The use of low carbon concept steel with a niobium content of up to 0.11%, submerged arc welding (SAW) during pipeline fabrication, and lower heat input processes such as gas metal arc welding (GMAW) during field seam welding can improve the performance of the welded area of the pipeline. These significant advantages are mainly due to the resistance of HAZ grain coarsening during welding.
The microstructure formed in the heat-affected zone (HAZ) immediately adjacent to the weld is a function of many variables. These factors include the composition of the steel, the thickness of the plate or coil, the choice of welding process, heat input, welding speed and number of passes. All of these factors affect the rate of heating and cooling during and after welding. Ultimately, all of these variables determine the grain size formed in the coarser HAZ and determine the mechanical properties of this important region.
This grain refinement can be further understood by using electron back scattering diffraction (EBSD) analysis of welded joints of the following steels. This effect is maximized over a large heat input range when niobium content is in the range 0.08% to 0.11%. This important role, combined with other unique properties of niobium in solution, ensures optimal heat-affected zone microstructure and toughness during pipe fabrication and subsequent ring seam welding under all relevant conditions.
Electron back scattering diffraction (EBSD) images show the microstructure changes of steels A and B from the weld metal to the base material. The article from these reproduced images shows that an increase in niobium content from 0.07% to 0.10%, coupled with an appropriate "best practice" rolling process, ensures an optimal size distribution of precipitates in B steel with a higher niobium content. This reduces the range of coarse crystalline heat affected zone (CGHAZ) from 275μm in 0.07% Nb steel to 125μm in 0.10% Nb steel. These finer grains in coarse-grained heat affected zones (CGHAZ) result in finer microstructure (finer lath bundle size).
