Thread Content
The burst pattern identification method is an important means for determining the cause of pipe rupture at the scene. The characteristics of the crack opening mainly refer to: (1) Crack location: the specific part of the heated surface where it occurs, whether on the fire-facing side or the backfire side. (2) Shape of the fracture: 1) Whether the fracture surface is perpendicular to the axis; 2) Whether there is significant thinning at the edges of the fracture, and whether they are sharp or blunt; 3) Whether there is fouling on the inner wall of the fracture, as well as oxidation on the outer wall, and any macroscopic cracks in the area near the fracture; 4) Whether there are obvious corrosion pits on the inner and outer walls near the fracture; 5) The direction of the cracks on the inner and outer walls in the area near the fracture. (3) Microstructure near the crack: including the composition, quantity, shape, size, and distribution of phases, as well as various types of microstructural cracks (their nature, size, shape, direction, and relationship with the microstructure), the size and distribution of microscopic pores, the degree of spheroidization of pearlite and the degree of graphitization, as well as issues such as decarburization, overburning, and overheating. 2 Overheating and tube rupture: Overheating can be divided into two categories: short-term overheating and long-term overheating. Prolonged overheating: In cases of tube failure due to prolonged overheating, the crack size is usually small; the cross-section of the crack is rough and uneven. The wall thickness of the tube does not decrease significantly, and the edges of the crack are blunt rather than sharp. There are numerous axial cracks in the tube that run parallel to the crack site. Due to long-term operation at high temperatures, there is often a relatively thick layer of black scale near the burst point. From the perspective of creep mechanics, the fracture should be a plastic fracture; however, creep-induced tube failure is often accompanied by stress corrosion, which causes the fracture to exhibit characteristics of brittle fracture. When the tube overheats, its diameter expands at an accelerated rate due to creep; obvious intergranular creep cracks can typically be seen in the metallographic view of the cracked area, along with severe spheroidization. Due to prolonged operation at high temperatures, oxidation occurs within the cracks as they develop; as a result, an oxide layer is formed on the inner walls of the cracks. This oxide layer is particularly evident in the larger creep cracks. Short-term overheating occurs when the mechanical properties of pipes deteriorate significantly under conditions of severe over-temperature, causing the pipes to undergo plastic deformation under pressure and eventually burst. Short-term overheating-induced tube failure can be further classified according to the degree of overheating as follows: (1) instantaneous overheating-induced tube failure, with temperatures above Ac3; (2) short-term direct overheating-induced tube failure; (3) tube failure due to small bulges. The fracture surface at the site of instantaneous overheating-induced tube rupture is trumpet-shaped; the tube is severely thinned and enlarged, with sharp edges, indicating a ductile fracture, and its surface features blue-black oxidized tissue. Due to the rapid ejection of the steam-water mixture from inside the pipe, the inner wall of the rupture appears very smooth; the pipe has also undergone significant bulging. The outer surface of the pipe is generally blue-black; there are no numerous axial cracks parallel to the crack near the fracture site, and the microstructure at the fracture site is feathery bainite. The fracture surface of short-term direct overheating tube failure is large, appearing as an irregular rhombus in shape; the microstructure features spheroidized carbides, with relatively sharp edges at the fracture site. There is some swelling near the fracture, and the tube also shows varying degrees of swelling at locations further away from the fracture. The fracture zone consists of ferrite along with globular pearlite, which has reached a certain degree of spheroidization. A small bulge resulting from tube rupture is caused by localized overheating; the areas that have not ruptured do not show significant swelling, while there is a noticeable small bulge at the point of rupture, which is also sharp and smooth. The fracture zone consists of ferrite along with massive pearlite; the pearlite is to some extent spheroidized, and there are also cementite spheres at the grain boundaries. 3 Wear-induced tube rupture: The characteristic of a wear-induced rupture is significant thinning of the tube wall near the rupture site; there are no obvious changes in the microstructure of the area affected by the rupture, and it represents a type of ductile fracture. The edge of the rupture is thin. Four-tube explosion is caused by wear on the heating surfaces, and depending on the mechanism of wear, it can be classified into the following categories: (1) Fly ash wear; fly ash wear is one of the most important causes of wear, leakage, and tube explosion in low-temperature heating surfaces. Tests have shown that on carbon steel surfaces, the areas with an impact angle of 30° to 50° suffer the most severe wear, resulting in worn edges on the pipe wall surface. Meanwhile, in the boiler, fly ash wear causes tube failures in the low-temperature heating surfaces, mainly due to the presence of flue gas channels in those areas. (2) Mechanical wear; Mechanical wear occurs because the tube clamps on the heat-exposing tube rows often deform due to overheating or become loose as a result of poor welding, which leads to vibration of the tubes and friction against those clamps. Alternatively, collisions or friction between the water wall and other adjacent components can cause the tube walls to wear down. When the wall thickness reaches a certain level, the tubes burst under the effect of internal pressure. Therefore, obvious traces of mechanical