On the intergranular embrittlement of a solid-solution strengthened Fe-base alloy by internal oxidation

A study was conducted to demonstrate the effect of as-welded structures on the relationship between mechanical strength and oxidation resistance of an iron-based alloy under actual service conditions. welded tubes in the same condition to be used in practice were used in a benzene plant and exposed to an environment, consisting of non-aromatics, benzene, aromatic compounds, and diphenyl at 600 °C. Light metallography and scanning electron microscopy were used to characterize the microstructure of the alloy. The maximum principal stress developed in the tube during the field test, was less than one-half the respective yield strength of the material. It was observed that the tube fractured in the weld heat-affected zone after about 1000 hours. It was concluded that the the observed difference in hardness could be related to residual stress within the heat-affected zone, as secondary precipitates were not detected down to the scale of transmission electron microscopy.