Abstract
The microstructural change in a 25% Cr-20% Ni 0.41% N steel during aging at 700°, 800° and 900°C for up to 10 000 hr was investigated through optical microscopy and measurements of lattice parameters of both austenitic matrix and precipitated nitrides.
A dichromium nitride, Cr2N, precipitated both as a cellular precipitation and as a precipitation on grain boundaries. When nitrogen content in the austenitic matrix reached at certain lower concentration due to the precipitation of Cr2N after long aging, π. phase (a nitride with the structure of β-manganese type) began to precipitate. The rate of decrease in the lattice parameter of the austenitic matrix of the steel in the present investigation was an order of magnitude faster at earlier stages of aging (e. g. up to 100 hr at 800°C) than that of a 25% Cr-28% Ni-2% Mo-0.31% N steel where only a precipitation on grain boundaries was observed. This implies that the rate of nitrogen precipitation from the austenitic matrix is enhanced by the cellular precipitation. The lattice parameters, a and c, of precipitated Cr2N decreased with time of aging. A correlation of the decrease in the lattice parameter of the austenitic matrix with that in the lattice parameters of precipitated Cr2N was discussed.
