How do columnar grains form in a one-pass weld?

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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 a one-pass weld, the formation of columnar grains is a direct result of the solidification process that occurs after the welding arc passes. As the molten pool cools rapidly, the metal solidifies by growing grains that extend from the cooler edges of the fusion zone toward the center of the weld.1

This specific grain structure is characteristic of the cast structure formed within the weld metal, which consists of the melted base metal and any added filler material. Because the cooling occurs from the fusion boundary inward, these grains develop in a columnar orientation, reflecting the direction of heat extraction through the surrounding base metal.1