The Effects of Tritium and Decay Helium on the Fracture Toughness Properties of Stainless Steels PDF Download

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Languages : en
Pages : 9

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J-integral fracture mechanics techniques and scanning electron microscopy observations were used to investigate the effects of tritium and its decay product, helium-3, on Types 304L, 316L, 21-6-9, A286, and JBK-75 (Modified A286) stainless steels. Tritium-exposed samples of each steel had lower fracture toughness values and less resistance to stable crack growth than control samples. Type 316L stainless steel was more resistant to the embrittling effects of tritium and decay helium than the other steels.

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Languages : en
Pages : 12

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J-integral fracture mechanics techniques and electron microscopy observations were used to investigate the effects of tritium and its radioactive decay product, 3He, on Types 316L, 304L and 21Cr-6Ni-9Mn stainless steels. Tritium-exposed-and-aged steels had lower fracture-toughness values and shallower sloped crack-growth-resistance curves than unexposed steels. Both fracture-toughness parameters decreased with increasing concentrations of 3He. The fracture-toughness reductions were accompanied by a change in fracture mode from microvoid-nucleation-and-growth processes in control samples to grain-and-twin-boundary fracture in tritium-charged-and-aged samples. Type 316L stainless steel had the highest fracture-toughness values and Type 21Cr-6Ni-9Mn had the lowest. Samples containing 3He but degassed of tritium had fracture toughness properties that were similar to uncharged samples. The results indicate that helium bubbles enhance the embrittlement effects of hydrogen by affecting the deformation properties and by increasing localized hydrogen concentrations through trapping effects.

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Languages : en
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Languages : en
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The deformation and fracture toughness properties of forged stainless steels pre-charged with tritium were compared to the deformation and fracture toughness properties of the same steels heat treated at 773 K or 873 K and precharged with hydrogen. Forged stainless steels pre-charged with tritium exhibit an aging effect: Fracture toughness values decrease with aging time after precharging because of the increase in concentration of helium from tritium decay. This study shows that forged stainless steels given a prior heat treatment and then pre-charged with hydrogen also exhibit an aging effect: Fracture toughness values decrease with increasing time at temperature. A microstructural analysis showed that the fracture toughness reduction in the heat-treated steels was due to patches of recrystallized grains that form within the forged matrix during the heat treatment. The combination of hydrogen and the patches of recrystallized grains resulted in more deformation twinning. Heavy deformation twinning on multiple slip planes was typical for the hydrogen-charged samples; whereas, in the non-charged samples, less twinning was observed and was generally limited to one slip plane. Similar effects occur in tritium pre-charged steels, but the deformation twinning is brought on by the hardening associated with decay helium bubbles in the microstructure.

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Languages : en
Pages : 5

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Studies to determine tritium exposure effects on the properties of welded steels are being conducted. In this investigation, the effects of tritium and decay helium on the fracture toughness properties of high-energy-rate-forged (HERF) Incoloy 903 were. Fracture toughness measurements were conducted for tritium-exposed samples in the as-forged condition and compared with welded samples. Tritium-exposed HERF Incoloy 903 had fracture toughness values that were 33% lower than those for unexposed HERF Incoloy 903. Tritium-exposed welded samples had fracture toughness values that were just 8% of the unexposed HERF alloys and 28% of unexposed welded alloys.

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Languages : en
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J-Integral fracture toughness tests were conducted on tritium-exposed-and-aged Types 304L and 21-6-9 stainless steel weldments in order to measure the combined effects of tritium and its decay product, helium-3 on the fracture toughness properties. Initially, weldments have fracture toughness values about three times higher than base-metal values. Delta-ferrite phase in the weld microstructure improved toughness provided no tritium was present in the microstructure. After a tritium-exposure-and-aging treatment that resulted in (almost equal to)1400 atomic parts per million (appm) dissolved tritium, both weldments and base metals had their fracture toughness values reduced to about the same level. The tritium effect was greater in weldments (67 % reduction vs. 37% reduction) largely because the ductile discontinuous delta-ferrite interfaces were embrittled by tritium and decay helium. Fracture toughness values decreased for both base metals and weldments with increasing decay helium content in the range tested (50-200 appm).

Author: M. Morgan
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Languages : en
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The fracture toughness data collected in this study are needed to assess the long-term effects of tritium and its decay product on tritium reservoirs. The results show that tritium and decay helium have negative effects on the fracture toughness properties of stainless steel and its weldments. The data and report from this study has been included in a material property database for use in tritium reservoir modeling efforts like the Technology Investment Program ''Lifecycle Engineering for Tritium Reservoirs''. A number of conclusions can be drawn from the data: (1) For unexposed Type 304L stainless steel, the fracture toughness of weldments was two to three times higher than the base metal toughness. (2) Tritium exposure lowered the fracture toughness properties of both base metals and weldments. This was characterized by lower J{sub Q} values and lower J-da curves. (3) Tritium-exposed-and-aged base metals and weldments had lower fracture toughness values than unexposed ones but still retained good toughness properties.

