In this study, the relationship between impact toughness and microstructure in Cr–Mo–V multi-pass weld metals has been systematically investigated. The Charpy impact energy of two weld metals with various alloy elements increased remarkably. The primary cause of the change of impact toughness was attributed to the difference of acicular ferrite (AF) content and prior-austenite grain size, and the size and distribution of necklace martensite–austenite (M–A) constituents. With increasing Ti content, Ti-containing inclusions were increased, which resulted in an increased number of nucleation sites for AF, a change in the microstructure from allotriomorphic ferrite to AF, and refined ferrite grain size. In addition, smaller and more dispersive M–A constituents were observed in the weld metal with higher impact toughness.
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