Welding dissimilar steels: filler choice and dilution
Joining two different steels is not a compromise between two procedures. The weld metal is a third alloy, made of both parents plus the filler, and the filler is chosen so that mixture lands somewhere safe.
Dilution is the whole problem
A weld melts parent metal from both sides and mixes it with the filler. The proportion that comes from the parents — the dilution — is typically 20 to 40 % depending on the process and the joint, more with deep-penetrating processes and less with low-current ones.
On a joint between like materials that hardly matters. On a dissimilar joint it decides everything: 30 % carbon steel mixed into a stainless deposit gives a composition neither the filler nor the parent was designed to be, and that composition can be martensitic and brittle.
The Schaeffler diagram in practice
The Schaeffler diagram plots chromium equivalent against nickel equivalent and marks out the regions: austenite, ferrite, martensite and the mixtures. Its practical use is straightforward. Mark the two parent metals, draw a line between them, mark the filler, and the weld composition sits on the line between the filler and the mixed parents, at the dilution fraction.
What you want is for that point to land in the austenite-plus-ferrite region, with a few per cent of ferrite: enough to resist hot cracking, not so much that the joint becomes brittle. What you want to avoid is the martensite region.
The common pairs
| Joint | Filler | Notes |
|---|---|---|
| Austenitic stainless to carbon steel | 309L / 309LMo | The standard answer. Over-alloyed to survive dilution. |
| Stainless to low-alloy Cr-Mo | 309L, or a nickel alloy | Nickel filler where the joint runs hot in service. |
| Carbon steel to low-alloy steel | Match the lower-strength parent | Preheat for the more hardenable of the two. |
| Duplex to carbon steel | 309LMo or 2209 | Watch the interpass temperature: duplex is sensitive. |
| Stainless to aluminium | — | Not weldable by fusion. Use a transition insert. |
Buttering
Where dilution cannot be kept low enough, one face is "buttered" first: a layer of the filler alloy is deposited onto the carbon steel side, ground flat, and then the actual joint is welded between the buttered layer and the stainless. The dilution then happens twice, and each time against a smaller composition difference.
It costs an extra operation and it is the standard answer on thick sections and on anything that will see service temperature or cyclic loading.
Practical rules
- Keep dilution low. Low current, stringer runs rather than weaving, and aim the arc at the filler pool rather than into the parent metal.
- Preheat for the more hardenable parent. Work its carbon equivalent out with the CE calculator and use the higher of the two preheat figures.
- Watch the service temperature. Above about 300 °C, carbon migrates from the carbon steel into the stainless over time and leaves a soft decarburised band. That is a design consideration, and it is why nickel-based fillers are specified on hot joints.
- Expect different expansion. Austenitic stainless expands about 50 % more than carbon steel, so a dissimilar joint that is thermally cycled is under stress every cycle whether it is welded well or not.
Frequently asked questions
Why 309L for stainless to carbon steel?
Because it is over-alloyed. When 309L is diluted by carbon steel from one side, the resulting mixture still lands in the austenitic-plus-ferrite region of the Schaeffler diagram rather than becoming brittle martensite. A matching 308L would be diluted below the safe composition and could crack.
What is dilution and why does it matter?
The proportion of parent metal melted into the weld. At 30 % dilution, nearly a third of the weld is parent metal, so the deposit composition is not what the filler wrapper says. On dissimilar joints that shift is what decides whether the weld is tough or brittle.
Can I weld stainless to aluminium?
Not by fusion welding. The two form brittle intermetallic compounds and the joint has essentially no strength. It is done with explosion-welded transition inserts, friction welding or mechanical joints — but not with an arc.
Updated: