Role of nitrogen in the cyclic deformation behavior of duplex stainless steels
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I. INTRODUCTION
DUPLEX stainless steels (DSS) consisting of austenite and ferrite have, up to now, expanded their applications to a lot of industries with more severe, corrosive environmental conditions such as oil, gas, and petrochemical processes, thanks to a combination of superior strength and corrosion resistance against standard austenitic steels.[1,2,3] Since it has been well known that nitrogen as an alloying element is beneficial enough to improve strength, toughness, and corrosion resistance, a variety of DSS containing different nitrogen contents, in this regard, have been developed for commercial purposes to satisfy industrial demands of new materials to withstand tougher conditions. In many applications, DSS used for structural materials can be exposed to cyclic loading under service conditions, which can bring about structural failures and even life loss. Accordingly, it is essential to thoroughly understand the cyclic deformation behavior of DSS, the knowledge of which can also be applied usefully to design and develop new materials with higher fatigue resistance. In a few decades, many studies on the cyclic deformation behavior of DSS were conducted, examining the cyclic hardening-softening and the cyclic stress-strain responses.[4–8] Dislocation substructural evolution was also investigated to clarify the cyclic behavior.[9–17] Hayden and Floreen[4] examined, in the early study, the cyclic hardening-softening response of a fine-grained duplex HYONG JIK LEE, Researcher, is with the Wire Rod Research Team, Technical Research Laboratories, POSCO, Pohang 790-785, Korea. Contact e-mail: [email protected] CHONG SOO LEE and YOUNG WON CHANG, Professors, are with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790784, Korea. Manuscript submitted March 29, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
alloy (43 pct -57 pct ), showing an appreciable hardening in a few cycles and, subsequently, continuous softening. However, their work is difficult to use for comparison purposes, because the precipitate formed in the annealing process can at least partially affect the cyclic hardening-softening response. The cyclic deformation behavior of a duplex alloy (50 pct -50 pct ) was also investigated by Magnin et al.[5,6] They concluded from the cyclic stress-strain curves (CSSCs) and Coffin–Manson curves that the materials are governed by the cyclic deformation behavior of ferrite at a high plasticstrain amplitude, whereas the alloys are controlled by that of austenite at a low one. Magnin and his co-workers, however, did not accomplish the studies on the dislocation structure evolution correlated to the cyclic response necessary to support their reasoning. Xia and Wang[9] studied not only the cyclic hardeningsoftening response, but also the dislocation structure, using an Fe-Cr-Mn-N duplex alloy developed for replacing a conventional Fe-Cr-Ni austenitic alloy. As reported by the aforementioned studies, initially, rapid cyclic hardening was followed
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