Research on the mechanism of attenuation of low-temperature impact toughness by microstructure evolution of 304 round tube welded joint
Publish Time: 2024-11-27
In many industrial application scenarios, 304 round tube is favored for its good comprehensive performance. However, the attenuation of impact toughness of its welded joint in low-temperature environment limits the expansion of its use in some low-temperature working conditions. In-depth analysis of the microstructure evolution mechanism behind this phenomenon is of great significance for optimizing welding technology and broadening the application boundary.The welding process is a complex thermal cycle process. The 304 round tube welded joint is first heated and melted and then solidified, and the original uniform austenite structure changes significantly. The metal in the weld zone cools rapidly to form a columnar crystal morphology, which is significantly different from the equiaxed crystal structure of the parent material. This columnar crystal has strong orientation and relatively small grain boundary area. Impurities and alloy elements are easily concentrated at the grain boundary, which constitutes the first "hidden danger" of impact toughness.The heat-affected zone also undergoes complex changes. The coarse-grained zone adjacent to the weld has a long high-temperature residence and rapid cooling, and the austenite grains grow rapidly and the grain boundaries coarsen. According to the Hall-Petch relationship, large grains reduce the resistance of grain boundaries to crack propagation, and cracks are more likely to penetrate at low temperatures, resulting in a loss of toughness. In some sensitized temperature ranges, chromium carbides precipitate along grain boundaries, causing chromium depletion near the grain boundaries, and the corrosion resistance and impact resistance are simultaneously weakened, making the already fragile grain boundaries "worse".As the temperature drops to a low temperature environment, the dislocation slip is hindered and aggravated, and the plastic deformation of the material is difficult. There are many stress concentration points in the uneven structure of the welded joint, and cracks are easy to initiate and expand rapidly at the grain boundaries and precipitates. The large-sized grains in the coarse-grained area and the special morphology of the columnar crystals in the weld, combined with the "mischief" of the precipitated phase at the grain boundary, synergistically reduce the absorption and dispersion capacity of the impact energy, and ultimately cause a significant attenuation of the impact toughness.With insight into this mechanism, in the future, we can use means such as finely controlling the welding heat input and optimizing the post-weld heat treatment specifications to refine the grains, improve the distribution of the precipitated phases, and reshape the microstructure of the welded joint, unlocking a broader low-temperature service potential for 304 round tube.