friction can be found on the surface of the tube. (3) Ash erosion; the use of ash blowers can cause wear on the pipe walls. The morphology of soot erosion is similar to that of fly ash erosion, and the metallography of the tube wall is also comparable; it is usually merely mechanical wear, resulting in plastic deformation, with a noticeable thinning at the worn areas of the tube wall. Wear and tube rupture generally occur in the soot blower tube arrays. (4) Coal particle erosion; coal particle erosion generally occurs when the tertiary air nozzle (or main burner) becomes damaged and deformed, causing the powder-laden airflow to erode the surrounding water-cooled walls. For coal particle wear, the fracture characteristics are as follows: the crack starts at the thinnest part of the wall on the side of the fire-facing surface along its centerline, and then spreads in a flapping manner with the other side of the fire-facing surface as the axis. The edge of the rupture is blade-like, with one end torn; the blast tube and the tubes on both sides show no thickening or bulging. On the side toward the centerline of the flame-facing surface, there is severe wear and thinning; near the burst hole, the pipe may have slagging. The microstructural changes at the edge of the crack and on the fire-facing side of the adjacent tube were minimal; there was no significant elongation of the ferrite grains at the crack edge, indicating that plastic deformation was limited when the water wall ruptured. (5) Spalling wear: There are relatively few instances of wear caused by falling slag; when it occurs, pinpoint perforations and leaks appear on the inclined surface of the cold ash hopper. 4 Corrosion-induced tube rupture: Tube rupture caused by chemical or electrochemical reactions between external media and the heated surface tubes is referred to as corrosion-induced tube rupture. Although corrosion-induced tube failures account for a low proportion of all tube failures, their suddenness and unpredictability mean that once corrosion occurs, the extent of damage is significant, often resulting in extensive damage to the heating surfaces. Based on the location where corrosion occurs, tube failures can be divided into flue gas side corrosion and water side corrosion tube failures. Flue gas side corrosion: Depending on the location and conditions in which it occurs, flue gas side corrosion can be divided into high-temperature corrosion and low-temperature corrosion. When high-moisture, high-sulfur fuels are burned, the tubes of the high-temperature heating surfaces are corroded, a phenomenon known as high-temperature corrosion. Low-temperature corrosion refers to the corrosion that occurs on the low-temperature heating surfaces at the tail. 1 High-temperature corrosion: Tube failure caused by high-temperature corrosion on the flue gas side, with the following corrosion mechanisms: (1) Action of corrosive gases such as SO2, SO3, and H2S ； (2) Mechanism of high-temperature corrosion in sulfate form ； (3) Mechanism of sulfide-type high-temperature corrosion. Near the crack, a large area of corrosion can be clearly identified; this corroded area is uneven in texture. The wall thickness decreases near the crack, and the rupture site appears to be torn, with long cracks. The difference between corrosion-induced tube failure and wear-induced tube failure is as follows: the pipe wall near a wear-induced failure is very smooth, with a worn edge, whereas the area around a corrosion-induced failure is uneven and lacks distinct edges; there is no significant change in the microstructure at the fracture site, the grains at that location are elongated, indicating a ductile fracture; on the flue gas side of tubes that fail due to corrosion, sulfur-containing deposits are present on the outer surface of the pipe wall, while on the side closer to the matrix, black deposits are usually found, which are tightly bonded to the pipe wall. 2. Low-temperature corrosion: Low-temperature corrosion-induced tube failure mainly occurs in economizers where the feedwater temperature is low. Near the fracture site of low-temperature corrosion-induced tube rupture, there are also uneven corrosion areas; the fracture is of a tearing type, with no significant changes in the microstructure. The grains at the fracture site are elongated, indicating a ductile fracture. Water-side corrosion: Boiler tubes can also experience tube rupture due to water-side corrosion. This type of corrosion includes corrosion that occurs during operation as a result of local concentration of water inside the boiler, oxygen corrosion caused by oxygen present in the feed water, and caustic embrittlement resulting from stress. Caustic embrittlement mainly occurs in expansion-jointed or riveted boilers, and is less common in large power station boilers. 1 Acidic corrosion and alkaline corrosion: The process of corrosion that occurs during operation due to localized concentration of water inside the boiler can be described as follows: The boiler water becomes locally concentrated beneath deposits in the tubes, in the gaps of the evaporation surfaces, and in areas where vapor plugs form within the tubes. This leads to the formation of concentrated acid or alkali, which destroys the Fe3O4 protective layer on the inner surface of the tubes. As a result, the metal surface of the tubes is corroded by acid or alkali, a phenomenon known respectively as acidic corrosion and alkaline corrosion. Alkaline corrosion often occurs beneath porous deposits; near the corrosion sites, the corrosion products have poor adhesion to the metal surface. These corrosion products contain furnace water components such as phosphates and silicates. After the corrosion products are removed, uneven corrosion pits remain. Due to the low concentration of hydrogen ions in highly alkaline conditions, the hydrogen produced can diffuse away easily and does not penetrate into the steel, thus avoiding carbon loss. The microstructure and mechanical properties of the metal beneath the pit remain unchanged; the metal retains its ductility. Tube