Effect of Niobium on the Weld Metal Toughness of Pipeline Steels
Niobium has been a well-recognized microalloying element to improve the strength and toughness of low carbon steels. However, concerns have been raised about the inconsistent weld metal toughness with high-Nb based steel after submerged arc welding (SAW). To understand the fundamental mechanisms of how Nb interacts with weld metal toughness, in the 1st phase of the project, Nb controlled SAW process is performed, where different levels of Nb contents are introduced into the weld through flux with mechanically mixed ferro-Nb powders. Charpy V-notch (CVN) specimen were prepared, where the notch is located in the center of the weld. According to the impact test results, high Nb weld metal (0.084wt%) can only absorb less than half of the energy as the weld with 0.014wt%Nb. On the other hand, the optical microscopic images revealed that under a higher concentration of Nb, the prior austenite grain size, the amount and width of ferrite at prior austenite grain boundary is smaller. In addition, the volume fraction of acicular ferrite is increased. This is in contradictory to conventional findings where promoted acicular ferrite formation should enhance toughness.
Industry Sponsor: Arcelor Mittal
Graduate Student: Tianzhao Wang
Industry Contact: Dr. Murali Manohar, Dr. Wenkao Hou