Metallurgical Analysis of Cracks Formed on Coal Fired Boiler Tube Related content Residual stress anisotropy in high pressure sodium lamp seals Metallurgical Analysis of Cracks Formed on Coal Fired Boiler Tube (original) (raw)

Stress-Assisted Corrosion in Boiler Tubes

2006

Weyerhaeuser Company for their advise and support in this project. Representative boiler tubes from the Weyerhaeuser Longview mill were provided by D. Rittenbach (Weyerhaeuser). A. Willoughby (ORNL) managed photography and specimen handling for the SAC investigation. Tube panels were flame cut longitudinally into manageable sections for handling by L. Smarsh (ORNL). Circumferential wave inspections were performed by M. Cooper, J. Gustafson, and S. Shankles (Longview Inspections). Longitudinal wave inspections were performed by M. Quarry (Lawrence Livermore National Laboratory) and J. Brignac (Alstom Power). Radiographic inspections were performed by R. Sweet and C. Culbertson (Longview Inspections). Following the various inspections, tubes were saw cut into sections to facilitate visual inspections and metallographic sampling by B. Gaskin (ORNL). H. Longmire (ORNL) prepared and photographed the metallography specimens. L. Walker (ORNL) performed the SEM and microprobe analyses. Along with P. Singh (Georgia Institute of Technology), D. Wilson and P. Tortorelli (ORNL) reviewed the draft manuscript, and F. Stooksbury and K. Choudhury (ORNL) helped prepare the final document. Help of Mr. Jamshad Mahmood, and Dr. Jorge Perdomo and other researchers at IPST @ GaTech, who worked on this project, is greatly appreciated. Content Page # LIST OF TABLES LIST OF FIGURES ABBREVIATED TERMS EXECUTIVE SUMMARY 3.5.2 Test Methods 3.5.3 Effect of Temperature and Dissolved Oxygen 106 3.5.4 Effect of Stress State 113 3.5.4.1 Effects of Cyclic Loading on SAC Cracking 3.5.5 Effects of Microstructure on SAC Cracking 3.5.6 Magnetite Film and Morphology 3.5.7 Mechanism of Stress Assisted Corrosion "Bulbous" Crack Growth 132 3.5.7.1 SAC Initiation and Propagation Mechanism 3.5.8 Conclusions on SAC Controlling Factors 137 3.5.9 References on SAC Controlling Factors 137 4 ACCOMPLISHMENTS 4.1 JOURNAL ARTICLES PUBLISHED 4.2 CONFERENCE PUBLICATIONS 139 4.3 PRESENTATIONS 5 CONCLUSIONS 6 RECOMMENDATIONS 7 APPENDICES 7.1 Bibliography on SAC Related Topics (Alphabetic Order) 146

Analysis of End Crack in Boiler Tube

Advanced Materials Research, 2012

Boiler tube failures usually result in frequent forced outages, and ultimately in costly extended outages for major tubing replacement in a plant. There are several failure modes that may occur in a boiler tube, i.e. stress-corrosion cracking, pitting, water-side corrosion, fire-side corrosion, fatigue failure, overheating, dissimilar metal weld fatigue, mechanical fatigue and erosion. In this research the failure modes of boiler and its end cracks due to the dissimilar metal weld is analyzed. Hence data are collected and explored to determine the cause of failure and as a solution ceramic ferrule is suggested. The use of ceramic ferrule in boiler tube can eliminate the crack occurrence or delay the process due to thermal properties. The prevention of crack will reduce frequent maintenance and thus the cost of operation can be minimized.

