Failure Analysis of Abnormal Bulging and Cracking for High-Pressure Steam Pipe

High-pressure steam pipes inevitably suffered from reciprocal interaction of high pressure and temperature during long-period service, causing deformation and cracking. To study the dominant influence factor, the bulging and cracking high-pressure steam pipes were investigated with a macro visual inspection, chemical composition analysis, metallographic microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). According to the analysis of the bulging and cracking samples, no effect of pearlite spheroidization was found for the bulging and cracking phenomenon. The simulation results show that the overlength distance between the steam drain valve and the main steam pipe, generating the alternating thermal stress and the subsequent thermal fatigue. And the Von-Mises modeling further illustrates the stress concentration located at the pipe orifice's fillet weld, causing the final creep cracking. It is concluded that the creep/fatigue interaction was the main factor for the pipe cracking combing with abnormal bulging.