Nitrogen distribution on rapid solidification in laser welded duplex stainless steels

Abstract

Microstructural features of LASER welded duplex stainless steels have been studied from viewpoints of nitrogen distribution and its effects on pitting corrosion resistance. The experiment was conducted by means of 25 kW CO2 LASER welding process without filler metals.Nitrogen supersaturation in ferrite phase due to rapid solidification of LASER weld metal was observed in the as-welded conditions. Pitting resistance was extremely improved in case of high nitrogen steels welded by LASER. On the other hand, local nitrogen depletion near the interface in the ferrite phase, where pitting occurred, was also observed. It was due to the migration of nitrogen from ferrite to austenite.Ferrite-austenite ratio and distributions of elements were restored as same as base metal after short time solution heat treatment. Pitting resistance was restored when the cooling rate was high by preventing nitrogen evolution.These nitrogen supersaturation, local depletion and evolution phenomena could be explained by the calculation based on nitrogen diffusion.