Failure Analysis of TP347H Austenitic Heat-Resistant Steel Tube in a 600 MW Subcritical Thermal Power Unit

The failure mechanism of TP347H austenitic heat-resistant steel tube serviced in boiler superheater in a 600 MW subcritical thermal power unit is presented in this paper. Through microstructure observation, energy-dispersive spectrometry analysis, and selected electron diffraction, the mechanism of TP347H superheater bend tube failure is found to be attributed to intergranular corrosion cracking, more M₂₃C₆ and primary NbC(N) phase precipitating and coarsening along the grain boundary and formation of high content of slip lines and α′-martensite structure. The α′-martensite kept the Kurdjumov–Sachs (K-S) orientation relationships with parent austenite phase (γ-parent), which generated in TP347H bend tube may be related to the deformation-induced martensite (DIM) during rolling and cold working. The α′-martensite nucleated mainly at the grain boundary intersection between the twin crystal and γ-parent and grew inwards along the twin band. The dislocation density at the intersection is larger, and the secondary NbC(N) phase inhibited the nucleation of α'-martensite.