Study of the influence of phase transformations on mechanical properties in a stabilized austenitic stainless steel (316nb)
Résumé
Stabilized austenitic stainless steels (316Nb type) are usually used for components exposed to significant thermo-mechanical stresses in corrosive environments, mostly in the chemical, nuclear and oil industries.
Different phases can be found in the austenitic matrix: retained $\delta$ ferrite, $\sigma$ phase, and primary and secondary niobium carbonitrides – Nb(N,C) (Figure 1). $\sigma$ phase can precipitate in $\delta$ ferrite islands depending on the cooling rate after high temperature heat treatment. It is known to induce embrittlement in austenitic stainless steels because of its very poor coherence with austenite and its own brittle behavior. It also lowers corrosion resistance by dropping chromium and molybdenum contents in the austenitic matrix. Nb(N,C) are the consequence of the reduced solubility of Nb in austenite. Nb prevents the formation of chromium carbides, and thus chromium depleted areas. Finally, the precipitation of fine intragranular Nb(N,C) also improves creep resistance, yield strength and ultimate strength.
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