Proceedings of COBEM 2005 18th International Congress of Mechanical Engineering Copyright © 2005 by ABCM November 6-11, 2005, Ouro Preto, MG SUPERPLASTIC BEHAVIOR OF A Fe-Mn-Al AUSTENITIC STAINLESS STEEL Paulo Guanabara Jr. PPG - CEM / DEMa – UFSCar Rod.Washington Luiz, km.235 – 13565-905 São Carlos(SP) gb@iris.ufscar.br Levi de Oliveira Bueno DEMa – UFSCar Rod.Washington Luiz, km.235 – 13565-905 São Carlos(SP) levi@power.ufscar.br Abstract. The superplastic behaviour of Fe-Ni-Cr duplex stainless steel system has been the subject of several investigations in the last two decades, with superplastic forming applications already existing in practice. A rare example of study of superplasticity on a Fe-Mn-Al steel was presented by Toscano (1983) showing some possibility of exploring the potential use of such materials in this regime for temperatures higher than 700°C. The present work was programmed to explore the occurrence of this behaviour in a similar steel with systematic hot tensile tests carried out in the range from 600 to 900°C and strain-rates varying from 8 x 10 -5 to 8 x 10 -2 s -1 . The variation in sensitivity of stress with strain rate (σ = C έ m ) was observed using distinct specimens pulled until rupture under different combinations of crosshead speed and temperature, as well as single specimens subjected to a sequence of crosshead speed changes during the hot tensile test. At 800 and 900°C the maximum values of m ( m = dLogσ/dLogέ ) were found to be about 0.57 and 0.66 respectively, which confirms the material susceptibility to superplastic behaviour. The maximum elongation values were observed to stay around 300% obtained with the lowest strain rate level of 8 x 10 -5 s -1 at 850°C. Keywords: Fe-Mn-Al steel, hot tensile test, strain-rate sensitivity, superplasticity. 1. Introduction It is well known that superplastic materials exhibit usually a three-stage relationship in the steady-state strain rate (έ) dependence of the applied stress (σ) [1]. These three ranges are named regions I, II and III, respectively. The most important superplastic characteristics associated to high elongations occur in region II, with progressive drop in superplastic characteristics in both regions I and III, as shown schematically in Figs. 1a and 1b, according to Langdon (1982). There are two kinds of mechanical tests to explore the superplastic properties of metals: a) tensile tests on constant crosshead speed (or constant strain rate) machines where the flow stresses are measured as function of the strain rates and are related by the expression: σ = C έ m , where C is a constant including the temperature dependence, and m is the strain rate sensitivity exponent (m = dLogσ/dLogέ); b) tensile tests on creep machines at constant load (or constant stress), where the strain rates are measured as function of the imposed stresses, being related by the expression: έ = A σ n , with n = 1/m. Values of m ranging from 0.35 to 0.8 are generally considered to produce superplastic behaviour. Figure 1 – Schematic illustration of the two different procedures used to plot the mechanical data of superplastic materials: a) stress vs strain rate and b) strain rate vs stress. Log έ Log σ I SP region II III m 1 Log σ Log έ I SP region II III n 1