The Effect of Niobium on Austenite Decomposition in Line Pipe Steels Available to Purchase

Niobium is a common micro-alloying addition in high-strength low-alloy (HSLA) steels primarily to add strength to the final product. Detailed quantification of the various effects of niobium is critical for understanding the microstructure evolution in the heat affected zone (HAZ) of welds. Laser ultrasonics for metallurgy (LUMet) was used to measure austenite grain growth kinetics in two experimental HSLA steels during continuous heating. For higher heating rates that are of relevance for the HAZ, grain growth during heating is negligible and grain size is determined during the brief time at the peak temperature. Thermal histories were designed based on these tests to produce a variety of microstructures relevant for various positions in the HAZ i.e. coarse and fine grain regions. It was found that the dissolution of niobium carbonitrides has a strong effect on austenite decomposition, particularily in the case of large austenite grain sizes. Nb in solution significantly depresses transformation temperatures in refinement of bainitic microstructures, significantly increasing the hardness in the coarse grained HAZ.