Changes in microstructure and properties of weld heat-affected zone of high-strength low-alloy steel

The evolution of the microstructure and toughness of APL5L X80 pipeline steel after thermal welding simulation was investigated by X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. The results indicated that primary heat-affected zones can be divided into weld, coarse-grained, fine-grained, intercritical, and subcritical zones. The microstructure of the weld zone is mainly composed of bainitic ferrite and a small amount of granular bainite; however, the original austenite grains are distributed in the columnar grains. The structure of the coarse-grained zone is similar to that of the weld zone, but the original austenite grains are equiaxed. In contrast, the microstructure in the fine-grained zone is dominated by fine granular bainite, and the effective grain size is only 8.15 mu m, thus providing the highest toughness in the entire heat-affected zone. The intercritical and subcritical zones were brittle valley regions, and the microstructure was dominated by granular bainite. However, the martensite-austenite (M/A) constituents are present in island chains along the grain boundaries, and the coarse size of the M/A constituents seriously reduces the toughness. The results of the crack propagation analyzes revealed that high-angle grain boundaries can significantly slow down crack growth and change the crack direction, thereby increasing the material toughness. The impact toughness of the low-temperature tempering zone was equivalent to that of the columnar grain zone, and the impact toughness was between those of the critical and fine-grained zones.