1985 Volume 26 Issue 12 Pages 876-885
In situ high temperature electron microscopic observations of 1.5 mass%C martensitic steel plates show that at the third stage of tempering a small θ-Fe3C region is formed at first on the surface of θ′-particle, which is formed below 550 K and composed of microsyntactically intergrown various thin carbide layers, and then grows to a nearly perfect θ-particle at the expense of θ′-particles. At the matrix grain boundary the θ-region is formed at 550 K or lower, while in the grain it is formed at about 700 K. During the tempering, the coalescence and disappearance of θ′-particles take place in a complicated way. Some of the θ-particles in the grain dissolve into the matrix above about 800 K to make other θ-particles grow. The matrix recovery starts at about 700 K. The heat evolution at the third stage, which finishes at 730–750 K, can be ascribed to the transition from θ′-particles to θ-particles and the matrix recovery. In Appendix, high resolution study of planar defects in a θ-particle remaining in a tempered specimen at 870 K is described.