WSRC-MS-2001-00040

M. R. Louthan, Jr.
Westinghouse Savannah River Company
Aiken, SC 29808

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Abstract

The deleterious effects of hydrogen on the mechanical properties of austenitic stainless steels are frequently magnified by the presence of helium. Helium can be introduced into steel by either tritium absorption, diffusion and decay (tritium-tricked) or by implantation during irradiation. Displacement damage, which accompanies irradiation-induced hydrogen/helium implantation, is not associated with tritium, deuterium and/or protium absorption and tritium decay. Therefore, the microstructure of irradiated steel containing any given level of transmutation induced hydrogen and/or helium will differ significantly from the microstructure of a tritium tricked steel because of the interactions of hydrogen and helium with the irradiation induced defect structure. The defect structures (black spot damage, bubbles and/or faulted Frank loops) developed during low temperature (T<200°C) irradiation trap the implanted hydrogen and helium. The trapping reduces atom mobility and hence the ability of the hydrogen and helium to either agglomerate (form bubbles) or accumulate at potential crack sites. The absence of such trapping in tritium-tricked steel causes the response to thermal/mechanical parameters to differ considerably from the response of irradiated steel containing similar hydrogen and helium concentrations. This difference in response minimizes the effects of hydrogen and helium on the low temperature mechanical properties of irradiated steels. Additionally, this difference causes irradiated steel to be less weldable than a tritium-tricked steel containing the same amount of helium.

Introduction

The deleterious effects of hydrogen and helium on the behavior of metals and alloys are well known and extensively documented. Unfortunately, the investigation of these effects is divided into several camps and there is only minimal interaction among the camps. Investigations of the effects of radiation-induced hydrogen and helium rarely consider the tritium in metals literature. Similarly, investigations focussed on hydrogen in metals seldom consider the irradiation effects literature. Additionally, the investigations using ion implantation to study hydrogen/helium effects on metals frequently consider the effects of irradiation-induced hydrogen and helium but basically ignore the hydrogen/tritium in metals literature. This lack of interaction among the various hydrogen/helium in metals camps may have lead to the assumption that "helium is helium". This assumption indirectly supports the conclusion that one may establish the effects of irradiation induced helium on the behavior of metals and alloys by measuring the impact of helium introduced by tritium decay. Numerous investigators, including this author, have used tritium-tricked metals to establish the effects of helium on the weldability of reactor components even though there is evidence that helium trapping at sites of displacement damage and the subsequent release of tritium from the trap sites increases the susceptibility of a steel to helium-induced reductions in weldability (1).

The U. S. Department of Energy considered the use of an accelerator to produce tritium for national defense purposes. The Accelerator Production of Tritium, APT, project included the irradiation of selected structural materials in a high-energy proton and spallation neutron flux, produced by exposing a tungsten target to a 800 MeV, 1mA Gaussian proton beam at the Los Alamos Neutron Science Center (LANSCE) (2). Irradiation with these high-energy particles causes displacement damage and the production and implantation of large quantities of hydrogen and helium in the irradiated materials. The amount of hydrogen and helium produced by nuclear spallation reactions per displacement per atom (dpa) of exposure is significantly greater than the amount produced by irradiation in a typical fission reactor. Additionally, the effect of the displacement damage associated with any given quantity of hydrogen and/or helium in any irradiated sample may differ significantly from the effect in samples that contain helium as a result of the decay of thermally charged tritium and have no displacement damage. Type 316L stainless steel is a structural material that has been tested after irradiation in the APT program (3), after irradiation in fission reactors (4) and after tritium tricking (5,6). This paper compares test results from APT irradiated samples, fission reactor samples and "tritium tricked" samples to demonstrate that hydrogen/helium trapping at irradiation induced defects significantly alters the susceptibility of Type 316L stainless steel to hydrogen/helium embrittlement.

Background

A 1984 review of the influence of helium on the bulk properties of fusion reactor structural material emphasized that the behavior of helium in metals is governed by trapping, the formation of defect clusters and bubble nucleation and growth (7). The review also showed that over the 300 to 700°C range, tensile samples of Type 316 stainless steel irradiated under conditions that produced a high helium to dpa ratio (a mixed spectrum reactor) showed less elongation to fracture than samples from steel irradiated under conditions that produced a low helium to dpa ratio (a fast reactor). The helium content of the tested samples varied from nearly zero to several thousand atomic parts per million depending on the irradiation conditions.

