Liquid Metal Induced Embrittlement in Fuel Line Braze Joints

This paper documents an instance of liquid metal induced embrittlement (LMIE) that occurred during the manufacture of automobile engine fuel lines. The fuel lines were constructed of low-carbon steel fittings press fit onto 304 stainless steel tube. The joints were subsequently furnace brazed with copper braze paste. The carbon steel fittings were occasionally observed lying separately as the part exited the brazing furnace. Failure analysis using metallographic techniques, scanning electron microscopy, and energy-dispersive spectroscopy indicated that the fractures were intergranular with no observable ductility. Copper from the braze filler metal was observed on the stainless steel tube fracture surfaces and within the tube wall grain boundaries. Copper and austenitic stainless steel are a well-known LMIE couple. Subsequent investigation indicated the stress required for rapid fracture was due to impacts of the parts against one another and the brazing furnace stationary components while the braze metal was liquid.