MICROSTRUCTURAL IMPRINTS IN FAILURE OF POWER PLANT BOILER TUBES

ABSTRACTPower plant boiler tubes fail in service due to a number of reasons and throughvarious mechanisms. Overheating is a prime cause, and creep, corrosion, erosionand hydrogen damage constitute major mechanisms of failure. Long-termoverheating brings about microstructural changes like grain growth, disintegration ofpearlite, spheroidization of carbides, graphitization and decarburization leading toloss of strength of the tube material, eventually resulting in stress rupturing or creeprupture through grain boundary void formation. A tube burst resulting from short-termoverheating also often bears the imprint of microstructural changes occurring due totransformation of the high temperature phase. Overheating also accelerates theprocess of hydrogen damage, where the presence of grain boundary fissures bearsthe evidence of such type of failure. The paper deals with the theoretical aspects ofmicrostructural changes as encountered in boiler tube failures and presents a fewcase histories to highlight the metallographic features in failures due to overheatingand hydrogen damage.