In situ observation of recovery of lath structure in 9% chromium creep resistant steel

The recovery process of the lath structure during tempering was investigated by in situ observations of 9% chromium creep resistant steel using TEM. A decrease of dislocation density inside the lath grains and an increase in lath width takes place during tempering. In situ observations revealed lath boundary migration and the disappearance of laths during tempering. Lath boundary migration is caused by repeat bulging and migration of local parts of the lath boundaries. Relatively small laths tend to disappear, leading to a decrease in the total energy of lath boundaries. All laths do not start to disappear at the same time; the rate of lath boundary migration differs with boundaries. The driving force of these microstructural changes is the strain accumulated by the martensitic transformation. Moreover, the recovery of the lath structure does not occur homogeneously during tempering because the strain does not distribute homogeneously in the lath structure.