Epsilon carbide precipitation during tempering of plain carbon martensite

Abstract

Epsilon carbide precipitation in steel martensite has been investigated by means of transmission electron microscopy. The first stage of tempering initiates with the nucleation of very fine ε-carbide particles on the closely spaced parallel line defects, the morphology being so-called “cross-hatched” ε-carbide needles. The ε-carbide particles which produce the well-defined dif-fraction patterns are related to the martensite matrix with a Pitsch and Schrader orientation relationship.^[32] These particles subsequently grow into rods elongated in the direction parallel to the$\langle 100\rangle _\alpha \parallel \langle 11\bar 20\rangle _\varepsilon$ within the matrix. The final reaction in the first stage is the rearrangement of ε-carbide rods into a disklike morphology. The e-carbide rods elongated in the 〈100〉_ε di-rections coalesce on

planes in a raftlike manner, as in the case of those formed in the quench-aged low-carbon ferrite, the tetragonality of martensite being completely lost. Although the deviation from hexagonal symmetry about the [0001]_ε axis exists, no evidence of orthorhombic η-carbide formation was obtained.