Makoto Kikuchi1, Si-Kyung Choi2, Yasutsugu Ogura3 and Ryohei Tanaka1
1Department of Metallurgical Engineering, Tokyo Institute of Technology, Tokyo
To study cellular precipitations with carbide or nitride as the precipitate, high chromium-high nickel austenitic steels with 0.4 per cent nitrogen were chosen as a model system. Two austenitic steels with the composition of 25%Cr-28%Ni-0.40%N and 25%Cr-20%Ni-0.41%N were aged at 1073 K and growth characteristics of cellular precipitation of Cr2N were investigated.
All the following growth characteristics are quite different from the well known growth characteristics of cellular precipitations in binary substitutional systems. (A) Average migration rate of the free interface between untransformed matrix and cell decreases with reaction time. Cell growth stops even when the large portion of the specimen still remains untransformed with substantial supersaturation. This leads to the result that only a part of the specimen decomposes into cell. The rest remains untransformed even after a prolonged aging. (B) The observed volume fraction of Cr2N precipitates inside the cell is much larger than the calculated one based on the matrix-precipitate equilibrium in the original solid solution. Average nitrogen concentration inside the cell becomes higher than that of the original solid solution. In contrast to this, nitrogen concentration in the untransformed matrix outside the cell decreases with reaction time. (C) The volume fraction of Cr2N precipitates inside the cell decreases with reaction time.
These unusual characteristics can be ascribed to the long range diffusion of nitrogen. It is suggested that the redistribution of solutes accompanying the cellular precipitation of Cr2N takes place through grain boundary diffusion of chromium and long range volume diffusion of nitrogen. The present behaviors are expected to be generally observed in any cellular precipitation of carbide or nitride.
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