Microstructural Evolution of Secondary Phases in the Cast Duplex Stainless Steels CD3MN and CD3MWCuN
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I.
INTRODUCTION
DUPLEX stainless steels (DSS) possessing both ferrite (d) and austenite (c) as primary phases are produced by adding Cr, Mo, Si, Ni, Mn, and N to a base Fe-C system. The addition of such alloying elements affects the relative stability of existing phases and also causes new phases to form. Elements such as Cr, Si, and Mo are called ferrite stabilizers because they promote ferrite phase formation over wider composition and temperature ranges as compared to the Fe-C system. Concurrently, the austenite region is contracted, i.e., the c loop shrinks. By contrast, elements such as C, Ni, Mn, and N promote austenite formation and are called austenite stabilizers. Both ferrite and austenite co-exist in DSS by adjusting the ratio of ferrite-to-austenite stabilizers. The two-phase microstructure of such steels confers a favorable combination of mechanical and corrosion properties. Because DSS are highly alloyed, many secondary phases other than d and c may also form during either service or fabrication. Typical secondary phases that have been reported[1] are listed in Table I and include secondary austenite (c2), carbides such as M7C3 and M23C6, and various types of intermetallic phases such as r, v, R, and p. Secondary austenite (c2) has the same crystal structure as primary austenite (c1); however, the lattice parameter and the formation temperature are slightly different from the primary form. In spite of their differences, the physical and mechanical properties of c Y.-J. KIM, Research Engineer, is with the Hyundai-Kia Motors, Seoul, South Korea. O. UGURLU and C. JIANG, Research Scientists, are with the Los Alamos National Laboratory, Los Alamos, NM, USA. B. GLEESON and L. SCOTT CHUMBLEY, Professors, are with the Ames Laboratory, Iowa State University, IA 50011, USA. Contact e-mail: [email protected] Manuscript submitted August 11, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
and c2 are quite similar.[1] Austenite is known to improve mechanical properties such as fracture toughness[1] and is beneficial to DSS performance. The carbon content in DSS is usually very low (less than 0.03 wt pct), and, as a consequence, the amount of carbide is typically too low to significantly affect the mechanical properties. The carbides usually form in conjunction with c2 and r[2] to result in dual-phase morphologies. The formation mechanism of nitrides such as Cr2N in DSS has also been reported.[3] The phases most deleterious to mechanical and corrosion properties of DSS are the Cr- and Mo-rich intermetallics, because they are brittle and reduce the steel’s resistance to pitting corrosion due to depletion of Cr and Mo from the d matrix.[1,4,5,14] The Cr-rich r phase has a tetragonal lattice structure and its nucleation is usually observed at the interphase boundary between d and c.[1–12] The Mo-rich v phase has a cubic crystal structure and its formation may be detrimental to the impact toughness of DSS.[15,16] The v phase is reported to be more brittle than r.[4] Time-temperature-transformation (TTT) and continuous-co
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