rupture is caused by corrosion-induced damage that leads to thinning of the tube wall and subsequent overheating and bulging. Acidic corrosion often occurs beneath relatively dense deposits; due to the high concentration of hydrogen ions in concentrated acid environments, the generated hydrogen cannot diffuse easily. Some of this hydrogen penetrates into the steel and reacts with the cementite Fe3C present in it. Therefore, acidic corrosion and hydrogen embrittlement always occur together. Near the crack, the corrosion products are firmly bonded to the metal surface, and there are corrosion pits on the metal surface. The inner wall surface of most cracks is decarburized, and the pipe wall is often covered with microcracks that form a network; these cracks are mostly intergranular fractures. The swelling at the fracture site is not significant; the fracture surface is flat and blunt, showing characteristics of brittle fracture. 2 Oxygen corrosion of boilers: Oxygen corrosion in boilers is a type of electrochemical corrosion. Oxygen corrosion mainly occurs on the inner wall of the heating surface in the inlet section of the economizer; in severe cases, it can extend to the middle part of the economizer and even to the boiler water wall. Its main characteristic is ulcerative corrosion, which results in the formation of many small bulges on the corroded metal surface; these bulges vary greatly in diameter. The color of their surfaces ranges from yellow-brown to brick red, with a black powder layer beneath them, and the metal surface features corrosion pits. The microstructural changes at the fracture surface are not significant, indicating a ductile fracture. 5 Fatigue failure: Boiler tubes are subjected to alternating thermal and mechanical stresses due to startups, shutdowns, or changes in load. Additionally, they are under the effect of gravity caused by the weight of the tubes themselves and the fluid inside them. When the tubes vibrate for various reasons, the stresses within them also change periodically, resulting in thermal fatigue and vibration fatigue of the boiler tubes. 1 Vibration fatigue: Vibration fatigue is often caused by the failure of supports or improper installation; there is no obvious thinning at the fracture site, and it is a transverse fracture. 2 Thermal fatigue: Thermal fatigue can be caused by periodic cooling of the metal on the water side due to intermittent steam stagnation or sudden cooling. There is generally no significant thinning at the fracture site; it is a transverse fracture. Below the fracture, multiple cracks of varying sizes parallel to the fracture plane can be observed. Microscopic examination of the crack tips reveals that the cracks propagate along the grain boundaries, with secondary grain-boundary cracks present; the crack tips are discontinuous, and the direction of crack propagation is perpendicular to the tube axis. Rapid cooling during soot blowing, or intermittent wetting of the heated surface tubes by slag layers, can also cause periodic temperature changes, leading to fatigue damage of the furnace tubes and the formation of numerous transverse cracks on their outer surfaces, resembling an \"elephant skin\" pattern. 3 Corrosion fatigue: Various corrosion phenomena occur within boilers, and the fatigue failure of components resulting from the action of corrosive agents is known as corrosion fatigue. The outer or inner surface of a burst crack due to corrosion fatigue usually has a corrosion layer or oxide layer attached to it. Generally, there is no bulging or swelling at the fracture site; there is no thinning of the wall thickness, nor any plastic deformation; instead, it exhibits brittle fracture. The fracture surface of the crack opening is relatively flat and perpendicular to the wall thickness of the tube, but it is not smooth. At the onset of corrosion fatigue failure, it often occurs through multiple crack sources; as a result, the fracture surface typically exhibits distinct multi-toothed characteristics. 6 Welding quality and welding of dissimilar steels: Welding defects remain a significant issue in pipe failures in China. Taking shielded metal arc welding as an example, common defects include undercutting, porosity, weld beads, undercuts, lack of penetration, slag inclusions, pores, and cracks (including thermal cracks, cold cracks, and reheat cracks). These welding defects lead to stress concentration at the welded areas as well as a reduction in the mechanical properties of the joints, thereby making the welded sections weak points that can cause pipe failures. The quality issues related to welding that lead to tube rupture are easy to identify, as the fracture always occurs at the welded area; the crack generally forms along the defective part, and it is relatively straight. The welded joint of dissimilar steels is also a location prone to tube burst, as circumferential cracking can occur at the weld due to differences in thermal expansion. Bursts in the four tubes of the boiler occur frequently. Identifying the causes of these damages helps to reveal potential hazards in the boiler; if left unaddressed, they can lead to more serious problems. Most causes of damage can be attributed to one of several fundamental reasons. Comprehensive metallographic failure analysis can usually reveal its root cause ； However, it is not necessary to perform metallographic analysis on all damaged tubes. The damaged appearance of the tube can provide intertwined information about the cause of the damage. This information helps narrow down the range of possible causes for pipe burst; sometimes, combined with some knowledge of boiler operation, it is sufficient to determine the cause of the damage. It must be noted, however, that many types of damage cannot be distinguished based solely on their visual appearance; sometimes different causes can result in damages that appear identical in terms of their visual features. Frequent or severe tube damage must be subjected to a thorough analysis in a qualified metallographic laboratory in order to identify the true cause.