Failure Analysis of Duct and Panel Samples from A New Electric Arc Furnace and Off-Gas Duct System

A metallurgical failure analysis was performed on several pipe sections from an off-gas duct system exhibiting significant wall thinning. The pipe sections formed part of the duct walls consisting of a series of parallel tubes that are seal welded together, creating a structure similar to a water wall in a power plant. The entire electric arc furnace and duct system were fairly new; the new equipment was failing at a rate of 1/3 the life of the previous equipment when it was originally expected to increase the service lifetime. The exhaust gases from the furnace pass through the duct, while cooling water is circulated through the ID of the tubes. The pipes examined exhibited significant wastage on the OD.

The physical damage present included severe wall thinning on the OD, the scale deposits (both over the OD and ID), gouge-like pitting over the ID of the pipe, and cracking initiating at the weld membranes. The physical damage causing the most impact and leakage was the wastage or severe wall thinning on the OD. Characterization of the scale with SEM/EDS and XRD confirmed chemical species of Ca, Zn, Pb, Cl, S and a crystalline compound of Pb and S, anglesite. The Pb constituent in a liquid state can embrittle carbon steel, though no evidence of liquid-metal embrittlement was observed in the microstructure or hardness results. The pipe base metal did not exhibit any microstructural changes associated with elevated temperature exposure such as increased grain sizes or spheroidization of the pearlite. This confirmed the absence of sufficient elevated temperature required for molten phases to become the aggressive chemical species. The accelerated corrosion was attributed to species of S and Cl in the hot gas environment.