Influence of Initial Microstructure on Warm Deformation Processability and Microstructure of an Ultrahigh Carbon Steel

Various isothermal compression tests are carried out on an ultrahigh carbon steel (1.2% C in mass percent), initially quenched or spheroidized, using a Gleeble-3500 system. The true stress is observed to decrease with increasing temperature and decreasing strain rate. The true stress of the initially quenched steel is lower than that of the initially spheroidized steel at high deformation temperature (700 degrees C) and low deformation strain rate (0.001 s(-1)). The value of the deformation activation energy (Q) of the initially quenched steel (331.56 kJ/mol) is higher than that of the initially spheroidized steel (297.94 kJ/mol). The initially quenched steel has lower efficiency of power dissipation and better processability than the initially spheroidized steel. The warm compression promotes the fragmentation and the spheroidization of lamellar cementites in the initially quenched steel. The fragmentation of lamellar cementites is the spheroidizing mechanism of the cementites in the initially quenched steel. Results of tranmission electron microscope investigation showed that fine grains with high angle boundaries are obtained by deformation of the initially quenched steel.