The effects of thermo-mechanical treatments on superplasticity of Fe-24Cr-7Ni-3Mo-0.14N duplex stainless steel

Abstract

The duplex stainless steels are defined as a family of stainless steels consisting of two phase microstructure of a-ferrite and y-austenite. The duplex stainless steels have attractive combination of good mechanical properties with excellent corrosion resistance, and are suitable for marine and petrochemical applications( I-3). Furthermore, the duplex stainless steels having fine grained microstructure show superplasticity up to 2000% elongation, since the grain growth is suppressed at high temperature due to their two phase aggregated microstructure. The first investigation on the superplasticity of duplex stainless steels was reported by Hayden et a1.(4-5). They investigated the superplastic behavior of hot rolled duplex stainless steels with various Fe-Cr-Ni ternary compositions at temperature near lOOO”C, and reported an elongation of 500% in a commercial 2.5Cr-6.5Ni-0.6Ti duplex stainless steel. Gibson et aL(6) reported that the superplasticity of 25Cr-6.5Ni-0.6Ti duplex stainless steel was enhanced by additional cold working of hot rolled sheet. Maehaaa(7-10) recently reported an elongation above 1000% was obtained in a wide temperature range from 800°C to 1100°C by the thermo-mechanical treatment in 25Cr-7Ni-3Mo-0.14N duplex stainless steel. Hayden and Gibson(4-6) obtained a fine grained duplex microstructure through the recrystallization during thermo-mechanical treatment. However, Maehara(7-9) obtained a fine grained duple:x microstructure through the precipitation of second phase at test temperature after solution treatment above 125O’C followed by cold rolling of 50% reduction. Duplex stainless steels exhibited a single phase just below the liquidus temperature, but the phase transformation of 01 single phase into a t- y duplex phases occurred below 1320°C. Since the volume fraction of y phase increases with decreasing temperature below 132O”C, the precipitation of fine y phase in a matrix phase improved the hot ductility at test temperature between 950°C and 1100°C. The Q phase is known to be formed through the eutectoid decomposition of a phase into y and cr phases at temperatures between 700°C and 950°C. Especially, it was reported that the precipitation of 0 phase played an important role in improving the hot ductility below 95O”C(8). However, most of the previous investigations on superplasticity were focused on the duplex stainless steels having a/y duplex microstructure near lOOO”Cw, hile the superplasticity below 950°C was not understood yet. In this study, the effect of thermo-mechanical treatment on superplasticity of 24Cr-7Ni-3Mo-0.14N alloy was investigated at 850°C in three phase regime consisting of a, y and <T phases. In order to examine the effect of thermo-mechanical treatment conditions on superplasticity, the solution treatment temperature and cold reduction ratio were varied. The effects of thermo-mechanical treatment conditions on the microstructural factors of constituent phases were analyzed. The relationships between the microstructural factors and superplasticity in Fe-24Cr-7Ni-3Mo-0.14N duplex stainless steel were discussed.