Effect of Ti and N concentrations on microstructure and mechanical properties of microalloyed high strength line pipe steel welds

A selection of nine heats of line pipe steel from the same project with different Ti and N content were used to evaluate the effects of Ti/N ratio on weld zone microstructure, hardness and prior austenite grain size (PAGS) in the coarse grained heat-affected zone (CGHAZ). It was shown that the microstructure of the base metal (BM) mainly consisted of fine grained polygonal ferrite with fine islands of pearlite. The weld metal (WM) microstructure was predominantly acicular ferrite with a small volume fraction of pro-eutectoid ferrite. The CGHAZ consisted of bainitic ferrite and islands of martensite-austenite. The inside region of the pipe weld zone and base pipe was slightly harder due to the aging effects of the outer weld pass. The prior austenite grain size was shown to be consistent over the range of seam welding conditions and the range of Ti/N from 2 to 4.2.