Investigation on the microstructure and toughness of coarse grained heat affected zone in X-100 multi-phase pipeline steel with high Nb content

Effect of increasing heat input on microstructure evolution and impact toughness of coarse grained heat affected zone (CGHAZ) in high Nb X-100 multi-phase pipeline steel was investigated by means of Gleeble simulator, optical microscope (OM), scanning electron microscope (SEM) and electron backscattering diffraction (EBSD). Charpy impact test confirmed the optimum toughness of CGHAZ was achieved at heat input of 20 kJ/cm, equivalent to the excellent toughness of the base plate. Observations performed by OM, SEM and EBSD show that the microstructure of CGHAZ varies dramatically with heat input without a noticeable changing in prior austenite grain size, and the optimum toughness achieved at the heat input of 20 kJ/cm is related to the cumulative contribution of its well-refined martensite/austenite (M/A) constituent and the highest density of high angle boundaries. Analysis on crystallography shows that high angle boundaries are mainly the boundaries between the products from different Bain groups produced from the fcc to bcc coherent transformation within prior austenite grain, and the density of high angle boundary is controlled by the configuration of Bain groups within the crystallographic packet in each austenite grain. With the ideal configuration, the density of high angle boundary can be optimized to be beneficial to keep high toughness in CGHAZ, together with well-refined M/A constituent. This indicates that in addition to M/A refinement, the characteristic in crystallography of the crystallographic packet (the configuration of Bain groups within it) is related to the mechanical properties of CGHAZ and should be controlled to be optimum.