电厂锅炉末级过热器爆管原因分析

在长期过热的运行工况下,锅炉高温过热器极易产生材料的劣化以及材料组织和性能的下降,最终引发爆管事故。通过宏观检查、布氏硬度测试、抗拉强度测试和金相组织检验对某电厂末级过热器爆管原因进行分析。结果表明,该末级过热器管由于在长期高温高压下超温运行,导致碳化物在晶界富集,晶界强度下降,在拉应力的作用下,晶界产生蠕变空洞及蠕变裂纹,使得珠光体耐热钢的持久强度和抗拉强度下降,最终产生断裂失效。     

Failure analysis on a primary superheater tube of a power plant

In this paper failure analysis on the SA213-T12 superheater tube by visual inspection, in situ measurements of hardness and finite element analyses is presented. A primary superheater tube has failed with a wide open burst after running at around 28,194 h. Heavy clinkers were found to almost entirely cover the primary superheater region. In situ hardness measurements were carried out on the selected primary superheater first row tubes at the middle region between furnace rear screen tube and primary superheater blower flow path. Hardness measurements are also taken on the as-received failed tube. Finite element analyses on possible features prior to failure are also conducted in order to illustrate and deduce the failure mechanism and failure root cause. Localized short-term overheating of the tube due to localized and concentrated flue gas flow resulted in a failure of the primary superheater tube.     

Failure analysis on high temperature superheater Inconel® 800 tube

This paper presents failure analysis on a super alloy Inconel^® 800 superheater tube in Kapar Power Station Malaysia. Visual inspection, microscopic examinations and creep analysis utilizing available related data are carried out to evaluate the failure mechanism and its root cause. The failed high temperature superheater (HTSH) tube was found snapped into two parts, heavily distorted shape and bent at several points. Microstructures of the failed tube showed that creep crack initiated at both external and internal surfaces of the tube and propagated as grain boundary creep cavities coalesced to form intergranular cracks. The severe geometry of tube causing steam flow starvation is identified to have caused increasing tube metal temperature resulting in overheating of the failed tube. Creep rupture is revealed as the cause of failure of the superheater tube.     

Intergranular cracking of Hastelloy C-276 in the socket welded assemblies

Abstract

The cause of thermal cracking about a socket welded tube was investigated by stress simulation and its comparison to hardness distribution of welding seam, HAZ and matrix of the tube. In the condition of heat-input in melting range of 1349°C, the stress was concentrated in HAZ at the opposite side of welding area by ANSYS program, which corresponded to the distribution of real hardness values of the tube. The high stress concentration in HAZ might be induced from fine carbide precipitation which formed during welding process. It could cause, therefore, tube cracking in operating condition simultaneously followed by bulging inside of the welded tube. In the crack propagation through all the tube originated from inside HAZ, it might be accelerated by thermal fatigue, and propagated along grain boundaries accompanied by the metal dust formation.     

Synthesis of high-speed tool steel surfaces on mild steel

Wear-resistant, hard surfaces of high-speed tool steel were synthesized on mild steel specimens. Discs of mild steel were subjected to carburization to a depth of 2.5 mm. Thin strips of tungsten were spot welded and the specimen was subjected to electron beam surface melting. The beam power was varied from 60 kV, 10 mA to 60 kV, 20 mA. Oscillation frequency and the specimen translation velocity were kept at 1000 Hz and 2 cm s-1, respectively. The width of the modified layer was 10 mm while the depth varied from 0.7–2.3 mm. A concentration of up to 30 wt% tungsten could be achieved in the surface layer by varying the thickness of the foil spot welded prior to electron-beam melting. Tungsten concentration was uniform along the depth. The hardness achieved in the as-solidified layer was uniform along the depth and reached 800 Hv. The reprocessing of the alloyed layer with the beam promoted fine carbide precipitation which then resulted in refinement of martensite plates.     

