The controlled, _ne-grained microstructure of thermomechanically processed Nb microalloyed linepipe steels is destroyed in the vicinity of welds used in fabricating pipelines. There are conflicting views on the influence of niobium in the `heat-affected zone', particularly in the region closest to the weld fusion line which is most dramatically impacted by the thermal cycling that occurs during welding. Consequently, there is a need to fully characterise the influence of niobium on the evolution of structures and properties in this zone.

The aim of the work presented in this thesis was to quantify and characterise precipitates of niobium and dissolved niobium across sub-zones of the weld heat-affected zone, in order to develop a better understanding of the effects of niobium across the region. In order to achieve this, heat treatments were undertaken for the first time to simulate each sub-zone of the heat affected zone such that unique states of dissolved niobium and precipitated niobium was developed. A novel technique as designed and applied for the first time to measure and quantify the precipitate sizes and size distributions in bulk samples of Nb micro-alloyed steels. In addition, measurements of the dissolved niobium across the heat-affected zone were completed in order to ensure that the discrete effects of all states of niobium were subjected to analysis.

Weld simulations of the coarse-grained heat-affected zone, the region closest to the weld fusion line, were conducted and assessed against the measured states of niobium. This was followed by the manufacture of commercial welds in order to assess the variation of structures and properties across the heat-affected zone for different plate conditions generated by heat treatment prior to welding. This work established that that a wide range of niobium carbide precipitate sizes were crucial in assuring the excellent mechanical properties in the line-pipe steel, coarser precipitates were found to control the austenite grain size that evolved in the coarse-grain heat affected zone, while fine precipitates dissolved in the thermal cycles close to the weld fusion line, and produced finer microstructures.