2022 Fiscal Year Research-status Report
Oxide Accelerating Primary Ferrite Nucleation of Austenitic Stainless Steel Weldment
| Project/Area Number |
22K14510
|
|---|
| Research Institution | Osaka University |
|---|
Principal Investigator |
HOU YUYANG 大阪大学, 接合科学研究所, 助教 (60905933)
|
|---|
| Project Period (FY) |
2022-04-01 – 2024-03-31
|
| Keywords | Stainless steel / Weld metal / Nucleation / Oxide / TiN / δ-ferrite / Austenite |
|---|
| Outline of Annual Research Achievements |
In this study, we evaluate the accelerating effect of Ti2O3, (Ti,Al)2O3, and MgAl2O4 on TiN formation to promote primary ferrite nucleation. Ti2O3, (Ti,Al)2O3, and MgAl2O4 could all accelerate TiN formation to nucleate δ-ferrite, the specimen with MgAl2O4 showed the optimal effect of δ-ferrite refinement. Ti2O3 and (Ti,Al)2O3 could act as heterogeneous nuclei for TiN because low lattice misfits of 0.6% and 0.6~8.1% could be reached as for Ti2O3 and (Ti,Al)2O3 with orientation relationship of {0001}oxide//{111}TiN and [10-10] oxide//[110]TiN. Spinel type MgAl2O4 was identified to have a fully paralleled orientation relationship with TiN and lead to a 4.8% misfit with TiN in each crystal plane. MgAl2O4 could significantly accelerate TiN formation to form MgAl2O4-TiN with a higher area density of above 250 counts/mm2 compared to 75 counts/mm2 regarding Ti2O3-TiN and (Ti,Al)2O3-TiN. Baker-Nutting (B-N) orientation relationship was confirmed between TiN and δ-ferrite. The formed TiN could nucleate δ-ferrite.
The effect of primary ferrite refinement on austenite formation was investigated. When MgAl2O4-TiN was introduced, the primary ferrite was greatly refined with random and multiple orientations, having little relationship with the temperature gradient. The morphology of residual ferrite showed vermicular morphology with fine-dispersed character. The austenite shows columnar morphology because the formation temperature of austenite is higher than the solidus and the austenite nucleated and grew independently to primary ferrite.
|
| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
The objective of this study is to promote primary ferrite nucleation accelerated by effective oxide and evaluate the corresponding evolution of mechanical properties. In the last year, the reported oxides including Ti2O3, (Ti,Al)2O3, and MgAl2O4 were evaluated in the simulated welding condition of stainless steel. MgAl2O4 was confirmed as the optimal oxide for the TiN formation to promote δ-ferrite nucleation. The component, density, and distribution of the effective nucleus were evaluated, and the microstructure and phase orientation relationship were investigated to reveal the nucleation mechanism. The effect of elemental composition on equilibrium phase formation was examined by thermodynamic analysis to design the solidification sequence of oxide, TiN, and δ-ferrite. After the addition of MgAl2O4-TiN, the primary ferrite with fine-dispersed vermicular was achieved with a no-clear orientation relationship between austenite. The austenite keeps columnar morphology even though the primary ferrite was refined by nucleation. It was assumed that austenite nucleated and grew independently during solidification. The solidification sequence and the effects of microstructure evolution on mechanical properties will be investigated based on the current findings as scheduled. Therefore, according to the above views, we consider that the study is progressing smoothly.
|
| Strategy for Future Research Activity |
In the last year, the optimal oxide for primary ferrite nucleation was revealed and the residual ferrite was greatly refined to fine-dispersed vermicular morphology by the addition of MgAl2O4-TiN. For the next step, the nucleus formation, nucleation, solidification, and ferrite-austenite transformation will be investigated by the liquid-tin quenching method to elucidate the mechanism. The quenched microstructure will be systematically evaluated by optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The plate with primary ferrite nucleation will be fabricated by oxide acceleration. Mechanical properties including tensile properties and toughness properties of the fabricated plates are planned to be evaluated. Furthermore, the fracture mechanism of the modified specimen will be systematically investigated by SEM/EBSD observation. These contents will not only reveal how the MgAl2O4-TiN forms and influence the nucleation and phase transformation but also establish a relationship between microstructure modification and mechanical properties with the corresponding mechanism. This research will provide an innovative strategy to improve the mechanical properties of weld metal utilizing the oxide acceleration and nucleation theory.
|
Research Products
(3 results)
