窒素と Mo とを含む 25%Cr-28%Ni オーステナイト鋼の時効組織 : 高温クリープ破断強度向上に寄与する組織学的因子の検討

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Two austenitic steels, designated as steel 22N (25%Cr-28%Ni-1%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℃ for up to 5500hr. 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 Cr_2N takes place above 700℃. In steel 31N, Cr_2N 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 Cr_2N 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)_8N_<4-y>, where y represents a deviation of nitrogen concentration from the stoichiometric composition. The lattice parameters of both Cr_2N 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℃ 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 1000hr. 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 Cr_2N precipitation on grain boundaries was also discussed.
社団法人日本鉄鋼協会の論文
1975-08-01

窒素と Mo とを含む 25%Cr-28%Ni オーステナイト鋼の時効組織 : 高温クリープ破断強度向上に寄与する組織学的因子の検討

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