Weld metal alloy systems for seam welding of niobium micro-alloyed pipe steels

The mechanical properties of niobium alloyed line pipe steels are achieved during thermomechanical controlled processing (TMCP) and through the formation of niobium carbides and nitrides. More recent developments of higher strength pipe utilize levels of niobium up to 0.11%, achieving exceptional mechanical properties in the presence of additional conventional alloying with elements such as molybdenum and chromium. However, the cooling rates associated with submerged arc welding will not allow the development of the fine grained bainite and ferrite microstructure created during TMCP of the base metal. The weld metal strength and toughness will depend primarily on the influence of alloy concentration upon solidification. Two-run seam welding of pipe often results in 60 percent admixture of the pipe body in the weld metal. The concentration and type of micro-alloying agents introduced from the base metal can have significant effects on weld metal strength, toughness and hardness. In order to achieve the desired balance of weld metal mechanical properties, the alloying additions from the welding consumables are critical. (AU) Copyright © 2013 Companhia Brasileira de Metalurgia e Mineração (CBMM) All Rights Reserved