Microstructural Characterisation of Pearlitic and Complex Phase Steels Using Image Analysis Methods by Xi Liu A thesis submitted to The University of Birmingham For the degree of DOCTOR OF PHILOSOPHY Metallurgy and Materials Science School of Engineering The University of Birmingham January 2014 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Acknowledgements A big thank you must go to my supervisors Dr R. M. Ward and Dr M. Strangwood, for all their guidance, encouragement, kind support and help throughout this project. I would like to thank Professor P. Bowen for the provision of research facilities within the School of Metallurgy and Materials. Thanks are also given to all the staff and students that helped me during my time in the department. Thanks to all the colleagues in the research group and my friends at the university. You make me really enjoy my 4 year time in Birmingham. Finally, I want to thank my parents for their patience and support in every situation. Abstract The properties of materials are a primary factor to determine their applications, which makes the measurement of properties very important for both material design and quality control. As materials’ properties are determined by the microstructure of the materials such as grain size or the volume fraction of the present phases, microstructural characterisation is a powerful tool for property prediction. Unfortunately, microstructural characterisation has not been widely applied with all steels such as pearlitic steels or complex multi-phase steels due to their complex microstructures. These microstructures may contain features that cannot be resolved by optical microscopy, and in which important information is contained in their texture rather than simply their grey level. These microstructures were investigated in this study using image texture analysis. Fourier transform-based analysis was applied to pearlitic microstructures to extract the image orientation information. The orientation information as well as the grey value of low pass filtered image was used as predicates in a split-merge algorithm to segment the pearlitic colonies. A supervised classification method based on various statistical measures including a number of 2-point statistics (Grey Level Co-occurrence Matrix measures) was developed to distinguish the bainite (upper bainite and lower bainite), martensite and ferrite phases in steels. The influence of etching on the analysis results was also investigated. The pearlite colony segmentation was fairly accurate on a synthetic image that contains idealised pearlitic structures, but it was also found that there are fundamental difficulties with estimating colony boundaries from a real single 2-D pearlitic image. The multi-phase analysis was found to be fairly accurate for overall phase fraction under the conditions investigated here. The investigation of etching effect on image analysis show that the method used in this study is not very sensitive to the etching degree. Table of Contents Chapter 1. General Introduction .................................................................................... 1 Chapter 2. Literature Review on Steel Metallurgy, Microstructure and Properties ....... 6 2.1 Brief Background of Steel ................................................................................... 6 2.2 Strengthening Mechanisms .................................................................................. 7 2.2.1 Work Hardening ............................................................................................ 7 2.2.2 Grain Refinement .......................................................................................... 7 2.2.3 Solid Solution Strengthening ........................................................................ 9 2.2.4 Precipitation Hardening and Dispersion Hardening ..................................... 9 2.2.5 Transformation Hardening .......................................................................... 10 2.2.6 Effects of Carbon ........................................................................................ 12 2.3 Phases in Steel.................................................................................................... 13 2.3.1 Pearlite ........................................................................................................ 13 2.3.2 Bainite ......................................................................................................... 18 2.3.3 Martensite ................................................................................................... 21 2.4 Effects of Mixed Microstructures on Properties ................................................ 26 2.5 Quantitative Characterisation of Microstructure ............................................... 29 Chapter 3. Literature Review on Image Analysis ........................................................ 31 3.1 Background of Digital Image Analysis .............................................................. 31 3.1.1 Microscopy and Image Acquisition ............................................................ 31 3.1.2 Types of Digital Image Analysis ................................................................. 32 3.2 Image Shade Correction ..................................................................................... 33 3.2.1 Shading Problem in Image Acquisition ...................................................... 33 3.2.2 Shading Correction Method ........................................................................ 35 3.3 Conventional Intensity Based Image Analysis .................................................. 35 3.4 Image Texture Analysis ...................................................................................... 37 3.4.1 Approaches of Estimating the Local Orientation of Images ....................... 40 3.4.2 Fourier Transform (FT) ............................................................................... 41 3.4.3 Grey Level Co-occurrence Matrix (GLCM) – Based Methods .................. 52 3.4.4 Texture Classification ................................................................................. 66 3.5 Image Segmentation Techniques ....................................................................... 68 3.5.1 Types of Segmentation Techniques ............................................................. 68 3.5.2 Measures for Image Segmentation Evaluation ........................................... 75 Chapter 4. Literature Review on the Applications of Image Analysis to Steel and Metallurgy .................................................................................................................... 77 4.1 Non-image-based Microstructural Characterisation Methods ........................... 77 4.2 Image Analysis Applications in Steel and Other Metallurgical Study ............... 80 4.3 Aims and objectives ........................................................................................... 88 Chapter 5. Experimental Methods ............................................................................... 90 5.1 Sample Preparations........................................................................................... 90 5.1.1 Cutting......................................................................................................... 90 5.1.2 Mounting ..................................................................................................... 90 5.1.3 Grinding, Polishing and Etching ................................................................. 90 5.2 Characterisation Methods .................................................................................. 91 5.2.1 Optical Microscopy ..................................................................................... 91 5.2.2 Scanning Electron Microscopy (SEM) ....................................................... 91 5.3 Shading Correction ............................................................................................ 92 5.4 Pearlite Colony Characterisation ....................................................................... 93 5.5 Phase Classification for Complex Steel ............................................................. 94 5.6 Study of Influence from Etching Degree on Image Analysis ............................ 97 Chapter 6. Image Analysis Models Used in This Study .............................................