Critical Failure Analysis of Superheater Tubes of Coal-Based Boiler

This paper highlights a methodology for failure investigation of superheater tubes made of the material T-22 of a coal-based boiler. The process includes visual observation, the identification of sampling locations, the determination of the bulk chemical composition of the base alloy, microstructural investigation using optical microscopy, the exploration of finer structural details using a scanning electron microscope (SEM), the evaluation of hardness over samples obtained from different locations, the fractographic analysis of different failed locations, the X-ray diffraction (XRD) study of corrosion products adhered to inner surfaces, and the determination of the nature of the failure. Within a span of four months, three successive failures of superheater tubes were reported. The tubes were observed to have undergone significant wall thinning. Microscopic examinations using SEM on the failed region and a region some distance away on the as-received tubes were conducted in order to determine the failure mechanism. Layer-wise oxidation corrosion (exfoliation) in the inner surface was observed. Apart from major cracking, a number of nearly straight line crackings were observed in the longitudinal direction of both tubes. Close to cracking/bulging, void formation/de-cohesion of grain boundary indicated creep deformation under service exploitation. The failure mechanism was identified to be a result of excessive oxidation corrosion along the inside wall to reduce thickness, the spheroidization of alloy carbides and the coarsening of precipitate as well as creep void formation along grain boundary leading to inter-granular cracking with material flow near regions covered with thick scales. Moreover, there was a drastic reduction in bulk hardness of alloy and finally ‘thin lip fish mouth’ fractures. Keywords: coal based boiler, corrosion, creep, fractographic analysis, SEM, superheater tube Highlights • Bulk chemical composition of base alloy has been determined. • Optical microscopy has been applied for microstructural investigation. • Details of finer structural have been explored by applying scanning electron microscope (SEM). • Hardness of samples obtained from different locations of failed tubes has been evaluated. • Fractographic & X-ray diffraction XRD analysis has been performed at different failure locations. • Nature/type of failure to highlight the cause of failure ae has been examined.

BOILER TUBE FAILURE ANALYSIS IN 210MW POWER PLANT STEAM BOILER

IRJET, 2023

This study is aimed at analyzing the reasons associated with boiler tube failure. Inspection and data collected from MTPS - I serve as the basis for this analysis. Three samples (damaged LTSH tube, failed Economizer tube and failed RH tube) were collected from MTPS. Metallographic investigations were carried out in the samples collected to understand, interpret and substantiate the probable reasons that led to tube failure which occur when the effective strength falls below a critical level. Apart from visual inspection, optical microscopy, micro- hardness tests, SEM investigations, chemical analysis and EDS tests were carried out on various regions of the failed tubes and the results have been elaborately discussed. Upon visual inspection, formation of oxide layers on the inner side of the tube has been noticed which indicated that the inner side of the tube is subjected to corrosion. Formation of oxide layers has led to inhomogeneous overheating which has thereby affected the homogeneity of the tube. The main reason for rupture (hole formation) in the failed region can be attributed to steam erosion either from an adjacent failed tube or from soot blower. Besides steam erosion, flue gas erosion has also occurred due to unevenvelocity of flue gas which could be catalyzed by the presence of unburnt coal particles. Graphitization leading to formation of elongated as well as spherical graphite nodules and spheroidization over a period of time were identified as the major failure mechanisms involved from a microscopic perspective which could be related with overheating accompanied by creep leading to softening of the tube at the failed region thereby causing ductile fracture from a mechanical perspective. SEM micrographs showed the formation of graphite nodules, micro-cracks and void coalescence. Though occurrences of tube failures in boiler couldn’t be completely eradicated, they can be considerably reduced by adopting certain remedial measuressuggested at the end

Effects of Long-Time Service on the Material of a Steam Boiler Fire Tube

Materials Characterization, 1998

After 23 years of operation, the fire tube of a 2000 kg steam/h boiler broke down in a waterfilling operation. Because of the intense vaporization processes, the boiler body blew up at about 250 m. The study of metallographic features, of mechanical properties in various areas of the fire tube pointed out microstructural transformations that caused the material's embrittlement and damage.

