The Effect of Mo Content on the Multi-Scale Martensitic Structure and Mechanical Properties of Ultra-High-Strength and -Toughness Oil Well Pipes

The study systematically investigates the effect of molybdenum (Mo) content (0.70–1.57 wt.%) on the microstructure and mechanical properties of quenched and tempered martensitic steel for ultra-high-strength and -toughness oil well pipes. The results demonstrate that increasing the Mo content substantially enhances the strength of the steel. The yield strength (YS) increases from 1135 MPa to 1233 MPa, the ultimate tensile strength (UTS) rises from 1176 MPa to 1285 MPa, and the elongation after fracture is marginally improved to 19%. However, the low-temperature impact energy (AKV2) of the steel at −20 °C exhibits a pronounced decrease, from 117 J to 36 J. Mo refines the multi-scale martensitic microstructure, increases the fraction of high-angle grain boundaries (HAGBs) and dislocation density, and promotes the precipitation of three types of carbides. Quantitative analysis indicates that grain refinement strengthening is the predominant factor contributing to the enhancement of steel strength. The decline in the steel’s resistance to low temperatures is attributed to the separation of coarse, blocky M3C-type carbides at the grain boundaries. This results in the accumulation of stress at these boundaries, leading to a transformation in the steel’s fracture mode from ductile to brittle.