Development of high strength ductile hypereutectoid steel by cyclic heat treatment process

In this work a homogenizing annealed 1.24 wt% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 894 °C (above Acm temperature) followed by forced air cooling. During short-duration holding at 894 °C, fragmentation (owing to partial dissolution) and thickening of cementite lamella occurred at the preferred high energy sites of lamellar faults. These undissolved fragmented cementite acted as pre-existing nuclei promoting the divorced eutectoidal reaction (DET) during forced air cooling that generated non lamellar region (NLR) in the microstructure apart from the presence of lamellar pearlite region (LPR) and cementite network (CN). The CN was also fragmented with repeated holding and cooling cycles. Besides, new lamellar fault sites were generated during forced air cooling to promote lamellar fragmentation. Accordingly, as the number of heat treatment cycles increases, more uniformly dispersed finer cementite particles (pre-existing nuclei) in austenite matrix was obtained at 894 °C. Therefore, the proportion of NLR increased and the proportions of LPR and CN decreased with the progress of cyclic heat treatment. After 8 cycles of heat treatment the microstructure mostly consisted of a combination of spheroids (0.55 ± 0.24 μm in size) and non-spheroids (mainly the plate shaped particles of 0.22 ± 0.10 μm in size) of cementite in 1:5 proportion by area% dispersed in ferrite matrix. The generation of cementite plates occurred through the motion of transformation disconnections. This provided an excellent combination of strength (UTS = 1086 MPa) and ductility (%elongation = 13) in this 1.24 wt% C steel. The property obtained had similarity to what was reported for a thermo-mechanically processed steel of much higher carbon content (1.8 wt%) where the microstructure consisted of solely the cementite spheroids of 0.65 μm average size dispersed in ferrite matrix.