Weld overlay (cladding)

Weld overlay and cladding name the same thing done for the same reason: put a different, more expensive alloy on top of a cheaper structural base metal, so the finished part gets the corrosion or wear resistance of the expensive alloy without being built from it entirely. Nothing is being joined in the sense a butt or fillet weld joins two parts — the base metal is the substrate, and the deposit is the product.

That purpose makes dilution — the fraction of the deposit that actually comes from melted base metal rather than filler — the single variable an overlay procedure exists to control. Every other welding variable that matters elsewhere still matters here, but it matters through dilution: heat input, travel speed and process choice change dilution first, and the deposit’s corrosion or wear performance only second, as a consequence.

The decision rule that follows needs no number attached to a standard to be usable. A corrosion-resistant overlay specification fixes the alloy chemistry that has to be present at a stated depth below the finished surface — not a dilution percentage on its own — because that depth is what the corrosive process fluid actually reaches. ASME Section IX qualifies corrosion-resistant overlay procedures by chemical analysis taken at that specified depth, exactly because a single dilution figure without a depth attached does not say whether the protective chemistry survives where it needs to. If one deposited layer cannot reach the required chemistry at that depth, the answer is not to chase a lower dilution on that same layer — it is to add a second layer, because a second pass dilutes against the first layer’s already-close chemistry, not against the base metal, and reaches the target far more reliably than trying to force one pass to do it alone. Hardfacing overlay, by contrast, is qualified on macro-examination and hardness rather than chemistry at depth — the wear mechanism it resists cares about surface hardness, not bulk composition — so this depth-chemistry rule applies to the corrosion-resistant case, not to every overlay.

Dilution is controlled through the same mechanism that governs penetration: energy into the weld pool and how long that energy has to act. A single-pass, high-energy process dilutes heavily; multi-layer strip cladding, where later passes barely touch the original base metal, dilutes far less — which is why the process and layer count, not just the parameter set within one process, are part of the same procedure decision.