Microstructure evolution in warm forged sintered ultrahigh carbon steel

What is it about?

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.

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Why is it important?

Optimizing the microstructure of ultrahigh carbon steels is key to achieving the desired balance of strength, ductility, and processing efficiency for demanding engineering applications. Traditional heat treatment routes—especially spheroidizing anneals—are time-consuming, energy-intensive, and costly. Recent research shows that applying warm forging or thermomechanical processing to powder metallurgy ultrahigh carbon steels can directly transform the as-quenched or as-sintered microstructure into a fine dispersion of spheroidized carbides in a ferritic matrix, without the need for prolonged heat treatments. This not only accelerates processing and reduces manufacturing costs, but also enables full densification, improved microstructural uniformity, and mechanical properties comparable to or better than conventional methods. Such process innovations are significant for the production of advanced steel components, as they allow for shorter and more efficient manufacturing cycles, reduced energy consumption, and new possibilities for high-performance materials in automotive, tooling, and structural applications.