Peritectoid carbide transformation based on ε-carbide Fe2C in Fe-C system alloys. Part 2. Metallographic studies

Abstract

Original metallographic studies of annealed medium carbon steel (steel 45, steel 40X and steel 35XGA) are carried out in order to reveal leakages in pearlite of these steels of low-temperature carbide transformation of peritectoid type, in which solid solutions of ferrite and cement form a solid solution of wide area of homogeneity based on ε-carbide Fe2C. Most of the inclusions of pearlite cement 45 steel are almost entirely covered with a light grey “foam” shell of nanoglobular crystals ε-сarbide Fe2C. In the process of crystallization of ε-сarbide Fe2C on the cement plates of pearlite steel 45 three morphological types of structure of ε-сarbide Fe2C can be distinguished: “foam” globular shell, granular “outgrowths” and wrapping of particles of decomposed and partially dissolved cement plates. Chromium, which is a part of steel 40X, abruptly inhibits peritectoid transformation. On many cement plates pearlite surface is smooth. In areas where the concentration of chromium is low, the process of formation of ε-сarbide Fe2C is active, with the formation of individual sites with light gray “foam” shell of nanoglobular crystals ε-сarbide Fe2C. It can be expected that in high-alloy chrome steels, the peritectic transformation can be completely blocked through the stabilization of chrome cement or its transformation into thermodynamically stable high chrome carbides. In 30ХГСA steel a sharp intensification of the process of peritectoid transformation of solid solutions of ferrite and cement which are part of pearlite is fixed. The reason for acceleration of the disintegration process of pearlite cement into individual fragments and intensification of release of ε-сarbide Fe2C in the form of column-shaped crystals between the plates of pearlite cement is silicon and manganese, which are part of steel 30ХГСА. As a result of the acceleration of martensite decomposition, the morphology of the released crystals of ε-сarbide Fe2C has changed from “foam” nanocrystals of ε-сarbide Fe2C, typical for steel 45 and steel 40X, to granular. There was also intensive fragmentation or disintegration of cement plates with the appearance of plane-parallel boundaries between the fragments and the formation of large longitudinal flat inclusions of ε-сarbide Fe2C above 100 nm, whose axis is mainly perpendicular to the axis of the cement plate. On the basis of the performed experiments it is possible to consider as proved the presence of low-temperature carbide peritectoid phase transformation in the Fe-C alloy system as a result of interaction of solid solutions of ferrite and cement at 3820C with formation of solid solution on the basis of ε-сarbide Fe2C with wide area of homogeneity. The influence of the chemical composition of steel on the peritectoid transformation between ferrite and cement slurries opens up additional possibilities for regulating the microstructure of pearlite, such as the degree of dispersion of pearlite, which has a determining influence on a number of performance characteristics of steel, such as wear resistance, yield strength and others.