How can multiple welding passes change the structure of weld metal?

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Welder in a blue uniform and protective helmet using shielded metal arc welding to join metal pieces, with bright sparks and smoke visible at the welding point. — welding Shielded metal arc welding.

In fusion welding, the metallurgy of the joint is significantly influenced by the number of passes used to create the weld.

A single-pass weld typically exhibits a cast structure characterized by columnar grains that grow from the edges of the molten pool toward the center of the weld. When multiple welding passes are employed, this initial cast structure can be modified. The specific nature of this modification depends on the particular metal being welded, as subsequent passes subject the previously deposited weld metal to additional thermal cycles.1

These cycles can alter the microstructure of the underlying layers, potentially refining the grain structure or changing the mechanical properties of the fusion zone. Understanding these changes is essential, as the composition and solidification conditions of the weld metal directly determine whether a joint will meet its required service specifications.1