Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung) Stuttgart Gas Nitriding of Iron-Based Alloys Benjamin Schwarz Dissertation an der Universität Stuttgart Bericht Nr. 248 Juli 2014 G N O I B A AS ITRIDING F RON- ASED LLOYS Von der Fakultät Chemie der Universität Stuttgart genehmigte Abhandlung zur Erlangung der Würde eines Doktors der Naturwissenschaften (Dr. rer. nat.) Vorgelegt von B S ENJAMIN CHWARZ aus Waiblingen Hauptberichter: Prof. Dr. Ir. E. J. Mittemeijer Mitberichter: Prof. Dr. J. Bill Prüfungsvorsitzender: Prof. Dr. T. Schleid Tag der Einreichung: 23. Mai 2014 Tag der mündlichen Prüfung: 30. Juli 2014 MAX-PLANCK-INSTITUT FÜR INTELLIGENTE SYSTEME (EHEMALS MAX-PLANCK-INSTITUT FÜR METALLFORSCHUNG) INSTITUT FÜR MATERIALWISSENSCHAFT DER UNIVERSITÄT STUTTGART 2014 Indes sie forschten, röntgten, filmten, funkten, entstand von selbst die köstlichste Erfindung: der Umweg als die kürzeste Verbindung zwischen zwei Punkten. Erich Kästner Contents Chapter 1: Introduction ............................................................................................................... 13 1.1. General introduction ............................................................................................................ 13 1.2. The gas nitriding facility ...................................................................................................... 15 1.3. Thermodynamics of gas nitriding ........................................................................................ 16 1.4. Microstructural evolution upon nitriding ............................................................................ 18 1.4.1. The compound layer ..................................................................................................... 21 1.4.2. The diffusion zone ........................................................................................................ 23 1.5. Outlook of the thesis ............................................................................................................ 30 Chapter 2: The Process of Tungsten-Nitride Precipitation upon Nitriding Ferritic Fe-0.5 at.% W Alloy ..................................................................................................................... 33 2.1. Introduction ......................................................................................................................... 34 2.2. Experimental procedures ..................................................................................................... 35 2.2.1. Specimen preparation and nitriding/denitriding ........................................................... 35 2.2.2. Specimen characterization ............................................................................................ 36 2.3. Results and discussion ......................................................................................................... 37 2.3.1. Effect of surface preparation; the development of surface nitrides .............................. 37 2.3.2. Development of nitrides in the bulk .............................................................................. 39 2.4. Conclusions ......................................................................................................................... 43 Chapter 3: Pore Formation upon Nitriding Iron and Iron-based Alloys; The Role Alloying Elements and Grain Boundaries ................................................................................................. 45 3.1. Introduction ......................................................................................................................... 46 3.2. Thermodynamics of gas nitriding and pore formation ........................................................ 47 3.3. Experimental procedures ..................................................................................................... 53 3.3.1. Specimen preparation .................................................................................................... 53 3.3.2. Nitriding ........................................................................................................................ 55 3.3.3. Weight measurement..................................................................................................... 55 3.3.4. X-ray diffraction ........................................................................................................... 55 3.3.5. Light microscopy .......................................................................................................... 56 3.3.6. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) ... 56 3.3.7. Electron probe microanalysis (EPMA) ......................................................................... 57 3.4. Results ................................................................................................................................. 57 3.4.1. Pore formation in nitrogen ferrite and the role of alloying elements ............................ 57 3.4.2 Pore formation in nitrogen austenite and the role of alloying elements ........................ 61 3.5. Discussion ............................................................................................................................ 66 3.5.1. Pore formation in nitrogen ferrite and the role of alloying elements ............................ 66 3.5.2. Pore formation in nitrogen austenite and the role of alloying elements ....................... 72 3.6. Conclusions ......................................................................................................................... 74 Chapter 4: Nitriding of Iron-based Ternary Fe-V-Si Alloy; The Precipitation Process of Separate Nitrides .......................................................................................................................... 77 4.1. Introduction ......................................................................................................................... 78 4.2. Experimental procedures ..................................................................................................... 79 4.2.1. Specimen preparation .................................................................................................... 79 4.2.2. Nitriding ........................................................................................................................ 80 4.2.3. Weighing ....................................................................................................................... 81 4.2.4. X-ray diffraction ........................................................................................................... 81 4.2.5. Microhardness measurement ........................................................................................ 82 4.2.6. Transmission electron microscopy ............................................................................... 82 4.3. Results and evaluation ......................................................................................................... 83 4.3.1. Nitrogen uptake and precipitate development .............................................................. 83 4.3.2. Role of the initially precipitated VN on the kinetics of the later precipitating Si N ... 91 3 4 4.4. General discussion ............................................................................................................... 94 4.4.1. Separate and successive precipitation of VN and Si N ............................................... 94 3 4 4.4.2. Kinetics of Si N precipitation ...................................................................................... 98 3 4 4.5. Conclusions ....................................................................................................................... 100