Stress relaxation cracking failure in a high pressure steam pipeline in an ammonia plant

In this study, a failure in a high pressure steam pipeline, made of ASTM A335 P11 steel grade, has been investigated. The failure in this case appeared in the form of cracking in a 16 inch pipe, containing high pressure steam (47 bar) at 400 degrees C after eight years of service, which is considered as a relatively premature failure in comparison to the design service-life of the pipeline. Optical and electron microscopies were employed to investigate the microstructure of the failed pipe. Hardness measurement was also conducted to measure the hardness of samples. Results indicated that the main mechanism of the failure was stress-relaxation cracking (SRC). Formation of coarse carbide precipitates at grain boundaries appears to have a vital attribution to this failure. Failure mechanism and mitigation strategies are discussed in this paper.