.J. Kotecki, "Welding Metallurgy and Weldability of Stainless Steels," Wiley.

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