Kinetics of isothermal phase transformations in premium and standard rail steels.

Abstract

Recently published articles have enhanced the importance of understanding phase transformations in rail eutectoid steels in an attempt to improve their manufacturing and welding processes. Herein, the kinetics of isothermal phase transformations of three eutectoid steels used in railways around the world are evaluated. Computer simulations are performed to evaluate the effect of steel chemical compositions on phase transformations considering thermodynamic equilibrium conditions. Austenitizing temperatures under continuous heating and the time intervals of isothermal austenite decomposition are measured by dilatometry. The time–temperature–transformation diagrams are determined. Qualitative and quantitative microscopy techniques are applied to characterize the phase transformation products. It is shown that by applying a well-planned heat treatment, it is possible to obtain microstructural characteristics for a standard steel as refined as those achieved for premium steels. Empirical equations proposed in the literature to predict the Bs temperatures are applied, and the experimental values are compared. Using the dilatometry data, it is possible to determine the global activation energy for austenite–bainite transformation. Results show that premium steels have higher overall activation energy for austenite–bainite decomposition than what occurs in standard. This article presents unpublished data useful for steel researchers as well for rail manufacturers and welders.