An electron microscopic study of spheroidal graphite nodules formed in a medium-carbon steel by annealing

Abstract

The detailed evolution of the graphitized microstructure of carbon steel during annealing has been studied by thin-foil transmission electron microscopy, enabled by the refined dispersion of small graphite nodules produced. The graphitization annealing of 0.38 wt.% carbon steel in the quenched martensitic state resulted in the formation of graphite nodules, of the order of 2–5 μm in diameter, after 2–4 h. Two types of graphite nodules were observed to form: one possesses an irregular morphology and exhibits a coring particle (either a nitride or an oxide), whilst the other is more regularly spheroidal and apparently without a nucleating particle at its core. Analytical electron microscopy has been employed to characterize the microstructure of the graphite nodules formed, in order to provide a better understanding of their nucleation and growth, in particular, the mechanism of formation of the spheroidal graphite nodules without an apparent nucleating particle. For these more spherical nodules, the results showed that the outer mantles had a strong graphitic character, whilst the core regions exhibited a more amorphous carbon structure. It is deduced from the available experimental evidence that the nucleation sites most likely to result in this structure are dissolving cementite particles in the martensite.