Abstract: The steam leakage of the three-way tubes on the steam pipeline of a catalytic device happened in the welding line.The leakage analysis was made by means of chemical composition,hardness,macrostructure and microstructure examination etc.The results showed that the gap corrosion and stress corrosion was occurred at the welding line of the three-way tubes in long-term corrosion environments,by no welding on gaps caused during welding.