Development of Duplex Stainless Steels by Field-Assisted Hot Pressing: Influence of the Particle Size and Morphology of
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TRODUCTION
THE importance of powder metallurgy (PM) duplex stainless steel (DSS) components is continuously growing in the market, particularly in the automotive industry, since these materials exhibit an outstanding combination of strength, toughness, corrosion resistance, and cost, compared to austenitic stainless steels.[1] The first DSSs were commercially introduced in 1970.[2] DSS are usually characterized by a combination of austenitic and ferritic stainless steel grades in almost equal amounts of ferrite and austenite, in which the ferritic phase is the main phase responsible for the strength and the austenitic phase confers the toughness and corrosion resistance.[1] Conventional routes to produce DSS require a tight control of the chemical composition and temperature to obtain the desired duplex microstructure.[3] Heat treatments may produce undesirable secondary brittle phases, such as sigma phase, chromium carbides, and other intermetallic compounds.[4–6] In this sense, PM technology is an attractive alternative to avoid the presence of these secondary phases, providing an economic manufacturing process of small components with complex shapes and
A. GARCI´A-JUNCEDA is with the IMDEA Materials Institute, Calle Eric Kandel 2, 28906 Getafe, Madrid, Spain. Contact e-mail: [email protected] M. RINCO´N and J.M. TORRALBA are with the Universidad Carlos III Madrid, Av. Universidad 30, 28911 Legane´s, Madrid, Spain. Manuscript submitted April 24, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
good dimensional tolerances. Moreover, PM enables the use of different routes to sinter duplex stainless steels, including the use of prealloyed powders with a required composition,[3,7] master alloys,[7] or the mixture of austenitic and ferritic stainless steel powders available on the market.[8–10] In the latter case, the duplex microstructure is achieved due to the interdiffusion of the principal alloying elements between both phases during the sintering of the steel. The main advantage of this option is the possibility of sintering DSS with the desired austenite/ferrite ratio and good levels of corrosion resistance,[11] and for this reason this is the option chosen in this work. In previous investigations, the presence of a hard constituent between the ferritic and the austenitic phase was reported for DSS obtained by PM.[7,12–14] Thus, Campos et al. sintered different DSS at 1523 K (1250 C) for 30 minutes using mixtures with different proportions of austenitic and ferritic atomized powders, reporting that a Ni concentration gradient was developed from austenite to ferrite particles, generating an acicular interface with a thickness of around 5 to 10 lm that increased the tensile behavior with respect to ultimate tensile strength (UTS) and ductility, despite a small reduction of the yield strength.[9,12] Garcı´ a et al. sintered mixtures of austenitic and ferritic stainless steels at 1523 K (1250 C) for 60 minutes under nitrogen and vacuum atmospheres and also observed by optical microscopy the existence of
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