Sigma phase precipitation on welded SAF 2205 Duplex Stainless Steels after isothermal heat treatment
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Sigma phase precipitation on welded SAF 2205 Duplex Stainless Steels after isothermal heat treatment A. F. Miranda Pérez a, R. Sandström b, I. Calliari c, F. A. Reyes Valdés a Corporación Mexicana de Investigación en Materiales, COMIMSA, Ciencia y Tecnología N° 790. Fracc. Saltillo 400. Saltillo, Coahuila, México b Materials Science and Engineering, Royal Institute of Technology, Brinellvägen 23, Stockholm, Sweden c Industrial Engineering Department, University of Padua, Via Marzolo 9, Padua, Italy
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ABSTRACT Duplex stainless steels are commonly used for various applications owing to their superior corrosion resistance and/or strength. They have ferromagnetic behavior together with a good thermal conductivity and a lower thermal expansion as a result of higher ferrite content than austenitic steels. Their ferrite matrix suffers a decomposition process during aging in the temperature range 650-950° C producing precipitation of austenite, σ and χ, carbides and nitrides. These intermetallic phases are known to be deleterious for corrosion resistance and mechanical properties. In this work the effect of aging time during isothermal treatment at 850°C and 900°C on the microstructure of SAF 2205 Duplex Stainless Steels welded plates has been investigated. The aim of this work is to determine the morphology of σ phase, and perform a quantitative analysis of the precipitation process. Submerged Arc Welding is used for processing. It produces a high content of δ ferrite in the heat affected zone and low content of austenite in the weld. Microstructural examination shows that the σ phase precipitates at δ ferrite/γ interphases. Longer aging treatments give rise to an increase of volume fraction together with a coarser morphology. INTRODUCTION Duplex stainless steels (DSS) are extensively used e.g. petrochemical, pharmaceutics, nuclear industries and, lately for marine applications due to their special characteristics [1-2]. Their chemical composition develops a particular microstructure (50% ferrite, 50% austenite) which confers an excellent corrosion resistance and strength at elevated temperature as well as, good toughness and ductility [3-4]. In the fabrication of components or equipment, manufacturers employ welding as the principal joining method, frequently followed for an isothermal heat treatment. Such manufacturing process produces changes in the microstructure that modify the desired properties [5]. These steels are prone to precipitation of second phases in certain temperature ranges, or even during manufacturing and welding. Precipitation of Sigma (σ) and Chi (χ) phases has been related to detrimental effects on mechanical properties and corrosion resistance [6]. However, secondary phases as nitrides, carbides, R and ϵ can precipitate as well. Isothermal treatment can also result in recrystallization that gives rise to their crystallographic textures in each of the two phases. Submerged Arc Welding (SAW) is commonly used for joining DSS, previous studies evidence that DSS 2205 submerged arc weldings with proper
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