Title Page

Contents

CHAPTER 1. INTRODUCTION 22

1.1. Motivation of Study 22

1.1.1. The future of steel 22

1.1.2. The challenge 24

1.1.3. An alternative approach of high-strength steel alloys 26

1.2. Aim of study 29

CHAPTER 2. LITERATURE REVIEW 32

2.1. Common steel-grades 32

2.1. Strengthening methods in steel 34

2.1.1. Solid solution strengthening 35

2.1.2. Grain boundary strengthening 38

2.1.3. Work hardening 39

2.1.4. Precipitation strengthening 40

2.2. A brief discussion of lath martensite 46

2.3. Maraging steel 49

2.3.1. Alloying elements 51

2.3.2. Precipitation type in the existing maraging steel 52

2.4. Formation of nanoprecipitates 58

2.5. Reverted austenite phenomenon 62

CHAPTER 3. ALLOY DESIGN STRATEGY 65

3.1. Chemical composition design criteria 65

3.2. A theoretical approach to intermetallic compound formation 69

3.3. The alloy design process with CALPHAD assistance 72

CHAPTER 4. EXPERIMENTAL METHODS 79

4.1. Alloy compositions and fabrication 79

4.2. Thermodynamic calculations of composition candidates 81

4.2.1. Alloy 1 (Fe-Ni-Mo-Cr-Ti) 81

4.2.2. Alloy 2 (Fe-Ni-Mo-Mn-Ti) 82

4.2.3. Alloy 3 (Fe-Cr-Ni-Mo-Ti, Stainless steel) 83

4.2. Thermomechanical treatment 85

4.3. Mechanical properties 87

4.2.1. Hardness test 87

4.2.2. Tensile test 87

4.4. Phase identification and microstructure analysis 88

4.4.1. X-ray diffraction (XRD) 88

4.4.2. Scanning electron microscopy (SEM) 88

4.4.3. Transmission electron microscopy (TEM) 89

4.4.4. Atom probe tomography (APT) 90

CHAPTER 5. COMPREHENSIVE STUDY OF THE EXPERIMENTAL MARAGING STEELS 92

5.1. Introduction 92

5.2. Preliminary heat treatment 92

5.2.1. Homogenization and cold rolling treatment 93

5.2.2. Solution treatment (ReX) 94

5.2.3. Hardness results 96

5.3. Aging treatment 99

5.3.1. Hardness results 100

5.3.2. Tensile test results 104

5.3.3. Phase identification and microstructure 109

5.4. Modification of heat treatment 114

5.4.1. Alloy 1 (Fe-11.9Ni-5.1Mo-3.2Cr-1Ti) 118

5.4.2. Alloy 2 (Fe-12.7Ni-6Mo-2.9Mn-1Ti) 119

5.4.3. Alloy 3 (Fe-11.5Cr-10.5Ni-1.2Mo-1.5Ti) 121

5.5. Extended investigation of the effect of aging temperature 124

5.5.1. Microstructure of Alloy 1 125

5.5.2. Microstructure of Alloy 3 128

5.6. Summary 130

CHAPTER 6. EFFECT OF AGING TEMPERATURE on Fe-Ni-Mo-Mn-Ti MARAGING STEEL 133

6.1. Introduction 133

6.2. Results 135

6.2.1. Mechanical properties 135

6.2.2. Phase identification and microstructure 138

6.2.3. Characterization of precipitates 148

6.3. Discussion 163

6.3.1. Precipitates formation 163

6.3.2. Precipitation strengthening mechanism 165

6.3.3. Ductility behavior of A450 and embrittlement of A400 173

6.4. Summary 175

CHAPTER 7. CONCLUSIONS AND FUTURE WORK 178

7.1. Conclusions 178

7.2. Future work 180

APPENDIX 184

REFERENCES 200

ATTENDED CONFERENCES 236

LIST OF PUBLICATIONS 238

ABSTRACT 241

요약 244

Table 1. Summary of mechanical properties of the commercialized maraging steel. 50

Table 2. Summary of atomic information elements used for the alloy design. 70

Table 3. Chemical composition of the experimental maraging steel in the present study. 80

