Microstructural investigation on the failure in APMT/KHR45A dissimilar weld interface after long-term service at high temperature

Abstract

We report our findings of a detailed investigation into the frequent failures of ethylene furnace radiant tubes at a petrochemical plant. One of the failed samples having gas tungsten arc welded joint between Kanthal-APMT(Febased) and KHR45A(Ni-based) which had served for 2 years at 950 C as an actual ethylene furnace tube was analyzed and turned out to have fracture occurred at the welding interface between the different metals. The detailed microstructural study revealed that the failure was induced by the massive formation of NiAl intermetallic phase and Cr23C6 carbide along the weld interface. To further discuss the failure mechanism, three additional specimens were made in as-welded and heat-treated (at 950 C for 170 and 1000 h) conditions. Microstructural observation on the additional specimens revealed the enhanced diffusion of Al from APMT weld metal into KHR45A and, as a result, the early-stage precipitation of NiAl intermetallic phase in Al-enriched KHR45A side almost covering the weld interface was successfully detected, which plausibly explains the premature failures in the petrochemical plant.