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I.

INTRODUCTION

HIGHLY corrosive environments in chemical, pulp, and paper plants; desalination installations; and oil and gas industry require cast metallic components with long life and little maintenance. Among the possible materials for these applications, cast duplex stainless steels (DSS) have become attractive due to a combination of high specific strength, superior resistance to stress corrosion cracking compared to ferritic stainless steels, as well as better pitting/crevice corrosion and lower cost, compared to austenitic stainless steels.[1–4] DSS are based on their two-phase microstructure, which exhibits nearly equal volume fractions of ferrite (d) and austenite (c) due to their unique chemical compositions and heat treatment conditions. To provide superior resistance to pitting and crevice corrosion, DSS are alloyed with various elements, such as Cr, Mo, N, W, and Cu.[1,5,6] As a consequence of the high alloy content, these DSS grades are prone to the formation of detrimental R.O. SOUSA and L.M.M. RIBEIRO are with the Department of Metallurgical and Materials Engineering, University of Porto, Porto, Portugal and also with the INEGI, Institute of Science and Innovation in Mechanical and Industrial Engineering, Porto, Portugal. Contact e-mail: [email protected] P. LACERDA is with the FERESPE Fundic¸a˜o de Ac¸o e Ferro, Lda, V.N. Famalica˜o, Fradelos, Portugal. P.J. FERREIRA is with the Materials Science and Engineering Program, University of Texas at Austin, Austin, TX 78712 and also with the INL International Iberian Nanotechnology Laboratory, Av. Mestre Jose´ Veiga s/n, 4715-330 Braga, Portugal and also with the Mechanical Engineering Department and IDMEC, Instituto Superior Te´cnico, University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal. Manuscript submitted September 21, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS A

complex phases, such as sigma (r), chi (v), tertiary austenite (c3), and/or chromium nitrides[5,7,8] during cooling. The microstructural changes can be drastic, considering that short periods of time at temperatures ranging from 600 C to 1000 C affect severely the balanced duplex microstructure.[6,7,9] In this context, the sequence of formation of the various secondary phases, as well as the mechanisms and kinetics of precipitation depend intimately on the chemical composition of the alloy and thermal conditions during processing. In any case, the effect of the secondary phases on the mechanical and corrosion properties is significant and critical for the performance of these materials. Among the various secondary phases, r and v appear to be the most deleterious ones. In particular, the presence of r-phase results in a severe loss of ductility, toughness, and pitting corrosion resistance.[8–12] The current investigation was undertaken in order to understand the formation of secondary phases in a 25Cr-7Ni-Mo-N cast super duplex stainless steel (SDSS) (CD3MWCuN grade according to ASTM A 890). This SDSS is one of the highest alloyed standard cast grades, with a growing deman