2001 Volume 41 Issue 6 Pages 580-589
The influence of nitrogen on the microstructure and properties of austenitic and duplex stainless steel welds is reviewed. Three aspects are covered: intermetallic phase formation, nucleation and growth of austenite in ferrite and nitrogen balance in TIG welding.
The effect of nitrogen on the precipitation of deleterious intermetallic phases is analysed for duplex and austenitic steels by thermodynamic means and the differences in response of nitrogen alloying illuminated. Also the replacement of molybdenum by tungsten is discussed.
Nitrogen alloying made the duplex grades weldable. The duplex steels, designed to comprise approximately equal amounts of ferrite and austenite, solidify in a ferritic mode and a rapid transformation of ferrite to austenite is a prerequisite. Nitrogen has a beneficial effect on the austenite reformation in the HAZ (Heat Affected Zones) on cooling. This has been confirmed by both experiments and calculations. Attempts have been made to improve the nucleation of austenite and thus to increase the possibilities for welding by more demanding techniques which involve rapid cooling through the critical temperature range 1350°C to 1000°C.
Nitrogen is added to many of the modern stainless steels because of its favourable effects on both strength and corrosion resistance. This implies that the welding procedure must consider the risk of nitrogen escape from the weld pool. Avoiding nitrogen losses may be accomplished by applying e.g. nitrogencontaining shielding gas. Applying balanced shielding gases is generally successful but on occasion reproducibility problems may arise. Correcting measures require insights in the process and the mechanisms controlling the nitrogen fluxes in TIG welding. These can be illuminated using a simple process model that accounts for alloy composition, shielding gas nitrogen content, weld pool shape and size. It is suggested that the main reasons for lack of reproducibility may be found in the batch to batch variation in surfactant content that controls the weld pool shape viathe Marangoni effect and surface mass transfer reactions.
