V. A. Shabashov, L. G. Korshunov, A. V. Litvinov, N. V. Kataeva, A. E. Zamatovsky

INCREASING THE DEPTH OF THE NITRIDED LAYER IN THE SURFACE OF AUSTENITIC ALLOYS USING FRICTION TREATMENT

In the Fe-12Cr-30Ni and Fe-15Cr-38Ni ion-plasma nitrided austenitic alloys the depth of the surface, gradient in concentration and phase composition, has been increased from 5 µm to 20 µm using dry sliding friction and high-pressure torsion in Bridgman anvils. The treatment is based on deformation-induced chromium nitride dissolution-precipitation cyclic phase transitions. As a result of 1 h nitriding at 500 ºC, a nitrogen solution in the Fe-Ni-Cr-N matrix and disperse nitrides of chromium CrN and iron Fe₄N are formed in the alloy surface layers. Subsequent dry sliding friction or high-pressure torsion leads to nanostructuring, deformation-induced dissolution of chromium and iron nitrides, as well as to the mechanical alloying of the surface and the inner non-nitrided alloy layers. Besides, in the deformed austenitic matrix secondary chromium nitrides are formed. With additional 2 h annealing at 600 ºC, the additional exit of nitrogen from the austenitic Fe-Ni-Cr-N solid solution occurs, with the preferential formation of Cr₂N nitride and the increase of the volume (depth) of the alloy matrix changed in terms of its structure and composition.

The reverse sequence, that is, preliminary friction and subsequent nitriding of the surface, does not noticeably increase the depth of the nitrided layer. It is explained by the accelerated migration of non-equilibrium grain boundaries in the surface nanostructured by friction, which reduces the time required for complete recrystallization to much less than the time of 1 h nitriding at 500 ºC. The result of fast recrystallization is the slight effect of fast diffusion in the surface nanostructured by friction.

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