Abstract: In this paper, we mainly analyze the types of fracture faults that often occur in the bolt fittings of power station boiler boilers, focusing on the root cause of sudden failure. In order to let the reader understand the cause of fracture in a clear perspective, I hereby start with the macroscopic inspection and detailed observation. The area where the valve bolt breaks is excluded from the thread area and is delineated at the base of the end cap. Then it analyzes the fracture character and evolution process, and deduces the process of short layer fracture, extension fracture and complete fracture. At the same time, in order to refine the fracture factors, the article also conducted various forms of analysis (chemical, microscopic examination, metallographic, corrosion, etc.) for the fracture structure. Finally, based on the actual fault factors identified, Formed the reasons and the relative formulation of countermeasures, hope that through this article to play a warning to the relevant manufacturing industry personnel, improve the quality standards of the industry.
Keywords: power station boiler; valve bolt; countermeasure Introduction:
In the normal operation of power plant boilers, because of its long-term internal components in high temperature and pressure and other unconventional work environment, it often results in micro-firmware performance failure or reduction and so on. Including boiler valve bolt failure probability is relatively high, and the role of power plant boilers in the inevitable conventional routine, once the small firmware failure to contain the overall operating efficiency, ranging from interference with the boiler safety operation index, the weight will be a direct role in The overall power plant, not only affect the normal transmission process, but also cause serious power problems. Therefore, the boiler troubleshooting, the use of the effectiveness of the valve bolts and troubleshooting great investigation value. In order to make the reader more intuitive and clear the reason for the fracture, I hereby refer to the investigation files as an example, hope to pass the details of the description of peer or technical staff play a supporting role.
A selected case is described in a petrochemical plant boilers, condensers drop line a throttle valve bonnet fastening bolt suddenly broken, causing a large steam leakage, causing an emergency shutdown, the valve replacement. The condenser medium is 108 ℃ high temperature water and 200 ℃ steam mixing pressure about 36kg / cm. Valve failure model JZ46W, size DN50, put into use in June 2006, the use of nearly four and a half years cycle. Valve bonnet and other components found no exception, only the valve cover is connected to the body of the fastening bolt fracture. The fastening bolts of the valve are hexagonal flange type bolts, three of the four fastening bolts are broken, and one is not broken.
Second, the macro inspection Remove the four bolts from the valve cover cleaning, and macroscopic examination. The bolt break is not at the thread but at the root of the bolt's end cap. As can be seen from the fracture surface of the rupture bolt, the dark gray area is larger and the fresh area is smaller. Dark gray areas should be pre-cracking fractures subject to oxidation and corrosion, more fresh areas should be the new fracture of the sudden fracture. The end cap portion of the bolt should be forged. There are several obvious forging cracks in the outer edge of the end cap near the fracture.
Third, the bolt fracture microscopic test analysis The three broken bolts and a broken end cap repeatedly cleaned with an ultrasonic cleaner, the use of imported scanning electron microscopy and energy dispersive spectroscopy analysis of four fracture surface morphology, analysis of corrosion Product composition. It can be seen from the micro-morphology of the fracture surface that the micro-morphology of most of the fracture areas is along the grain boundary, that is, the crack is cracked along the grain boundary and belongs to the typical brittle fracture. Scanning the dark gray area of ​​the fracture by 200 times electron microscopy, it can be seen that the surface of the fracture along the fracture is covered with a layer of corrosion products. According to the composition analysis, the corrosion products mainly consist of iron oxide, of which c1 and S are present in the corrosion products.
Then, the fresh area of ​​the fracture was examined by 200 times electron microscopy. The microscopic appearance of a small part of the fresh area of ​​the fracture was not the orthorhombic feature, but dimple feature, and the dimple was the fracture feature of normal plastic fracture. This shows that the bolt fracture belongs to the delayed fracture, that is, the first initiation of the crack, followed by the crack propagation and finally the sudden fracture. It can be seen from the microscopic appearance of the outer edge region of the bolt cap that the forging forming process of the bolt is not standardized, the forging temperature is lower, the thick deformation of the pier at the bolt end cap part is larger, which leads to more forging cracks, No secondary processing was done to remove these cracks.
