Failure investigation of a convection line elbow

Failure investigation was carried out on a cracked 5″ 304L stainless steel elbow tube for the convection line of an ethylene furnace. The crack was observed in the heat-affected zone (HAZ) at the end leading to the elbow parent material. The crack was along the circumference and was about 12″ in length. The crack started from the toe of the weld to the parent metal on the elbow side. The failure investigation revealed that the HAZ of the elbow tube suffered from stress corrosion cracking and mild grain boundary carbide precipitation during welding. The carbide precipitation was possibly due to improper welding practices such as high heat input and/or extended welding time. The tube thickness, which is relatively thick, was a contributing factor in making proper welding more difficult.