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

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.

Investigation of Shape Recovery Characteristics on Ferrous Shape Memory Alloy

Aims of this study are to clarify the shape recovery characteristics on ferrous shape memory alloy in order to utilize one as a many kinds of mechanical components. In this study, Fe-Mn-Si-Cr shape memory alloy is used. The fundamental characteristics and the shape recovery characteristics are investigated in this alloy. From the result of this investigation, shape recovery strain without training process reaches to maximum value of 2% at the amount of 5∼7% work-strain. That with the training process reaches 3.5% in the maximum, which is 1.8 times of that without training process. In addition, the shape recovery characteristic under constant stress which is given during the heating process is investigated. The microstructure of the deformed material is observed. The Widmanstatten structure is generated. This structure is attributed to the transformation from austenite to ε-martensite. In order to enable the prediction analysis of the shape recovery behavior, the relationship between shape recovery strain and work-strain is formulated by the regression analysis in cubic equation.