Failure analysis of a heat-exchanger serpentine

The failure analysis of a ASTM A 178 grade A steel serpentine, belonging to a vertical heat-exchanger, has been carried out. Parallel transversal wedge cracks were observed in the concave region of the outer serpentine after just two years of use. The results indicated that the cracking was caused by thermal fatigue (corrosion-fatigue) due to thermal cycling. The presence of graphite nodules and spherodised Fe3C particles (dominant) indicates a localised overheating of the outer serpentine in temperatures between 550 and 650 °C, which further catalysed the thermal fatigue mechanism. The overheating can be explained by the local presence of a thick deposit (rich in S and Mg) on the internal surface the serpentine, composed of hematite, magnetite and iron sulphide. Finally, the results did not show evidences of the action of a creep mechanism associated to the failure.