Physical simulation of structural transformations in the welded joint of high-carbon rail steel

Abstract This article presents the results of physical modelling of the heat treatment process and investigates the effect of its parameters on the welded joint metal of K76F rail steel. Dilatometric tests were performed to determine the critical temperatures of the structural and phase transformations at different cooling rates. The temperature and time intervals of the transformations were determined and a CCT diagram of the austenite transformations was constructed. It was found that the polymorphic α→γ transformation for the weld metal begins at a temperature of 739 °C (Ac1) and ends at a temperature of ∼790 °C (Ac3). The onset of structural transformations occurs in the temperature range of 600–650 °C and decreases with an increase in the cooling rate. The effect of the cooling rate on the mechanical properties and the influence of the cooling characteristics on the structural and phase composition of the welded joint metal were investigated. During the heat treatment of the welded joint, ferrite-pearlite and pearlite-martensite structures are formed. With an increase in the cooling rate above 16 °C s −1 , the amount and size of the martensitic component in the weld metal increased. It was found that the optimum cooling rates that meet the requirements of standards for the distribution of hardness and microstructure are in the range of 8–12 °C s −1 . The obtained results can be used to improve existing and developed new modes of heat treatment of welded joints and to improve the accuracy of predicting the phase composition of the weld metal of high-carbon rail steels.