Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.758369
Title: Investigations on the role of Cr, Mn and Ni on the formation, structure, and metastability of nitrogen-expanded austenite on Fe-based austenitic alloys under triode-plasma nitriding
Author: Tao, Xiao
ISNI:       0000 0004 7431 1426
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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Abstract:
Thermochemical diffusion treatments (such as nitriding and carburising) are widely applied to metallic components to improve material surface hardness and wear/tribological performance. AISI 304 (Fe-18Cr-8Ni, in wt.%) and AISI 316 (Fe-18Cr-11Ni-3Mo, in wt.%) type ASS, as two of the most commonly used ASS, have been investigated extensively under low-temperature thermochemical diffusion treatments (e.g. < ~450°C for nitriding and < ~550°C for carburising). The unusual crystallographic structures - i.e. anisotropicallyexpanded austenite (or so-called S phase) - observed on ASS (or other alloys) under lowtemperature thermochemical diffusion treatments and the remarkable enhancement of surface mechanical/tribological properties that these structures provided (without loss of corrosion resistance) led to a drive to try to understand their origin. The principal aim of this study was to contribute to a roadmap for design of 'nitridable' austenitic alloys (capable of being nitrogen interstitially-supersaturated under lowtemperature nitriding). The objectives were i) to study the nitrogen-expanded structure and decomposition of ASS with either high-Ni or high-Mn content (other than AISI 304 or 316 type ASS) after nitriding, ii) to access the performance of the nitrided ASS, and iii) to explore the possibility of nitrogen-supersaturation on Cr-free high-Ni or high-Mn austenitic steels under low-temperature nitriding. AISI 304 and four other selected steels (with different Ni/Mn level and with/without Cr) were triode-plasma nitrided under low (and close to monoenergetic) ion energy of ~200 eV at different temperatures for 4-20hrs. Auxiliary radiant heating was used to facilitate different treatment temperatures under a deliberately controlled substrate current density (e.g. at ~0.13 A/cm2 ), in which case material surface crystallographic structure obtained under nitriding was mainly influenced by the different treatment temperatures and times applied. Firstly, the role of SFE on the crystallographic structure and stability of γN has been revealed and discussed with the vastly available literature. At low treatment temperature (400°C), low SFE ASS (with high-Mn content) tended to exhibit superior nitrogen absorption and relative high lattice expansion, while high SFE ASS (with high-Ni content) tended to absorb less nitrogen - reaching a saturated lattice expansion quite quickly during treatment. Dislocation dissociation and dislocation glide were proposed and discussed for the different plastic deformation structures observed. Nevertheless, at elevated treatment temperatures (425 and 450°C), lattice instabilities (SFs and SF-associated local lattice distortions) developed on the low SFE alloy under nitrogen 'over-supersaturation', tending to accelerate the decomposition of γN, while low SF density (or SF-free) γN developed on high SFE ASS, tending to exhibit relatively sluggish decomposition. Secondly, the wear and corrosion performance of the nitrided high-Mn/Ni ASS were investigated. The surface hardened ASS after lowtemperature TPN exhibited superior wear performance under dry sliding condition owing to enhanced load-bearing capacity and reduced adhesive wear, but not under selected wet SiC slurry abrasive wear conditions potentially owing to layer brittleness and insufficient layer thickness under the selected nitriding parameters. Improved corrosion performance was observed for ASS under nitrogen-supersaturation and loss of corrosion resistance was evident after CrN formation in γN. Last but not the lease, the necessity of Cr in substrate alloy on the formation of γN after nitriding has been demonstrated. Formation of iron nitrides (rather than γN) was evident on both of the Cr-free austenitic steels (Invar and Hadfield steel) under TPN at selected low treatment temperatures, which indicates that nitride-forming elements (such as Cr) played an important role in the formation of expanded austenite.
Supervisor: Leyland, Adrian ; Matthews, Allan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.758369  DOI: Not available
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