Study on the mechanism of heat input on the grain boundary distribution and impact toughness in CGHAZ of X100 pipeline steel from the aspect of variant

Highlights: • Variant selection of CGHAZ is stronger than 9 and 21 kJ/cm, and the V1/V1 becomes the main variant pair at 4 kJ /cm. • As the heat input decreases, the variant pairs transform from V1/V4/V8 to V1/V2 and finally to V1/V1. • Compared with the V1/V2, V1/V1 and V1/V4/V8 decrease the density of HAGB, thereby reducing the impact toughness in CGHAZ. • The effective grain boundary is composed of all the Block boundaries and a part of Packet boundaries. The variant selection and pairing directly affect the grain boundary distribution in the coarse-grained heat-affected zone (CGHAZ) of pipeline steel and consequently impact toughness. In this study, we attempted to establish a direct correlation between heat input, variant, grain boundary distribution, and impact toughness in CGHAZ of pipeline steel by means of electron backscatter diffraction (EBSD) analysis. The results showed that the variant selection is strong at 4 kJ/cm, such that V1/V1 becomes the dominant variant pair in CGHAZ. As the heat input increases, the transformation strain decreases, such that the mean variant pair in CGHAZ is V1/V2 and V1/V4/V8 at heat inputs of 9 and 21 kJ/cm, respectively. Compared with V1/V1 and V1/V4/V8 variant pairs at 4 and 21 kJ/cm, respectively, V1/V2 variant pair promotes the formation of high-angle grain boundaries (HAGB) and decreases the residual stress. When the heat input is 9 kJ/cm, the microstructure of CGHAZ has maximum resistance to crack propagation and lower crack nucleation ability, leading to the highest impact energy.