Abstract
A pipe rack assembly uses hot-dip galvanized trusses joined together by screw connections. The welded trusses were manufactured with ASTM A572 gr.50 W steel profiles. Two of them fractured during the galvanizing bath. The fractures presented typical characteristics of the LMAC — liquid metal-assisted cracking mechanism. In both cases, the fracture was located in the structural element base metal and followed the fillet weld to the neighboring elements at a distance beyond the heat-affected zone. The trusses height was greater than the depth of the molten zinc vat, and double immersion is required for complete galvanization. The failed trusses had a particular geometry, distinct from the others. The design of the trusses was verified taking into account the thermal stresses generated by the partial immersion in the zinc bath. Among all, only the two broken trusses were exposed to stresses of thermal origin substantially higher than the material yield strength. Metallographic analysis revealed secondary intergranular cracks near the fracture surface. The crack roots contained zinc. In addition, the secondary cracks had a large opening in relation to the depth, which indicates material plasticization in the root region.
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