Dynamic Flow Stress Behavior of Hypo-Eutectoid Ferrite-Pearlite Steels Under Rapid Heating

In the machining process, the workpiece undergoes large plastic deformation at high strain rate and is heated rapidly by plastic work and friction. Rapid temperature excursions brought about during this process may result in non-typical microstructures whose mechanical behavior differs from what has traditionally been observed and modeled. This paper presents dynamic stress-strain measurements on three hypo-eutectoid ferrite-pearlite carbon steels of increasing carbon content (AISI 1018, 1045 and 1075) under rapidly heated conditions, with total heating times less under 4 s, up to 1000 °C. The mechanical behavior of these steels is broken down into four regions: low temperature thermal softening, followed by dynamic strain aging, pearlite decomposition and, finally, ferrite-austenite thermal softening. The present rapidly heated high strain rate results are generally commensurate with literature data up through dynamic strain aging to about 700 °C, indicating limited effects of short heating times below the pearlite decomposition temperature (A1). Above A1, however, the results diverge significantly, owing to the limited time for diffusion processes that govern the transformation from ferrite-pearlite to ferrite-austenite and finally austenite. The divergence includes an inversion of the effect of carbon content on flow stress above A1 compared to previous studies with longer heating times.