Gas Nitriding of Iron-Based Alloys Benjamin Schwarz

160 Pages·2014·5.04 MB·English

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