Failure Analysis of Welded Reformer Tubes of a Fertilizer Unit

A fertiliser unit experienced cracking of the circumferential weld of H39WM heater tubes at the hottest region along the vertical direction and particularly those closer to the wall. The cracks occurred in the internal side and observed to grow along the circumference. Longitudinal cracks were also seen in the inner wall. A few tube samples were examined to understand the cracking problem. The service exposed tubes show changes in carbide morphology and isolated creep voids are seen in the base metal and regions closer to the weld fusion line (heat affected zone). The longitudinal cracks are very close to the weld and creep void alignment away from the weld is very less. The circumferential crack show creep voids ahead of the crack and in regions close and parallel to the main crack. Isolated creep voids were observed mostly in the inner wall region and on the outer wall creep voids are absent. The creep cracking occurred in the weld of the hotter section as weld is the weakest link in the tube column.