Microstructure evolution in warm forged sintered ultrahigh carbon steel

Fe–1.4C–0.65Si–0.85Mo ultrahigh carbon steel was liquid phase sintered in 10%H2–90%N2 at 1300 °C from Höganas Astaloy 85 Mo HP base iron, fine graphite and silicon carbide powders mixed with polypropylene glycol. The microstructure then comprised fine pearlite and grain boundary cementite networks and the density increased from ∼6.8 g cm−3 to ∼7.7 g cm−3. A group of specimens then underwent austenitisation, isothermal quenching/autotempering at M(10%) temperature, followed by cooling to room temperature. This produced a crack-free martensitic microstructure, which transformed to ferrite plus fine spheroidised carbides by annealing for 3 h at 750 °C. To attain full density and well-distributed submicron carbides, these specimens were warm forged at 700–750 °C. To ascertain if some processing steps can be discarded, as-sintered and quenched samples were similarly thermo-mechanically processed. The required stresses and resultant microstructures depended on temperature and strain rate, with optimum microstructure, for Bähr processing at 775 °C of quenched material, fully comparable with that of prior spheroidised specimens. Microstructures and hardness values are presented for all processing routes.