Failure analysis on weld joints between the elbow and straight pipes of a vacuum evaporator outlet

This study presents an analysis on a weld joint leakage accident between elbow and straight pipes that connect the inlet of a catalyst plant vacuum pump and the outlet of a vacuum evaporator tank. Moreover, the causes and mechanisms of material degradation are investigated by analyzing the chemical compositions and observing the microstructures with the use of a metallographic microscope and a scanning electron microscope. Energy dispersive spectroscopy was used on the test materials and corrosion products to examine the changes in the elements during material degradation. The results show that stress corrosion cracking caused the cracks and the leakage on the joints. The fracture morphologies were dominated by river cleavage and fan-shaped cleavage patterns. The existence of Cl− and residual stress from the installation of the elbow pipe and the heat effect of the welding process contributed to the stress corrosion cracking.