ML20003F926
| ML20003F926 | |
| Person / Time | |
|---|---|
| Issue date: | 08/05/1980 |
| From: | Budnitz R NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| RIL-095, RIL-95, NUDOCS 8104230896 | |
| Download: ML20003F926 (8) | |
Text
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V, UNITED STATES neo NUCLEAR REGULATORY COMMisslON o
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bf MEMORANDUM FOR: Harold R. Denton, Director Office of Nuclear Reactor Regulation FROM:
Robert J. Budnitz, Director Office of Nuclear Regulatory Research
SUBJECT:
RESEARCH INFORMATION LETTER #
96 F05:TP3 A'WIHI-LATION FOR NON-DESTRUCTIVE EXAMINATION
REFERENCE:
J. A. Van Den Avyle, W. B. Jones, W. B. Gauster, and W. R. Wampler, " Assessment of Positron Annihilation as a Non-destructive Examination Technique,: NUREG/CR-ll29 March 1980.
Introduction and Sumary This memorandum transmits the results of completed research on the ability of positron annihilation (PA) to measure " damage" quantitatively in metals. Although optimistic claims had been made in the literature for PA as an NDE technique, this research shows that it cannot be recom-l mended for in-service inspection of components which have been operated at elevated temperatures. The availability of extensive equipment at Sandia from non NRC projects and fatigue test specimens from the NRC Elevated Temperature Program enabled a very cost effective study of the PA technique to be undertaken for temperatures to 1100*F for 316SS. The research results from this study show that for temperatures above S700*F, where vacancies in 31655 become mobile, adequate sensitivity does not exist in the PA technique to recommend it as a NDE technique for-316SS that has been defomed or fatigued in the temperature range of 700*F to 1100*F. The reference documents the PA technique, quantifies the sensitivity, provides a good basis for assessing PA as an engineering l
l tool, and shows that it is not applicable to stainless steels for the i
operating temperature range of breeder reactors.
The ability of positron annihilation measurements to measure metal
" damage" from fatigue is a result of the fomations of vacancies and dislocations in the crystalline lattice structure. The positrons are i
l preferentially annihilated by the higher density of electrons at such ll 1
8104230 PPr
Harold R. Denton sites, thus providing a source of coincident photons which may be detected and counted. Correlation of the numbers of such sites with fatigue or plastic defomation would potentially provide a HDE measurement of 1
component life.
Structural design rules which utilize " damage accumulation" assign life fractions to segments of creep and fatigue. During creep defomation, voids can fann and during fatigue, dislocation densities can increase or decrease. Local accumulation of such damage results in crack initiation.
The difficulties encountered in utilizing ultrasonics to evaluate crack growth in 316SS emphasized the need for a new method for monitoring damage at elevated temperatures. The PA technique had been used success-fully to measure dislocation density changes in cold worked metals, and changes of vacancy concentrations and vacancy clustering in quenched metals.
The Nuclear Regulatory Comission (NRC) program at Sandia Laboratories on " Elevated Temperature.'esign Assessment" provided metallic specimens (31655) with precise defomation histories and thorough metallography.
Equipment was also available from another project for the PA measurements.
Thus, precise tools and specimens were available at Sandia to assess PA as a ncn-destructive examination method for 31655 at elevated tamperatures.
An evaluation of the technique was undertaken and ccepleted.
The goals of tne Sandia research were:
(1) to assess the sensitivity of the technique for stainless steels and the applicability at elevated temperatures; (2) to correlate positron annihilation readings with observed microstructural changes; and (3) to detennine correlations among positron annihilation measurements and fraction of life or damage.
The results of the research showed that PA measurements of fatigue could be measured to 886 K (1100*F) but that the sensitivity decreased to an impracticable level. Practical sensitivity levels are limited to the temperature at which vacancies become mobile, approximately 650 K (710'F) for 316S5. Also, the accumulation of " damage" which modifies the positron annihilation is limited to about 10% of fatigue life. However, with the design factor of safety of 20 on cycles, it is noted that this is beyond expected fatigue application for the nomal case.
Results pA measurement results are reported in tems of change of the "lineshape" parameter. This parameter is a measure of the shape of the coincident photon peak at 611 kev. A higher value means the peak is higher with respect to the total count sumation over all energy et;annelsto 10-6 The technique is sensitive to defect concentrations of 10
Harold R. Denton Initial scoping measurements were made by cold rolling 3165S in stages to 75% reduction.
Room temperature pA measurements with counting statistics error limits of +2% showed almost identical behavior with measurements on pure nickel. %asurements from low cycle fatigue specimens of 3165S at, room temperature and 866 K (1100*F) were made next (Figure 1).