The helium-induced reduction in ductility was established from the same data set that demonstrated that the yield strength is not changed by the helium to dpa ratio in material tested at low temperature (<300°C) (8). Furthermore, as the test temperature increased above 300°C, yielding became easier in the samples with the higher helium contents (Figure 1). A similar helium-induced reduction in strength was found in material that was 20% cold worked prior to irradiation (8). Additionally, the yield strength of the 20% cold worked and irradiated material decreased with increasing helium concentration in samples tested at 575°C. These helium-induced decreases in strength contrast the effect helium has on the yield strength of tritium-tricked, high-energy rate forged (HERF) Type 316 stainless steel (5).

Figure 1. Effect of Helium on the Temperature Dependence of Yield Strength in
Irradiated Type 316 Stainless Steel (after Reference 8)

Figure 2. Effect of Helium on the Temperature Dependence of Yield
Strength in Tritium Tricked Type 316 Stainless Steel

Tensile tests of tritium-tricked, HERF Type 316 stainless showed that the yield strength increased with increasing helium content. The hydrogen content of these HERF test specimens was not reported; however, with the exception of the decay-induced losses, the hydrogen content should have been approximately the same in all the specimens. The largest helium-induced strength increases were observed in tests conducted between room temperature and 400°C and the yield strength was only marginally increased at the test temperatures above 700°C (Figure 2). However, even though helium had contradictory effects on the strengths of irradiated and of tritium-tricked samples, helium accumulations reduced the ductility in both types of material.

The helium-induced reduction in ductility in the tritium-tricked material was, considering the difference in implantation techniques, remarkably consistent with the observations on irradiated materials. The elongation-to-fracture was decreased by the accumulation of helium and the extent of the decrease increased with increasing test temperature, especially at temperatures above 600°C (Figure 3).

Hardness measurements on ion implanted Type 316LN stainless steel (9) showed that when iron implantation was used to develop displacement damage, the simultaneous implantation of helium and/or hydrogen with iron, increased the strength of the steel (Figure 4). The measurements also showed that hydrogen ion implantation alone "produced extremely fine black dots and bubbles. Despite the high number density of defects (0.3 dpa) it produced, H⁺ contributed to hardening only to a small extent"(9). Implantation of iron or helium alone increased the hardness more than hydrogen and simultaneous implantation of hydrogen with either iron or helium did not significantly effect the hardness above the increases associated with implantation of iron or helium alone. The displacement damage associated with helium and hydrogen implantation was approximately 0.9 and 0.3 dpa, respectively while the damage associated with iron implantation was approximately 50 dpa (9). Bubbles were produced whenever helium and/or hydrogen ions were implanted.

Figure 3. Effect of Helium on the Temperature Dependence of Elongation to
Fracture in Tritium Tricked Type 316 Stainless Steel

Figure 4. Hardness of Type 316 Stainless Steel after Various Single,
Dual and Triple Ion Implantations (after Reference 9)

Bubbles have also been observed in tritium-tricked Type 316 stainless steel, even when the total helium concentration was as low as 0.18 appm (10). When the helium concentration was 2.5 appm or higher bubbles about 1.8 to 2 nm in diameter were observed on grain boundaries and slightly smaller bubbles (1.6 nm diameter) were intragranular, primarily along dislocations (10). However, even though bubbles are observed in the tritium-tricked material, only a "fraction of the ³He can be inventoried by normal through-focus TEM techniques" and "much of the ³He is trapped in small clusters less than 0.8 nm in diameter" (11). Bubble formation in the ion implanted and tritium-tricked samples is contrasted by the absence of bubbles in LANSCE irradiated, Type 316 stainless steel samples (12). Some of the bubble-free, irradiated samples contained approximately 2000 appm helium and roughly 5000 appm hydrogen (13) while bubbles developed in other irradiated material containing less than 10 appm He. The absence of bubbles in the LANSCE irradiated samples, coupled with the knowledge that microstructure plays a major role in establishing the susceptibility of a metal or alloy to hydrogen/helium effects, clearly demonstrates the difficulty in using helium implantation to simulate the effects of helium/hydrogen on a material exposed to a different set of conditions. In spite of this difficulty and the demonstration that the weldability of Type 304 stainless steels was significantly different for tritium tricked and irradiated samples having identical helium contents (14), efforts to assess the weldability of irradiated materials from measurements with tritium-tricked alloys (15) and ion-implanted materials (16) have continued.

