EBSD characterization of HAZ from single and multipass welding of niobium microalloyed linepipe steels

EBSD studies on the HAZ have confirmed that there is significant loss in the density (number count per unit area) of high angle boundaries in HAZ regions in single-pass welding from high heat inputs (>35 kJ/cm). The microstructure is deprived of microcrack arresters due to loss of high angle boundaries at slow cooling rates (high t8/5), when transformation is controlled by a thermally activated diffusion mechanism. Thus, in the absence of crack arresters, any microcrack nucleated by hard and brittle MA product could grow to attain the Griffith critical crack length to initiate brittle fracture in accordance with the Cottrell-Petch model. The research has identified the target microstructure with optimum density and dispersion of crystallographic high angle boundaries required to obtain maximum toughness at low temperature in the HAZ region in single-pass welding associated with a low temperature window of transformation in higher niobium bearing higher grade linepipe steels. The effect of increasing heat input of welding on the hierarchical evolution of microstructure and crystallographic high angle boundaries within austenite grains in the HAZ was investigated in higher Nb microalloyed higher grade (X80 and X100) linepipe steels. Pole figure analysis of crystallographic data captured by EBSD analysis is used to identify crystallographic relationships of coherent transformation products with the parent austenite grains. It is shown that high angle boundaries are formed between crystallographic units with different Bain groups within each packet upon coherent transformation of austenite grain in the target microstructure with maximum toughness. This work has demonstrated that a uniform dispersion of high density of high angle boundaries, due to large misorientations between different Bain groups occurring within austenite grains, could be promoted by a low temperature window of transformation associated with an optimum cooling rate. Niobium addition is effective in lowering the transformation temperature because niobium dissolved in the matrix inhibits ferrite nucleation at austenite grain boundaries. Further, interphase precipitation of NbC retards growth of ferrite grains. Thus niobium addition is beneficial in lowering the transformation temperature to obtain a high density of high angle boundaries due to large misorientations between different Bain groups occurring within austenite grains in single-pass welding.In multi-pass low heat input welds typical of the pipeline girth welding process, the effect of nickel addition on structure and toughness of inter-critically reheated coarse grained austenite, IRCGHAZ was investigated with simulated weld thermal cycles. High Ni addition to Nb microalloyed steel is shown to increase the toughness compared with low Ni in both CGHAZ and IRCGHAZ. This is attributed to suppression of both MA formed from continuous cooling in the CGHAZ and massive MA along prior austenite grain boundaries in the IRCGHAZ. (AU)