Shape Recovery Characteristics of Pipes With Heavy Wall Thickness Made by Ferrous Shape Memory Alloy

Abstract

Recently, ferrous shape memory alloys have been developed. Shape recovery strains of ferrous shape memory alloys are smaller than those of Ti-Ni shape memory alloys[1,2]. Strength, ductility and workability of the former alloy are higher than those of the latter alloy. Therefore, ferrous shape memory alloys are tried to apply for several kinds of pipe joints, as an example, joints of support pipes in the tunnel[3]. One of the other applications, the alloys is used as the material of simulation model to analyze the deformation behavior of the core tube in the fast breeder reactor. In this study, we investigated the shape recovery characteristics of pipes with heavy wall thickness made by ferrous shape memory alloy. Chemical compositions of this alloy are Fe, 28%Mn, 6%Si and 5%Cr. The alloy is melted, and round bars are manufactured by rolling, and pipes are machined from them. Tensile strength is 1100MPa, and yield strength is 320MPa. Ratios of wall thickness to central diameter of pipes are 10, 15, 20 and 25%. We insert tapered punch in the pipe, and expanded it by test machine. Then the circumferential strain of the center diameter of the pipes is 7%. And finally, the heat treatment is conducted at 350 degrees C in order to induce the shape recovery strain, and the pipe diameter decreases by means of austenitic transformation. The results obtained by the experiment are shown as follows. As the value of ratios of wall thickness to center diameter decreases, shape recovery strain increases and seems to approach the shape recovery strain obtained by uni-axial tension test, which was conducted in the past time.