A Study on Wear Failure Analysis of Tungsten Carbide Hardfacing on Carbon Steel Blade in a Digester Tank

This paper addresses wear failure analysis of tungsten carbide (WC) hardfacing on a carbon steel blade known as the continuous digester blade (CD blade). The CD blade was placed in a digester tank to mix ilmenite ore with sulphuric acid as part of a production process. Tungsten carbide hardfacing was applied on the CD blade to improve its wear resistance while the CD blade was exposed to an abrasive and acidic environment. Failure analysis was carried out on the hardfaced CD blade in order to improve its wear resistance and lifetime. A thickness and hardness comparison study was conducted on worn and unworn specimens from the CD blades. The carbide distribution along with elemental composition analysis of the hardfaced CD blade specimens was examined using scanning electron microscopy and energy-dispersive spectroscopy. The investigation revealed that an inconsistent hardfacing thickness was welded around the CD blade. Minimum coating thickness was found at the edges of the blade surfaces causing failure to the blades as the bare carbon steel blades were exposed to the mixed environment. The wear resistance of the CD blade can be improved by distributing the carbide uniformly on the hardfaced coating. Applying extra coating coverage at the critical edge will prevent the exposure of bare carbon steel blade, thus increasing the CD blade lifetime.     

Investigation of Premature Failure of a Coal-Fired Boiler Superheater Tube

Journal of Failure Analysis and Prevention - A metallurgical investigation has been carried out to ascertain the cause of premature failure of boiler superheater tube. The tube material was...     

Investigation into the Probable Cause of Failure of Economizer Tube of a Thermal Power Plant

In this investigation, we examined the probable cause of failure of a welded joint of a economizer tube of a 210 MW thermal power plant. X-ray radiography and chemical analysis along with detailed optical and scanning electron microscopic (SEM) examination of the economizer tube were carried out. It was concluded that severe blowholes, pipings, and porosity along with brittle fracture were responsible for the failure.     

Failure investigation of a convection line elbow

Failure investigation was carried out on a cracked 5″ 304L stainless steel elbow tube for the convection line of an ethylene furnace. The crack was observed in the heat-affected zone (HAZ) at the end leading to the elbow parent material. The crack was along the circumference and was about 12″ in length. The crack started from the toe of the weld to the parent metal on the elbow side. The failure investigation revealed that the HAZ of the elbow tube suffered from stress corrosion cracking and mild grain boundary carbide precipitation during welding. The carbide precipitation was possibly due to improper welding practices such as high heat input and/or extended welding time. The tube thickness, which is relatively thick, was a contributing factor in making proper welding more difficult.     

Failure Analysis of Secondary Superheater Tube in a 600-MW Coal Power Plant

Journal of Failure Analysis and Prevention - A failure analysis was carried out on an A213 T91 secondary superheater tube after 4 months of overhaul. The failure caused the 600-MW coal...     

Failure Analysis of Ethylene Cracking Tube

Radiant tubes of an ethane furnace at a petrochemical plant fabricated from an austenitic heat resistant steel casting (HP grade) failed along longitudinal direction after a fraction of anticipated service life. To study the cause of failure, microstructures of as-received and used tubes were investigated by optical and scanning electron microscopy and the microchemical composition of tubes and precipitated carbide were determined by energy dispersive X-ray (EDS). Also, the morphology of deposited coke particles was determined by SEM/EDS. Finally to measure the extent of carbon penetration, hardness testing was performed on the inner and outer surface of tube. The experimental results show that the improper coking and decoking cycles remove the protective oxide layer (Cr₂O₃) that forms on the exposed surfaces and that, with this layer removed, the coke could easily deposit on inner, non-protected surface. The carbon diffusion into the metal was accelerated with deposited coke and caused microstructural degradation and drastically reduced the ductility of material at high temperatures.     

Failure analysis of a boiler tube in USC coal power plant

This paper presents failure analysis of final superheater tube in ultra-supercritical (USC) coal power plant. Visual inspection was performed to find out the characteristics of fracture of the as-received material. And the micro-structural changes such as grain growth and carbide coarsening was examined by scanning electron microscope. Detailed microscopic studies were made to find out the behavior of the scale exfoliation on the waterside of tubes. From those investigations, the creep rupture may be caused by the softened structure induced by carbide coarsening and accelerated by the metal temperature increase by the impediment of heat transfer due to voids.     

Metallographic investigations of the failed superheater tubes used in liquid hydrogen plant

2.25Cr–1Mo low alloy steel tubes of diameter 42.5 mm ID and 46.5 mm OD were used in superheater of liquid hydrogen plant. This superheater had 10 rows of tubes for carrying naphtha and steam and is heated by flue gas. The flue gas directly impinges on the first three tubes of the tube bank. A failure occurred in the first tube leading to the removal of a portion of the material causing to shut down of the plant.Detailed metallurgical investigations were carried out to understand the cause of failure. This paper brings out the metallographic investigations on the failed tubes and the necessary remedial actions thereon.     

