Liquid Metal Embrittlement in Shell and Tube Heat Exchangers

Heat exchangers are ubiquitous in industrial processing environments including chemical and petrochemical refining and production, as well as in power generation. One of the most common types of heat exchangers is the “shell and tube” design that generally consists of numerous (can be hundreds or thousands) long tubes joined to tubesheets (plates) on either end. Tubes are generally connected by either welding or mechanically rolling the tube into the tubesheet. The processing fluid to be heated or cooled passes through the tubes and heads of the vessel, while the heating or cooling media flows around the tubes and is contained between the tubesheets within the shell. It is critical that the tube-to-tubesheet and tubesheet-to-shell joints do not allow leakage and mixing of the fluids. In this investigation, intergranular cracks were identified in the ends of tubes near welded joints that were exposed to operating conditions, as well as new (un-used) replacement heat exchangers from the same manufacturer. Metallurgical analysis revealed that the cracks in the austenitic stainless steel tubes were due to copper liquid metal embrittlement of the grain boundaries caused by inadequate control of welding parameters during fabrication with the orbital TIG welding machine. The tube end cracks are discussed with respect to allowable imperfections within ASME BPVC, the risk of propagation, and required rectification measures.