Microstructural Evolution of Low Carbon Steel Tubes with Variable Wall Thickness in Dieless Mandrel Drawing

In this study, a carbon steel tube with a variable wall thickness was fabricated under various conditions by dieless mandrel drawing to evaluate the effect of the Zener-Hollomon parameter on its microstructure. STKM13C low carbon steel tubes with an outer diameter of 6.0 mm and inner diameter of 4.0 mm were drawn under various conditions of heating temperature, feeding speed, and the reduction in area. The feeding speed was set as constant, and the drawing speed was increased or decreased during dieless mandrel drawing to obtain tubes with a variable wall thickness. Thereafter, the wall thickness and microstructure of the drawn tubes were evaluated. Furthermore, the Zener-Hollomon parameter was calculated under various dieless mandrel drawing conditions. Consequently, a tube with variable wall thicknesses, with a wall thickness reduction range of 0 – 21%, was obtained by the dieless mandrel drawing. Furthermore, the Zener-Hollomon parameter increased as the heating temperature decreased, and as the feeding velocity and reduction in area increased. As a result of an increase in the Zener-Hollomon parameter, the average grain size decreased to less than 2 μm in a single pass of the dieless mandrel drawing.