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The α-ferrite-phase in nickel and in manganese containing stainless steel alloys is very brittle. Its formation is connected with a huge decrease in volume, which also causes stresses in the alloys that are very dangerous if the materials cannot accommodate them. The α-ferrite-phase is formed in stainless steel alloys only if nucleation sites are provided, α′-martensites are nucleation sites for the formation of α-ferrite, even at temperatures at which α-ferrite is not stable. If α′-martensite is dissolved during an anneal, no new α-ferrite is formed, and the remaining α-ferrite transforms back into γ-austenite.
In “pure” nickel or in “pure” manganese containing stainless steel alloys the martensitic temperature is above 100°C and thus α′-martensite is always present at ambient temperature in these materials, giving rise to the formation of α-ferrite during a subsequent anneal. It was established in the present work that the γ↔γ+α-phase boundary in iron-chromium-nickel alloys is also almost independent of the temperature as in iron-chromium-manganese alloys. Thus the existing phase diagram for nickel containing stainless steels has to be revised. The various elements added to “pure” stainless steel alloys, as we find them in EUR-316L, US-316L, US-PCA, and in AMCR, cause a drastic decrease of the martensitic temperature so that neither α′martensite nor α-ferrite is found in these alloys.
However, in all these four alloys α-ferrite is formed readily during irradiation with high energy particles and a tentative γ↔γ+α-phase boundary, valid during irradiation with high energy particles, is derived. The amount of α-ferrite formed during irradiation increases with decreasing irradiation temperature and with decreasing applied stress. The alloys EUR-316L, US-316L, US-PCA, and AMCR do not survive one reactor cycle, if irradiated at 100°C.
Phase diagram, nickel and manganese containing stainless steels, nucleation, α-ferrite, α′-martensite, dislocations, volume change, brittleness
Professor, Institut für Angewandte Physik der Johann Wolfgang-Goethe Universität, Frankfurt,