Stress relaxation cracking failure in heat exchanger connection pipes in a petrochemical plant

The purpose of this investigation is to give insights into stress relaxation cracking (SRC) failure, observed in heat exchanger pipes in a petrochemical plant. In this study, a failed heat exchanger pipe/nozzle (SA-336 Gr. F22 grade) was analyzed. The pipe was continuously used in an ammonia production complex for almost one year. The pressure of the steam inside the pipe was 173 bar at a temperature of 235 degrees C. The detected leakage was due to a crack of roughly 4 cm, perpendicular to the hoop stress in the axial direction. It appears that crack was formed as the consequence of void formation and coalescence during service time. Also, it was observed that the main crack was surrounded with many branches of smaller-sized cracks and that the crack has propagated alongside grain boundaries. To analyze the microstructure of the failed pipe, optical and electron microscopes were put to use. Hardness measurement was performed using Vickers hardness tester. It appeared that the coarse carbide precipitates that form at the grain boundaries crucially impacted this failure. The failure was attributed to the stress relaxation cracking (SRC). Details over the activation of SRC failure mechanism and possible preventive measures are addressed in this investigation.