Hydrogen-Assisted Cracking in Boiler Steam Drums
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A statutory inspection of a boiler steam drum that had seen 10 years of service revealed a few scattered cracks on the inner surface of the drum. The drum was used in the refinery and petrochemical industry. Remnant life assessment, including inspection of other drums, showed presence of several cracks in two out of seven boiler inspected. In situ microstructural analysis revealed a variation in the microstructure in the cracked regions compared to the microstructure in uncracked regions. Additionally, a wide variation in hardness was associated with the microstructural variation. Detailed study on a sample extracted from a cracked region demonstrated that the cracking occurred in a crescent zone similar in appearance to that of a heat affected zone (HAZ) generally associated with a spot weld or other sources of hot spots in the material. Subsequent examination of more samples confirmed that repair welding was carried out at several places on the inner surface of the drums before installation and commissioning and all the cracks were around the repair welds. The failure occurred by crack initiation in the HAZ of the repair welds and the cracks then propagated progressively across the inner surface of the drums. The failure mechanism was identified to be cold cracking and the failure analysis showed that all the factors required for cold cracking, namely, vulnerable microstructure, residual stresses and hydrogen atmosphere, during welding had been present in the drum material.
KeywordsSteam drum failure Weld Cold cracking Hydrogen-assisted cracking
The author thanks Head, Materials Science Division and Director, NAL, for granting permission to publish this paper.
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