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.