Effect of the Strain Rate on (D)CCT Transformation Diagrams of Steel 25CrMo4

The transformation kinetics during cooling of steels is most often documented in CCT and DCCT transformation diagrams, in cases where austenite was deformed prior to cooling. In such cases, attention is paid to austenite transformations in steel 25CrMo4. Within the research work, one CCT diagram and two DCCT diagrams with different strain rates (1 and 20 s-1) were developed based on dilatometric tests and combined with metallographic analyses and hardness measurements. The work also involved the verification of the deformation effect shifting the onset of ferritic and pearlitic transformation to the left, i.e. towards higher cooling rates as well as the investigation of the effect of two strain rates. It was found that, compared to the lower strain rate (1 s-1), the larger austenitic grain size combined with the higher strain rate (20 s-1) delayed the transformation of ferrite and bainite. The above-named effect was explained by insufficient time for the full dynamic recrystallization process to take place during austenite deformation.