Cracking of Alloy 625 Bellows due to SAGBO / Risse in Wellrohrkompensatoren aus Alloy 625 infolge SAGBO

Abstract

Bellows in the combustion system of a large gas turbine engine used for power generation exhibited cracking in their outer plies. These components are designed to compensate for relative movement of combustor parts upon start-up and shut-down of the engine, and in transient operating conditions. The subject bellows was made of Alloy 625, a wrought nickel-base superalloy, possessing excellent corrosion and oxidation resistance. Five different failure hypotheses were discussed. Laboratory investigations were performed to verify or falsify these hypotheses. It was concluded that the subject bellows cracked due to dynamic strain-assisted grain boundary oxidation (SAGBO), a failure mechanism this particular alloy is not immune against.