Abstract
The microstructural change in a 25%Cr-28%Ni-0.4%N austenitic stainless steel during aging at 800 °C for up to 10 000 h was studied by means of optical microscopy and lattice parameter measurement. A dichromium nitride, Cr2N, precipitated at an earlier stage of aging both as intergranular precipitation and as cellular precipitation. Further aging caused π phase (a nitride with the structure of β-manganese type) precipitation, accompanied by redissolution of Cr2N. A nitride transformation from Cr2N to π phase started at 100 h and completed after 10 000 h aging in solution specimens without prior cold deformation. Cold rolling prior to aging accelerated the nitride reaction. A coarse dispersion of Cr2N produced by aging at 900°C retarded considerably the nitride reaction at 800°C. The chemical composition of the π phase formed after 10 000 h aging was determined to be Cr12(Ni0.5Fe0.4Cr0.1)8N4. The solubility of nitrogen corresponding to Cr2N and π phase was 0.15 and 0.125 wt%, respectively, in this austenitic steel at 800°C.
