Failure evolution of steam‐side oxide scales grown on T91 superheater tubes by acoustic emission technique

Abstract

The damage of steam‐side oxide scales grown on the high temperature components of boiler can seriously influence the safety of power plants. In this paper, the uniaxial tension test and the acoustic emission (AE) technique were used to investigate the damage evolution and the failure characteristics of steam‐side oxide scales with varying thicknesses grown on T91 alloy tubes. The AE features associated with the metallographic investigation were adopted to detect the onset of each failure type of oxide scales. The frequency components of AE signals were extracted by the wavelet based method, and the quantitative correlation between failure types and AE signals was established. The surface and interface fracture toughness of the oxide were assessed by the critical state of the failure process. The results showed the quantitative damage evolution of steam‐side oxide scales, and found the oxide thickness had more significant effect on critical strain than the fracture toughness of oxide scales.

Numerical simulation of the failure behavior of steam‐side oxide scale considering oxide creep and physical defects

“…Thus, it would be difficult to observe the interface crack after cooling. In our previous work, the tensile test is used to simulate the oxide failure behavior on cooling . The occurrence of interface crack is detected by acoustic emission technique, and the interface crack formed at the Fe‐Cr spinel/substrate boundary is observed by the metalloscope, as shown in Figure .…”

“…Under such stress, oxide scale is susceptible to mechanical failure as they do not have the ductility of underlying metal . Scale exfoliation can cause plugging, steam starvation, and local overheating of tubes, which would even lead to tube burst in boiler . Thus, the ability to assess the failure behavior of oxide scale at high temperature is an important requirement for evaluating the endurance of boiler tubes …”

Fracture characterization of AlSi coating at high temperatures

“…Therefore, the area under voltage-time graph, as shown in Figure 2.1 is considered a suitable parameter to detect coating cracks. In the case of deformation, the output AE signals usually consist of a variety of different waveforms such as background, continuous, burst and reflective waveforms [27]. Low intensity AE signals usually represent uninterrupted events such as plastic deformation, while burst-type high intensity signals represent a unique event, like fracture or delamination [28].…”

“…The burst-type AE activity is usually accompanied by relatively low strength reflection waves, which along with background noise can be ignored via an appropriate voltage threshold [27]. Consequently, the output AE signals can be exploited to characterize different fracture types: Koshimizu and Otsuka examined ductile to brittle fracture transitions via AE sensors and noticed higher Investigating fracture of AlSi coating at elevated temperatures activity at brittle fracture events than at ductile ones [29].…”

“…To sum up, although AE sensors are extensively used to detect delamination of fibers in composite materials, applying them to utilize AE sensors and investigate fracture in metallic coatings at high temperatures is a novel approach. , also known as the method of arrival time difference [27]. In this setup, the AE sensors, situated at two opposite ends, are assumed to be connected via a 1-dimensional signal framework, with each sensor digitally located at L AE = 0 and 1.7 m (Fig.…”