Study on nonuniform microstructure and strength-toughness mechanism of thick-walled hot induction seamless bend

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.