Effect of Annealing Temperature on Mechanical Behavior, Pitting Resistance and Grain Boundary Character of a 2304 Lean D
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DUPLEX stainless steels are a category of high-alloyed steels characterized by a biphasic austenitic-ferritic microstructure, which provides an attractive combination of mechanical properties and corrosion resistance.[1–8]
PAULA O. MALTA, BARBARA L. CONDE´, and DAGOBERTO B. SANTOS are with the Department of Metallurgical and Materials Engineering, Universidade Federal de Minas Gerais, Antoˆnio Carlos Avenue, 6627, Pampulha, Belo Horizonte, Minas Gerais, CEP 31270-901, Brazil. Contact e-mail: [email protected] RAPHAEL F. ASSUMPC¸A˜O, DANIELA B. PERASOLI, and DALILA C. SICUPIRA are with the Department of Chemistry, Universidade Federal de Ouro Preto, Campus Universita´rio Morro do Cruzeiro, Bauxita, Ouro Preto, Minas Gerais, CEP 35400-000, Brazil. Manuscript submitted October 23, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The mechanical performance of duplex alloys is strongly dependent on the behavior of their individual phases. While the ferritic phase is responsible for the good strength of such steels, the austenitic phase is responsible for their high toughness.[8–11] Increasing the amount of ferrite increases the yield strength of duplex steels but decreases their ultimate tensile stress (UTS). The latter occurs because many of the benefits provided by the austenite, such as its high work-hardening ability and its non-cleavage type fracture, are lost with increased ferrite volume fraction.[10,12] The mechanical properties of duplex stainless steels are also a function of the deformation behavior of ferrite and austenite. Plastic deformation of ferrite is mainly dominated by dislocation glide due to high stacking fault energy (SFE) and the numerous slip systems. In austenite, the plastic deformation may involve either transformation induced plasticity (TRIP) or twinning induced plasticity (TWIP), as well as a planar glide of dislocations. It has been reported that during the deformation of lean duplex steels, austenite undergoes deformation-induced
martensitic transformation (DIMT).[13,14] DIMT usually leads to a strong hardening and, potentially, to excellent elongation, characterizing the occurrence of the TRIP effect.[13–16] Although duplex grades have attractive mechanical properties, it is because of their corrosion properties that these alloys are largely applied in various industrial sectors. The pitting resistance of stainless steels in aqueous environments arises from the existence of a thin passive film on their surfaces and is usually associated with the pitting resistance equivalent number (PREN).[1,2] The PREN index is related to the content of Cr, Mo, and N, the three main elements responsible for providing localized corrosion resistance, according to the following expression: PREN ¼ pct Cr þ 3:3 pct Mo þ 16 pct N
½1
Increasing annealing temperature makes the passive film thinner and more prone to localized attack, due to higher rates of oxide dissolution if compared to the growth rate of a new protective layer.[3] Additionally, as the annealing temperature increases, Cr and Mo
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