Welding Dissimilar Metals: A Complete Guide

Welding dissimilar metals – joining two base metals of different chemistry, such as carbon steel to stainless steel, or stainless to a nickel superalloy – is one of the most common sources of avoidable weld failure in industry. The joint often looks perfectly normal after welding and only fails weeks or months later from brittle cracking or accelerated corrosion, because the real problem was never visible on the surface: it was decided by the resulting weld metal chemistry the moment dilution mixed the two base metals with the filler.

This guide walks through the tools engineers actually use to plan these joints – the Schaeffler and WRC-1992 diagrams, dilution estimation, and galvanic compatibility – along with practical filler selection tables for the combinations you will meet most often: carbon steel to stainless steel, stainless to duplex, and steel to nickel alloy. We will also cover the buttering technique used when the two sides of a joint need different heat treatments.

If you already work with duplex stainless steel or creep-strength enhanced ferritic steels like P91, you have likely run into dissimilar joints already without necessarily framing the filler choice in these terms. This guide gives you the underlying logic so you can apply it to any combination, not just the ones covered by a standard’s lookup table.

Why Dissimilar Joints Fail Differently

A similar-metal weld fails, when it fails, for reasons you would predict from the base metal alone – hydrogen cracking, lack of fusion, porosity. A dissimilar joint adds three failure modes that do not exist in similar welding at all.

Dilution changes the weld metal’s identity

Every arc welding pass melts some of both base metals into the filler. Dilution is calculated as the base metal contribution divided by the total weld metal, expressed as a percentage. If you weld carbon steel to stainless steel using ordinary stainless filler, the iron from the carbon steel side dilutes the chromium and nickel content of the pool enough that the resulting deposit can land in the brittle martensite region of a constitution diagram rather than the ductile austenitic region the filler was chosen for.

Brittle and low-melting phases at the fusion boundary

When two dissimilar alloys such as low-carbon steel and austenitic steel are fusion welded, the joints or heat-affected zones often exhibit low ductility and poor creep properties, since brittle martensite is more prone to form and the weld becomes more susceptible to hot cracking from low-melting impurities introduced by the carbon steel. An additional structure called an unmixed zone can form directly at the fusion line, where base metal melts and resolidifies without actually blending with the filler, sometimes producing hard, untempered martensite even inside an otherwise correctly-selected weld.

Galvanic corrosion risk

In a conductive environment, joining two metals with different electrochemical nobility creates a galvanic cell. The less noble metal becomes the anode and corrodes preferentially. A filler metal chosen only for mechanical compatibility can inadvertently become the anode relative to the surrounding base metals, concentrating corrosion into a thin weld bead – a very different, and often more dangerous, failure mode than uniform corrosion of a plate.

The Schaeffler and WRC-1992 Diagrams

Welding engineers do not guess when it comes to dissimilar metals; they use predictive diagrams. The Schaeffler diagram, and the more modern WRC-1992 diagram, plot chromium equivalent (ferrite-forming elements) against nickel equivalent (austenite-forming elements) to predict the microstructure of the finished weld.

By plotting the composition of the filler wire and the composition of the base metal and connecting them with a line, the resulting weld composition falls somewhere along that line depending on the dilution ratio. This lets an engineer confirm, before any welding happens, that the finished weld will land in a safe region of the diagram rather than the brittle martensite zone.

Filler Metal Selection by Combination

AS/NZS 1554.6 lookup approach

Industry standards frequently simplify this into a direct lookup: carbon steel to 304(L) uses 309L filler, and carbon steel to 316(L) uses 309LMo filler. These pre-validated combinations are a reasonable starting point for common joints, but any combination outside a standard table should be checked with a constitution diagram rather than assumed safe.

Controlling Dilution in Practice

Dilution is primarily controlled by weld geometry and welding parameters; for a given filler metal feed rate, dilution increases with higher heat input. Practical dilution control techniques include:

• Process selection: GTAW gives the lowest, most controllable dilution and is the default choice for root passes on dissimilar joints; SAW and other high heat-input processes push dilution higher and need closer monitoring.
• Arc placement: The welding arc can be manipulated so it primarily impinges on the base metal nearest in composition to the filler, reducing dilution from the more dissimilar side of the joint.
• Arc drift toward steel: When joining nickel-base alloys to carbon or low-alloy steels, the arc may tend to drift toward the steel side, so short arc length and careful torch or electrode manipulation are needed to compensate.
• Multi-layer buildup: Building up the first layer or two with lower current before increasing deposition rate on fill passes limits how much of the dissimilar base metal enters the final weld composition.

The Buttering Technique

Buttering solves a specific problem: two base metals that each need a different, sometimes incompatible, heat treatment. A layer of compatible filler metal is deposited onto the more demanding side (commonly a low-alloy or creep steel requiring PWHT) and that buttered layer is heat treated on its own before the dissimilar joint is finally completed and welded together.

Galvanic Compatibility

Beyond metallurgical compatibility in the as-welded structure, dissimilar joints in wet, humid, or immersed service need a galvanic check. Selecting a filler that sits close to the more noble of the two base metals on the galvanic series, rather than close to the less noble one, avoids concentrating corrosion attack into the weld bead itself.

Inspection Considerations for Dissimilar Joints

• Hardness surveys: A hardness traverse across the fusion boundary confirms no untempered martensite or unmixed zone formed at unacceptable hardness levels.
• Ferrite number checks: On stainless-to-steel joints, a ferrite number check (magnetic gauge or metallographic point count) confirms the weld deposit landed in the targeted 3-8 FN window rather than drifting into a brittle or corrosion-prone extreme.
• Volumetric and surface NDT: Radiography or UT per the governing code, supplemented with liquid penetrant testing, since fine cracking at the fusion boundary can be easy to miss on radiographs alone.
• Corrosion testing: Where service is aggressive, ASTM G48-type testing can confirm the actual weld deposit chemistry delivers the corrosion resistance the diagram-based prediction assumed.

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