Prevention of High-Temperature Surface Degradation in SiMo Cast Irons by Cr and Al Alloying
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INTRODUCTION
HIGH alloyed by silicon and molybdenum cast irons (SiMo) with spherical graphite are widely used for high-temperature engine components, including exhaust manifolds and turbocharger housings.[1] The ASTM A1095-15 specification for ferritic SiMo cast irons includes castings with microstructures of spheroidal graphite (SG), compacted graphite (CG), and mixed graphite or medium-nodularity graphite (MG) cast alloys.[2] There are many other grades of SiMo ductile irons used in commercial applications. SAE J2582 standard ‘‘Automotive ductile iron castings for high-temperature applications’’ designated three basic grades of SiMo cast iron with spherical graphite with different amounts of the significant elements, which included 3.5 to 4.5 pct Si and 0.5 to 1.0 pct Mo.[3] The most commonly used in the automotive industry SiMo SIMON N. LEKAKH, ASEBI BOFAH and WEI-TING CHEN are with the Missouri University of Science and Technology, Rolla, MO 65409. Contact e-mail: [email protected]. LARRY GODLEWSKI and MEI LI are with the Research and Innovation Center, Ford Motor Company, Dearborn, MI 48124. Manuscript submitted July 8, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
cast iron with spherical graphite is designated as SiMo51 and contains 3.1 pct C, 4.1 pct Si, 0.4 pct Mn, 0.8 pct Mo, and 0.05 pct Mg.[4] The high Si content increases oxidation resistance of SiMo cast iron through the formation of a silicon-rich oxide surface layer, which prevents oxidation of the ferritic matrix at elevated temperatures.[5–7] Alloying by Si also elevates the temperature at which ferrite transforms to austenite, thereby raising the useful temperature range for ferritic SiMo cast irons, avoiding stress related to volume change during the eutectoid transformation.[5] Increasing Si from 4 pct to up to 5 pct has been shown to improve resistance to thermal fatigue.[8] The addition of Mo is known to increase the high-temperature tensile strength and resistance to creep rupture due to the formation of a network of complex Fe-Mo carbides at grain boundary.[9] During the operation of exhaust manifolds, SiMo cast irons are exposed to high temperatures at different atmospheres which include exhaust gases contained water vapor and carbon dioxide. It has been reported that SiMo cast iron exhibits poor resistance to oxidation in the atmosphere with 10 pct water vapor at a temperature range of 650 C to 800 C.[10] A study[11] showed that the formation of the protective SiO2 layer was slower in exhaust gases.
Several experimental studies have been done in an attempt to improve high-temperature performance of SiMo cast iron by applying alloying with elements well known to advance oxidation resistance of steel.[11–13] For example, Cr is a well-known alloying element proven to impart oxidation resistance of austenitic and ferritic steels.[14] However, strong carbide forming ability of Cr in cast iron limits a level of Cr alloying because the need to avoid the formation of undesirable fully carbidic microstructure. Therefore, only relatively small qu
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