Table 4. Examples of specimen nomenclature. 86

Table 5. Detail modification on the heat treatment of all the experimental alloys at peak hardness. 116

Table 6. Summary of tensile properties of all experimental alloys with various heat treatment conditions. 122

Table 7. Summary of tensile properties of all specimens. 137

Table 8. Chemical composition of all precipitates obtained from APT and ThermoCalc in the A450 specimen. 158

Table 9. summary of the statistical analysis of all precipitates in the A450 specimen. 159

Table 10. Summary of microstructural features observed in the present study. 162

Table 11. Summary of strength contributions from η-Ni₃Ti and Mn-rich phase. 170

Figure 1. The class of steel showing IF (Interstitial free steels), MILD (medium strength steels), IF-HS (interstitial free high strength steels), BH... 32

Figure 2. Flow strength of martensitic steel as a function of the square root of carbon content. 37

Figure 3. Schematic of Orowan looping mechanism (a) bowing between particles (b) by-passing the particles by leaving a dislocation loop. 41

Figure 4. Microstructure of plate martensite in Fe-1.86 wt. % C alloy. 47

Figure 5. Lath martensite structure in 0.2 wt. % C steel alloy and its illustration of prior austenite grain, packet, and block boundaries. 48

Figure 6. Precipitate size evolution during aging treatment. 60

Figure 7. calculation of mixing enthalpy between elements by Miedema's model. 71

Figure 8. Design schematic to obtain a chemical composition for experimental candidates. 72

Figure 9. Phase equilibrium diagram of Fe-18.9Ni-4.1Mo-1.9Ti-0.22Co (left) and isopleth phase diagram as a function of Ni wt. %. 73

Figure 10. Isopleth phase diagram of Fe-18.9Ni-4.1Mo-1.9Ti-0.22Co as a function of wt. % Ti (left) wt. % Mo (right). 74

Figure 11. Phase equilibrium of Fe-12Ni-6Mo-1Ti. 76

Figure 12. Phase equilibrium diagram of Fe-11.9Ni-5.1Mo-3.2Cr-1Ti. 81

Figure 13. Phase equilibrium diagram of Fe-12.7Ni-6.1Mo-2.9Mn-1Ti. 82

Figure 14. Phase equilibrium diagram of Fe-11.5Cr-10.5Ni-1.2Mo-1.5Ti. 83

Figure 15. Schematic of the heat treatment for all alloy compositions. 85

Figure 16. Illustration of the tensile specimen size. 88

Figure 17. APT specimen fabrication with FIB techniques. 91

Figure 18. XRD results of Alloy 1, 2, and 3 after (a) homogenization and (b) cold rolling. 93

Figure 19. SEM images showing the microstructure of solution-treated specimen (a) Alloy 1, (b) Alloy 2, and (c) Alloy 3. (d) Corresponding XRD... 94

Figure 20. SEM image of Alloy 3 after solution treatment at 900 ℃ for 1 h. 96

Figure 21. Hardness of all alloys on every step of the preliminary heat treatment process. 96

Figure 22. Plot of the hardness of all experimental alloys at ReX (solution treatment) condition and hardenability based on Ref [113]. 98

Figure 23. Hardness of Alloy 1 after aging at 400, 450, 500, and 550 ℃ for 1, 3, 6, 12, and 24 h. 100

Figure 24. Hardness of Alloy 2 after aging at 400, 450, 500, and 550 ℃ for 1, 3, 6, 12, and 24 h. 101

Figure 25. Hardness of Alloy 3 after aging at 400, 450, 500, and 550 ℃ for 1, 3, 6, 12, and 24 h. 102

Figure 26. Engineering stress-strain curve of Alloy 1 with solution treatment condition at 900 ℃ for 1h (A1-ReX) and aged condition at 500 ℃ for 6 h (A1-A500). 105

Figure 27. Engineering stress-strain curve of Alloy 2 with solution treatment condition at 900 ℃ for 1h (A2-ReX) and aged condition at 450 ℃ for 6 h (A2-A450). 106