Failure evaluation of SA 210C riffle water wall tubes in 70 MW CFBC boiler

Engineering Failure Analysis, 2019

Boiler tubes used as water walls made up of ferritic steels is having some finite life, because of prolonged exposure in the furnace at elevated temperature, stress and aggressive environment, tube failure taking place. Now a day's premature failure of the boiler tube mainly in water walls is one of the very common phenomena in the thermal power plants. The present investigation was done on the as-received ASTM SA 210C failed boiler tube steel used as water walls in the Circulating Fluidized Bed Combustion (CFBC) coal fired thermal power plant. An attempt has been made to understand the cause, because identifying correct failure mechanism often helps to perform meaningful life assessment and also to prevent the future Boiler Tube Failure (BTF). The as-received of failed tube along with the parent (unused) tube from the water wall raiser panel were selected to study for mechanical and metallurgical properties. Tensile test and micro hardness examinations were carried out on both the parent tube and failed tube. Visual examination reveals "fish-mouth" appearance of the as-received failed tube because of short-term overheating. The micro structure of the parent metal has the conventional structure of ferrite (white constituent) and pearlite (dark constituent), whereas for the as-received failed tube reveals Widmanstatten ferrite. A detailed structure-property relationship has been made by using the combined techniques of Optical Microscopy (OM), SEM/EDAX and XRD. Tensile fractography depicts the presence of micro-voids coalescence in the fibrous network shows ductile mode failure in the as-received failed boiler tubes.

CORROSION AND CRACKING IN RECOVERY BOILERS

The different chemical environments in different parts of a recovery boiler cause many different types of corrosion. They can also aggravate thermal and mechanical cracking. This paper discusses how tubes corrode and crack in each environment. Implementing proven strategies developed to reduce corrosion rates and inhibit cracking reduces the risks of smelt-water explosions, extends the service life of a boiler and reduces its operating cost.

High Temperature Corrosion Failure of a Secondary Superheater Tube in a Thermal Power Plant Boiler

High Temperature Materials and Processes, 2009

Failures of boiler tubes are a foremost cause of the unscheduled outages of the thermal power plant boiler. The failure of the boiler tubes occurs due to various reasons like creep, fatigue , corrosion and erosion. This paper highlights a case study of typical premature failure of a platen superheater tube of thermal power plant boiler. Visual examination, dimensional measurement, chemical analysis, oxide scale thickness measurement, microstructural examination, Energy Dispersive spectroscopy (EDS) were carried to ascertain the probable cause/causes of failure of inner leg of platen super heater tube. From the investigation, it was finally concluded that the presence of potassium (K) and sulphur (S) in the deposited ash may be responsible for formation of low melting compound alkali-iron trisulfate. The combination of localized high tube metal temperature and wall thinning due to high temperature fireside corrosion led to the premature tube failure.

FAILURE ANALYSIS OF BOILER TUBES OF A THERMAL POWER PLANT

A major portion of the total electricity generated in our country is through thermal power plants using direct combustion of pulverized coal. The majority of forced outages of these thermal power stations are due to premature failure of vital components such as boiler tubes. Case studies pertaining to the failure analysis of various kinds of boiler tubes such as super heater tubes, reheater tubes, and water wall tubes that have failed involving creep deformation and damage have been studied. In the present study the metallurgical investigations revealed microstructural degradations through the formation of creep voids at the grain boundaries and intercrystalline cracks due to continued exposure to higher temperatures. The microstructure of the lip portion of the burst has been found to change depending upon the temperature of the rupture. Rupture taking place between Ac and Ac 3 has revealed a mixed structure consisting of bainite due to the quenching effects of the steel. Similarly rupture taking place below temperature Ac 1 have been marked by divorced /degenerated pearlite and or spheroidised carbides in the ferrite matrix. Analysis made regarding the overheating (creep) failure of pendant reheater tubes indicates that surrounding temperature of the tube exceed several degrees higher than the components are designed for and also due to factors like erosion of tube surface by Fly ash, short supply of water through the boiler tubes caused by internal deposits.

Factors Involved in Stress Corrosion Cracking of Tubes from a Nuclear Power Plant Feedwater Heater

Revista de Chimie

This study has been carried out to identify the operating factors involved in the failure of two tubes made of 304 stainless steel removed from a high pressure feedwater heater working in a nuclear power plant. The samples cut from tubes have been analyzed by different methods: visual examination, optical (metallographic) microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Following analyses, on the surface of tubes was highlighted the presence of many pits in which cracks have started. Inside the pits, impurities of the type S, Cl, K, Ca were detected too. The branched cracks most likely have occurred as a result of precipitation of small amounts of chlorides deposited and concentrated on the surface of the tubes over a long period of operation. The stresses that favored this type of corrosion cracking were both residual stresses and stresses occurring at the torsion and bending of the tubes, while the high water temperature from feed heater was also...