In this paper, the microstructure and mechanical properties of heavy-wall seamless bend pipe after quenching at different tempering temperatures, including 550 °C, 600 °C, 650 °C, and 700 °C, were studied. Microstructure and dislocations observations were characterized by means of an optical microscope, a scanning electron microscope, a transmission electron microscope, and X-ray diffraction. As the tempering temperature increases, the dislocation density in the test steel gradually decreases, and the precipitation behavior of (Nb, V)(C, N) increases. The sample tempered at 650 °C exhibits a granular bainite structure with a dislocation cell structure and a large number of smaller precipitates. The yield platforms of tempered samples at 650 °C and 700 °C are attributed to the pinning effect of the Cottrell atmosphere on dislocations. The sample tempered at 650 °C not only presents the highest strength, but also the highest uniform elongation, which is attributed to the higher strain-hardening rate and instantaneous work-hardening index. This is closely related to the multiplication of dislocations, the interaction between dislocations and dislocations, and the interaction between dislocations and precipitates during plastic deformation of the 650 °C-tempered samples with low dislocation density, which delays the occurrence of necking.

The relationship between the microstructure and the low-temperature toughness of an X80 × D1422 mm heavy-wall heat-induced seamless bend was investigated, including the influence of microstructure on crack initiation and crack propagation. Using optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron backscattered diffraction, the microstructure and crystallographic orientation characteristics were studied. An instrumented impact tester was used to investigate the impact toughness. The results showed that during the hot bending process, there was a difference in the induction heating temperature and the cooling rate results in the uneven microstructure of the inner surface, center position, and outer surface of the bend. The center position was mainly composed of granular bainite and exhibited the best combination of strength and toughness. The ductile–brittle transition temperatures of the inner surface, center position, and outer surface were −88, −85, and −60 °C, respectively. In the process of impact deformation, the non-uniformly distributed strain concentration regions are likely to cause uneven distribution of plastic deformation and the nucleation of microcracks. The high ratio of high-angle grain boundaries and the smaller effective grain size of the inner surface and center position lead to higher crack growth absorption energy. The low crack propagation energy of the outer surface is attributed to the fact that the high-angle grain boundary does not effectively deviate or arrest the crack propagation, and multiple microcracks are connected to one another and cause fracture failure.

This paper investigated the continuous transformation cooling curve of thick-walled seamless pipe steel, microstructure nonuniformity of thick-walled hot induction seamless bend along wall thickness direction caused by hot induction bending, and their influences on the mechanical properties. The differences in microstructure, dislocation, precipitate characteristic, local misorientation angle, and grain boundary distribution were investigated acocording to scanning electron microscopy, optical microscopy, transmission electron microscopy, X-ray diffraction, and Electron back-scattering diffraction. The tensile tests and low-temperature impact tests were used to determine the strength and impact toughness of thick-walled hot induction seamless bend. The results demonstrate that thick-walled seamless pipe steel can obtain bainite structure in a wide range of cooling rate, which is beneficial to the adjustment of bending process. The nonuniform microstructure is mainly caused by the difference in heating temperature and cooling rate. The structure in the center of wall thickness is dominated by granular bainite, showing the excellent combination of strength and toughness. Among all the samples, dislocation strengthening and fine-grain strengthening are the two most important strengthening mechanisms. However, for the outer surface samples, precipitation strengthening mechanism also plays an significant role, which is attributed to the high heating temperature that causes a large amount of carbonitrides to dissolve and a large amount of precipitation during the subsequent tempering process. The high density dislocations and the large average local misorientation result in dislocation pile-up and strain concentration, which promotes crack initiation and reduces toughness. Large average misorientation and high density high-angle grain boundaries help to consume more energy in crack propagation and improve impact toughness.