The major observations from thcle tests are:
(1) Saturation in lineshape parameter (S) occurs at about 10% of fatigue life.
At room temperature, the change in lineshape parameter from (2) the initial condition to saturation is less for small-strain range tests than for the high-strain range tests.
(3) Much less change in lineshape parameter is measured for 866 K (1100*F) tests than for room temperature tests.
(4) Tests with several cccbinations of hold periods show pA saturation values equivalent to non-hold period runs al-though fatigue life was reduced by a factor of 4 for the tensile hold case.
Examination of the samples from the saturation regime of each test cen-dition of Figure 1 shewed that the lineshape parameter was not a unique function of the disiccation density; the final dislocation density at the higher temperature is equal to or greater than it is for room tempera-The following studies established the basis for changes or lack of ture change.
Annealing response of cold worked material. pure nickel and 31655 were Two evaluated to sort oE efTe~ cts of alloying elements in the 3165S.
stages of annealing were defined (Figure 2). The first, lower tempera-ture, annealing stage occurs when vacancies become mobile; the second stage is due to the disappearance of dislocations (recryst3111zation).
The interpretations were inferred from lineshape parameter.easurements j
and_ confirmed by transmission election microscopy.
I Annealing response of fatigued 316SS. The annealing curve generated at room temperature (FWure 3a) exhibits a response similar to the room temperature curve for 25% cold rolled 31655. However, the curves for cold rolled 31655 steadily decrease starting at 500 K and reach the well annealed level at 800 to 900 K.
In contrast, annealing of a fatigued sample cycled at 866 K (Figure 3b) does not produce any significant decrease in AS (a nomalization to the room temperature, large strain, lineshape parameter value). This indicates that vacancies created during fatigue cyclying at 866 K are mobile and diffuse to sinks leaving i
few residual vacancies to anneal.
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4-Harold R. Denton Discussion oj[ Results The lack of PA response to dislocations in 316SS implies that disloca-tions either trap positrons weakly or the presence of alloying elements cause the lineshape parameter for positron annihilation at disiccations to be similar to that in the undisturbed lattice. The additional work required to investigate this phenomena is considered to be outside the objectives of this study. Alternate potential positron traps are small
(.02 um) carbides which precipitate heterogeneous 1y during fatigue of this alloy at 866 K.
PA response to low temperature fatigue and monotonic deformation shows good sensitivity to 10% of life or 20% cold work. Since the nominal factor of safety on cyclic life is 20, knowledge of truly exceeding 10%
of fatigue could be useful in low (room) temperature applications.
The experiments performed here (Figure 1) with various hold periods show that PA does not correlate with reduction of life due to cree,;/ fatigue interaction. Samples with or without hold times produce the same PA results. This is consistent with the observation that creep alone does not provide large numbers of defects.
Conclusions 1.
In 31655 cold worked or fatigued at room temperature, positron annihilation (PA) measurements primarily show sensitivity to vacancies generated during deformation.
2.
In 316SS PA is insensitive to dislocation density.
3.
For 3165S the PA lineshape parameter increases monotonically with damage and saturates at 20% strain for cold work and at 10% of fatigue life for low or elevated temperature. However, much lower sensitivity in terms of change in S is observed for elevated tem-i l
perature than for room temperature.
l
- 4. ~ Non-destructive monitoring of early stages of deformation near the surface is practicable, particularly at lower temperature. Potentially such near surface damage could be correlated to fatigue crack initiation.
i 5.
The PA technique is limited to the investigation of specific points (i.e. small volumes). Measurement at a point requires about l
30 minutes and thus large volumetric surveys are not practical.
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Harold R. Denton Recomendations The positron annihilation technique is not recomended for the purpose of monitoring fatigue damage or creep-fatigue interaction at elevated temperature. The enclosed report, NUREG/CR-ll29, quantitatively defines the limited ability at elevated temperatures. Alternate applications, from room temperature to the temperature at which vacancies become mobile, 650 K (710*F) for 316SS, would produce results with practical sensitivity in the measured change of the lineshape parameter.
For further infonnation on these results, please contact Dr. T. J. Walker of my staff.
Robert J. Budnitz, irector Office of Nuclear Regulatory Research
Enclosure:
NUREG/CR-il29 6
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Positron annihilation response of 316 stainless steel fatigued at 293 K and 866 K.
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(a) Isochronal annealing response of pure nickel; S-parameter measurements of cold worked nickel with resistivity data for quenched nickel (b) Isochronal annealing response of 25% cold worked 316 stainless steel; S-parameter versus annealing temperature l
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