Damaged Long Seam Weld on an Outlet Elbow outside Hotbox of an Ethane Cracking Furnace

The damaged long seam weld was located on the inside radius of 90° elbow coming out of a hotbox of an ethane cracking furnace. Severe material loss was observed on the surface of this weld in the form of voids, inside of which contained powdery dusts and metallic balls. Two nearby circumferential welds, however, maintained their integrity without any visible damage.

The damage was determined to be the result of metal dusting and erosion basing on the metallurgical analysis. Erosion played an important role to form confined spaces for the coke deposit, initiating carburization and resulting in metal dusting.

The longitudinal seam weld was fabricated by the pipe manufacturer using Inconel 82 (AWS ERNiCr-3) at the root pass and Inconel 117 (AWS ENiCrCoMo-1) at the fill pass. The circumferential weld was fabricated onsite using Inconel 117 (AWS ENiCrCoMo-1) for both the root and fill passes. Damage-free on the circumferential weld was benefited from the formation of a Mo/Si rich layer roughly 50μm thick on the surface of the root pass, preventing further penetration of oxidation and carburization. The oxides on the base metal had very high content of Nb, which enhanced the oxidation resistance. The root pass of the long seam weld, however, was a potential weak point due to the lowest chromium content and lack of Mo/Si rich layer underneath the surface oxides. Therefore damage was only restricted on the long seam weld, rather than on the base metal or the circumferential weld. It is thus recommended that the root passes of the longitudinal seam weld be replaced with Inconel 117 or other filler metals with higher Cr contents in order to take advantage of the protective characteristics it can provide to the weld/process interface.