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The girth weld tensile properties of API X80 grade high-frequency electric resistance welded (HFW) steel pipe for surface casing with the chemical composition of 0.05C–1.6Mn–0.06Nb (mass %) and the diameter of 558.8 mm and wall thickness of 25.4 mm were investigated by simulated postweld heat-treatment (PWHT). The tensile specimens taken from girth butt welded pipe were heat-treated under the conditions of 625 °C × 2 h and 675 °C × 2 h in an air furnace in order to simulate PWHT of casing products. The result of the girth weld tensile test of the heat-treated specimens showed that yield strength and tensile strength decreased very little and these properties sufficiently satisfied the API X80 specification. The change in strength due to heat treatment was discussed based on microscopic observation of the submicrostructures of the base metal by the electron back-scattered diffraction (EBSD) technique, transmission electron microscopy, X-ray diffraction (XRD), and the extraction residue precipitate classification method. The authors concluded that the fine NbC with a diameter of 12–18 nm, which precipitated during the heat treatment, prevented the decrease of strength due to the slight grain growth and dislocation recovery associated with PWHT. Additionally, the effect of PWHT conditions was evaluated by using small-scale laboratory specimens obtained from the base metal. Tensile properties were summarized as a function of the tempering parameter. As a result, strength remained almost constant at the tempering parameter equivalent to the PWHT conditions of 625 °C × 16 h.