Tritium Aging Effects on Fracture Toughness of Stainless Steel Weldments

Abstract

The long-term embrittlement effects of tritium and decay helium on the structural properties of stainless steels have been studied for years at Savannah River National Laboratory (Savannah River) to provide required data for establishing safe operating conditions and the lifetimes of the pressure vessels used to contain tritium gas. In this study, the fracture toughness properties of the longest-aged tritium-precharged stainless steel base metals and weldments tested at Savannah River were measured and compared to earlier results. The fracture toughness values were the lowest recorded here for tritium-exposed stainless steel. As-forged and as-welded specimens were thermally precharged with tritium gas at 34.5 MPa and 623 K, then aged for up to 17 years to build in decay helium prior to testing. American Society for Testing and Materials J-integral fracture mechanics analyses, transmission electron microscopy (TEM), and small-angle neutron scattering (SANS) examinations were conducted to characterize the effects of tritium and its radioactive decay product ³He. Results show that the fracture toughness values were reduced to less than 2% to 4% of the as-forged values for specimens with more than 1300 atomic parts per million helium from tritium decay. The trend of decreasing fracture toughness values with increasing helium content was consistent with earlier observations, and the data show that Type 304L stainless steel is more resistant to tritium-induced cracking than Type 21-6-9 stainless steel at similar decay helium levels. The fracture toughness properties of long-aged weldments were also affected, but the reductions were not as severe over time because the weldments did not retain as much tritium as did the base metals. TEM observations were used to characterize the effects of decay helium bubbles on the deformation substructures, but nanometer-sized helium bubbles were not easily resolved because of high dislocation densities within the forged microstructures. SANS results are presented that suggest the technique can provide information on decay helium bubble size, spacing, and distribution in these steels.

Keywords:

• Tritium
• fracture mechanics
• toughness
• embrittlement, steel

Acknowledgments

This work was prepared under an agreement with and funded by the U.S. government. Neither the U.S. government or its employees, nor any of its contractors, subcontractors, or their employees, make any expressed or implied: (1) warranty or assumes any legal liability for the accuracy, completeness, or for the use or results of such use of any information, product, or process disclosed, or (2) representation that such use or results of such use would not infringe privately owned rights, or (3) endorsement or recommendation of any specifically identified commercial product, process, or service. Any views and opinions of authors expressed in this work do not necessarily state or reflect those of the U.S. government or its contractors or subcontractors. This document was prepared in connection with work done under contract DE-AC09-96R18500 with the U.S. Department of Energy. By acceptance of this document, the publisher and/or recipient acknowledges the U. S. government’s right to retain a nonexclusive, royalty-free license in and to any copyright covering this document, along with the right to reproduce and authorize others to reproduce all or part of the copy righted material.

Henry Ajo conducted the fractography examinations, Mike Tosten the TEM, and Stephen Crossland assisted in conducting the fracture toughness tests. This work benefited from the use of the SasView application, originally developed under National Science Foundation award DMR-0520547. SasView contains code developed with funding from the European Union’s Horizon 2020 research and innovation programme under the SINE2020 project, grant agreement 654000.