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Development of advanced high nitrogen steels

The studies aimed at the clarification of mechanisms responsible for the beneficial effect of nitrogen in steels and at obtaining fundamental knowledge for the development of new promising grades of nitrogen steels. The research programme included studies of the electronic structure of nitrogen iron-based solid solutions, the thermodynamical simulation of solid solutions and the experimental determination of nitrogen distribution in austenite and martensite, the nature of phases precipitated during tempering of high nitrogen martensitic steels, relaxation phenomena induced by nitrogen, mechanical properties of nitrogen steels including the behaviour under hydrogen attack.

It was shown that nitrogen alloying of austenitic steels increases concentration of conduction electrons, i.e. enhances the metallic interatomic bonds, whereas carbon alloying induces localisation of electrons (assists the covalent bonds). Combining Mössbauer measurements and the simulation of solid solutions by Monte Carlo method, it was shown that the interatomic potentials for nitrogen-nitrogen interaction in the first two co-ordination spheres corresponds to short range atomic ordering of nitrogen atoms in austenite as well as to the formation of ordered structures like Fe4N. No ordering was found in binary Fe-C austenitic alloys. From the measurements of small angle neutron scattering in FeCrNiMn and FeCrNi austenites, it was shown that nitrogen increases the chemical homogeneity of solid solutions while carbon enhances the inhomogeneity. A concept of homogeneous distribution of the substitutional solutes in nitrogen austenites and martensites due to the nitrogen-enhanced metallic interatomic bonds and the correlation between the metallic character of bonding and atomic ordering is proposed as an explanation for the high thermodynamical stability of nitrogen austenitic steels.

A delay in the precipitation of nitrides during tempering of nitrogen and nitrogen+carbon chromium martensitic steels is proven to be a consequence of nitrogen-caused short range ordering of chromium atoms inherited from austenite. The largest effect was revealed in the mixed nitrogen+carbon steels, which is consistent with the excellent mechanical and corrosion properties of these grades of nitrogen martensitic steels.

The strengthening of austenitic steels by nitrogen is interpreted from the results about the electron structure and it was found that the abnormal strengthening of austenitic steels due to nitrogen at low temperatures is caused by the striking temperature dependence of stacking fault energy arising from the nitrogen-caused increase in states density at the Fermi surface. A strong interaction between nitrogen atoms and dislocations is also shown to arise from the enhanced metallic character of interatomic interactions. A strong pinning of dislocation sources by nitrogen atoms is also proven to be a reason for the increase in the grain-boundary strengthening of austenitic steels with an increasing nitrogen content.

The good damping and shape memory properties of nitrogen austenitic steels are obtained from the studies of the relaxation phenomena and mechanical properties in nitrogen austenites, which creates a basis for development of new industrial high-damping materials.

Alloying of stable austenitic steels with nitrogen is shown to decrease the deviations from the cubic symmetry of non-cubic defects created by hydrogen atoms and substitutional solutes and to prevent the hydrogen-induced γ transformation, which is suggested to be a reason for favourable nitrogen effect on hydrogen embrittlement.

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J FOCT
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