Austenitic and duplex stainless steel weld metal is not single phase. A controlled amount of delta ferrite retained in the austenite matrix is what stops solidification cracking, because ferrite dissolves sulphur and phosphorus that would otherwise segregate to the austenite grain boundaries as low-melting films. Too little ferrite and the weld cracks; too much and toughness, corrosion resistance and — after long service above roughly 400 °C — ductility all fall away as sigma phase forms.

How much ferrite forms is set by the balance between the ferrite-forming and austenite-forming elements. The WRC-1992 diagram condenses that balance into two coordinates: Creq = Cr + Mo + 0.7Nb and Nieq = Ni + 35C + 20N + 0.25Cu. Note what the coefficients say about the physics — carbon is 35 times as potent an austenite stabiliser as nickel, and nitrogen 20 times, which is why a small pickup of atmospheric nitrogen from poor gas shielding can move a weld out of its intended ferrite window. WRC-1992 superseded the Schaeffler and DeLong diagrams because it handles nitrogen and copper properly and predicts Ferrite Number rather than a ferrite percentage.

The Ferrite Number itself is read from the diagram at the plotted point, or measured directly on the weld with a magnetic instrument calibrated to AWS A4.2M. This tool gives the coordinates to plot and the ratio that governs solidification mode; it does not read the diagram for you, because the iso-ferrite lines are a graphical construction rather than a published equation.

The Creq/Nieq ratio predicts how the weld pool freezes, and that matters more than the final ferrite content for hot cracking. Ratios below about 1.25 solidify as primary austenite (mode A) and are crack sensitive. Between 1.25 and 1.48 the mode is AF. From 1.48 to 1.95 solidification starts as primary ferrite (mode FA) — this is the target for standard 308/316 type welds and gives the best cracking resistance. Above 1.95 the weld freezes fully ferritic (mode F), typical of duplex and superduplex, where the ferrite-to-austenite transformation on cooling then has to be controlled by nitrogen content and cooling rate.

Worked example

Carbon C	0.02 wt%
Chromium Cr	19.5 wt%
Nickel Ni	10 wt%
Molybdenum Mo	0.1 wt%
Niobium Nb (columbium)	0 wt%
Nitrogen N	0.06 wt%
Copper Cu	0.1 wt%
Chromium equivalent Creq	19.6
Nickel equivalent Nieq	11.925
Creq / Nieq	1.644
Solidification mode code	3

Typical 308L deposit. Creq = 19.5 + 0.10 + 0.7x0 = 19.60. Nieq = 10.0 + 35(0.02) + 20(0.06) + 0.25(0.10) = 10.0 + 0.70 + 1.20 + 0.025 = 11.925. Ratio = 19.60/11.925 = 1.6436 -> 1.644, which lands in the 1.48-1.95 band, so mode code 3 (FA, primary ferrite). That is the expected and desired behaviour for a 308L weld.

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