Thread Content
Serious cracks were found on the welded neck flange (WNF) of a pressure vessel at a certain plant. The cracks are parallel to the weld joining the flange to the vessel nozzle, but there are no cracks on either the nozzle or the vessel. The same is true for the other container. Flange diameter: 50 cm, material: 304L stainless steel. Metallographic analysis showed that the corrosion failure was intergranular fracture of 304L stainless steel, and microprobe analysis indicated the presence of zinc in the cracks; it was therefore concluded that this was liquid metal brittleness (LMC) failure caused by molten zinc. Where does zinc come from? Tracking the container manufacturing process, it was found that the neck welding flanges used by the container manufacturer were purchased from external suppliers. Due to their size and weight, the two WNFs were strapped to flatbed frames and placed outside the container workshop. Innocuous zinc-rich paint (with a zinc content of over 90%) is being sprayed near the storage area; the paint powder is blown by the wind onto the two WNF units, where it settles on the windward side. When WNF is welded to the container nozzles, the welding heat in the heat-affected zone provides the temperature necessary to melt zinc as well as the stresses that cause severe cracks. Stainless steel at higher temperatures should be protected from contact with zinc and cadmium. This issue should be taken into account both during the manufacture and use of stainless steel equipment. In this case, the WNF should be covered up or removed.