锅炉过热器管爆裂原因分析

针对某热电厂4号锅炉12CrlMoV钢过热器管的爆裂破坏,进行了宏观形貌、化学成分、金相组织、扫描电镜等的分析。结果表明,炉管爆裂的原因是炉内局部温度过高,该处炉管长期过热,管壁组织珠光体球化达到5级．晶界出现蠕变裂纹,导致炉管强度降低所致。     

锅炉顶棚过热器管开裂原因分析

对某热电厂3号锅炉的无损探伤检查,发现该炉部分顶棚过热器管的背火面上存在裂纹。通过对过热器管的裂纹形貌、化学成分、金相组织和断口等的分析,确认3号锅炉顶棚过热器管的开裂失效性质属于热疲劳破坏。     

Failure analysis of styrene reactor tubes

The analysis of failure of the styrene reactor tubes made of stainless steel has been reported where the tubes contained catalyst as well as ethyl benzene in vapour form along with superheated steam at 570 °C. The tubes were exposed to flue gases causing heating of tubes to 790 °C in the upper part where cracking and failure were mostly noticed. The studies included microstructural examinations of cracked and uncracked tubes, fracture surface investigations and estimation of creep rupture strength, etc. The degradation in microstructure such as extensive grain coarsening, sigma phase formation, carbide formation along grain boundaries, etc., led to development of cracks/voids within the grains and along the grain boundaries and also excessive oxidation of tubes. Possible chemical attack by the feed on the inner surface of tubes further deteriorated the tube life. Suggestions for avoidance of failure have been listed.     

锅炉高温过热器爆管原因分析

某电厂高温过热器在运行期间发生爆管事故,爆口具有短时超温和长时超温特征.通过对爆管管样及相邻对比管样进行宏观检查、化学成分、金相及力学性能等分析,确定此次爆管是由于锅炉运行时间较长,管子长期在高温下服役,金相组织慢慢出现碳化物析出等老化现象,导致材料本身的性能下降,加之管子内部存有异物,造成管子介质流量减小及管子壁温上升,使管子在高温下的环向应力超过其材料本身强度而发生短时超温爆管.     

Role of oxide notching and degraded alloy microstructure in remarkably premature failure of steam generator tubes

Failure analysis investigation was carried out on a ruptured primary superheater tube of a boiler unit in a power plant. The rupture of 1Cr–0.5Mo steel tube had caused steam leakage and led to boiler shutdown after only 17,520 h of operation. The physical and mechanical characteristics of the alloy and the oxide scales in the area of failure were characterized by visual inspection, optical microscopy and microhardness testing, whereas the chemical variations were examined by optical emission spectrometry and energy dispersive spectrometry (EDS). A distinct surface/sub-surface zone of decarburization that was observed both on the inner and outer sides of the tube was developed before the tubes were put to the service. The fracture was attributed to a prolonged overheating as a result of the oxide scale build-up, causing loss of creep strength. The decarburized surface/sub-surface facilitated crack initiation/propagation.     

Failure analysis of superheater tube

The failure analysis of a ruptured superheater tube after 20 years service in the oil-fueled boiler, as the typical problems in power plants, was investigated. A thin-lipped rupture at failed region was observed in superheater tube. By measuring the tube’s wall thicknesses far from failed region, non-uniformity was seen. The suggested main root cause of failure was fireside corrosion of the tube during the service. Because of low grade of used fuel, sodium, sulfur, and vanadium elements were observed at the outer surface, which caused continuously scale formation and reduction of wall thickness, by metal consumption. In addition, it seems that it has been worsened by occurrence of long-term overheating. Coagulation of carbides at both outer and inner regions of tube was observed that could prove the occurrence of overheating during the service. In addition, the formation of sigma-phase particles was revealed because of being in the susceptible temperature after 20 years in service. At the end, in order to prevent or decelerate such failure, some recommended remedies were suggested.     

过渡合金层对激光多层堆焊组织性能影响

采用45钢作为基体材料,在不预热情况下通过增加过渡合金粉堆焊层,再选用专用焊丝进行堆焊试验。由于过渡合金粉中的Ni、Cr提高的奥氏体的稳定性,产生FeNi缓解了高硬度焊丝中碳化物内产生的残余应力,多次堆焊加热使其热应力在该区域得到缓解,减少了裂纹倾向,同时焊丝中的C、V等形成碳化物,因此提高了堆焊层的硬度。