Abstract
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.
Highlights
It shows that the microstructure was made up of mainly lath bainite (LB), very little granular bainite (GB), and some martensite–austenite constituents (MA constituents)
transmission electron microscopy (TEM) results show that the ferrite structure of the plate after high-temperature tempering does not completely disappear, which is consistent with the decrease in the strength of the sample tempered at 700 ◦ C
When to 650 °C after quenching, with the increase in tempering temperature, the microstructure gradually changed from with the increase in tempering temperature, the microstructure gradually changed from lath bainite to granular bainite
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. It is urgent to develop a new type of large-diameter and thick-walled seamless bend pipes with high strength and toughness, which has become a new direction of bend pipe development It can effectively balance the contradiction between the induction heating bending process and the deterioration of mechanical properties in welding the heat-affected zone. Reported a new design of X80 steel for thick-walled thermal induction bend pipe, and investigated the effects of hot bending parameters (including heating temperature, cooling rate, tempering temperature) on microstructure and mechanical properties. Zhou et al [17] reported the effect of the tempering process at different temperatures on the microstructure and mechanical properties of thick-walled bend pipes produced by the ultra-fast cooling technology, which systematically expounded the relationship between strength toughness and dislocation, misorientation angle, effective grain size, as well as microcrack propagation. The effects of different tempering temperatures on the yield behavior, work-hardening behavior and uniform plastic deformation of seamless bend pipe, and the relationship between strengthening mechanism and work-hardening behavior were systematically discussed
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