Enhanced Radiation Tolerance in Austenitic Stainless Steel

Compared to ferritic/martensitic steels, austenitic stainless steels (SS) possess good creep and fatigue resistance at elevated temperatures, and better toughness at low temperature. However, a major disadvantage of austenitic SS is that they are vulnerable to significant void swelling in nuclear reactors. The lack of resistance to void swelling in austenitic alloys led to the switch to ferritic/martensitic steels as the preferred material for fast reactor cladding applications. The recently developed alloy 709 is a creep-resistant austenitic steel expected to show improvement to void swelling resistance through the trapping of point defects at nanometer-sized carbides and nitrides. Further improvement to void swelling resistance would permit GenIV reactors to operate with similar lifetimes as currently operating LWRs. Shear Form’s technical approach is to increase radiation tolerance in austenitic steel through improvements to precipitate dispersions, grain size, and grain boundary (GB) characteristics. The focus will be on nanocrystalline austenitic Fe-Cr-Ni alloy with an FCC crystal structure. The long-term goal is to design and develop bulk nanostructured austenitic steels with enhanced void swelling resistance, substantial ductility, enhanced creep resistance, and preserved corrosion resistance via GB engineering. The advanced thermomechanical processing permitted by SFI’s technology allows us to tailor the material strength, ductility, and resistance to swelling by 1) changing the sink strength for point defects, 2) by increasing the nucleation barriers for bubble formation at, and 3) by changing the precipitate distributions. Shear Form will use equal channel Angular extrusion (ECAE) to create GB- engineered bars with dimensions 2.5 x 2.5 x 20 cm. Shear Form will process specimens for ion implantation, tension, creep, and thermal stability studies in collaboration with Texas A&M University, Department of Material Science and Engineering. The thrust of Phase II work will be scale-up and the demonstration of enhanced radiation tolerance. This will be accomplished by ECAE processing plates and tubes, engineering and evaluation of component production, neutron irradiation coupled with ion implantation, and long term creep studies. A new technique will be employed to fabricate stainless steel for use in high radiation environments. Severe Plastic Deformation processing will be employed to achieve this goal. The result will be a stronger, more uniform material with enhanced radiation tolerance for use in Gen III and IV reactors. Commercial Applications and Other Benefits: The feasibility and success of advanced reactor designs rely on the evaluation and commercialization of novel materials and manufacturing methods. ECAE can improve existing metallic alloys to meet material performance needs without prohibitive cost. In addition to advanced reactor structural materials, smaller and non-structural components for existing designs, including cladding material, are targeted. Creep and swelling resistant stainless steel is needed for many existing and future applications.