Cause and mechanism analysis of cracking and leakage of water wall tube

: In this paper, the cracking and leakage cause of the water wall tube was studied by means of macro morphology analysis, chemical composition analysis, microstructure detection, mechanical properties test and energy dispersive X-ray spectroscopy (EDS) analysis. The results indicated that the cracking of water wall reducer is mainly related to high temperature fatigue and sulfur corrosion. During operation, under the combined action of thermal expansion and contraction caused by frequent boiler load changes and axial alternating stress caused by pipe wall vibration, thermal fatigue cracks are gradually formed on the outer wall of the steel pipe toward the fire side. Under the combined action of the alternating stress and S corrosion, the cracks are gradually spread from outer to inner wall, and finally penetrate the tube wall, causing leakage. Finally, the corresponding preventive measures of water wall leakage are put forward to improve the safety and economy of power plant boiler.


Introduction
As an important energy industry, thermal power generation industry is closely related to people's daily necessities and is also an important original driving force for the development of national economy. In the current power industry, thermal power generation has always been in the absolute dominant position of China's power industry. Therefore, the thermal power industry has an important impact on the modernization development. The continuous optimization and efficient development of thermal power generation is an important guarantee for our country to complete the socialist modernization drive. In thermal power generating units, utility boiler is one of the unstable equipment, the safety condition of which limits the safety of the whole unit. However, the operation status of the water wall determines the safety of the boiler to a great extent. Because the water wall of utility boiler is not only affected by the high temperature and high pressure inside the pipe, but also by the wear and corrosion on the both sides of flue gas and working medium, thus the working environment the water wall is very bad. In recent years, with the rapid development of thermal power in China, more and more supercritical and ultra supercritical units have been put into use. With the extension of operation time, the problem of explosion and leakage caused by early failure and cracking of water wall gradually appears and becomes more and more prominent. Some power plants even shut down frequently and unplanned, which has become an important factor affecting the safe and stable operation of ultra supercritical units [1][2]. Water wall is the heating surface distributed on the inner wall of boiler furnace and cooled by water. It is also the main heating surface in the furnace. Its reliability is directly related to the safe operation of power plant units [3][4][5][6]. With the development of boiler to large capacity and high parameters, the temperature of boiler water wall increases accordingly. In May 2021, a water wall tube leakage failure occurred in a power plant, and the specification of the water wall tube in question was Φ6×7 mm, 20G. The boiler described in this paper is a single drum natural circulation, ultra-high pressure parameter and one-time intermediate reheat solid slag discharge pulverized coal boiler manufactured by Harbin Boiler Factory with the model of HG-670/13.7-YM13. The high-temperature reheater is of W-shaped structure and arranged in the horizontal flue downstream, with a total of 114 rows. The design working pressure is 2.4 MPa, the design working temperature is 540 ℃ and the boiler was put into operation in 2002. In order to find out the causes of water wall tube leakage and avoid the recurrence of similar accidents, this paper makes a comprehensive inspection and Analysis on the leaking tube section and its adjacent tube section of water wall tube.

Experimental results and analysis 2.1 Macroscopic morphology observation and analysis
It can be seen in Fig. 1 (a) that there is one leakage point on the tube wall on the fire side of the water-cooled wall tube. The size of the leakage point is small, and there is no obvious expansion of the pipe diameter near the explosion opening, while a large number of transverse cracks can be seen on the outer wall surface of the steel pipe on the opposite side of the explosion opening. In addition, there is a certain thickness of oxide skin on the tube wall of the water wall, and there are no signs of wear and other mechanical damage, as shown in Fig. 1 (b).

Chemical composition detection and analysis
According to the requirements of GB/T 4336-2016 [7], the SPECZROMAXx desktop direct reading spectrometer is used to analyze the element chemical composition of water wall tubes, and the results are shown in Table 1. It can be seen that the content of each element in the water wall tube meets the requirements of the standard.

Microstructure detection and analysis
According to the requirements of DL/T 884-2019 [9], the microstructure of water wall tube was detected by Axio Observer.Alm type metallographic microscope. It can be seen from the metallographic structure observation that the water wall tube has a small leakage point, and the cut decreases from the outer wall to the inner wall, presenting funnel-shaped. The matrix structure near the explosion opening is consisted of ferrites and pearlites, and no deformation and elongation grain are found the tip. The matrix structure near the blast hole is consisted of ferrites and pearlites, the carbides in the pearlite area are begun to disperse, while the pearlites morphology is still obvious, and the spheroidization level of which is rated as level 2, as shown in Fig. 2(a)~Fig. 2(b). In addition, a lot of transverse cracks are found near the burst, and the cracks are initiated from the outer wall and expanded to the inner wall. The shape of the crack is nearly wedge-shaped with a sharp tail end, and the interior is full of corrosions. The matrix structure of the water wall tube is mainly consisted of ferrites and pearlites, and the carbides in pearlite area are begun to disperse, while the pearlite morphology is still obvious, the spheroidization level of whicu is rated to level 2, as shown in Fig. 2(c) and Fig. 2(d).

SEM observation of micro morphology and EDS analysis
It can be seen form the observation results of SEM micro morphology that the transverse crack of water wall tube wall is mainly transgranular, and the crack is filled with corrosion products, as shown in Figure 3. The chemical elements of corrosion products are then analyzed by EDS energy spectrometer, and the results are shown in Table 2. It can be seen from the table that the corrosion products rare mainly consisted of Fe and O, a small amount of S elements are also detected [10][11][12].

Testing and analysis of mechanical properties
Take samples from the water wall tube for tensile performance test, and the results are shown in Table 3. It can be seen that the mechanical properties of the exploded water wall tube are all met the requirements of standard values.

Discussion and analysis
The macroscopic morphology observation results shown that the water wall tube has one leakage point, and the diameter near the leakage point is not expanded. Multiple transverse cracks can be seen on the surface of water wall. The chemical composition detection results revealed that the material of water wall tube meets the standard requirements. The results of the microstructure observation indicate that there are no elongated grains formed at the burst tip of the water wall tube. In addition, there are multiple transverse cracks extending from the outer wall to the inner wallwwater wall are found far from the burst, and the cracks are filled with amount of corrosion products containing a small amount of S element, indicating that the formation of cracks is related to high-temperature sulfur corrosion. The test results of mechanical properties show that the mechanical properties of the exploded water wall tube meet the standard requirements. To sum up, the cracking and leakage of water wall pipe are mainly related to high temperature fatigue and S corrosion.

Conclusions and suggestions
The cracking and leakage of water wall tube are mainly related to thermal fatigue and S corrosion. During the operation of the boiler, transverse fatigue cracks are gradually formed at the outer wall of the water wall tube under the long-term action of axial alternating thermal stresses. Then the cracks are gradually expand from the outer wall to the inner wall under the combined action of S element corrosion and alternating stresses, and finally penetrate the tube wall to cause leakage. At last, it is suggested that the nearby water wall pipes should be inspected first, and the water wall tubes with similar cracks should be replaced in time. In addition, regularly clean the scale layer on the fire side of the water wall to reduce the high-temperature S corrosion. Finally, the metal technical supervision of heating surfaces at all levels should be strengthened to effectively avoid the recurrence of the same type of failure happening again.