Hydrogen/Helium Embrittlement

Microstructure and Trapping Sites

The keynote lecture at 1994 Hydrogen Effects in Materials Conference (17) stated that "the thermo-mechanical development of varying microstructures is the more (most) fundamental determinator of susceptibility" to hydrogen embrittlement. Hydrogen trapping at various microstructural features (grain & twin boundaries, dislocations, precipitates, dispersoids and even clusters of helium atoms) has been both inferred and confirmed in numerous tests and analytical assessments. Most investigators agree that hydrogen trapping, detrapping and/or redistribution play dominant roles in the virtually any hydrogen embrittlement scenario. These facts place a severe burden of proof on any attempt to use one set of environmental exposure conditions to simulate the effects of hydrogen on a metal or alloy exposed to another set of conditions. For example, before electrolytic charging of hydrogen can be used to simulate the effects of exposure to a gaseous hydrogen environment, the user must first demonstrate that the charging process did not alter the microstructure of the charged alloy. Similarly, the use of ion implantation or tritium trick technologies to simulate hydrogen/helium implantation in fission or fusion reactor materials should be limited to conditions where trapping and redistribution are fully understood or where trapping and redistribution are not important to the processes evaluated.

Microstructural damage associated with tritium-tricking of 300 series austenitic stainless steels includes: a) black spot defects generally attributed to the development of helium atom clusters and b) the formation of small (~2 to 3 nm diameter) bubbles on grain boundaries and on dislocations within the grain interiors (1, 10). These microstructural defects are effective room temperature strengtheners as is apparent from tensile tests of 304L and 309S stainless steels (Table 1) which show that the yield strength of Type 309S is increased by >50% with the accumulation of only 44 appm helium (18). This strengthening results because the helium atom clusters act as misfit inclusions in the austenite matrix and the small bubbles have pinned dislocations by precipitation along the dislocation line. Annealing, even the tritium removal anneals that often accompany a tritium tricking process, and/or plastic deformation cause the size and number density of helium bubbles to increase presumably by causing the clusters of helium atoms to relocate to either existing or newly formed bubbles. However, helium-bubble pinning of dislocations remains an effective strengthening mechanism over a range of helium bubble sizes despite extensive deformation and vacuum annealing at a moderate temperature (11). Calculations suggest that the helium bubbles act as soft inclusions with small misfit strains and that dislocation pinning by the bubbles is the major contributor to strengthening of tritium-tricked austenitic steel (11).

Table 1. Effect of Tritium and Helium on the Yield Strength
of 304L and 309S Stainless Steel

Radiation damage in austenitic stainless steels irradiated in fission (fast and mixed spectrum) reactors at temperatures below ~200°C results in small defect clusters (~1 nm in diameter), stacking fault tetrahedra and faulted Frank loops. The loops and clusters are very effective in strengthening the austenite lattice, raising the yield from about 250 MPa to a level of approximately 800 Mpa. The yield strength does not change appreciably after a dose of approximately 2-4 dpa, regardless of the helium production levels accompanying the irradiation. The cluster density exceeds 10¹⁷ cm^-3 at a small fraction of a dpa (19) and increases by an order of magnitude before the yield strength reaches its maximum. A cluster density of 10¹⁸ cm^-3 is equivalent to approximately one cluster per 10⁵ atoms in the steel. Therefore, steels containing 10 appm helium uniformly trapped at the irradiation-induced defect clusters would contain only one helium atom per cluster. Similar trapping at the faulted Frank loops would further disperse the helium. Microstructural observations from Type 316 stainless steel irradiated at T< 200°C in the APT program showed that the efficiency of helium trapping at the clusters and loops was such that no bubbles could be observed in samples that contained approximately 2000 appm helium and 5000 appm hydrogen (12, 13).

Helium Embrittlement during Welding

Helium embrittlement during welding is manifested by intergranular cracking in the weld heat affected zone (HAZ). This cracking is caused by the coalescence of cavities initiated at helium bubbles precipitated along boundaries that were perpendicular to the tensile stresses induced during weld solidification and cooling (1, 10). The coalescence of cavities requires the redistribution of helium as demonstrated by TEM measurements on welded, tritium tricked samples. The size and spacing of helium bubbles in the HAZ increases as the weld fusion zone is approached. The only way the spacing between bubbles could increase is by the movement of the precipitated bubbles. The extremely low solubility of helium precludes its absorption in, and diffusion through, the austenite lattice.

