Effect of Cr microalloying on microstructure evolution and toughness in SCGHAZ of X80 pipeline steel

Two experimental X80 steels with different Cr contents (0.13, 0.40 wt.%) were designed to study the influence of Cr content on the microstructure transformation and properties in the coarse-grained heat-affected zone by using a Formastor-F II thermal dilatometer and to simulate the microstructure of the subcritically reheated coarse-grained heat-affected zone (SCGHAZ) by means of the Gleeble-3500 thermal simulator, along with the scanning electron microscope, transmission electron microscope, and electron backscattering diffraction test methods to characterize the microstructures at 650 °C. The findings indicate that a higher Cr content can promote the formation of bainitic ferrite (BF), while the microhardness and impact toughness of SCGHAZ are improved. Granular bainite and BF dominated the microstructures formed by the two experimental steels at 650 °C, respectively. For experimental steels mainly composed of BF, they have a high proportion of high-angle grain boundaries (HAGB), and the misorientation angle of HAGB is mostly greater than 55°. Moreover, the distribution of martensite–austenite (M-A) constituents in SCGHAZ altered from dense to sparse, and the form altered from elongated to island-like when the Cr concentration was changed from 0.13 to 0.40 wt.%. Consequently, by suitably increasing the concentration of Cr, it is possible to raise the density of HAGB and improve the shape of M-A constituents, allowing SCGHAZ to have a higher toughness.