Effect of Cr Content on the Microstructure and Toughness of the Supercritically Coarse-Grained Heat-Affected Zone in X80 Pipeline Steel

The existing studies mainly focus on the coarse-grained heat-affected zone and the inter-critically reheated coarse-grained heat-affected zone, while the studies on other sub-zones are relatively low. Meanwhile, the studies on the Cr element in steel mainly focus on the influence of the Cr element on strength and hardness; however, its mechanism is not very clear. Therefore, three kinds of X80 experimental steels with different Cr contents (0 wt.%, 0.13 wt.%, and 0.40 wt.%) were designed in this paper. The thermal simulation experiments on the supercritically coarse-grained heat-affected zone (SCCGHAZ) were carried out using a Gleeble-3500 thermal simulator. The effects of Cr on the microstructure and toughness of SCCGHAZ were systematically investigated through Charpy impact tests and microstructural characterization techniques. The results indicate that the microstructures of the three Cr-containing X80 experimental steels in SCCGHAZ are predominantly composed of fine granular bainite. However, impact tests at -10 °C show that the SCCGHAZs of 0 wt.% and 0.13 wt.% Cr steel exhibit higher impact energy, while that of the 0.40 wt.% Cr steel demonstrates significantly reduced energy impact (<100 J). Microstructural characterization reveals that the impact toughness of the SCCGHAZ in X80 steel is correlated with microstructural features, including effective grain size, grain boundary angles, and the volume fraction and shape of martensite-austenite (M-A) constituents. Among these factors, the volume fraction of M-A constituents substantially influences toughness. It was found that island-shaped M-A constituents inhibit crack propagation, whereas blocky M-A constituents impair toughness.