The Morphology of the Volume Plasticity Zones at the Gas-Filled Pores in Cast and Powder Steels under Stress Corrosion

Abstract

Purpose of research . The purpose of this work is to develop a method for calculating parameters of volumetric zones of plasticity in powder and cast steels, occurring in the vicinity of pores under external tensile stress and internal gas pressure. Methods . The problem was solved on the basis of the analysis of the distribution of stress tensor components in the vicinity of discontinuities (pores) of different shapes. A sample under tensile stresses ( σ ), containing a single stress raiser - a spherical pore with radius a, and a pore in the form of a biconvex lens, was studied stepwise. The choice of pores morphology was determined by their experimental observation in the structure of real commercial steels, and the presence of a solution for estimating the stress field near the spherical pore, performed by L. D. Landau and E. M. Lifshitz. In comparison with such stress raisers as a tight crack or a pore in the form of a biconvex lens, the stress concentration near a spherical pore is weaker. However, for the processes of diffusion of atoms through the zone of increased stress, not only the intensity of stress is important, but also the size of the zone itself. Near the spherical cavity, the size of the overstressed zone is the largest, so the analysis of its morphology was taken as a basis. In this paper, we used the modeling of functions describing the stress distribution around the pores similarly to the velocity field of an ideal fluid. Results . As a volume object, the described zones of plasticity are the surfaces of a spheroid and an ellipsoid surrounding spherical and lenticular pores. It is obvious that the smaller the ratio h/l for the pore is, the further in the transverse direction the plasticity zone (more than p max -l) spreads, becoming tighter and tighter (z 1 is approximately proportional to h). The following geometric parameters of the plasticity zones near the lenticular pore were determined as characteristic ones: its greatest length in the radial direction from the apex; the characteristic thickness of the zone ( ρ max – l)l/a; the area of the lens; the volume of the zone and its share of the volume of the lens. The development of plasticity zones near pores of different morphologies in steels under stress-corrosion conditions stimulates the change (increase) of the kinetic characteristics of the metal and the creation of favorable conditions for accelerated diffusion (decarburization). In the area of plasticity zones in the vicinity of pores under external and internal (gas pressure) stresses, the creation of channels of facilitated diffusion forms places of nucleation of submicrocracks at points D, B and C. Juvenile free surfaces formed in radial directions create zones of realized accommodation opportunities of the medium - zones of plasticity. Along with the growth of radial cracks to their tops from the pore under high pressure, gases diffuse rapidly. Conclusion . The stress distribution analysis algorithm proposed in this paper allows predicting the intensity of plasticity zones (crack formation) development depending on the ratio of parameters β ( β = σ T/ σ ) and s (s = p/ σ ), that is, on the ratio of external stress values, steel yield stress and gas pressure in the pore. The calculation made it possible to clarify the place of nucleation, the shape and scale of the development of zones of plasticity (cracking) in the vicinity of pores of different morphology depending on the ratio of external stress and gas pressure in the pores.