Causes of Surface Cracks on Outer Surface of Inner Arc of Transposition Bend of Partition Platen Superheater

The causes of surface cracks on the outer surface of inner arc of a transposition bend of partition platen superheater tube of a 600 MW subcritical unit were analyzed through macroscopy examination, chemical composition analysis, mechanical property testing, microstructure observation, etc. The results showed that the enrichment concentration of corrosive S element corroded the substrate of the tube, forming “micro notches” and initiating microcracks. The presence of a decarburized layer on the outer surface provided favorable conditions for crack propagation. Microcracks continued to propagate under the synergistic and interactive effects of S corrosion environment and thermal fatigue cyclic loads, ultimately resulting in corrosive thermal fatigue cracks.