Effect of ultra-fine martensite and retained austenite induced by slow quenching on the mechanical properties of 9Cr 1.5Mo1Co(FB2) steel

The effect of ultra-fine martensite (ultra-fine M) and retained austenite (RA) induced by slow quenching (furnace cooling) on the mechanical properties of FB2 steel was investigated using experimental observations and phase-field simulation. The results show that the partition of C from martensite into untransformed austensite (UA) during the slow quenching process gives rise to the split (two-stage martensitic transformation) and incompletion of martensitic transformation. And most C-riched UA with high internal stress transforms into ultra-fine M during the second stage martensitic transformation, while other UA remains at room temperature to form banded/blocky RA. After tempering at 700 °C, lathy M and ultra-fine M are decorated with large and fine M23C6 particles, respectively. Meanwhile, banded and blocky RA transforms into globular M23C6 and ferrite. The slow quenching treated steel has higher or better tensile strength and impact toughness of tempered state when compared to the tempered steel with fast and medium quenching (water and air cooling) treatments. This improved strength and impact toughness attributes to the combined effect of effective grain refinement, the carbides density increment, and ferrite. Further quantitative assessment of the effect of ultra-fine M and RA on the mechanical properties of FB2 steel indicates that the strength and impact toughness of 9-12Cr steels may be tailored by optimizing the ultra-fine M and RA.