Structure of Iron-Carbon Martensite in the Transition State from the First to the Third Stage of Tempering Studied by Electron Microscopy and Diffraction

The structure of 1.5 mass%C martensite tempered at 470 K for 180 ks has been investigated. In the matrix α-iron, η-Fe2C and θ′-particles coexist. The η-Fe2C particle is the precipitate at the first stage, while the θ′-particle is the precipitate at the early third stage and made of thin layers of θ-Fe3C and χ-Fe5C2 grown microsyntactically. At the transition from the first to the third stage, the θ′-particle nucleates on the surface of the η-Fe2C particle and then grows at the expense of carbon in the matrix. The η-Fe2C particle disappears after the θ′-particle has grown sufficiently large. The carbides making the θ′-particle have slightly larger lattice parameters than the ideal values. Owing to this good coherency is kept with the matrix. When the θ-Fe3C particle appears at the later third stage, it has the ideal lattice parameters, but the coherency with the matrix is not so good as the θ′-particle. Since the θ′-particle and the θ-particle are different in nature from each other, they should be distinguished in discussing the martensite tempering.