The effect of first and repetitive inter-pass annealing during severe plastic deformation on microstructure and mechanical properties of low carbon steel

In this study, we used the severe plastic deformation (SPD) technique, called constrained groove pressing (CGP), to investigate the effect of first and repetitive inter-pass annealing on the mechanical and microstructural properties of a low carbon steel. The main purpose of this study was to increase the imposed strain while reducing the reported decline in ultimate tensile stress (UTS) in the CGP process of the low carbon steel sheet. To achieve the objectives of this study, two distinct annealing modes—repetitive inter-pass annealing and first-stage inter-pass annealing—were applied between the CGP passes. These annealing treatments were conducted at 300 °C, 400 °C, and 500 °C for 20 min. The differences between these annealing modes were then thoroughly investigated. Using hardness and tensile tests, we found that, by performing both the repetitive and first inter-pass annealing process, the hardness remains approximately constant, 160–170 Vickers, by increasing the CGP number of passes; in addition, the rate of reduction of elongation (%) decreases with an increase in the number of passes. Besides, as the number of CGP passes increases, the UTS reduction following the second CGP pass becomes less significant compared to the non-annealed condition. The UTS of the inter-pass annealed samples after the second CGP pass ranges between 350 and 400 MPa, which is 10–20 % higher than that of the non-annealed condition (326 MPa). Notably, in the study of first inter-pass annealing, we could process up to four passes for low carbon steel without tearing the sample, a significant achievement for low carbon steel. Moreover, for the first time, the differences between first and repetitive inter-pass annealing were investigated, revealing that first inter-pass annealing yields a higher UTS, up to 7 %, and greater elongation (%), up to 41 %, compared to repetitive inter-pass annealing. As a matter of fact, inter-pass annealing during the CGP process could lead to an improvement in the microstructure and mechanical properties of low carbon steel.