Structure of the diffusion layer after low-temperature carbonitriding
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The formation of pores results from the large metastability of ε-phase in which the pressure of molecular nitrogen in equilibrium with the nitrogen dissolved in the lattice of ε-phase reaches several hundred thousand atmospheres.
Pores are evidently nucleated on dislocations and the nuclei of pores form when nitrogen atoms in the center of the dislocation combine to form a molecule of larger volume, dissociation of which does not occur, since the internal pressure of molecular nitrogen exceeds the local compressive stress induced by distortion of the crystal lattice.
The growth of pores occurs by slip under the influence of the internal pressure of molecular nitrogen.
Nitrogen, active in pores under high pressure, tends to leave the metal. With a high density of pores the bridges between individual pores collapse and channels are created in the pores for diffusion of biatomic gases. If air acting on the diffusion layer at high temperatures oxidizes the inner surface of the pores then the pores will be filled to a greater or lesser extent with iron oxide.
Studies of the theory and laws of the phenomenon considered make it possible to determine the optimal conditions for carbonitriding to guarantee strict limitation of the pore density and the depth of the zone in which pores are formed.
KeywordsOxide Nitrogen Iron Atmosphere High Pressure
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