Microstructural features and pitting corrosion properties of duplex stainless steel weld metal have been studied by means of 25kW CO2 LASER welding process without filler metals from viewpoints of nitrogen distribution and nitrides precipitation. Two types of pitting mechanisms have been clarified depending upon the cooling rate of the weld metal.
One is the local nitrogen depletion in the ferrite phase, where pitting occurred, due to the migration of nitrogen from ferrite to austenite when the cooling rate was higher. Pitting resistance was improved by supersaturated nitrogen in the ferrite phase when the cooling rate was extremely high. These behaviors could be explained by the calculation based on nitrogen diffusion.
The other is Cr depletion due to chromium nitrides precipitation in the ferrite phase when the cooling rate was not so high. The composition was Cr2N, which lowered the pitting corrosion resistance, although CrN precipitated when the cooling rate was higher, which did not affect the pitting corrosion resistance.
