ABSTRACT
Power plant boiler tubes fail in service due to a number of reasons and through
various mechanisms. Overheating is a prime cause, and creep, corrosion, erosion
and hydrogen damage constitute major mechanisms of failure. Long-term
overheating brings about microstructural changes like grain growth, disintegration of
pearlite, spheroidization of carbides, graphitization and decarburization leading to
loss of strength of the tube material, eventually resulting in stress rupturing or creep
rupture through grain boundary void formation. A tube burst resulting from short-term
overheating also often bears the imprint of microstructural changes occurring due to
transformation of the high temperature phase. Overheating also accelerates the
process of hydrogen damage, where the presence of grain boundary fissures bears
the evidence of such type of failure. The paper deals with the theoretical aspects of
microstructural changes as encountered in boiler tube failures and presents a few
case histories to highlight the metallographic features in failures due to overheating
and hydrogen damage.