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Languages : en
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Tritium reservoirs are constructed from welded stainless steel forgings. While these steels are highly resistant to the embrittling effects of hydrogen isotopes and helium from tritium decay; they are not immune. Tritium embrittlement is an enhanced form of hydrogen embrittlement because of the presence of helium-3 from tritium decay which nucleates as nanometer-sized bubbles on dislocations, grain boundaries, and other microstructural defects. Steels with decay helium bubble microstructures are hardened and less able to deform plastically and become more susceptible to embrittlement by hydrogen and its isotopes. Ductility, elongation-to-failure, and fracture toughness are reduced by exposures to tritium and the reductions increase with time as helium-3 builds into the material from tritium permeation and radioactive decay. Material and forging specifications have been developed for optimal material compatibility with tritium. These specifications cover composition, mechanical properties, and select microstructural characteristics like grain size, flow-line orientation, inclusion content, and ferrite distribution. For many years, the forming process of choice for reservoir manufacturing was high-energy-rate forging (HERF), principally because the DOE forging facility owned only HERF hammers. Today, some reservoir forgings are being made that use a conventional, more common process known as press forging (PF or CF). One of the chief differences between the two forging processes is strain rate: Conventional hydraulic or mechanical forging presses deform the metal at 4-8 ft/s, about ten-fold slower than the HERF process. The material specifications continue to provide successful stockpile performance by ensuring that the two forging processes produce similar reservoir microstructures. While long-term life storage tests have demonstrated the general tritium compatibility of tritium reservoirs, fracture-toughness properties of both conventionally forged and high-energy-rate forged are needed for designing and establishing longer tritium-reservoir lifetimes, ranking materials, and, potentially, for qualifying new forging vendors or processes. Measurements on the effects of tritium and decay helium on the fracture toughness properties of CF stainless steels having similar composition, grain size, and mechanical properties to previously studied HERF steels are needed and have not been conducted until now. The compatibility of stainless steel welds with tritium represents another concern for long-term reservoir performance. Weldments have not been well-characterized with respect to tritium embrittlement, although a recent study was completed on the effect of tritium and decay helium on the fracture toughness properties of Type 304L weldments. This study expands the characterization of weldments through measurements of tritium and decay helium effects on the fracture toughness properties of Type 21-6-9 stainless steel. The purpose of this study was to measure and compare the fracture toughness properties of Type 21-6-9 stainless steel for conventional forgings and weldments in the non-charged, hydrogen-charged and tritium-charged-and-aged conditions.

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Languages : en
Pages : 28

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High-energy-rate-forged (HERF) austenitic stainless steels are used for the containment of hydrogen and its isotopes. Embrittlement of these materials by hydrogen has been a source of concern for some time. The nature and the degree of embrittlement by hydrogen varies considerably and, among other factors, is a complicated function of material composition and processing variations. Helium, the radioactive decay product of tritium, will also embrittle stainless steels. Precipitation of microscopic helium bubbles tends to increase the material's flow stress, through dislocation pinning, as well as weaken interfaces like grain and twin boundaries. Since fracture toughness tends to decrease with increasing yield strength, at least part of the helium-embrittlement problem may be due to strength effects. The relationship between a material's yield strength and toughness and, the incremental strength increase and corresponding toughness decrease imparted by helium is not known. The purpose of this study was to measure the combined effects of strength, hydrogen isotopes, and helium on the room temperature mechanical and fracture toughness properties of HERF 21-6-9 stainless steel.

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Languages : en
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Forged stainless steels are used as the materials of construction for tritium reservoirs. During service, tritium diffuses into the reservoir walls and radioactively decays to helium-3. Tritium and decay helium cause a higher propensity for cracking which could lead to a tritium leak or delayed failure of a tritium reservoir. The factors that affect the tendency for crack formation and propagation include: Environment; steel type and microstructure; and, vessel configuration (geometry, pressure, residual stress). Fracture toughness properties are needed for evaluating the long-term effects of tritium on their structural properties. Until now, these effects have been characterized by measuring the effects of tritium on the tensile and fracture toughness properties of specimens fabricated from experimental forgings in the form of forward-extruded cylinders. A key result of those studies is that the long-term cracking resistance of stainless steels in tritium service depends greatly on the interaction between decay helium and the steels' forged microstructure. New experimental research programs are underway and are designed to measure tritium and decay helium effects on the cracking properties of stainless steels using actual tritium reservoir forgings instead of the experimental forgings of past programs. The properties measured should be more representative of actual reservoir properties because the microstructure of the specimens tested will be more like that of the tritium reservoirs. The programs are designed to measure the effects of key forging variables on tritium compatibility and include three stainless steels, multiple yield strengths, and four different forging processes. The effects on fracture toughness of hydrogen and crack orientation were measured for type 316L forgings. In addition, hydrogen effects on toughness were measured for Type 304L block forgings having two different yield strengths. Finally, fracture toughness properties of type 304L stainless steel were measured for four different forging strain rates which and two forging temperatures. Tritium exposures have been and are being conducted on companion specimens for property measurements in the upcoming years.