Impingement-Corrosion Failure of a Ferritic Malleable Iron Elbow
Leakage was detected in a malleable iron elbow (ASTM A 47, grade 35018) after only three months in service. Life expectancy for the elbow was 12 to 24 months. The piping alternately supplied steam and cooling water to a tire-curing press. The supply line and elbow were subjected to 14 heating and cooling cycles per hour for at least 16 h/day, or a minimum of 224 cycles/day. Steam and water pressure were 1035 kPa (150 psi) and 895 kPa (130 psi) respectively, and water-flow rate was estimated to be 1325 L/min (350 gal/min) based on pump capacity. Water-inlet temperature was 10 to 15 deg C (50 to 60 deg F) and outlet temperature was 50 to 60 deg C (120 to 140 deg F). The pH of the water was 6.9. Investigation (visual inspection, chemical analysis, and 67x nital etched micrographs) supported the conclusion that the elbows had been given the usual annealing and normalizing treatment for ferritizing malleable iron. This resulted in lower resistance to erosion and corrosion than pearlitic malleable iron. Recommendations included replacing the elbows with heat-treated fittings with a pearlitic malleable microstructure.