Hydrogen Embrittlement of a Boiler Water Wall Tube in a District Heating System

Abstract

A district heating system is an eco-friendly power generation facility with high energy efficiency. The boiler water wall tube used in the district heating system is exposed to extremely harsh conditions, and unexpected fractures often occur during operation. In this study, a corrosion failure analysis of the boiler water wall tube was performed to elucidate the failure mechanisms. The study revealed that overheating by flames was the cause of the failure of the boiler water wall tube. With an increase in temperature in a localized region the microstructure not only changed from ferrite/pearlite to martensite/bainite, which made it more susceptible to brittleness, but it also developed tensile residual stresses in the water-facing side by generating cavities or microcracks along the grain boundaries inside the tube. High-temperature hydrogen embrittlement combined with stress corrosion cracking initiated many microcracks inside the tube and created an intergranular fracture.

Deciphering Hydrogen Embrittlement Mechanisms in Ti6Al4V Alloy: Role of Solute Hydrogen and Hydride Phase

“…The β phase has much higher hydrogen diffusivity than the α phase and, therefore, hydrogen can easily accumulate at the α phase side near the α/β interface, exceeding its threshold value of solubility [ 7 , 11 , 18 , 25 ]. Moreover, GBs are reported as optimal pathways for accelerating the diffusion and accumulation of hydrogen [ 26 , 27 , 28 , 29 ]. As a result, the precipitation of HP primarily occurs either at the α–β interface or GBs.…”

“…The uncharged sample displays a colony structure ( Figure 5 a), while an intergranular cracking was clearly observed in the SH sample, as the crack exclusively propagated along the grain boundaries as well as the α/β interface, supporting the notion that the nano/micro-voids generated at the GBs or the α/β interface could be a preferred site for HE. Indeed, GBs are reported as preferred pathways for the diffusion and accumulation of solute hydrogen [ 26 , 27 , 28 , 29 ]. Furthermore, the differences in hydrogen solubility and diffusion rates of α and β phases also contribute to the α/β interface serving as a hydrogen trap [ 7 , 11 , 18 , 25 ].…”

Failure Analysis of a Deep Pit on the Inner Wall of Water Wall Tube in a Power Plant

“…Otherwise, it will affect the heat transfer efficiency. In severe cases, it will cause deterioration of tube material [11][12], overheating [13], hydrogen damage [8,[14][15], corrosion [8-9, 13-14, 16-18, 19-22] and fatigue [10], etc. Furthermore, a tube explosion occurs.…”