The welding of SAF 2507 steel for nuclear power plants

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. How to obtain welded joints with good seawater corrosion resistance is very important. 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. Mainly used in harsh media, especially the chlorine-containing environment, such as sea water. The balanced composition of high chromium, high molybdenum and high nitrogen makes the steel offering high resistance to stress corrosion cracking, pitting and crevice. Here we will introduce the welding of SAF 2507 super duplex stainless steel, please keep reading on.

 

Welding methods

Due to its limited metallurgical properties of duplex stainless steel, the following principles should be followed when selecting welding methods:

1. 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.
2. 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. The table below is the welding parameters recommended by Sandvik of Sweden.

Grade	heat input/KJ mm	Interpass temperature/ ℃
SAF2507	0.2 —1.5	<150

Welding material

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 weld metal, austenite-dominated weld metal is the ideal choice for duplex stainless steel welding. Its advantages are: grain refinement, reduction of nitride precipitation, improvement of plastic toughness and wear resistance, enhancement of crack resistance and reduction of inhomogeneity of upper and lower layers in multi-layer 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. Both elements can increase the proportion of austenite phase and use stable, but nitrogen addition can not only delay the precipitation of intermetallic phase, but also improve the strength and corrosion resistance of welding metal.

At present, the resistant filler materials are generally added with the same nitrogen content as the base material on the basis of improving nickel, so as to ensure the austenite content of the weld is 60% to 70%. The sandvik25.10.4.l wire was used for TIG welding of SAF2507 steel. The typical chemical composition is shown below:

Grade	C	Si	Mn	S max	P max	Cr	Ni	Mo	N
Sandvik25.10.4.L	0.02	0.3	0.4	0.02	0.020	25	10	4	0.25

 

It is shown that 20%N2 in pure Ar gas is lost in surface-weld bead during TIG welding of super duplex steel. The addition of nitrogen to the shielding gas effectively avoids the loss of nitrogen, because too much nitrogen will make the weld metal produce porosity. In general. When SAF2507 steel was welded by TIG, Sandvik25.10.4 was selected.L wire shielding gas Ar +2% N2 was used to obtain the welded joints with good corrosion resistance to seawater.