Failure analysis of high-temperature reformer tube of Incoloy 800H in synthetic ammonia installation

Failure analysis was conducted for a cracked Incoloy 800H reformer tube serviced at 580celcius in an ammonia installation based on the microstructure observation, fracture analysis, mechanical properties testing and finite element analysis. Results showed that the stress relaxation cracking (SRC) during operation was the primary cause of the repeated cracking, and the assembling malposition had a significant impact on the SRC by raising local stress. The larger the malposition was restrained, the earlier the cracking time was. A method for estimating the failure time due to malposition was proposed, which could be used to determine the malposition criteria during fabrication. In addition, the aged Incoloy 800H tube was much more sensitive to the SRC than virgin material since the ductility of the material decreased obviously after about 5 years' service at high temperature. This work confirms that "free-stress deflection angle" method is of choice to reduce SRC in case large malposition cannot be avoided.