Phase transitions in the Fe/NH3/H2 and Fe-N systems

Abstract

In the article, based on the literature, the phase changes in iron nitrides on iron powders and on solid samples were discussed. Phase transformations in NH3/H2 atmosphere and in inert atmospheres are discussed. The similarity of phase transformations in different atmospheres used during annealing were indicated. The conditions of phase transformations in iron nitrides during annealing in NH3/H2 atmosphere, argon and vacuum were discussed. Phase transformations occurring during annealing in the NH3/H2 atmosphere are reversible and there is a hysteresis phenomenon. During the phase transformation ɛγ' in the NH3/H2 atmosphere until the transformation is completed, nitrogen emission to the atmosphere takes place. On the other hand, the condition for the course of the transformation of γ'ɛ is the nitrogen flow from the atmosphere to the surface. Phase changes during heating in vacuum and argon are irreversible. During continuous heating at a rate of 30 K / min in vacuum and argon, nitrided iron powders, two phase transformations may occur, which are not accompanied by weight loss, the first (α+γ')γN in the temperature range 540÷550°C in a vacuum and 620÷630°C in argon and the second (γ+γ')ɛ in the range of 610÷620°C in vacuum and 690÷710°C in argon. In the case of heating in argon, the onset of weight loss was recorded at a temperature of about 860°C. Whereas in vacuum the denitration of nitrogen austenite γN ends at this temperature. During annealing at the temperature of 360°C, the phase change ɛγ′ in the ɛ/γ′ layer is accompanied by an increase in the thickness of the γ′ phase, which is at the expense of the thickness of the ɛ zone, while the total thickness of the layer after the transformation is the same as its initial thickness. At the temperature of 420°C, after the completion of the γ′ transformation, the formed monophasic layer γ′ is thicker than the ɛ/γ′ layers in the initial state.