NITROGEN AND CARBON REDISTRIBUTIONS IN FERRITE AUSTENITE AND AUSTENITE FERRITE STEEL WELDED-JOINTS

The paper deals with an original model of parallel carbon and nitrogen redistributions in ferrite/austenite and austenite/ferrite welded joints. The model assumes quasistationary diffusion of both interstitionary elements with respect to substitutional elements, and enables qualified estimation of their redistribution in welded joints in dependence on isothermal annealing time and temperature. Thermodynamical, diffusional and phase data were selected, ordered and verified for this purpose. To verify the model, both low-alloy AREMA (0.05 mas. % C) - austenitic steel welded joints and carbon (0.47 mas. % C) - austenitic steel welded joints were used. AREMA and carbon steels were longterm nitrided before welding. Welded joints were annealed at 500-degrees-C for 255.5 h. Carbon, nitrogen and chromium distributions were measured at a joint transient zone cros-section. Qualitative and semiquantitative agreements of the model and experiments were established by the collection of predicted and measured courses of carbon, nitrogen and chromium concentration curves for model steels. It was proved in both cases that nitrogen played the leading role in reciprocal competition between interstitionary elements (carbon, nitrogen) during their "up hill" diffusion from ferritic to austenitic steel, while carbon tended to adapt to nitrogen distribution.