SUPERCOOLED AUSTENITE TRANSFORMATIONS IN A CONSIDERABLE STEEL FOR A SPECIAL ENGINEERING PRODUCTS

Abstract

The advantages, disadvantages and some features of a dilatometric method for studying the transformation of supercooled austenite in steels under continuous cooling are briefly discussed. The continuous cooling transformation diagram of the steel under consideration containing 0.50 wt. % C, 0.25 Si, 0.65 Mn, 0.75 Cr, 1.30 Ni and 0.15 Mo, respectively was plotted based on experimental data obtained upon dilatometer measurements carried out with different cooling rates (0.1, 0.3, 1, 5, 30 K/s) on the Linseis L78 RITA dilatometer. According to the obtained dilatometer curves, the degree of transformation was calculated in accordance with the standard method, its disadvantages and possible ways to eliminate them are noted. Additionally for the proper design of the continuous cooling transformation diagram of supercooled austenite metallographic examination and hardness tests were carried out after various heat treatment modes. A qualitative description of the formed microstructures was performed in order to confirm the obtained dilatometer data. The “tangent method” was also employed to determine the critical temperatures upon heating at the rate of 0.1 K/s and the temperature of the of the martensite start temperature upon cooling from the austenitization temperature of 880 °C, based on the literature data the application of the implemented method is justified. Some recommendations are presented as possible methods for selecting the dimensions of parts for which the steel under consideration was planned to be used. Using the example of a billet with diameter of 50 mm, from which samples were machined for research, the possibility of practical application of one of the methods for determining the type of structures formed along the cross section of steel products under continuous cooling from the austenitic region is shown.