Fourth, the bolt material composition analysis of the sample from a fracture bolt, the use of direct-reading spectrometer and carbon-sulfur analyzer analysis of bolt material composition. Commonly used in bolts of stainless steel 1Crl7Ni7 and 1Crl8Ni9, but in accordance with the national standard GB / T1220-1992 (stainless steel grades and chemical composition) standards, fracture bolt material composition does not meet 1Crl7Ni7 and 1Crl8Ni9, and closer to 1Crl8Mn8Ni5N. However, according to the national standard 1Crl8Mn8Ni5N requirements, fracture bolt material composition, the high content of C, Cr content is low, which greatly reduces its corrosion resistance.
V. Measurement and Analysis of Bolt Metallography The two samples were intercepted from the fracture bolts and pre-ground, ground, polished, polished and aqua regia etched with water sandpaper to prepare a metallographic sample. The microstructure of one sample was examined by metallographic microscope. The other sample was observed and analyzed for its forging defects by imported scanning electron microscope. After testing, the metallographic microstructure of the austenite, grain size rating of 6, relatively coarse tissue.
It can be seen from the micro-morphology of the forging defects of bolts that larger forging cracks and inclusions are found on the surface and the interior of the bolt, and obvious intergranular cracking morphology is found near the fracture surface and the crack, which indicates that the intergranular cracking of the bolt is mainly related to Forging defects related.
Sixth, the investigation of chloride ion source After sampling analysis, the final confirmation of Cl- from insulation materials coated waterproof asphalt. The waterproof asphalt is applied to the exterior of the thermal insulation material to prevent the rain and moisture from entering the interior of the thermal insulation material. The water-proof asphalt sample into the water, immediately qualitative detection of water containing Cl-, indicating that the water-soluble asphalt CI - highly soluble. Due to the presence of Cl- in water-proof asphalt, moisture in the summer air is high and during the rainy season, chlorine vapor-containing water vapor is exposed to equipment and pipelines, causing corrosion.
Seven, the bolt fracture analysis and countermeasures After a series of testing process technicians can confirm that the case of the valve bolt failure occurred in the basic part of the end cap can be identified, and is a continuum of sudden rupture properties, the overall process of slow destruction , The fracture at the Department of Obviously consistent with the crystalline state.
(1) The reason for the fracture of the valve bolt is mainly based on the lack of material, and the material problems can be divided into two categories: First, the bolt quality standards and material specifications adopted by the international quality standards for bolts Obviously deviate from the standard specifications. So its corrosion resistance and firmness will inevitably also not support the normal operation of the boiler; Second, the top of the nut requires careful process to support the structure of its utility, once there is deviation in the manufacturing process, it will inevitably crack in high temperature environment causing fracture risk ; The secondary factor of bolt fracture is the environmental temperature fluctuation and the medium corrosion factor. The concrete manifestation is that the valve withstands the thermal fatigue stress caused by the temperature fluctuation and accelerates the initiation and propagation of the crack.
For the quality of the problem of deviation, manufacturers can focus on the process of manufacturing process to explore the quality requirements of the status quo, the requirements of lax or size standard deviation of the process stage, we must immediately carry out the whole eye, for all the production process may exist Hidden factors to exclude all. Treat the problem valve that has been installed in the same factory for the same batch, promptly recall it and replace it without compensation, so as to avoid affecting the industry reputation due to technical error. At the same time learn lessons to strengthen the quality inspection of the arrival of the valve, the key parts of the valve parts sampling, to ensure the safety and reliability of the process.
Eight, lessons learned and lessons in the boiler valve structure is the most prone to rupture in the high residual stress areas, such as welding, bolts or rivets at the junction, as well as those that can stress concentration or concentration of the environment, such as the gap and Weld at. The most effective way to prevent this kind of corrosion is to use non-destructive testing technology to monitor these sensitive parts, find the source of cracks as early as possible, and take precautionary measures to avoid accidents.
references:
[1] LUO Guanghui, JU Linli, LIU Houchao, et al.Analysis and countermeasures of fracture of valve bolts of power station boiler [J] .Petroleum Chemical Design, 2011, 28 (3): 31-32.
