The Substructure of Quenched High-Carbon Pearlite in Fe-C Alloys

After a brief review of the history of pearlite structures in carbon steels, particularly on the pearlite formation mechanism, recent experimental investigations on the pearlite substructure are presented to express a distinct point of view. The water-quenched high-carbon pearlite substructure is investigated in detail by means of scanning electron microscopy and transmission electron microscopy. In the experimental observation results, it is shown that the cementite layer or ferrite layer in pearlite is composed of fine grains, which cannot be simply explained by traditional nucleation and grain growth mechanisms. However, the fine grain structure can be explained by the martensitic transformation products (twinned martensite with ultrafine grains of alpha-Fe and twinning boundaries omega-Fe (or omega-Fe3C)) and detwinning process. Upon tempering or detwinning, recrystallization of the ultrafine grains of both crystalline phases occurs to form the initial pearlite structure, while the grain size of both phases is still fine. The twinned martensite can be treated as the precursor of pearlite structure (pearlite nucleation stage), and the detwinning process can be regarded as the growth of the pearlite structure. Thus, the pearlite reaction can be described as follows: austenite -> twinned martensite -> pearlite. Scanning electron microscopy images of the pearlite structure in the boiling-water-quenched Fe-1.4C (wt%) alloy: a) low-magnification image and b) high-magnification image. The bright "layers" (cementite layers) are formed by the alignment of fine cementite particles, which have a uniform size distribution and almost the same orientation. Each bright-contrast dot in the high-magnification image corresponds to one cementite particle.image (c) 2024 WILEY-VCH GmbH