Most nuclear power plants in the world use sea water as its cooling medium; seawater is the most corrosive medium in the natural environment which are prone to produce pitting for common stainless steel materials. Super duplex stainless steel SAF2507 offers good pitting resistance, is used in cooling pipes for most modern nuclear power plants.
SS2328 (00Cr25Ni7Mo4N) steel is a type of super duplex stainless steel developed in Sweden in recent years with a commercial brand SAF2507(UNSS32750), which belongs to the third generation of duplex stainless steel. The balanced composition of high chromium, high molybdenum, and high nitrogen makes the steel offering high resistance to stress corrosion cracking, pitting and crevice.
Due to its limited metallurgical properties of duplex stainless steel, the following principles should be followed when selecting welding methods: Avoid using too high or too low welding heat input. Too low heat input will greatly reduce the austenite precipitate, leading to the process and performance reduction. However, too high heat input will result in the precipitation of harmful phase and coarse grain in the welded joint which produces the reduction of corrosion resistance and toughness.
Suitable for multi-layer welding: multi-times, low melting rate. TIG welding is chosen as the ideal welding method according to the practical site construction.
Due to the thermal cycle of welding, the ferrite of the weld metal increases sharply during the self-fusion welding of duplex stainless steel, and the precipitation of nitride and secondary Austenite at the same time, leading to the decrease of toughness and corrosion resistance. In order to restrain the increase of ferrite in the weld metal, austenite-dominated weld metal is the ideal choice for duplex stainless steel welding.
It is feasible and useful to increase the content of nickel or nitrogen while decreasing the Cr in welding materials. The content of nickel in the filler material is usually 2%- 4% higher than its base material. The filling material containing nitrogen is better than which containing nickel only.
