Microstructure and mechanical behavior of AISI 4340 steel fabricated via spark plasma sintering and post-heat treatment

We investigated the evolution of microstructure and mechanical properties AISI 4340 steel during high-energy ball milling, spark plasma sintering (SPS), and post heat treatments. High-energy ball milling of the powder mixture led to the formation of a nanocrystalline (∼10 nm) bcc Fe matrix containing some segregation of alloying elements and oxide particles. As-sintered alloy consisted of martensite-austenite (MA) constituent and fine pearlite with an average grain size of 2.4 μm along with oxide particles and M₂₃C₆ particles. The quenching after austenitization formed a microstructure composed of martensite and MA constituent, thereby increasing the fraction of retained austenite and decreasing the average grain size (∼0.5 μm) compared to the as-sintered alloy. The M₂₃C₆ particles were fully dissolved at the high austenitization temperature, leading to the increase in retained austenite fraction after quenching. Tempering induced decomposition of retained austenite and precipitation of cementite particles, while it has negligible effects on the grain size, undissolved M₂₃C₆ particles, and oxide particles. As-sintered alloy exhibited a compressive yield strength of ∼2 GPa due to primary strengthening by dislocations and grain boundaries/cementite lamellae and secondary strengthening by oxide particles. Changes in dislocation strengthening played a dominant role in determining the yield strength of quenched and tempered steels.