Study of the Hot Ductility and Fracture Mechanisms of a Medium Carbon Steel

The hot ductility behavior of a medium carbon steel (0.48%C, 0.742%Mn, 0.183%Si, 0.028%A1) was studied by tensile tests carried out at strains rates varying from 1.10^-4 to 1.10^-3 s^-1 and temperatures ranging from 590°C to 960°C. Before testing, samples were treated at 1200°C for 5 minutes. Then they were cooled down to the experimental testing temperature using a cooling rate of 1 °C/s. Ductility was evaluated by measuring the reduction in area. SEM examination was also performed to characterize the fractographic aspect of the fracture surfaces. A continues diminution in ductility with decreasing temperatures was observed. This diminution was also sensitive to the strain rates tested. This ductility behavior was promoted by the different deformation modes and fracture mechanisms active and steel phases present. In the austenitic range of temperatures, two behaviors were detected. At high temperature (high ductility zone) deformation occurred by grain sliding without significant void coalescence. In the low ductility zone (low temperature), a clear intergranular fracture (grain decohesion) was apparent. At temperatures below A₃, where austenite transform into ferrite or ferrite plus pearlite, ductility is still low due to strain concentration at the ferrite or pearlite networks. In contrast with low carbon steel, ductility remained low with decreasing temperatures. The typical improvement in ductility of low carbon steels at low temperatures (i.e., in the ferrite regime) was not observed in the present case.