Assessment of steel deformability in terms of energy dissipation

The deformability of steel grades with different strength is analyzed, from the perspective that deformation is dissipative in thermodynamic terms: some of the kinetic energy of the external mechanical perturbation is converted to internal energy of the metal being deformed, with the creation of particular dislocational structure. Accordingly, energy criteria are proposed for the deformability of steel. These criteria may be determined in standard tensile tests. They are based on the work of deformation, which is determined by the area of the extension diagram. The absorbed energy determines the work of deformation, while the rate of energy absorption determines the resistance of the steel to deformation (the pliability in plastic deformation). The energy dissipation is quantitatively estimated, with comparison of the unit work and the pliability. The research is based on standard tensile tests of samples made from steels that differ in strength as a result of alloying (changes in chemical composition) and heat treatment and are used for different purposes. The steels vary in yield point from 210 to 1660 MPa and in strength from 840 to 1940 MPa. The unit work of point deformation is found to exceed the unit work of uniform deformation by an order of magnitude. The pliability in point deformation is markedly less than the pliability in uniform deformation. A clear correlation between these quantities is noted. This may be regarded as the expression of the structural evolution of the metal at both stages of deformation. In particular, in self-organization of the dissipative system—that is, the deformable steel—the dislocation density acts as an internal parameter regulating the evolutionary transformations. A correlation is established between the pliability criteria and the limiting loads in uniform deformation and failure. Thus, steels that differ in strength may be ranked in terms of the energy absorbed in deformation. In practice, the numerical values of the unit work and the pliability may be used to predict the behavior of structural steels belonging to different steel classes under mechanical perturbations in the course of operation and machining.