NSUF 19-1619: Effect of laser weld repairs on deformation mechanisms of neutron irradiated austenitic steels
The objective of this project is to understand the change in deformation mechanisms due to laser weld repairs of neutron irradiated austenitic stainless steels, as a function of strain rate. Austenitic steels are the major structural components of the current fleet of light water reactors (LWRs), and life-extension of LWRs requires low-heat-input laser welding replaces conventional gas tungsten arc welding (GTAW) to repair the reactor internals, which can cause He-induced cracking at the weld boundary. Understanding microstructural and mechanical evolution of these welds is a key step towards commercial deployment of advanced laser welding technologies, but the mechanical behavior in these welds has been understudied and is not well understood. Recent micropillar compression results from a laser weld on neutron irradiated 304L SS indicate a transition from slip and martensitic transformation in the irradiated base metal to twinning in the heat affected zone (HAZ). In the proposed work, we seek to attain a mechanistic understanding of this phenomenon. We hypothesize that laser welding induces thermal annealing of irradiation-produced defects as well as dendritic structures, thereby altering the strain rate regimes at which specific deformation mechanisms are active. Hence, a critical need exists to understand the influence of laser welding on the strain rate thresholds at which twinning or phase transformation are active.We propose to address this critical need by conducting a scanning electron microscopic (SEM) in-situ micropillar compression test study of laser welds on irradiated 304L SS. Samples irradiated at 415°C with 23 dpa and 3 appm He will be studied. The condition is relevant to end-life conditions of LWRs and the different strain rates help us to determine the transition criteria from slip and phase transformation to twinning and in the HAZ. We are requesting access to the Idaho National Laboratory (INL) Irradiated Materials Characterization Laboratory (IMCL) to prepare micropillars, use the recently installed PI-88 micromechanical testing system to perform micropillar compression tests, and utilize transmission electron microscopy (TEM) to characterize the post-compression microstructures. Results will reveal underlying mechanisms of laser-weld induced deformation twinning and phase transformation. The large body of data we will generate can also be leveraged to validate non-linear mechanics models in the DOE-NEAMS program as well as more classical strain-induced martensitic transformation kinetics models. The project outcome is a systematic and mechanistic understanding of laser welding effect on the mechanical integrity and deformation mechanisms (e.g. twinning and phase transformation) in austenitic steels; the broader impact of this work is that it will both improve the laser welding repair technology and enable a science-based design and selection of austenitic steels and to maximize mechanical and phase stability under irradiation and various external stress conditions.
Additional Info
| Field | Value |
|---|---|
| Abstract | The objective of this project is to understand the change in deformation mechanisms due to laser weld repairs of neutron irradiated austenitic stainless steels, as a function of strain rate. Austenitic steels are the major structural components of the current fleet of light water reactors (LWRs), and life-extension of LWRs requires low-heat-input laser welding replaces conventional gas tungsten arc welding (GTAW) to repair the reactor internals, which can cause He-induced cracking at the weld boundary. Understanding microstructural and mechanical evolution of these welds is a key step towards commercial deployment of advanced laser welding technologies, but the mechanical behavior in these welds has been understudied and is not well understood. Recent micropillar compression results from a laser weld on neutron irradiated 304L SS indicate a transition from slip and martensitic transformation in the irradiated base metal to twinning in the heat affected zone (HAZ). In the proposed work, we seek to attain a mechanistic understanding of this phenomenon. We hypothesize that laser welding induces thermal annealing of irradiation-produced defects as well as dendritic structures, thereby altering the strain rate regimes at which specific deformation mechanisms are active. Hence, a critical need exists to understand the influence of laser welding on the strain rate thresholds at which twinning or phase transformation are active.We propose to address this critical need by conducting a scanning electron microscopic (SEM) in-situ micropillar compression test study of laser welds on irradiated 304L SS. Samples irradiated at 415°C with 23 dpa and 3 appm He will be studied. The condition is relevant to end-life conditions of LWRs and the different strain rates help us to determine the transition criteria from slip and phase transformation to twinning and in the HAZ. We are requesting access to the Idaho National Laboratory (INL) Irradiated Materials Characterization Laboratory (IMCL) to prepare micropillars, use the recently installed PI-88 micromechanical testing system to perform micropillar compression tests, and utilize transmission electron microscopy (TEM) to characterize the post-compression microstructures. Results will reveal underlying mechanisms of laser-weld induced deformation twinning and phase transformation. The large body of data we will generate can also be leveraged to validate non-linear mechanics models in the DOE-NEAMS program as well as more classical strain-induced martensitic transformation kinetics models. The project outcome is a systematic and mechanistic understanding of laser welding effect on the mechanical integrity and deformation mechanisms (e.g. twinning and phase transformation) in austenitic steels; the broader impact of this work is that it will both improve the laser welding repair technology and enable a science-based design and selection of austenitic steels and to maximize mechanical and phase stability under irradiation and various external stress conditions. |
| Award Announced Date | 2019-02-08T00:00:00 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Alina Zackrone |
| Irradiation Facility | None |
| PI | Keyou Mao |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 1619 |