Abstract
An acceleration of bainitic transformation was observed with the specimens of two stage austempered spheroidal graphite cast iron by the analysis of dilatometric measurement and the microstructural observation. The result of dilatometric analysis indicates that the nucleation of upper bainitic ferrite is activated in the beginning of the transformation. Microstructural observation shows two types of upper bainitic ferrite αU and αS growing during the second isothermal holding; αU nucleates close to the lower bainitic ferrite, and αS nucleates in pre-transformed austenite matrix independently. And both dilatometric and microstructural studies shows that the acceleration of transformation is caused by the increasing αU with decreasing of αS simultaneously.