Figure 28. Engineering stress-strain curve of Alloy 3 with solution treatment condition at 1000 ℃ for 1h (A3-ReX) and aged condition at 500 ℃ for 6 h (A3-A450). 107

Figure 29. XRD result of all alloys after aging at peak hardness. 109

Figure 30. EBSD image of Alloy 1 with the aged condition at 500 ℃ for 6 h (A1-A500) (a) IPF maps, (b) phase maps, and (c) KAM maps. 111

Figure 31. EBSD image of Alloy 1 with the aged condition at 450 ℃ for 6 h (A2-A450) (a) IPF maps, (b) phase maps, and (c) KAM maps. 112

Figure 32. EBSD image of Alloy 1 with the aged condition at 500 ℃ for 6 h (A3-A500) (a) IPF maps, (b) phase maps, and (c) KAM maps. 113

Figure 33. Engineering stress-strain curve of Alloy 1 with heat treatment modification. Solution treated (A1-ReX) and Aged condition... 118

Figure 34. Engineering stress-strain curve of Alloy 2 with heat treatment modification. Solution-treated (A2-ReX) and Aged condition... 119

Figure 35. Engineering stress-strain curve of Alloy 3 with heat treatment modification. Solution treated (A1-ReX) and Aged condition... 121

Figure 36. Electron backscattered diffraction of the Alloy 1 with ReX, A500, and A550 specimens. Image quality map, inverse pole figure map, and... 125

Figure 37. Distribution of misorientation angle of the ReX, A500, and A550 specimens taken from the EBSD maps. 126

Figure 38. Electron backscattered diffraction of Alloy 3 with ReX, A400, A450, A500, and A550 specimens. Image quality map, inverse pole figure... 128

Figure 39. Distribution of misorientation angle of the ReX, A400, A450, A500, and A550 specimens taken from the EBSD maps. 129

Figure 40. Stress-strain curve pattern of Alloy 2 with solution treatment (ReX) and the different aging temperatures at 400 ℃, 450 ℃, 500 ℃, and... 135

Figure 41. XRD pattern of Alloy 2 with solution treatment (ReX) and the different aging temperatures at 400 ℃, 450 ℃, 500 ℃, and 550 ℃ for 6 h... 138

Figure 42. Electron backscattered diffraction of the ReX, A400, A450, A500, and A550 specimens. Image quality map, inverse pole figure map, and... 140

Figure 43. Distribution of misorientation angle of the ReX, A400, A450, A500, and A550 specimens taken from the EBSD maps. 142

Figure 44. SEM images and their corresponding point EDS measurement (in wt. % and at. %) along with their energy spectrum of (a,b) A400, (c,d) A450,... 143

Figure 45. (a,b) SEM images of A550 specimen; (c) high magnification taken from (a) marked with a blue rectangle; (d-h) EDS mapping; (i) high... 145

Figure 46. TEM micrograph of A450 specimen to observe the Laves phase (a) ABF-STEM image; (b) HAADF-STEM; (c) corresponding SADP from... 148

Figure 47. TEM micrograph of A450 to observe η-Ni₃Ti phase (a) BF-TEM micrograph; (b) high magnification of an area marked by the red square in... 150

Figure 48. Three-dimensional APT maps of A450 specimen with the atomic reconstruction of Fe (grey), Ni (green), Mn (yellow), Mo (blue), Ti (purple),... 152

Figure 49. Three-dimensional APT maps of A450 specimen based on isoconcentration of (a) 22 at. % Mo and (b) proximity histogram taken from... 153

Figure 50. Three-dimensional APT maps of A450 specimen based on isoconcentration of (a) 30 at. % Ni; (b) 8 at. % Ti; (c) 12 at. % Mn; (d) 35... 155

Figure 51. Particle size distribution of the Laves, η-Ni₃Ti, and Mn-rich precipitates in A450 specimen. 160

Figure 52. Three-dimensional APT maps of A450 specimen based on isoconcentration of (a) 70 at. % Fe and (b) proximity histograms taken from... 161

Figure 53. Calculated precipitation strengthening contribution of all precipitates in the A450 specimen with the experimental value for... 171