Abstract: Cracking occurred in the reboiler tube plate of a certain ethylene cracking unit. The cracking reasons of the tube plate were analyzed using macroscopic observation, chemical composition analysis, hardness testing, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that improper welding process control led to the formation of martensitic quenched microstructure in the tube sheet material. Under the combined action of stress and quenched microstructure, welding cold cracks occurred in the weld zone and heat affected zone at the gap between the tube sheet and the bundle. The entry of process water or rapid cooling oil into the crack, under the action of corrosive elements such as S and O, the material at the crack underwent stress corrosion cracking, further expanded the crack, and ultimately led to cracking of the hole bridge or tube bundle.