2006 Volume 92 Issue 10 Pages 609-617
The microstructure of a Type IV damage, a ductile and early creep failure at outer edge of Heat Affected Zone of weld, was precisely researched. The rupture life of a simulated fine grain area of HAZ was the shortest compared with those of the other microstructures, coarse grain HAZ and dual phase HAZ, according to the temperature acceleration creep test at 700°C. The tensile strength at room temperature was the lowest at around Ac1 transformation point, determined by rapid heating diratometry. Therefore, the Type IV damage did not coincident the conventional HAZ softening phenomenon often observed in low carbon steels.
Dislocation substructure of the fine grain zone is composed of the globular sub-grain microstructure and the coarsened carbide through the Transmission Electron Microscope observation of thin foils. They were possibly explained to be formed through the thermal cycle of HAZ and Post Weld Heat Treatment as follows: a base metal with lath martensite microstructure is warmed above Ac3 point once, and immediately cooled and transformed. Such "weak" lath martensite structure, apparent ambiguous lath martensite, was easily recovered to the globular sub-grain microstructure. Based on the hypothesis above mentioned, globular sub-grain microstructure at fine grain HAZ possibly resulted in the decrease of the creep life.