Abstract
Alloy 33 is a recently developed Cr-Fe-Ni alloy with nominally 33 wt.% Cr, 32 wt.% Fe, 31 wt.% Ni which exhibits a fully austenitie structure due to the nitrogen content of 0.4 wt.%. In this work we describe the microstructure of Alloy 33 samples aged at 700 °C and 900 °C for 100 h as characterized by electron microscopy (SEM, EPMA, TEM) and several microanalytical methods (EDS, WDS, EELS). Aging at 900 °C leads solely to the precipitation of σ-phase at the grain boundaries. The microstructural evolution at 700 °C is much more complex, the main features being the precipitation of intermetallic phases at the grain boundaries and the discontinuous precipitation of Cr-rich phases at migrating boundaries starting at grain boundaries. Previous results concerning the stable corrosion resistance of this new alloy in boiling azeotropic nitric acid as well as the changes in the mechanical properties after long term aging are discussed in view of the observed microstructures. Finally, a first assessment of the microstructure of welded joints is given.
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