LOW-TEMPERATURE GAS-PHASE NITRIDING AND NITROCARBURIZING OF 316L AUSTENITIC STAINLESS STEEL by DANDAN WU Submitted in partial fulfillment of the requirements For the degree of Doctor of Philosophy Dissertation Advisor: Dr. Arthur H. Heuer Department of Materials Science and Engineering CASE WESTERN RESERVE UNIVERSITY January, 2013 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the thesis/dissertation of Dandan Wu ______________________________________________________ Doctor of Philosophy candidate for the ________________________________degree *. Arthur H. Heuer (signed)_______________________________________________ (chair of the committee) James D. McGuffin-Cawley ________________________________________________ Frank Ernst ________________________________________________ John J. Lewandowski ________________________________________________ Farrel J. Martin ________________________________________________ 8/31/2012 (date) _______________________ *We also certify that written approval has been obtained for any proprietary material contained therein. Dedicated to my mother, Yuqin Wang, and my husband, Hongyi Yuan Table of Contents List of Tables .................................................................................................................... iv List of Figures ................................................................................................................... vi Acknowledgements ........................................................................................................ xiv Abstract ........................................................................................................................... xvi Chapter 1 Introduction ................................................................................................... 1 1.1 Background of expanded austenite .................................................................................... 1 1.2 Crystal structure of expanded austenite ............................................................................ 3 1.3 Diffusion of nitrogen in expanded austenite ...................................................................... 7 1.4 Orientation-dependent case depth in low-temperature nitriding ................................. 11 1.5 Effect of stress on solubility and diffusivity of interstitial atoms .................................. 12 1.6 Room-temperature ferromagnetism induced by low-temperature nitriding ............... 15 1.7 Dilation of austenite lattice from nitrogen and carbon interstitials .............................. 19 1.8 Precipitate formation and stability of expanded austenite ............................................ 24 1.9 Nitrocarburizing processes ............................................................................................... 27 References ................................................................................................................................ 31 Chapter 2 Experimental Methods ............................................................................... 36 2.1 Design of low-temperature gas-phase nitriding process ................................................ 36 2.2 Design of low-temperature gas-phase nitrocarburizing process ................................... 40 2.3 Characterization techniques ............................................................................................. 43 2.3.1 X-Ray Diffraction ........................................................................................................ 43 2.3.2 Auger Electron Spectroscopy ....................................................................................... 46 2.3.3 Scanning Electron Microscopy .................................................................................... 49 i 2.3.4 Electron Backscatter Diffraction .................................................................................. 50 2.3.5 Magnetic Force Microscopy ......................................................................................... 51 2.3.6 Microhardness tester .................................................................................................... 51 2.3.7 Transmission Electron Microscopy .............................................................................. 51 References ................................................................................................................................ 53 Chapter 3 Low-Temperature Gas-Phase Nitriding ................................................... 54 3.1 Effect of processing parameters ....................................................................................... 54 3.1.1 Effect of nitriding temperature ..................................................................................... 54 3.1.2 Effect of nitriding activity ............................................................................................ 63 3.1.3 Effect of nitriding duration ........................................................................................... 70 3.2 Ferromagnetism induced by low-temperature nitriding ............................................... 75 3.3 Orientation-dependent case depth ................................................................................... 82 3.4 Hardness and modulus measurements ............................................................................ 86 9 3.5 TEM results on nitrided 316L (aN2 = 4×10 ) .................................................................... 91 References .............................................................................................................................. 107 Chapter 4 Low-Temperature Gas-Phase Nitrocarburizing .................................... 108 4.1 Three scenarios of nitrocarburizing with NH3/CO/H2/N2 ............................................ 108 4.1.1 XRD analysis ............................................................................................................. 108 4.1.2 Metallography ............................................................................................................ 112 4.1.3 AES depth profiles ..................................................................................................... 115 4.1.4 Hardening effect ......................................................................................................... 118 4.1.5 Study of second phases after 20C+20N by TEM ....................................................... 119 4.2 Effect of nitriding activity on 20C+20N ......................................................................... 128 4.2.1 XRD analysis ............................................................................................................. 128 4.2.2 Metallography ............................................................................................................ 131 ii 4.2.3 AES depth profiles ..................................................................................................... 132 4.2.4 Hardness measurements ............................................................................................. 133 4.3 Effect of nitriding activity on 20 (N+C) ......................................................................... 134 4.3.1 XRD results ................................................................................................................ 135 4.3.2 Metallography ............................................................................................................ 137 4.3.3 AES depth profiles ..................................................................................................... 139 4.3.4 Hardening effect ......................................................................................................... 141 References .............................................................................................................................. 142 Chapter 5 Discussion................................................................................................... 143 5.1 Effect of temperature ...................................................................................................... 143 5.2 Effect of nitriding activity ............................................................................................... 146 5.3 Lattice parameter expansion induced by nitrogen ....................................................... 148 5.4 Anisotropy in lattice parameter ..................................................................................... 153 5.5 On the nitrogen content measured by AES ................................................................... 159 5.6 XRD peak splitting during stress measurements .......................................................... 161 5.7 Orientation-dependent case depth after low-temperature nitriding .......................... 164 References .............................................................................................................................. 166 Chapter 6 Conclusions ................................................................................................ 168 Appendices ..................................................................................................................... 171 Appendix I Nitro-carburizing by Urea (CO(NH2)2) ........................................................... 171 Appendix II Nitro-carburizing in NH3/C2H2/H2/N2 ............................................................ 178 iii List of Tables Table