Abstract
Super duplex stainless steels (SDSSs), exhibiting excellent strength and corrosion resistance, serve as the attractive materials in a variety of industries. However, improvements in their ductility and impact-toughness are required in extreme environments. In this study, the effects of gadolinium on the microstructures and Charpy impact properties of super duplex stainless steels were investigated. A base super duplex stainless steel (BDSS) and a gadolinium-added super duplex stainless steel (GDSS) were successfully fabricated using an air casting method. The oxygen content and grain size of SDSSs were found to decrease because of high reactivity of gadolinium with oxygen. Moreover, the average inclusion size and area of GDSS also decreased even with a slight decrease in the average distance between inclusions. Both the BDSS and GDSS exhibited typical Charpy impact transition behavior from −196 °C to 200 °C. Moreover, the GDSS impact energies using Charpy test were higher than those of BDSS over the entire temperature range. Moreover, the ductile-to-brittle transition temperature (DBTT) of SDSSs calculated from the fracture appearance transition temperature (FATT) significantly decreased by over 20 °C with the addition of gadolinium.
Highlights
Rare-earth metals (REMs) have been widely used in various materials to improve their performance or change their functionality [1]
Upon addition of Gd, the oxygen content of SDSSs decreased from 692 ppmw to 568 ppmw, which was about 20% lower
We demonstrated that Gd effectively reduced the oxygen content in SDSSs by reacting with oxygen during casting
Summary
Rare-earth metals (REMs) have been widely used in various materials to improve their performance or change their functionality [1]. REM (such as, ceria and lanthana)-based compounds are receiving increasing attention due to their automotive exhaust catalyst applications, high oxygen storage capacity and larger ionic size [1–3], which result in a significant enhancement of the catalyst performances. REMs can be used as additives in cast metals to improve their microstructures and mechanical properties. When they react with harmful non-metallic inclusion-forming elements, such as nitrogen, oxygen, and sulfur, they can modify MnS or MnO, resulting in the formation of stable REM compounds, such as REMOx Sy [8–11]. REMs can reduce the grain size of cast metals, Metals 2018, 8, 474; doi:10.3390/met8070474 www.mdpi.com/journal/metals
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