High Temperature Oxidation of Slurry Aluminized Deformable Austempered Ductile Iron (DADI)
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I.
INTRODUCTION
THE low-density deformable austempered ductile iron (DADI) has been designed to exhibit outstanding mechanical properties including high strength and toughness and good wear resistance, possibility to control its hardness values and easy machinability.[1–3] Therefore, DADI has potential applications for different engineering fields like in the automotive sector as a worthy substitution to the more costly steels.[4,5] However, its behaviour at high temperatures has not been studied yet because the formation of unprotective iron oxides is anticipated in relation to its chemical composition (Fe, 3.6C, 2Al, 0.8Si, 0.4Mn, 0.04Mg, wt pct). Further, the graphite inclusions derived from the very high C content (up to 4 wt pct) can be burnt by oxidation and provide easy paths for oxidation towards the deeper layers of the matrix. However, except for the recent works of Wang et al. on the pack aluminizing cast iron,[6] such phenomena have been rarely reported in the open literature, which is part of this investigation. Kiani Rashid and Edmonds demonstrated that the addition of Al to the bulk ductile cast alloy could increase the oxidation resistance while improving the resistance to decarburization at 550 C.[7] However, OLGA TSURTSUMIA, NUGZAR KHIDASHELI, and ELGUJA KUTELIA are with the Republic Center for Structure Research, Georgian Technical University, 77 Kostava Street, 0175, Tbilisi, Georgia. FERNANDO PEDRAZA and BENJAMIN GREGOIRE are with the Laboratoire des Sciences de l’Inge´nieur pour l’Environnement (LaSIE, UMR-CNRS 7356), Universite´ de La Rochelle, Avenue Michel Cre´peau, 17042 La Rochelle Cedex 1, France. Contact e-mail: [email protected] Manuscript submitted on 17 June 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
increasing the amount of Al makes casting of the alloy quite complex. Therefore, it derives that the application of a protective Al-containing coating could improve the oxidation behaviour of DADI. Among the different coating alternatives, slurry aluminizing has been proven as an alternative to other aluminizing techniques like pack cementation. The mechanisms of formation of these coatings have been thoroughly described on different metal substrates like pure Ni,[8] different steels[9,10] and Ni-based alloys.[11] Depending on the amount of slurry deposited and on the annealing treatment, FeAl-based intermetallics can be obtained,[12] which are reported to resist oxidation in a wide range of temperatures.[13] Therefore, this work reports on the aluminization of DADI steel and on its subsequent oxidation behaviour at high temperatures.
II.
EXPERIMENTAL PROCEDURE
Rectangular-shaped coupons of DADI with approximate dimensions 15 9 15 9 5 mm were cut from the casted ingots of the chemical composition of Fe, 3.6C, 2Al, 0.8Si, 0.4Mn and 0.04 Mg (wt pct). Two types of DADI steels were studied: the as-casted and the thermally treated one. For the latter, the samples went through austenization at up to 900 C in air for 20 minutes and subsequently quenched in water. They were then temp
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