Microstructure development of creep resistant ferritic steel during creep

Creep resistant steel samples were subjected to creep loading at 550 °C for different periods of lifetime in order to observe the development of the microstructure. After about 500 h, a local maximum was observed in the strain of the individual grains (monitored by electron back scattering diffraction, using the grain orientation spread and grain average misorientation parameters), after which a relaxation occurred. Based on transmission electron microscopic images, it is assumed that carbide precipitates caused pinning of the dislocations, and only after sufficient accumulation at the carbides could the dislocations move. After about 500 h, the sub-grain size increased, as well as the average distance between carbides. This caused enhanced movement of dislocations, and the slope of the creep curve increased.