NSUF 19-1687: Mechanical characterization of three lower dose HT-9 heats (ORNL, LANL and EBR II) after side-by-side neutron irradiation at LWR and fast reactor relevant temperatures
HT-9 is being considered as a candidate structural material for advanced reactors, due to their superior resistance to radiation induced void swelling, microstructural stability, and thermal properties. The progressive change in the microstructure with irradiation dose and temperature includes void formation, increases in dislocation density, second phase formation, and other changes that can lead to swelling, hardening, and embrittlement. The strongest hardening and embrittlement occurs at temperatures below ~425°C in HT-9 due to strong increases in dislocation density and the formation of several different populations of second phases that all act to reduce dislocation mobility. The extreme hardening and low fracture toughness that occur for irradiation temperatures below 425°C is a serious issue for the use of HT-9 because many reactor concepts call for core components to see temperatures as low as 320°C. Normalized and tempered HT-9 typically exhibit tempered martensitic structure characterized by larger M23C6 (M= Cr, Fe, Mo) carbides formed on lath and prior austenite grain boundaries, and smaller and low-density MC (M = V) carbides formed within the matrix. The heat treatability makes it possible to optimize the material for toughness, ductility and high temperature properties. The heat treatment can affect prior austenite grain size, lath and packet size, precipitate morphology and distribution. The mechanical properties of HT-9 after neutron irradiation varies based upon the manufacturing process, chemical composition, initial microstructure, irradiation dose and temperature. To address the issue of low-temperature neutron irradiation hardening and embrittlement, systematic investigations on the mechanical behavior of HT-9 with slight variations in chemical composition and heat treatment are needed over a range of doses and temperatures. As a part of the UW-Madison Irradiation Experiment, three HT-9 heats (ORNL, LANL and EBR II) with variations in manufacturing process, chemical composition and heat treatment were side-by-side neutron irradiated in the ATR at target temperatures of 300, 400 and 500°C to target doses of 3 and 6 dpa. Currently, efforts are ongoing under a funded FY18 RTE project to study the mechanical properties of higher dose HT-9 as a function of processing conditions and irradiation temperatures. For irradiated F-M steels, the increase in yield strength is quite steep up to around 10 dpa. Extrapolation of the information learned from the 6-8 dpa irradiations is best accomplished by also examining lower dose samples because having data at two doses results in a more accurate extrapolation to higher doses. The proposed project aims to evaluate the mechanical properties of three HT-9 variants at lower dose as a function of irradiation temperatures. The mechanical test data would be analyzed to understand the effects of radiation damage on these HT-9 heats at LWR and fast reactor relevant temperatures, and to develop appropriate processing-property-temperature-dose correlations. Limited low-temperature neutron irradiation studies had been performed to study the variation of manufacturing process, chemical composition and heat treatment on the mechanical behavior of irradiated HT-9. By doing this work, our team can contribute to filling the gap in the literature on understanding irradiation effects on HT-9 and F-M steels in general.
Additional Info
| Field | Value |
|---|---|
| Abstract | HT-9 is being considered as a candidate structural material for advanced reactors, due to their superior resistance to radiation induced void swelling, microstructural stability, and thermal properties. The progressive change in the microstructure with irradiation dose and temperature includes void formation, increases in dislocation density, second phase formation, and other changes that can lead to swelling, hardening, and embrittlement. The strongest hardening and embrittlement occurs at temperatures below ~425°C in HT-9 due to strong increases in dislocation density and the formation of several different populations of second phases that all act to reduce dislocation mobility. The extreme hardening and low fracture toughness that occur for irradiation temperatures below 425°C is a serious issue for the use of HT-9 because many reactor concepts call for core components to see temperatures as low as 320°C. Normalized and tempered HT-9 typically exhibit tempered martensitic structure characterized by larger M23C6 (M= Cr, Fe, Mo) carbides formed on lath and prior austenite grain boundaries, and smaller and low-density MC (M = V) carbides formed within the matrix. The heat treatability makes it possible to optimize the material for toughness, ductility and high temperature properties. The heat treatment can affect prior austenite grain size, lath and packet size, precipitate morphology and distribution. The mechanical properties of HT-9 after neutron irradiation varies based upon the manufacturing process, chemical composition, initial microstructure, irradiation dose and temperature. To address the issue of low-temperature neutron irradiation hardening and embrittlement, systematic investigations on the mechanical behavior of HT-9 with slight variations in chemical composition and heat treatment are needed over a range of doses and temperatures. As a part of the UW-Madison Irradiation Experiment, three HT-9 heats (ORNL, LANL and EBR II) with variations in manufacturing process, chemical composition and heat treatment were side-by-side neutron irradiated in the ATR at target temperatures of 300, 400 and 500°C to target doses of 3 and 6 dpa. Currently, efforts are ongoing under a funded FY18 RTE project to study the mechanical properties of higher dose HT-9 as a function of processing conditions and irradiation temperatures. For irradiated F-M steels, the increase in yield strength is quite steep up to around 10 dpa. Extrapolation of the information learned from the 6-8 dpa irradiations is best accomplished by also examining lower dose samples because having data at two doses results in a more accurate extrapolation to higher doses. The proposed project aims to evaluate the mechanical properties of three HT-9 variants at lower dose as a function of irradiation temperatures. The mechanical test data would be analyzed to understand the effects of radiation damage on these HT-9 heats at LWR and fast reactor relevant temperatures, and to develop appropriate processing-property-temperature-dose correlations. Limited low-temperature neutron irradiation studies had been performed to study the variation of manufacturing process, chemical composition and heat treatment on the mechanical behavior of irradiated HT-9. By doing this work, our team can contribute to filling the gap in the literature on understanding irradiation effects on HT-9 and F-M steels in general. |
| Award Announced Date | 2019-02-08T00:00:00 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Stuart Maloy |
| Irradiation Facility | None |
| PI | Ramprashad Prabhakaran |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 1687 |