Analysis of failed electron beam welds in ethylene cracking tubes

The occurrence of cracks at the electron beam (EB) welds of ethylene tubes in radiant chambers after 1–2 years in service has led to serious concerns. Failure analysis of the sample using optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) clearly reveals a centerline with dendritic crystals and blocky precipitates present at the welds, where the cracks occurred. Niobium-depletion at the EB weld centerline is mainly responsible for the carbide precipitates and cracks. Operation processes such as frequent start–stop cycling accelerated the cracks.

► Cracks occur at the centerline with dendritic crystals of electron beam (EB) welds of ethylene tubes.
► Precipitates, creep voids and Niobium-depletion at the centerline.
► Niobium-depletion is responsible for the cracks.