In Situ High Temperature Electron Microscopic Study of the Formation and Growth of Cementite Particles at the Third Stage of Tempering of Martensitic High Carbon Steel

抄録

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.