Metallurgical analysis of ASME SA213 T12 boiler vertical water-wall tubes failure

Abstract

Low carbon steel (SA213-T12) tubes were used as vertical water-walls of a boiler in a power plant of an electrical power generation, several tubes failed after serving for only 13,140 h. The failed tubes were located near the heat source (burner) at the entrance of the hot water before it was converted into steam. A failure analysis of the broken tubes was carried out to reveal the reasons behind their premature damage. The fractured surfaces of the failed tubes were examined by non-destructive testing. The metallurgical investigation of the failed tubes was carried out via optical and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction. The metallurgical characterization revealed that the structure of the sound part of the tubes consisted of proeutectoid ferrite and pearlite phases. However, the microstructure of the bulging area exhibited a complete decomposition of the pearlite, as well as formations of voids and cementite at the grain boundaries of the ferrite. It was concluded that the failure of the pipes happened due to creep damage associated with a localized overheating in the failed regions having outer and inner scales. Certain recommendations were also presented to prevent the repetition of this type of failure mechanism.