GOŁASZEWSKI Adam Warsaw University of Technology

Spoluautoři SZAWŁOWSKI Jerzy, ŚWIĄTNICKI Wiesław A.

The possibility of producing a multi-phase microstructure consisting of carbide-free bainite with residual austenite in ferritic matrix in different kinds of structural steels were analysed with a use of dilatometric measurements and computer simulations of phase transformations. This kind of microstructure occurs as a result of bainitic isothermal quenching after annealing in the intercritical region i.e. in the range between Ac1 and Ac3 temperatures. Austenite enrichment in carbon occurs within this range. It results from carbon partitioning between ferrite and austenite and from carbides dissolution. High carbon concentration in austenite after incomplete austenitization in the intercritical region enables to obtain residual austenite after the austempering process. The phase transformation simulations were carried out with a use of JMatPro programme, whilst the dilatometric measurements were conducted with a use of BaehrDIL805L dilatometer. Phase transformation simulations were used in order to determine the phase composition of the steel and to indicate carbon concentration in austenite in the intercritical region. Such simulations enabled to determine the range of temperature for incomplete austenitization. Changes in carbon concentration in austenite during the annealing in the intercritical region resulted in a change of Ms temperature. According to both the simulations and dilatometric tests, the intercritical region for the examined steels was determined. The studies revealed that a microstructure of carbide-free bainite with residual austenite in ferritic matrix may be obtained in some steels. The temperature range of annealing in the intercritical region, in which carbide dissolution and austenite enrichment in carbon occur, were determined.