Microstructural evolution in ultra-low-carbon steel weldments - Part I: Controlled thermal cycling and continuous cooling transformation diagram of the weld metal

The transformation behavior and microstructural evolution of the as-deposit ed weld metal from an ultra-low-carbon (ULC) weldment were characterized by dilatometry, optical microscopy, transmission electron microscopy, and mic rohardness measurements. These results were used to construct a continuous cooling transformation (CCT) diagram for this weld metal. The major microco nstituents observed in this ULC weldment were tin order of decreasing cooli ng rate) coarse autotempered martensite, fine lath martensite, lath ferrite , and degenerate lath ferrite. No polygonal ferrite was observed. These res ults were also used to develop criteria to differentiate between the two pr edominant microstructures in these ULC steels, lath martensite, and lath fe rrite, which can look quite similar but have very different properties.