CARBON, NITROGEN AND HYDROGEN IN STEELS: PLASTICITY AND BRITTLENESS

Abstract

Interstitial elements in steel, carbon, nitrogen and hydrogen, are analyzed in terms of their effect on the electron structure, properties of dislocations, strengthening, plasticity and fracture. It is shown that similarities and differences in the mechanical properties of corresponding solid solutions are controlled by the effect of the above mentioned elements on the density of electron states at the iron Fermi level and, as a result, on the concentration of free electrons. The latter is decreased by the carbon and increased due to nitrogen and hydrogen in the iron, which changes the character of interatomic bonds: carbon enhances their covalent component, whereas nitrogen and hydrogen strengthen the metallic one. The velocity of dislocations in the course of plastic deformation is discussed using the approach of mobile and immobile interstitial atoms. In the fi rst case, they are obstacles for dislocation slip, and mobility of dislocations is determined by the enthalpy of binding between interstitial atoms and dislocations. If interstitial atoms are suffi ciently mobile to accompany dislocations, the character of interatomic bonds within the interstitial clouds around the dislocations is locally changed. As a result, the specifi c energy of dislocations (line tension) and the distance between them in the pile-ups are changed in accordance with the local change of the shear modulus around the dislocations. Based on the performed studies, the effect of interstitial elements on the mechanical properties of steels is discussed. Particularly, the essential similarity between the hydrogen-caused brittleness and the nitrogen-induced ductile-to-brittle transition in the austenitic steels is interpreted.