Achieving low yield ratio and optimal strength-ductility synergy in F22M steel for blowout preventer in ultra-deep well through intercritical quenching

Pressure parts in blowout preventer (BOP), as the critical safety components in oil and gas drilling systems, should possess yield ratio (YR) below 0.85 to ensure higher fracture resistance before plastic instability. In this study, an intercritical quenching and tempering (QIT) treatment was applied to F22M steel to investigate the microstructural evolution and corresponding mechanical properties. Compared to conventional quenching and tempering (QT) process, with the introduction of intercritical quenching, the microstructures of F22M steel transform from the fully bainitic structure to a fine dual-phase structure consisting of intercritical ferrite (IF) and bainite, accompanied by precipitation of numerous carbides. As the intercritical quenching temperature increases from 790 °C to 840 °C, the volume fraction of bainite increases from 44% to 93%, the density of high-angle grain boundaries (HAGBs) increases from 0.14 μm -1 to 0.21 μm -1 . Accordingly, the yield strength increases from 412 MPa to 641 MPa, while the elongation decreases from 30.5% to 23%, and YR increases from 0.73 to 0.85. The key to achieving a desirable YR with optimal strength-ductility synergy lies not only in maximizing the “hard” bainite to improve strength, but also in incorporating sufficient “soft” IF to maintain great capacity for dislocation multiplication and storage, thereby elevating work hardening capacity. Compared to the QT treatment, the enhanced ductility of F22M subjected to QIT process is attributed to the coordinated deformation between the “soft” IF and the “hard” bainite and the increased density of HAGBs.