[2] LI Ling. Analysis of the Causes of Pipe Valve Bolt Fracture [J]. Mechanical Engineering Materials, 2013, 37 (5): 106-110.
[3] Xu Zhigang, Ju Linli, Wu Bing, et al. Analysis of the Causes of Valve Bolt Fracture [J]. Corrosion and Protection, 2010, 31 (7): 578-580.
Keywords: power station boiler; valve bolt; countermeasure Introduction:
In the normal operation of power plant boilers, because of its long-term internal components in high temperature and pressure and other unconventional work environment, it often results in micro-firmware performance failure or reduction and so on. Including boiler valve bolt failure probability is relatively high, and the role of power plant boilers in the inevitable conventional routine, once the small firmware failure to contain the overall operating efficiency, ranging from interference with the boiler safety operation index, the weight will be a direct role in The overall power plant, not only affect the normal transmission process, but also cause serious power problems. Therefore, the boiler troubleshooting, the use of the effectiveness of the valve bolts and troubleshooting great investigation value. In order to make the reader more intuitive and clear the reason for the fracture, I hereby refer to the investigation files as an example, hope to pass the details of the description of peer or technical staff play a supporting role.
A selected case is described in a petrochemical plant boilers, condensers drop line a throttle valve bonnet fastening bolt suddenly broken, causing a large steam leakage, causing an emergency shutdown, the valve replacement. The condenser medium is 108 ℃ high temperature water and 200 ℃ steam mixing pressure about 36kg / cm. Valve failure model JZ46W, size DN50, put into use in June 2006, the use of nearly four and a half years cycle. Valve bonnet and other components found no exception, only the valve cover is connected to the body of the fastening bolt fracture. The fastening bolts of the valve are hexagonal flange type bolts, three of the four fastening bolts are broken, and one is not broken.
Second, the macro inspection Remove the four bolts from the valve cover cleaning, and macroscopic examination. The bolt break is not at the thread but at the root of the bolt's end cap. As can be seen from the fracture surface of the rupture bolt, the dark gray area is larger and the fresh area is smaller. Dark gray areas should be pre-cracking fractures subject to oxidation and corrosion, more fresh areas should be the new fracture of the sudden fracture. The end cap portion of the bolt should be forged. There are several obvious forging cracks in the outer edge of the end cap near the fracture.
Third, the bolt fracture microscopic test analysis The three broken bolts and a broken end cap repeatedly cleaned with an ultrasonic cleaner, the use of imported scanning electron microscopy and energy dispersive spectroscopy analysis of four fracture surface morphology, analysis of corrosion Product composition. It can be seen from the micro-morphology of the fracture surface that the micro-morphology of most of the fracture areas is along the grain boundary, that is, the crack is cracked along the grain boundary and belongs to the typical brittle fracture. Scanning the dark gray area of ​​the fracture by 200 times electron microscopy, it can be seen that the surface of the fracture along the fracture is covered with a layer of corrosion products. According to the composition analysis, the corrosion products mainly consist of iron oxide, of which c1 and S are present in the corrosion products.
Then, the fresh area of ​​the fracture was examined by 200 times electron microscopy. The microscopic appearance of a small part of the fresh area of ​​the fracture was not the orthorhombic feature, but dimple feature, and the dimple was the fracture feature of normal plastic fracture. This shows that the bolt fracture belongs to the delayed fracture, that is, the first initiation of the crack, followed by the crack propagation and finally the sudden fracture. It can be seen from the microscopic appearance of the outer edge region of the bolt cap that the forging forming process of the bolt is not standardized, the forging temperature is lower, the thick deformation of the pier at the bolt end cap part is larger, which leads to more forging cracks, No secondary processing was done to remove these cracks.
Fourth, the bolt material composition analysis of the sample from a fracture bolt, the use of direct-reading spectrometer and carbon-sulfur analyzer analysis of bolt material composition. Commonly used in bolts of stainless steel 1Crl7Ni7 and 1Crl8Ni9, but in accordance with the national standard GB / T1220-1992 (stainless steel grades and chemical composition) standards, fracture bolt material composition does not meet 1Crl7Ni7 and 1Crl8Ni9, and closer to 1Crl8Mn8Ni5N. However, according to the national standard 1Crl8Mn8Ni5N requirements, fracture bolt material composition, the high content of C, Cr content is low, which greatly reduces its corrosion resistance.
