Nitrides Precipitation and Preferential Pitting Corrosion of Ferrite Phase in UNS S39274 Superduplex Stainless Steel
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TECHNICAL ARTICLE
Nitrides Precipitation and Preferential Pitting Corrosion of Ferrite Phase in UNS S39274 Superduplex Stainless Steel S. S. M. Tavares1,2 · A. C. Gonzaga2 · J. M. Pardal1,2 · J. N. Conceição3 · E. O. Correa3 Received: 26 June 2020 / Accepted: 16 September 2020 © ASM International 2020
Abstract A W-alloyed UNS S39274 was solution treated in three distinct temperatures, 1050, 1100, and 1150 °C, with water quenching, resulting in microstructures with different austenite and ferrite proportions, and chromium nitrides precipitated. The densities of chromium nitrides were evaluated qualitatively by optical (conventional and confocal) and scanning electron microscopy. These particles were found to precipitate inside ferrite grains and in the ferrite/ferrite and ferrite/austenite boundaries. The amount of chromium nitrides increased with the solution temperature, but the microhardness of ferrite was not affected by the increase of nitrides density. As a consequence of nitrides precipitation, ferrite was more susceptible to pitting nucleation and attack than austenite, according to polarization tests in 3.5%NaCl solution at 80 °C and in double loop electrochemical potentiodynamic (DL-EPR) tests. Keywords Corrosion · Heat treating · Microstructure · Precipitation phenomena · Stainless steels
Introduction Duplex (DSSs) and superduplex (SDSS) stainless steels are corrosion resistant alloys used in services where high strength and toughness are required [1, 2]. The difference between duplex and superduplex steels is based on the pitting resistant equivalent number (PREN) value given by Eqs 1 and 2 [1, 3–6]. Steels with PREN higher than 40 are called superduplex.
PREN = %Cr + 3.3(%Mo) + 16 (%N)
(1)
PREW = %Cr + 3.3 (%Mo + 0.5(%W)) + 16(%N)
(2)
* S. S. M. Tavares [email protected] 1
Universidade Federal Fluminense, Rua Passo da Pátria, 156, Niterói, RJ CEP 24210‑240, Brazil
2
Programa de Pós‑Graduação em Engenharia Mecânica e Tecnologia de Materiais, Centro Federal de Educação Tecnológica Celso Suckow da Fonseca, Rio de Janeiro, Brazil
3
Programa de Pós‑Graduação em Materiais para Engenharia, Universidade Federal de Itajubá, Itajubá, Brazil
It must be pointed that other PREN formulae proposed in the literature may attribute different weights to the elements Cr, Mo, W, and N, as also may include other elements. Equation (2) is often referred as “PREW” because also includes the element W. Haugan et al. [6] studied the effect of W addition on the corrosion resistance of 25%Cr SDSS, and found beneficial effects on the pitting and repassivation potentials. Park et al. [7] found that W addition minimizes the deleterious effects of aging at 475 °C on mechanical and corrosion resistance properties. As Eq 2 suggests, Cr, Mo, W, and N are the elements which most increase the localized corrosion resistance in austenitic-ferritic stainless steels. However, the effects of these elements are maximized when they are in solid solution in ferrite and austenite phases, and not forming other phases in
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