The distribution of hydrogen and helium in tritium tricked samples is significantly different than the distribution in irradiated samples. Helium bubbles precipitate on dislocations and grain boundaries in tritium tricked samples after very little tritium decay. The bubble density and size increases with increasing helium content. Redistribution of the helium from bubbles is much more difficult than the redistribution of helium that is trapped at defect clusters and/or faulted Frank loops. Bubble migration in a thermal gradient or by dislocation dragging is required for helium redistribution in tritium tricked samples while detrapping and diffusion can contribute to redistribution in irradiated materials. This difference in the ease of helium redistribution allows weld-induced helium relocation to occur much more rapidly in irradiated than in tritium-tricked materials. This difference in the ease of helium relocation causes helium induced reductions in weldability to be much greater in irradiated samples than in tritium tricked samples (Figure 5). This difference clearly illustrates the difficulty in using tritium-tricked material to simulate the response of irradiated materials to welding or any other thermal-mechanical treatment.

Figure 5. Quantitative Analysis of Weld Cracking in Samples with 10 appm Helium
Introduced by Irradiation and by Tritium Tricking (after Reference 1)

Hydrogen, Helium and the Yield Strength

Hydrogen and helium typically raise the yield strength of Type 316 stainless steel. However, strength increase is significantly less than the increase caused by the displacement damage that accompanies irradiation as seen by comparing Figure 2 with Figures 1. The helium bubbles present in the tritium-tricked samples strengthen the steel by acting as mechanically soft inclusions (11) and increasing the drag forces on dislocations (20). Under these conditions, increasing the helium concentration increases the yield strength of the materials, as shown in Figure 2. Additionally, the bubble microstructure is not stable at elevated temperatures. Dislocation dragging of the helium bubbles becomes easier as the temperature increases thus dislocation pinning becomes less effective in strengthening the alloy. Therefore, the amount of helium-induced strengthening decreases as the test temperature increases.

Irradiation-induced increases in yield strength of Type 316 stainless steel at temperatures above 300^oC are primarily due to the formation of Frank loops (19). Helium that is trapped at or near a loop at low temperatures (T<300°C) nucleate a bubble as the temperature increases (T>300°C) Frank loops and black spot damage serve as sites for bubble nucleation. The formation of helium bubbles at these sites converts the defect site from an effective strengthener to a less effective soft inclusion that may be dragged by the dislocation at elevated temperatures. This conversion, favored by high temperatures and by high helium contents, causes the yield strength of the irradiated samples containing high helium contents to be lower than irradiated samples with low helium contents.

It is no surprise that the effects of helium and hydrogen on the strength of Type 316 stainless steel are dependent on the defect structure of the alloy and on the interactions among the defects and the hydrogen and helium atoms. This dependence is clearly demonstrated in the hardness graph for ion implanted material shown in Figure 3. Hydrogen implantation caused only minimal increases in strength and the combination of hydrogen and helium was no more effective as a strengthener than helium alone. Yet, hydrogen implantation markedly increased the strength relatively to the strength increases due to the implantation of iron and helium. Clearly, the effectiveness of hydrogen as a strengthener depends on the microstructure of the material and on how hydrogen interacts with that microstructure.

Conclusions

The hydrogen/helium in metals studies referenced in this review simply illustrate the rather obvious fact that hydrogen/helium effects on type 316 stainless steel are strongly dependent on microstructure. This dependence is anticipated because of the importance of trapping and detrapping to hydrogen/helium embrittlement and strengthening processes. There are, however, two significant points that should be apparent from these discussions:

1. hydrogen/helium effects on metals are being evaluated in several different camps and increased interactions among the camps would facilitate a broad based understanding of those effects, and
2. hydrogen/helium effects on metals are dependent on the technique used to introduce the hydrogen and helium into the metal lattice. Thus, the use of one introduction technique to simulate effects resulting from another introduction technique should be avoided unless the simulation is accompanied by a comprehensive understanding of the interdependencies among the many variables affecting the behavior of the material in the situation of interest.

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