Deformation behavior of rolled and wire arc additively manufactured 316L stainless steel at cryogenic temperatures

다국어 초록 (Multilingual Abstract)

The deformation behavior and strengthening mechanisms of 316L stainless steel (SS) fabricated by rolling and wire arc additive manufacturing (WAAM) at room and cryogenic temperatures were investigated through phase transformation and microstructural observations. For the rolled 316L SS, real-time phase changes during tensile deformation at 298 K and 77 K were captured using in-situ synchrotron X-ray diffraction (SXRD). At 77 K, a multi-stage strain-hardening behavior driven by the sequential γ → ε → α′-martensite transformation was observed, whereas monotonic hardening occurred at 298 K.
Stacking faults acted as a major nucleation source for ε-martensite, which promoted the formation of α′-martensite and accelerated work hardening. Microstructural analysis near the fracture site revealed a complex architecture consisting of deformation twins and high-density α′-martensite. In the cross-section near the fracture plane, the FCC matrix was completely transformed into α'-martensite induced by cryogenic deformation. The results indicate that the primary mechanisms for the deformation capacity and excellent mechanical performance of 316L SS at cryogenic temperature come from Transformation-Induced Plasticity (TRIP) and multiple twinning.
The WAAM process, which deposits wires using a welding arc, has significant industrial potential due to its high deposition rate and its ability to manufacture large components. However, WAAM 316L (W-316L), due to layer-by-layer depositing and repeated thermal history, forms coarse columnar grains and a strong crystallographic texture, which can exhibit significant mechanical anisotropy. Research into the direction-dependent deformation behavior of W-316L SS at cryogenic temperatures such as 103 K remains limited. In this study, in addition to studying mechanical anisotropy, digital image correlation (DIC) was utilized to measure the surface strain field in real time, enabling the interpretation of local strain distribution and necking locations. Microstructural evolutions and deformation behavior were analyzed using electron backscatter diffraction (EBSD).
The comprehensive experimental and analytical evaluations conducted in this study are expected to enhance our understanding of the process-structure relationships of 316L SS.

목차 (Table of Contents)

• Abstract i
• ACKNOWLEDGEMENT iii
• Table of Contents iv
• List of Tables vi
• List of Figures vii
• Chapter 1. Introduction 1
• Chapter 2. Literature review 4
• 2.1. Austenitic stainless steels 4
• 2.2. Deformation and strengthening mechanisms 8
• 2.2.1. Stacking fault energy and temperature dependence 8
• 2.2.2. Phase transformation sequences and kinetics 13
• 2.3. Additive manufacturing 17
• 2.3.1. Process classification 17
• 2.3.2. Microstructure and solidification characteristics 20
• 2.3.3. Comparison of microstructures 22
• 2.4. WAAM 24
• 2.4.1. Process characteristics and thermal cycles 27
• 2.4.2. Mechanical properties of WAAM 316L SS 30
• Chapter 3. Deformation behavior and strengthening mechanisms at 77K of 316L
• SS probed by in-situ SXRD 32
• 3.1. Introduction 32
• 3.2. Experiment 33
• 3.2.1. In-situ experiment for cryogenic 33
• 3.3. Results and discussion 36
• 3.3.1. Intial microstructure. 36
• 3.3.2. Mechanical properties. 38
• 3.3.3. Fractography 40
• 3.3.4. In-situ SXRD analysis of phase evolution 42
• 3.3.5. Work-hardening performance. 46
• 3.3.6. EBSD analysis 49
• 3.4. Conclusions 52
• Chapter 4. Cryogenic deformation behavior of 316L SS processed by wire arc
• additive manufacturing 53
• 4.1. Introduction 53
• 4.2. Experiment 55
• 4.2.1. Sample preperation of WAAM. 55
• 4.2.2. Cryogenic tensile test with DIC set up. 59
• 4.2.3. Microstructure anlaysis. 59
• 4.3. Results and discussion 61
• 4.3.1. Intial microstructure. 61
• 4.3.2. Tensile deformation behavior. 65
• 4.3.3. Local strain distribution. 68
• 4.4. Conclusions 72
• Chapter 5. Conclusion 74
• Reference 76
• 국문초록 92