Phase and structural transformations when forming a welded joint from rail steel. Report 2. Isothermal diagram of decomposition of supercooled austenite of R350LHT rail steel

An isothermal diagram of decomposition of supercooled austenite of R350LHT steel was constructed based on the results of dilatometric, metallographic and hardness analysis of this decomposition during continuous cooling and under isothermal conditions. When comparing the thermokinetic and isothermal diagrams, it was found that the thermokinetic diagram plotted during continuous cooling shifts downward and to the right in comparison with the isothermal diagram. This result is fully consistent with the known regularities. During the research, the critical points of R350LHT steel were determined: Ас1 = 711 °С; Мn = 196 °С. This isothermal diagram was used to determine the temperature of the minimum stability of overcooled austenite, which was 500 °C. Under isothermal conditions, pearlite-type structures appear in the temperature range from 700 to 600 °C. At 550 °C, a mixture of pearlitic and bainitic structures is formed. In the temperature range from 500 to 250 °C, bainitic structures are formed: at 500 – 400 °C – upper bainite; at 350 ° C – a mixture of upper and lower bainite; at 300 – 250 °С – lower bainite. Almost in the entire studied temperature range of overcooled austenite isothermal decomposition, an increase in the hardness of the transformation products is observed with a decrease in the holding temperature from 246 HV (at 700 °C) to 689 HV (at 250 °C). However, at a temperature of 500 °C, a slight drop in hardness occurs, which is apparently caused by the appearance of retained austenite during the development of bainitic transformation.

A model describing the variation in free energy in compound and single phase regions in binary systems containing miscibility gaps

A model is derived to represent the variation of free energy of combination of one gram-atom of two components, represented by A and B, in intermediate single, or compound, phases in a binary system as a function of composition at constant temperature, and with a minimum of experimental data. The derivation of the model involves the, assumption that the straight lines representing the free energy of the two phase fields adjacent to a compound phase, on an isothermal integral free energy against atomic fraction diagram, intersect at the mid-point of the compound phase. A relation between logaA, logaB, and the atomic fraction NB is developed so as to conform with the preceding requirement and yield an almost horizontal tangent to the curve representing the compound phase at NB = � for a hypothetical symmetrical isothermal diagram. The equations developed on these bases are extended to non-symmetrical systems. These are shown to be successful in predicting the variation in free energy of compound phases, as a function of composition, in binary systems for which experimental data are available.