1975 Volume 61 Issue 10 Pages 2418-2432
Two austenitic steels, designated as steel 22N (25%Cr-28%Ni-11%Mo-0.011%C-0.22%N) and steel 31N (25%Cr-28%Ni-2%MO-0.009%C-0.31%N), were used to study the microstructural changes during aging in the temperature range from 700° to 950°C for up to 5500 hr. Optical micrographs as well as back scattered electron micrographs were taken. X-ray diffraction phase identification was made in potentiostatically extracted residues. Lattice parameters of both austenitic matrix and precipitated nitrides were measured.
In steel 22N, precipitated phases are mainly σ phase. No precipitation of Cr2N takes place above 700°C. In steel 31N, Cr2N and π phase (a nitride with the structure of β manganese type) precipitate on grain and twin boundaries. Volume fracion of π phase in the extracted residues increases with time of aging. Each precipitate can be identified either as π phase or as Cr2N in both back scattered electron micrographs and potentiostatically controlled etched optical micrographs. The chemical formula of π phase is derived from the composition of metallic elements to be (Cr, Mo)12 (Fe, Ni)8N4-y, where y represents a deviation of nitrogen concentration from the stoichiometric composition. The lattice parameters of both Cr2N and π phase decrease with time of aging. The lattice parameter of the austenitic matrix decreases also with time of aging. The lattice parameter decrease of steel 31N still continues at 800°C even after a few thousand hours aging. This implies that the precipitation of supersaturated nitrogen in this steel does not complete even after longer than 1000 hr.
The high creep-rupture strength of this type of austenitic steels alloyed with nitrogen was chiefly ascribed to a so called solid solution strengthening on the basis of the above metallographic observations. The strengthening effect of Cr2N precipitation on grain boundaries was also discussed.
