Study on In-Situ Nucleation, Growth Kinetics and Crystallographic Structure of Acicular Ferrite in X100 Pipeline Steel Welds

Acicular ferrite has appeared in various low carbon microalloyed steel welds (e.g., laser-arc hybrid welds) due to its high grain boundary density and toughness. However, the transformation kinetics and crystallographic structure characteristics of acicular ferrite in laser-arc hybrid welds are still unclear. In this paper, the nucleation and growth kinetics of acicular ferrite in laser-metal active gas hybrid welds with different cooling rates (1–150 K/s) were investigated and their crystallographic characteristics were revealed from the perspective of variant. The result showed that acicular ferrite can nucleate at all the test cooling rates in weld metal, and the microstructure coexisting with acicular ferrite changes from polygonal ferrite and granular bainite with a low cooling rate to lath martensite with a high cooling rate. As the cooling rate increases, the phase transformation temperature of acicular ferrite decreased, and the phase transformation mode changed from partitioning local equilibrium to negligible partitioning local equilibrium. The variety of the phase transformation mode and the higher phase transformation driving force led to an increase in the growth rate of acicular ferrite. Compared with the bainite and martensite in the base metal, the acicular ferrite had the characteristics of light variant selection, uniformly distributed operating factor and small variant size, which resulted in high-density effective grain boundaries. To obtain more acicular ferrite, the cooling rate should be increased as much as possible on the premise that the phase transformation termination temperature was higher than the Ms.