Effect of partitioning time on the microstructure and properties of chemical heterogeneous quenched and partitioned steels

Introducing chemical heterogeneity in high-temperature austenite has been proven to be an effective way to tailor retained austenite and achieve a good balance of strength and ductility. Here, we have studied the role of Mn heterogeneity in carbon diffusion from Mn-depleted martensite to Mn-enriched austenite in Si/Al-free quenching & partitioning (Q&P) steels. The Mn heterogeneity in high-temperature austenite is constructed through fast and short austenitization from Mn-partitioned pearlite; following quenching, alternative Mn-enriched film RA and Mn-depleted martensitic lath are obtained. The effect of partitioning time on carbon diffusion, microstructural evolution and tensile properties is systematically investigated. Interestingly, carbide precipitation is absent in Mn-depleted martensite; instead, carbon atoms diffuse from lath martensite to the neighboring austenite. After 5 min partitioning, the carbon distribution in austenite is uneven, making the austenite with low Mn concentration transform into martensite and forming untempered ghost pearlite. After 120 min partitioning, carbon distribution in austenite becomes more uniform, with average carbon concentration of 5.22 ± 0.22 at. %, which stabilizes the austenite at room temperature. This leads to the formation of tempered ghost pearlite. Consequently, increasing partitioning time from 5 to 240 min, the width of film RA increases from 42.1 ± 10.3 to 53.8 ± 14.1 nm; meanwhile, the RA fraction keeps around 20 %, which are beneficial to ductility. But the formation of fresh martensite after short-time partitioning and the decomposition of film RA after long-time partitioning are both detrimental to the ductility. Additionally, the decrease in yield strength with increasing partitioning time is attributed to the decrease in carbon concentration and dislocation density in martensitic matrix.