Welding Dissimilar Metals: A Complete Compatibility Guide

Joining two different base metals is routine in piping, pressure vessels, and cladding work, but it fails more often than a similar-metal weld because dilution, thermal mismatch, and diffusion at the interface all work against you at once. This guide sets out how to assess compatibility, select filler metal using the WRC-1992/Schaeffler approach, and control the three failure mechanisms that dominate dissimilar joints: hot cracking, carbon migration, and galvanic corrosion.

Why Dissimilar Metal Welds Fail More Often

A similar-metal weld only has to reconcile one alloy system across the fusion boundary. A dissimilar weld has to reconcile two, and the differences compound in the fusion zone itself, not just at the interface. Four properties matter most:

1. Coefficient of Thermal Expansion (CTE) Mismatch

Austenitic stainless steel expands roughly 40-50% more per degree than carbon or low-alloy steel. On cooling from welding temperature, or during thermal cycling in service, this mismatch generates residual stress concentrated right at the fusion line — the same location where microstructural weaknesses from dilution and carbon migration already exist.

2. Melting Point and Solidification Range

Pairs like copper-to-steel or aluminum-to-steel have melting points hundreds of degrees apart. The lower-melting-point metal can be substantially molten while the higher-melting-point metal is barely wetted, producing incomplete fusion, base metal cracking, or a weld pool that solidifies with segregated, brittle constituents.

3. Metallurgical Compatibility and Solid Solubility

Some pairs form stable solid solutions across a wide composition range (iron-nickel, iron-chromium). Others form brittle intermetallic compounds even in small quantities (aluminum-iron, titanium-iron, copper-aluminum). No amount of procedure control fixes a pair that is fundamentally metallurgically incompatible; a transition joint becomes necessary.

4. Post-Weld Heat Treatment (PWHT) Requirements

A low-alloy steel side may require PWHT for hydrogen removal and stress relief, while an austenitic stainless or nickel-alloy side does not need it and can be degraded by it (carbide precipitation, sensitization). The joint design has to satisfy the more demanding side without damaging the other.

Dilution and the WRC-1992/Schaeffler Approach

Dilution is the fraction of the finished weld metal that comes from melted base metal rather than filler metal. In dissimilar joints it typically runs 10-40% per pass depending on process, joint geometry, and heat input, and it is the single variable with the largest effect on final weld metal properties.

The practical method is to calculate chromium and nickel equivalents for both base metals and the filler, then use the iron-carbon phase relationships alongside the WRC-1992 diagram to trace where the diluted composition lands.

Cr_eq = %Cr + %Mo + 1.5(%Si) + 0.5(%Nb)
Ni_eq = %Ni + 30(%C) + 0.5(%Mn)

Diluted composition (element X) =
  (D × %X_basemetal) + [(1 − D) × %X_filler]

where D = fractional dilution (e.g. 0.30 for 30% dilution)

The target zone on the diagram is austenite-plus-ferrite (A+F): enough ferrite (typically 3-15 FN) to resist solidification hot cracking, but not so much that the joint becomes fully ferritic and loses toughness or corrosion resistance. A weld metal that lands in the pure martensite region is at high risk of cold cracking, particularly with hydrogen present, and normally requires preheat and controlled interpass temperature at minimum, with PWHT often needed as well.

Common Dissimilar Metal Combinations

Carbon Migration at the Interface

When a low-alloy steel is joined to a high-chromium alloy and later exposed to elevated temperature, whether during PWHT or in service above roughly 450°C, carbon diffuses from the low-alloy side toward the chromium-rich side. Chromium binds carbon as stable carbides and lowers its chemical activity, creating a steep activity gradient that drives long-range diffusion even though no melting is involved.

The result is a decarburized, softened band on the low-alloy steel side and a carburized, hardened band on the chromium-rich side, sitting directly at the location of peak residual stress from CTE mismatch. This combination — soft base metal, hard adjacent zone, high local stress — is a classic setup for creep cavitation and premature failure in elevated-temperature piping, which is why HAZ microstructure control is treated as a first-order design concern in these joints, not an afterthought.

A nickel-based filler (ERNiCr-3 or similar) is frequently substituted for a 309-type stainless filler specifically because nickel has a lower carbon activity gradient against low-alloy steel than chromium does, which slows the migration rate substantially compared with a straight stainless transition layer.

The Buttering Technique

Buttering means depositing a filler layer onto one base metal face before the joint is fitted up and finish-welded. It solves three problems at once in dissimilar joints:

Sequence

1. Deposit the buttering layer (typically 3-4 passes, 6-10 mm thick) onto the more heat-treatment-sensitive base metal, using a filler compatible with that base metal.
2. Perform any PWHT required by that base metal on the buttered piece alone, before the joint is assembled.
3. Machine the buttered face flat and fit up the joint.
4. Complete the final weld between the buttering layer and the second base metal, using a filler compatible with that interface.

Buttering is standard practice on Cr-Mo steel to austenitic stainless steel piping in refinery and power generation service, and on repair welds where a component with an unknown or difficult-to-match composition must be joined to a known alloy. It is covered under the same heat-treatment logic used for quenched-and-tempered steel repair welds, where controlling the thermal cycle at each stage is more important than the filler metal choice alone.

Galvanic Corrosion in Dissimilar Joints

Even a metallurgically sound dissimilar weld can fail in service from galvanic corrosion if the joint is exposed to an electrolyte (moisture, process fluid, seawater). The weld metal, HAZ, and both base metals typically occupy different positions in the galvanic series, and the further apart two connected metals sit, the higher the driving force for the less noble one to corrode.

This is closely related to the mechanisms covered in our corrosion mechanisms overview and in the discussion of pitting corrosion, both of which can initiate preferentially at a dissimilar weld interface even when galvanic driving force alone would be modest.

Pairs That Should Not Be Fusion Welded

Some combinations are metallurgically incompatible regardless of filler metal or procedure, because they form continuous brittle intermetallic layers at any practical fusion temperature:

Procedure Qualification for Dissimilar Joints

Under ASME IX and similar codes, a procedure qualified on one base metal combination does not automatically qualify a different combination — dissimilar joints are their own P-Number to P-Number qualification. Essential variables that carry extra weight for dissimilar joints include filler metal classification, preheat and interpass temperature, and PWHT temperature and time, since these directly control dilution behavior and carbon migration rate. Review procedures against the same rigor used for hydrogen cracking prevention in similar-metal high-strength steel welds, since preheat control serves an analogous crack-prevention role here.

Industrial Applications

Dissimilar metal welds appear throughout process industries: carbon steel to stainless steel transitions in refinery piping where corrosive service starts partway along a line; Cr-Mo steel to austenitic stainless steel welds at furnace outlet headers in power and petrochemical plants; nickel-alloy overlay welds (weld overlay/cladding) applied to carbon steel pressure vessel internals for corrosion resistance without solid alloy construction; and dissimilar pipe spools joining new alloy piping to existing carbon steel systems during plant modifications. In every case, the design intent is the same: get the corrosion or high-temperature resistance of the more expensive alloy only where it is needed, using dissimilar welding to transition back to carbon steel elsewhere.

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