Regularities in the structural formation of the chromed layer in iron-chromenickel alloys

Conclusions

• 1. The relation of the depth of chroming of iron-chrome-nickel alloys to the structure of an isothermic section of the ternary phase diagram has been indicated.

• 2. It has been established that during usual chroming of an alloy containing 35% Ni (KhN35VT steel) the change in chemical composition of the surface layer in the diffusion zone principally corresponds in the isothermic section of the diagram to the direction of a radial section joining the composition point of this alloy with the vertex of the concentration triangle belonging to chromium.

• 3. In chroming the same alloy (KhN35VT) under identical conditions but with the use of an accelerator, the appreciable acceleration of chromium diffusion is related to the fact that the chemical composition of the surface layer in the diffusion zone is characterized by a sharp rise in the ratio of iron to nickel as compared to this same ratio in the base metal. This leads to deviation of the composition points of the surface layer in the isothermic section of the diagram from the direction of the radial section and occasions the formation in the structure of the layer of a zone of α-phase in which the coefficient of chromium diffusion is greater than in the γ-phase.

• 4. It has been shown that during usual chroming of KhN35VT steel with powders in hermetic containers a transfer not only of atoms of iron but also of nickel atoms from the metal to ghe gas medium takes plase.

• 5. The existence of a zone of metastable austenite has been established when chroming an alloy containing 35% Ni (KhN35VT steel); this is explained by the absence on the chromium concentration curve of an abrupt change in concentration in the interval of values corresponding to the intermediate region in the isothermic section of the ternary diagram.