V. Measurement and Analysis of Bolt Metallography The two samples were intercepted from the fracture bolts and pre-ground, ground, polished, polished and aqua regia etched with water sandpaper to prepare a metallographic sample. The microstructure of one sample was examined by metallographic microscope. The other sample was observed and analyzed for its forging defects by imported scanning electron microscope. After testing, the metallographic microstructure of the austenite, grain size rating of 6, relatively coarse tissue.
It can be seen from the micro-morphology of the forging defects of bolts that larger forging cracks and inclusions are found on the surface and the interior of the bolt, and obvious intergranular cracking morphology is found near the fracture surface and the crack, which indicates that the intergranular cracking of the bolt is mainly related to Forging defects related.
Sixth, the investigation of chloride ion source After sampling analysis, the final confirmation of Cl- from insulation materials coated waterproof asphalt. The waterproof asphalt is applied to the exterior of the thermal insulation material to prevent the rain and moisture from entering the interior of the thermal insulation material. The water-proof asphalt sample into the water, immediately qualitative detection of water containing Cl-, indicating that the water-soluble asphalt CI - highly soluble. Due to the presence of Cl- in water-proof asphalt, moisture in the summer air is high and during the rainy season, chlorine vapor-containing water vapor is exposed to equipment and pipelines, causing corrosion.
Seven, the bolt fracture analysis and countermeasures After a series of testing process technicians can confirm that the case of the valve bolt failure occurred in the basic part of the end cap can be identified, and is a continuum of sudden rupture properties, the overall process of slow destruction , The fracture at the Department of Obviously consistent with the crystalline state.
(1) The reason for the fracture of the valve bolt is mainly based on the lack of material, and the material problems can be divided into two categories: First, the bolt quality standards and material specifications adopted by the international quality standards for bolts Obviously deviate from the standard specifications. So its corrosion resistance and firmness will inevitably also not support the normal operation of the boiler; Second, the top of the nut requires careful process to support the structure of its utility, once there is deviation in the manufacturing process, it will inevitably crack in high temperature environment causing fracture risk ; The secondary factor of bolt fracture is the environmental temperature fluctuation and the medium corrosion factor. The concrete manifestation is that the valve withstands the thermal fatigue stress caused by the temperature fluctuation and accelerates the initiation and propagation of the crack.
For the quality of the problem of deviation, manufacturers can focus on the process of manufacturing process to explore the quality requirements of the status quo, the requirements of lax or size standard deviation of the process stage, we must immediately carry out the whole eye, for all the production process may exist Hidden factors to exclude all. Treat the problem valve that has been installed in the same factory for the same batch, promptly recall it and replace it without compensation, so as to avoid affecting the industry reputation due to technical error. At the same time learn lessons to strengthen the quality inspection of the arrival of the valve, the key parts of the valve parts sampling, to ensure the safety and reliability of the process.
Eight, lessons learned and lessons in the boiler valve structure is the most prone to rupture in the high residual stress areas, such as welding, bolts or rivets at the junction, as well as those that can stress concentration or concentration of the environment, such as the gap and Weld at. The most effective way to prevent this kind of corrosion is to use non-destructive testing technology to monitor these sensitive parts, find the source of cracks as early as possible, and take precautionary measures to avoid accidents.
references:
[1] LUO Guanghui, JU Linli, LIU Houchao, et al.Analysis and countermeasures of fracture of valve bolts of power station boiler [J] .Petroleum Chemical Design, 2011, 28 (3): 31-32.
[2] LI Ling. Analysis of the Causes of Pipe Valve Bolt Fracture [J]. Mechanical Engineering Materials, 2013, 37 (5): 106-110.
[3] Xu Zhigang, Ju Linli, Wu Bing, et al. Analysis of the Causes of Valve Bolt Fracture [J]. Corrosion and Protection, 2010, 31 (7): 578-580.
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