b'Fracture Toughness of 9Cr and 12Cr Steels After New thermomechanical Treatments for Fast Reactor Cladding ApplicationPrincipal Investigator: Thak Sang Byun, PNNLTeam Members/ Collaborators: David A. Collins, Timothy G. Lach, Jung Pyung Choi, and Emily L. Barkley (PNNL)H development of a high-performanceAccomplishments: igh-Cr steels with fine ferritic/martensitic (FM) structuresreactor with higher thermal effi- This research aimed to find a new are candidate materials forciency. In this project a simultaneousprocessing route that can produce fast reactor core structures because ofimprovement of fracture toughnessan optimized microstructure of each their high creep strength and excellentand strength is specifically targetedhigh-Cr steel with improved mechanical resistance to high temperature radiationin the design of thermomechanicalproperties including excellent high damage such as void swelling. Afterprocessing for various 9Cr and 12Crtemperature fracture toughness. It was the traditional thermomechanicalsteels. Another principle that hasconsidered that (i) a highly controlled treatment consisting of normalizing,guided the process developmenttreatment, such as a rapid cooling, water quenching, and tempering,task is the common knowledge thatwhich can effectively refine the however, the high-Cr steels show limitedcontaining more defect recombinationquenched lath structure, and (ii) the mechanical properties, e.g., significantsites or higher defect sink strength isfollowing tempering process, should decrease of strength above ~450C.required for higher radiation resistance.be feasible for the practical processing Since the high-Cr steels are temperedThree 9Cr steels and two 12Cr steelsof thin core components such as fuel at high temperatures (typically aroundwith and without nitrogen additioncladding and fuel duct. In order to 750C), significant coarsening canwere selected for this research. A seriesdetermine the best processing route for occur during the final heat treatment.of new thermomechanical processthe high-Cr steels, the main task of the It is believed that the high temperaturetreatments (TMTs) were designed andyear was focused to thoroughly evaluate tempering can maximize ductilityapplied to those five steels to find atheir fracture toughness behavior after a but reduce high temperature strengthnew TMT route that can result in thevariety of thermomechanical treatments. and fracture toughness. It is also likelyincrease of both strength and fractureFor the testing campaign, twelve TMTs that such a fully-tempered structuretoughness. It was demonstrated forwere applied to two 12Cr alloys and a will experience a lowered radiationthe HT-9 steels that some tailoredselect of five TMT routes to three 9Cr resistance when compared to the steelsTMTs increased or maintained thealloys. Fracture toughness tests were with finer microstructures. This researchfracture toughness of the high-Cr steelscarried out over a wide temperature was planned to explore new processingwhile some of those treatments couldrange of 22600C, and the results were routes that can produce ultrafinesignificantly increase the strength of thecompared with reference data for the microstructures and thus can improvesteels to the higher levels comparable9Cr and 12Cr steels. Summarized here the high temperature mechanicalto those of nanostructured ferriticare the key observations and conclusions properties of the high-Cr steels. alloys. The fracture toughness data offrom the comparison of fracture tough-Project Description:both 9Cr and 12Cr steels after variousness data among the alloys and with This research aimed to improve thethermomechanical treatments werethe reference materials, which aimed mechanical properties of high-Cr steelsanalyzed and compared with those ofto answer to the question whether by developing new thermomechanicaltheir reference materials. A two-stepany of the new TMTs can improve the processes for application to advancedtempering route is selected for fracture resistance of the high-Cr steels. fast reactors. A reactor core structureapplication to the high-Cr steels forFirst, the two final TMTs with two-step with an improved high temperaturereactor core structures. temping, 500C/1h + 650C/15mi capability will be essential for theand 600C/1h + 750C/15min, consistently delivered high fracture toughness with the 12Cr (HT-9) steels 194 2019|AFC ACCOMPLISHMENTS'