Effect of Nb microalloying on the heat affected zone microstructure of X80 large diameter pipeline after in-field girth welding

The effect of Nb content in the range 0.07%-0.10% on the HAZ microstructure of X80 large diameter pipes is reported in this paper. Results show that, despite the difference in Nb content, no differences have been found in the prior austenitic grain size and consequently in the local hardenability. On the other hand, Nb is able to influence the size of the bainitic packets and cells in the heat affected zone, i.e. the microstructural parameters affecting impact toughness and hardness behavior. Such an effect leads to an improvement of both toughness and hardness for the steel with higher Nb content. (AU) Copyright © 2013 Companhia Brasileira de Metalurgia e Mineração (CBMM) All Rights Reserved