Abstract: High temperature superheater tube with TP347H steel membrane wall of one supercritical circulating fluidized bed boiler cracked frequently. The failure reasons were investigated by means of macroscopic examination, spectral analysis, mechanical property test, metallographic analysis and operation analysis. The results show that the chemical composition and mechanical properties of the sampled tubes meet the technical requirements of standard, and the metallographic structure is normal. The cracks originate from the weld toe between the tube and the fin and extend from the outer wall to the inner wall. The high-temperature superheater tube has a large-screen membrane wall structure. During the boiler start-up process, the temperature difference between adjacent tube walls is large and constantly changing, resulting in a large expansion difference in the axial direction of the tube and formation of alternating changes. So the thermal fatigue cracks initiate at the stress concentration position of weld toe. The welding residual stress in the weld toe and the straight groove on the outer surface of the tubes promote the formation and propagation of cracks.

Key words: circulating fluidized bed boiler, high temperature superheater, TP347H steel, membrane wall, temperature difference, thermal fatigue crack