*Corr. Author’s Address: Indian Institute of Technology (Indian School of Mines), Department of Mechanical Engineering, India, gagan_k_gupta@rediffmail.com 287 Strojniški vestnik - Journal of Mechanical Engineering 63(2017)5, 287-299 Received for review: 2016-11-13 © 2017 Journal of Mechanical Engineering. All rights reserved. Received revised form: 2017-01-22 DOI:10.5545/sv-jme.2016.4188 Original Scientific Paper Accepted for publication: 2017-02-20 0 INTRODUCTION A boiler is a vital element in power plants with regards to running cost and performance. A boiler generates steam via the optimized combustion of fuels (coal, gas & oil, etc.). It consists of several components of rotary equipment and pressure parts. The efficient and trouble-free operation of a boiler is difficult to maintain because the characteristics of input fuel vary over time. Deteriorated performance and repetitive failures of pressure parts of boiler is a very common issue. One of the critical components of pressure parts is the superheater, the failure of which leads to the forced outage of the working of a boiler, resulting in huge losses. A superheater is basically a heat exchanger in which heat is transferred from furnace gas to steam. Improper heat transfer between steam and furnace gas creates problems of localized heating. Uneven heat transfer is a result of non-uniform gas flow or non-uniform steam distribution in the superheater. Significant causes of failure in superheaters are localized prolonged heating, creep damage, thermal fatigue, excessive thermal stresses, water and fireside corrosion, erosion, etc. General modes of failures observed in superheaters are wall thinning, reduced thickness, fish-mouth opening type bursting, creep cracks on tube surfaces and puncturing. Tube failure in the superheater is sufficiently hazardous to lead to the forced evacuation of the whole plant. Therefore, it is important to take remedial actions to prevent technical as well as economic losses. It is highly necessary not only to critically identify areas of failures but also to critically determine the root cause of failures. Prolonged localized heating, which is considered a root cause of tube failures, is a result of improper operating procedures. Concentrated gas flow pattern over superheater and non-uniform steam distribution overheats localized portions of tubes. Proper distribution of furnace gas among the entire superheater tubes and uniform steam flow in each tube are advocated for the trouble-free operation of superheaters. A pictorial view of a boiler is presented in Fig. 1 with location of superheater. Dennies [1], Wulpi [2] and Brooks and Choudhury [3] has presented Critical Failure Analysis of Superheater Tubes of Coal-Based Boiler Gupta, G.K. – Chattopadhyaya, S. Gagan Kumar Gupta * – Somnath Chattopadhyaya 1 Indian Institute of Technology (Indian School of Mines), Department of Mechanical Engineering, India 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.