STRESS-CORROSION CRACKING IN A NICKEL-BASE ALLOY PRE-HEATER EXPANSION BELLOWS [Stress – Corrosion Cracking In A Nickel – Base Alloy Pre – Heater Expansion Bellows]

Abstract

This paper presents the results obtained from the failure analysis performed on a ruptured expansion bellows of a pre-heater. This bellows is a typical of single ply corrugated bellows consisting of five bellows convolutions and was made of Inconel 625, a standard specification for Ni-base alloy. The fluid circulated within the pre-heater on shell side was steam at the operating temperature and pressure of 265°C and 51.0 kg/cm2 g, respectively. The bellows was reportedly ruptured during an accident occurred after the pre-heater had been in service for more than five years. A number of specimens were prepared from the ruptured bellows for laboratory examinations including macroscopic examination, chemical analysis, metallographic examination, hardness test, and SEM (scanning electron microscopy) examination equipped with EDS (energy dispersive spectroscopy) analysis. Results of the failure analysis obtained showed that the ruptured expansion bellows had experienced predominantly to stress corrosion cracking (SCC) caused by the simultaneous presence of tensile stress and a corrosive agent in which sodium (Na) was found being the major caustic corroding agent. Most of the SCC occurred were concentrated on the upper or top side of the bellows convolutions and initiated from the inner wall of the outer bend convolutions and subsequently propagated outward in intergranular manner with extensive branching through the austenitic grain boundaries of the bellows material. The severity of cracking was very much affected by the level of tensile bending stresses present on the bend convolutions, and therefore most of the main crack rupture was found to take place at the peak bend convolutions where the tensile stress was maximum or highest. In some area having less or no tensile stress, the damage pattern was predominantly altered into the intergranular corrosion.