Chromium and nickel equivalents, with dilution
A weld does not have the composition of the filler: it is a mixture of filler with the melted parent metals. The calculator works out that mixture, derives the Cr and Ni equivalents and puts the point on the Schaeffler diagram — you see at once which region your weld lands in.
The formulas
Schaeffler:
Cr-eq = Cr + Mo + 1.5·Si + 0.5·Nb
Ni-eq = Ni + 30·C + 0.5·Mn
WRC-1992:
Cr-eq = Cr + Mo + 0.7·Nb
Ni-eq = Ni + 35·C + 20·N + 0.25·Cu
Carbon weighs thirty times more than nickel in these formulas — and that is the whole point. A tenth of a per cent of carbon shifts the point three units up, as much as three per cent of nickel does. So when welding unalloyed steel with a stainless filler, the number to watch is not the chromium but how much carbon the parent metal brings in.
Dilution
The weld composition is a weighted average. On a similar joint there are two terms: parent metal and filler. On a dissimilar one there are three, and that is exactly where mental arithmetic starts to slip.
The defaults in the calculator are that very case: 304 on one side, S235 on the other, 309L filler, 25 % dilution from each side. The raised chromium and nickel of 309L are no accident: the surplus is there so that after dilution with carbon steel the weld does not drop into the martensitic region. Put an ordinary 308L in place of the 309L and watch both equivalents fall.
Where it is needed
- Dissimilar joints. Choosing a filler for the stainless-to -carbon pair rests on exactly this arithmetic; the detail is in the article on welding dissimilar metals.
- Ferrite in an austenitic weld. A few per cent of ferrite guards against hot cracking; none at all and far too much are two different problems.
- Hardfacing and buttering. A buffer layer exists to cut the dilution — and the point of it only shows up in numbers.
Frequently asked questions
How accurate is the diagram?
It gives a region, not a number. Schaeffler built it in 1949 on arc welds in stainless steels, and between the iso-ferrite lines the reading comes out as a band — which is why the calculator says 5–10 % rather than 7 %. For the exact ferrite content of an austenitic weld use WRC-1992 or a ferritescope. Schaeffler’s strength lies elsewhere: it also covers the martensitic region, and that is what decides matters on dissimilar joints.
Where do the shares of each metal come from?
From the dilution, that is, the share of parent metal in the weld. It depends on process and settings: with a covered electrode usually 20–30 %, with MAG 30–50 %, under flux it can pass 60 %. On a dissimilar joint that share is split between the two sides, normally evenly, and the rest is filler.
Schaeffler or WRC-1992?
WRC-1992 is newer and more accurate for austenitic and duplex steels because it accounts for nitrogen and copper, and it reports ferrite as a Ferrite Number rather than a percentage. Schaeffler stays useful on dissimilar joints — it covers the martensitic region, which the WRC chart does not. Hence both are calculated.
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