Failure of Alloy 625 tube stub ends – Effect of primary nitrides

•Alloy failed due to the formation of carbide layers at grain boundaries.•Carbide precipitation was unusually faster and at low temperatures.•Presence of free carbon in the matrix has been attributed to the accelerated carbide formation.•Formation of primary nitrides has been held responsible for the presence of free carbon in the alloy.•Formation of nitrides has been related to improper control of processing parameters.

This paper reports a failure analysis of Alloy 625 stub ends of ammonia cracker tubes that failed during operation after 47,000 h of service operation. The failure occurred due to the formation of M23C6 carbides at grain boundaries, which made them very weak and brittle. However, the formation of M23C6 carbides at grain boundaries was surprising since they formed at temperatures around 550 °C, which is much below their expected temperature of formation and occurred in a period less than half the designed life. Precipitation of the grain boundary carbides has been attributed to the presence of primary nitride particles in the present alloy instead of primary carbides which are usually observed in these alloys. Formation of nitrides consumed Ti that binds C in the form of primary MC carbides in these alloys. This left free carbon in the alloy matrix for easy formation of M23C6 carbides which otherwise form due to degeneration of primary MC carbides.