Conclusions
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1.
Electron-microscopic examination showed that aluminum nitrides are formed in transformer steel with 0.02–0.03% residual aluminum, which are first precipitated as fine dendrites in the grain boundaries. The absence of residual aluminum in steel with an elevated oxygen content causes the formation of oxide films in the boundary areas.
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2.
The presence of nitride inclusions and oxide films in the grain boundaries prevents grain growth during annealing and raises the coercive force.
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3.
No substantial improvement of the magnetic properties of transformer steel containing oxide films and aluminum nitrides is possible by means of standard heat treatment.
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Literature cited
N. P. Zhetvin et al., Technically Pure Iron [in Russian], Metallurgizdat, Moscow (1962).
L. G. Dykstra, Structure of Metals and Alloys [Russian translation], Metallurgizdat, Moscow (1957).
L. M. Utevskii, Zavod, Lab., No. 10 (1967).
D. Ya. Povolotskii, Aluminum in Structural Steel [in Russian], Metallurgiya, Moscow (1970).
Additional information
Krasnyi Oktyabr' Factory, Volgograd. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 30–32, May, 1971.
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Okenko, A.P., Ivanenkov, M.I. & Kul'kova, M.N. Fine precipitates in the grain boundaries of transformer steel. Met Sci Heat Treat 13, 376–379 (1971). https://doi.org/10.1007/BF00652439
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DOI: https://doi.org/10.1007/BF00652439