Abstract
Investigation was made of the structure of crack region observed at the center of the first weld layer of AN 31 alloy steel. The weld crack originated in the boundaries of dendritic grains containing the primary and secondary precipitates. From the results of the X-ray, microphotographic and differential thermal analyses, it is clearly to be seen that the primary precipitates are Nb (C·N) intermetallic compound consisting of lattice constant: 4.404Å which does not dissolve in matrix even by heating at 1300°C, and that the secondary fine precipitates that appear in the grain boundary regions by tempering at about 900°C are the same as Z phase (MNb2) which were reported by Andrews et al. (1957).
From these results, it is evident that the weld crack had originated in the incoherent interfaces of the matrix and the primary precipitates at higher temperature than 1000°C in rapid cooling stage after solidification.
