Influence of Sintering under Nitrogen Atmosphere on Microstructures of Powder Metallurgy Duplex Stainless Steels
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WROUGHT and cast duplex ferritic-austenitic stainless steels (SS) are characterized by an outstanding combination of strength, toughness, and corrosion resistance in many environments and operating conditions. They show good weldability and are more economical than comparable austenitic grades. Therefore, they are successfully used in chemical, petrochemical, nuclear, fertilizer, and food industries.[1,2]. However, conventional manufacturing processes of duplex SS are complex, while the powder metallurgy (PM) route is almost trouble free, consequently becoming attractive.[3] Powder metallurgy enables productions of duplex SS by several methods: (1) using fully prealloyed powders with a required composition; (2) mixing a ferritic, austenitic, or martensitic powder with elemental powders, causing its destabilization and producing biphasic microstructures; or (3) mixing austenitic and ferritic powders in proper ratio to ensure required duplex microstructure.[4–12] The use of elemental powders needs precise control of the sintering steps. The final composition and structure are obtained by interdiffusion of the alloying elements during sintering. The third alternative shows certain advantage because of the feasibility of making duplex with the desired ferritic/austenitic ratio C. GARCI´A and M.P. DE TIEDRA, Profesora Titular de Escuela Universitaria, Y. BLANCO, Profesora Ayudante, F. MARTIN, Catedra´tico de Escuela Universitaria, and M.L. APARICIO, Catedra´tico de Universidad, are with Materials Engineering, E.T.S.I.I., Universidad de Valladolid, 47011 Valladolid, Spain. Contact e-mail: [email protected] Manuscript submitted on May 8, 2008. Article published online January 7, 2009 292—VOLUME 40A, FEBRUARY 2009
and good levels of corrosion resistance.[4] This last approach has been chosen for this work. Powder metallurgy SS applications are restricted due to the relatively poor mechanical and corrosion properties when compared to wrought SS. Therefore, there is always a thrust to improve their corrosion and mechanical properties. The limited corrosion resistance is related to the inherent residual open porosity and the problems that arise when sintering in industrial atmospheres.[6,7] Open porosity increases surface area of the material exposed to corrosive environments. Furthermore, crevice corrosion can take place due to electrolytic stagnation in the interconnected pores and formation of concentration cells.[13] Consequently, the passivity of the sintered alloy reduces. In addition, chromium-depletion processes due to precipitation of secondary phases act synergistically with morphological factors to reduce corrosion resistance.[6,7,10] Duplex SS sintered in atmospheres, such as vacuum or hydrogen, have been the goal of much research, though a few have dealt with sintering in nitrogen.[7–9,11,12] Non-PM duplex SS have been alloyed, especially with nitrogen, to optimize mechanical, corrosion, and wear properties, as well as weldability.[14–17] Consequently, sintering in nitrogen is potentially valuable due to the fac
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