Effect of the initial state of a low-carbon steel on nanostructure formation during high-pressure torsion at high strains and pressures

Abstract

The formation of nanocrystalline (d < 100 nm) and submicrocrystalline (100 nm < d < 1000 nm) structures in 09G2S and 10G2FT steels has been studied during cold and warm severe plastic deformation (SPD) by torsion at a hydrostatic pressure and subsequent heating. The steels were subjected to SPD in two initial structural states: ferritic-pearlitic and martensitic (bainitic) states. SPD at room temperature results in the formation of a cellular oriented nanostructure with individual equiaxed nanograins. As the deformation temperature increases to 500°C, the fraction of a grain structure and the average grain size increase. In the case of the initial ferritic-pearlitic state, the average grain size in the 10G2FT steel after SPD is larger (95 nm at 20°C and 120 nm at 500°C) than in the case of the initial martensitic structure (65 nm at 20°C and 85 nm at 500°C). The grain size in the 09G2S steel is slightly smaller than in the 10G2FT steel after SPD in both initial states. The coherent-domain sizes determined by X-ray diffraction agree well with the structural-element sizes determined by electron microscopy in the samples subjected to SPD. In the temperature range 20–500°C, torsional SPD results in the formation of a partly nanocrystalline structure and significant hardening of the metal. In both steels, the microhardness increases significantly with respect to the initial state: 2.5–3 times in the case of the initial ferritic-pearlitic state and 1.5–2 times in the case of the martensitic state.