Effect of Dual-Phase Heat Treatment on Microstructure and Mechanical Properties of S135 High-Strength Drill Pipe Steel

Effect of dual-phase heat treatment (DPHT) on microstructure and mechanical properties of S135 high-strength drill pipe steel was studied by means of optical microscope, scanning electron microscope and mechanical property testing. The results show that the ferrite–martensite dual-phase microstructures were obtained at 760-800 °C. With increase in DPHT temperature, the volume fraction of martensite increases and the ferrite decreases. When the DPHT temperature is low, martensite is distributed as an island on the ferrite matrix, and ferrite is distributed as an island on the martensite matrix when the DPHT temperature is high. The hardness and strength of the steel increase, the plasticity and toughness decrease, the steel changes from ductile fracture to brittle fracture and the micro-fracture morphology changes from dimple to cleavage with increase in DPHT temperature. The strain hardening exponent of the as-received steel shows one ‘n’ value, but exhibits two ‘n’ values after DPHT. When the DPHT temperature is 750 °C, the relative content of martensite in the duplex structure is 15-20%, and the impact energy of the steel is similar to that of the as-received S135 drill pipe steel, and the strength is increased by about 15%. Micropores and micro-cracks on the subsurface of tensile fracture surface are mainly formed at the martensite/ferrite phase boundary or in the martensite slit.