Effect of Tempering Temperature on Microstructure and Mechanical Properties of 165 ksi Grade Drill Pipe Steel

The study systematically investigates the effects of tempering temperatures ranging from 610 °C to 690 °C on the multi-scale martensitic microstructure evolution and strength–toughness matching characteristics of 165 ksi grade Cr–Mo–V martensitic steel for ultra-high-strength and high-toughness oil drill pipes. The intrinsic strengthening and toughening mechanisms of the developed steel were further clarified. It was established that an increase in the tempering temperature resulted in a reduction in strength that was found to be monotonic. Concurrently, an enhancement in ductility and low-temperature toughness was observed. The yield strength (YS) and ultimate tensile strength (UTS) decrease monotonically from 1338 MPa and 1397 MPa to 819 MPa and 891 MPa, respectively. Meanwhile, the −20 °C low-temperature impact absorbed energy rises significantly from 32 J to 148 J, and the fracture elongation increases from 16% to 21%. Combined multi-scale microstructural characterization via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and X-ray diffraction (XRD) demonstrates that the prior austenite grain size remains nearly unchanged during tempering. In contrast, the hierarchical martensitic substructures, including packets, blocks and laths, undergo continuous coalescence and coarsening. The matrix dislocation density drops sharply from 3.02 × 1015 m−2 to 1.20 × 1015 m−2. The gradual relaxation of internal lattice strain reduces the kernel average misorientation (KAM) value from 0.32° to 0.21°, and the nano-scale rod-shaped precipitates gradually transform into coarsened spherical carbides. Quantitative analysis of various strengthening mechanisms reveals that grain refinement strengthening and dislocation strengthening serve as the dominant strengthening contributors to the superior strength of the steel, while solid solution strengthening and precipitation strengthening play auxiliary roles. The remarkable improvement in low-temperature impact toughness is primarily attributed to the substantial increase in crack propagation energy dissipation.