Properties and High Temperature Dry Sliding Wear Behavior of Boronized Inconel 718
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RODUCTION
NI-BASED superalloys are indispensable materials for high-temperature and oxidizing environments because they can retain their mechanical properties and corrosion resistance at elevated temperatures.[1] Inconel 718 is a widely-used precipitation strengthening Ni-based superalloy with excellent tensile, fatigue, and creep strength. It is also known for its good weldability and outstanding resistance to post-weld cracking.[1,2] The combination of these properties have made Inconel 718 suitable for use in a number of applications including nuclear reactors, liquid-fueled rockets, pumps and blisks, casings and various formed sheet metal parts for aircraft and rings, turbine blades, land-based gas turbine engines, power generation units, extrusion dies, and containers and cryogenic storage, with service temperatures ranging from 250 C to 700 C.[2–5] Despite its unique combination of properties and widespread use, the wear characteristics of Inconel 718 (and many other superalloys) are often considered unsatisfactory,[6–8] especially at elevated temperatures.
ALI_ GU¨NEN is with the Iskenderun Technical University, Faculty of Engineering and Natural Sciences, Metallurgy and Materials Engineering Department, 31200 Hatay, Turkey. Contact e-mail: [email protected] Manuscript submitted April 19, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
When used in aggressive wear environments, Inconel 718 must be coated to improve its lifetime.[9] Thermochemical coatings are a good option for achieving this requirement, many of which are capable of maintaining surface stability and mechanical properties up to 1000 C.[10,11] Boronizing is a thermochemical process in which boron atoms with small atomic radius diffuse from the surface into the interior of the material at high temperatures to form hard boride layers. Boronizing is somewhat unique compared to other thermochemical processes as it generally provides harder coatings on steel and superalloys in comparison to other thermochemical coating processes such as nitriding, titanizing, chromizing etc.[8,12–14] It should be noted, however, that the boronizing of pure Ni and Ni-based alloys requires the use of Si-free boronizing powders. Otherwise, the formation of silicides or borosilicides in boride layers is inevitable. This is particularly undesirable for wear applications, due to the inherent porosity and low hardness of the resulting borosilicide layers.[7,9,15] It has been stated that the topmost silicide layer that forms with the use of Si-containing boronizing media has a hardness of only ~ 500 HV, and that it must be removed from the surface to eliminate its detrimental effects on wear resistance.[7,16,17] However, in recent years it has been found that silicide-free boride layers can be grown on Ni-based superalloys[18–20] using proprietary/
specialized SiC-free boronizing media such as Ekabor-Ni or by substituting the SiC filler with B4C. When considering boronized Ni-based superalloys, there are only few studies focused on the friction and wear behavior
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