The Phase Transformations in Hypoeutectoid Steels Mn-Cr-Ni

Abstract

Abstract The results of a microstructure and hardness investigations of the hypoeutectoid steels Mn-Cr-Ni, imitating by its chemical composition toughening steels, are presented in the paper. The analysis of the kinetics of phase transformations of undercooled austenite of steels containing different amounts of alloying elements in their chemical composition, constitutes the aim of investigations. Metallographic examinations were carried out on a Axiovert 200 MAT light microscope. Sections were etched with a 3% HNO3 solution in C2H5OH. Dilatometric tests were performed using L78 R.I.T.A dilatometer. Using dilatometer the changes of elongation (Δl) of the samples with dimensions Ø 3×10 mm as a function of temperature (T) were registered. Obtained heating curves were used to precisely determine the critical temperatures (critical points) for the tested steels, while the differentiation of obtained cooling curves allowed to precisely define the temperatures of the beginning and the end of particular transition to draw CCT diagrams. Four CCT diagrams worked out for the tested hypoeutectoid steels (for quenching of steel) are - in the majority of steels - separated by the undercooled austenitic range and are of the letter „C” shape. However, for steels with Mn and Ni the separation of diffusive transformations from the bainitic transformation by the stable austenitic range is not observed. Hardenability of four investigated hypoeutectoid steels is similar, but still not high. To obtain martensite in the microstructure of these steels, it is necessary to apply the cooling rate higher than 25°C/s. The exception constitutes the Mn - Ni steel, in which only cooling with the rate higher than 50°C/s allows to achieve the martensitic microstructure and to avoid diffusive transformations (pearlitic and ferritic).