Influence of heat input and solubilizing heat treatment on the impact properties of SAW joints in SAF 2205 duplex steel
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Influence of heat input and solubilizing heat treatment on the impact properties of SAW joints in SAF 2205 duplex steel M. Merlin*, R. Vazquez-Aguilar*, C. Soffritti*, A. Reyes-Valdes** * Engineering Department, University of Ferrara, via Saragat 1, 44122 Ferrara **Corporaciòn Mexicana de Investigaciòn en Materiales, Blvd. Oceanìa #190, Fracc. Saltillo 400, 25290 Saltillo, Coah. Mexico ABSTRACT In this study the influence of heat input (HI) and heat treatment on submerged arc welded duplex SAF 2205 steel joints has been evaluated. In particular, multi-pass welding operations have been performed on 18 mm thick plates using four different heat inputs; a post-weld solubilizing heat treatment has been carried out in order to reduce the microstructural effects on the structure of the heat affected zone (HAZ). Instrumented impact strength tests have been performed on Charpy samples machined from the welded joints; the total absorbed energy and the two complementary contributions of initiation and propagation energies have been evaluated and correlated to the percentages of ferrite and austenite. The microstructures and the fracture profiles have been observed using an optical microscope (OM) and quantitatively analyzed by means of an image analyzer. A scanning electron microscope (SEM) equipped by energy dispersive X-ray spectroscopy (EDS) has been used to study the fractured surfaces. Hardness profiles have been performed across the joints in order to verify the hardness variations. A total absence of secondary phases has been found on the joints due to the performing of a suitable solubilizing heat treatment after the welding process. The results have shown that the impact properties of the samples have been mostly affected by the different heat inputs; in some cases a partial welding penetration has been found. Keywords: Impact strength test, welding, microstructure
INTRODUCTION Duplex stainless steels (DSS) are characterised by approximately equal proportions of ferrite and austenite phases and they exhibit high resistance to stress corrosion cracking and high mechanical properties [1]. The most widely used grade is DSS SAF 2205 (UNS S31803), a grade accounting for about 80% of the duplex market [1,2]. The microstructure resulting from the slow cooling rate reveals a greater austenite and lower ferrite content, while that resulting from a fast cooling rate exhibits a greater amount of ferrite and lower austenite content [3]. The structural changes that occur in the heat-affected zone (HAZ) of stainless steel welds can potentially degrade strength, toughness, and corrosion resistance [1-5]. The potential problems associated with stainless steel HAZs include excessive grain growth, chromium carbide, intermetallic and compound precipitation, and improper phase balance. To alleviate these problems, careful control of the weld metal composition and temperature are often required during welding. In most cases, a post weld heat treatment is required to restore toughness and to achieve matching mechanical properties in the w
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