Fish-mouth opening of an ammonia cracker tube of Alloy 625

Abstract

Ammonia cracker tubes of Alloy 625 in Indian heavy water plants have consistently given their service for the designed life of 105 h at operating conditions of about 650 °C temperature and 14 MPa internal pressure. The alloy exhibits no creep deformation during service life under these operating conditions. However, premature failure of a tube after service of about 104 h occurred by a ‘fish-mouth opening’ of the tube. The thickness of the tube had reduced by about 50% before its failure. This work presents a detailed analysis to identify the likely conditions responsible for this failure. The microstructural analysis of the alloy around the ‘fish-mouth’ showed dynamic recrystallization of grains. Dynamic recrystallization of grains indicated that the tube had experienced above 1050 °C, at which the alloy’s strength dropped significantly. Independent finite element analysis supports the temperature excursion of the tube above 1075 °C, at which the hoop stress exceeds the alloy’s yield strength and fails the tube within a few hours.