In Situ Kinetic Study on Phase Transformations During the Annealing of Ductile Iron Pipes

Ductile iron pipes (DIPs) are widely used in water supply networks, and their microstructure and performance are strongly influenced by the annealing process. Previous studies have mainly focused on isothermal transformations during the holding stage, whereas the contribution of the heating stage to free cementite decomposition and graphite growth remains insufficiently understood. This study employs high‐temperature laser confocal microscopy to observe, in situ, the phase transformation behavior of DIPs during continuous annealing from 750 to 1000 °C. The results show that pearlite decomposition starts at 850 °C, while free cementite decomposition occurs above 900 °C during heating. When the temperature reached 1000 °C and was held for 5 min, most free cementite decomposed, while a certain amount of residual cementite remained. The average area reduction rate of free cementite was 2.00 μm 2 s −1 , while the graphite growth rate was 0.28 μm 2 s −1 . Based on these observations, a process‐specific solid‐state phase transformation kinetic description was established for the continuous annealing process of DIPs. This study addresses the limitations of isothermal phase transformation kinetic models in the annealing of DIPs, providing a kinetic reference for optimizing the heating and holding stages of DIP annealing.