Failure analysis of a machinable brass connector in a boiler unit installation

(in lingua inglese)

A machinable brass connector (33 mm Hex AF) failed in a boiler unit, after almost two years in function, caused leakage and heating unit shutdown. The collected evidence suggests that failure was induced by progressive crack-growth during service, initiated from inner thread root. A plausible failure mechanism is fatigue facilitated by the action of dezincification and secondary intergranular corrosion cracking. Crack emanated at high stress concentration area assisted by the presence of surface flaws created probably by shallow mechanical damage and/or dezincification pits. Potential improvement suggestions concern the following.