Metallurgical Analysis of Leakage Observed at EB Weld Joint Between SS321 Inner Cylinder and Flange of Air Heater-Mixing Zone Module.

Air heater is a ground test engine for generating and supplying hot vitiated air to scramjet engine combustor at designed parameters. It has different mixing zone modules consisting of an outer flange and an inner shell. Cooling channels are there in between the flange and inner shell. At the aft end and fore end of the mixing zone modules inner cylinder and the flange are joined by EB weld. After the ground test, the air heater was kept in the test bay for a long time. Subsequently during the cooling water flow trial through air heater, water leakage was noticed at the EB weld joint between the inner cylinder and flange. Scale like formation over the flange interface near the EB weld was also noticed. In situ metallographic analysis revealed that the cracks have initiated from the weld region and propagated in transgranular manner into the flange side as well as inner shell parent metal. The nature of cracks responsible for leakage and the chemical composition of the brown colored patches on the flange surface near the EB weld as well as at the middle region of inner shell were examined. Multiple cracks with extensive branching are observed. Cracks have been initiated from the corrosion products formed at the weld region. From the nature of the crack and presence of corrosion, cracking is attributed to Stress Corrosion Cracking. [ABSTRACT FROM AUTHOR]