The impact energy used for testing the KLST specimens was 15 J and the impact velocity 3.8 m/s. For the ISO-V specimens the corresponding values were 300 J and 5 m/s. A pneumatic mechanism was used to transfer the specimen from thermal bath to the proper striking position accurately and with minimum delay time. The transition temperatures were determined from a tanh- function fit obtained by minimising the squared sum of temperature deviations.
5. Results
The transition temperatures based on the mean USE ratio of the ISO-V and KLST specimens [2] are compared in Fig. 2 which shows also the mean and 95% prediction bounds calculated for the measured data with slope 1:1. The difference between the transition temperatures given by the KLST specimens (at 1.9 J and 3.1 J) and the ISO-V specimens (at 41 J and 68 J) was approximately 63oC. The standard deviation of the correlation was 18oC. The result also shows (Fig. 2) that the temperatures hold the linear 1:1 relationship moderately up to about 70oC 68 J level but remain below the mean dependence at higher temperatures. The temperature difference (and the scatter with remarks)
-100 -50 0 50 100 150 200 250 -100 -50 0 50 100 150 5% 95% n=2*22 41J / 1.9J 68J / 3.1J
ISO-V/KLST SPECIMEN - 41J/1.9J and 68J/3.1J TEMPERATURES
KLST SPECIMEN ISO-V SPECIMEN T(KLST) = T(ISO-V) - 63oC S.D. = 18oC T (1. 9 J / 3. 1 J ) ( o C) T(41J / 68 J) (oC)
Figure 2. Comparison of transition temperatures 41 J / 68 J and 1.9 J / 3.1 J measured with ISO-V and KLST-type impact specimens. The values of lateral expansion are excluded.
For comparison, the transition temperatures were determined also at the energy levels proportioned to the real (measured) USE value of each material instead of the average value. Neither the transition temperatures nor the mean dependence of them showed, however, any marked difference as compared with the results based on the mean USE ratio. Moreover, the standard deviations were very close to each other, suggesting that no improvement can be achieved by employing the USE ratio measured for each material rather than the mean value.
The transition temperatures based on the ratio of the ligament areas, i.e. the 3.15 J and 28 J criteria, are compared in Fig. 3. The difference between the transition temperatures given by the KLST specimens (at 3.15 J) and the ISO-V specimens (at 28 J) was in this case approximately 37oC. The standard deviation
of the correlation was 14oC, i.e. somewhat lower than that given by the USE-
based energy criteria (18oC). The result confirms that the thickness correction formula (2) [4] can be used for the KLST specimen (compare eq. 3) and also supports the validity of the formula (2) in general, i.e. the applicability of it for
(notched) impact specimens with various dimensions. The dependence of the transition temperatures (Fig. 3) is also distinctly linear and follows the 1:1 slope. -150 -100 -50 0 50 100 150 200 -100 -50 0 50 100 150 n=22 5% 95% T(KLST) = T(ISO-V) - 37oC S.D. = 14oC ISO-V SPECIMEN KLST SPECIMEN
ISO-V/KLST SPECIMEN - 35 J/cm2 TEMPERATURES
T( 3 .15 J) ( o C) T(28 J) (oC)
Figure 3. Comparison of transition temperatures 28 J and 3.15 J measured with ISO-V and KLST-type impact specimens.
The values of USE measured with the ISO-V and KLST specimens for the materials studied are shown together with the measurements published elsewhere for various steels [5] and for one 2 1/4Cr1Mo pressure vessel steel [6]
in Fig. 4. These data were used for deriving the correlation between the USEs of the ISO-V and KLST specimens. An appropriate function [3] was fitted to the measured USE data so as to solve the unknown factor (m) in eq. (2) as follows:
m E
A
ISO V n
Using these values, the formulas (2) and (5) are now available for calculating, e.g. iteratively, the USE of the ISO-V specimen from the value measured with the KLST specimen.
The straight line in Fig. 4 shows the mean (constant) USE ratio, according to which energy criteria 1.9 J/3.1 J/0.3 mm vs. 41 J/68 J/0.9 mm have been determined [2]. The relationship used describes thus also the materials discussed in this study satisfactorily though it is not clear if the correlation actually is linear as it has been assumed.
50 100 150 200 250 2 4.5 7 9.5 12 KLST E (J) CALCULATED CURVE: EKLST=EISO-V*B/10 mm*(b/8 mm) 1+m m=(EISO-V/188 J)0.32 E (J) ISO-V
Figure 4. Correlation of upper-shelf energies. The curve shows an estimate of given type and the straight line the relationship used for determining the proposed [2] transition criteria. Data from this work (o) and from ref. [5] ([) are included.
The USEs of the ISO-V specimens were also estimated from the USEs measured with the KLST specimens by using the derived relationship. It proved that the USE of the ISO-V specimen is predictable by this means with the accuracy of about ± 20%. Three measurements [5] diverged clearly outside this prediction.
The irradiated KLST and ISO-V specimens (15X2MFA-type steels) had been prepared so that the neutron fluences were equal, which made it possible to compare the transition temperature shifts of the base and weld materials exhibited by different specimens. The results indicate that the irradiation shift shown by the KLST specimen tends to be lower than the shift shown by the ISO-V specimen, with the same material condition. The largest difference was
measured to be as big as 51oC, and in one case was the difference to the
opposite direction, the shift shown by the KLST specimen being 5oC higher than the shift shown by the ISO-V specimen.
The neutron fluences of the KLST and ISO-V specimens fabricated from the irradiated FFA, JFL and JRQ steels were close to each other as well and thus the irradiation shifts of the transition temperatures measured with different specimens comparable. The shifts measured at different energy levels are compared in Fig. 5.
-40 -20 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 160
TT SHIFT, ISO-V SPECIMEN (oC)
TT SHIFT, KLST SPECIMEN ( o C) 28J/3.1J 41J/1.9J 68J/3.1J 1:1 Line JFL JRQ FFA
Figure 5. Comparison of the irradiation shifts of transition temperatures measured with ISO-V and KLST-type impact specimens at different energy levels
The degree of recovery was predicted quite accurately by the KLST specimen for those steels where the transition temperature shift in annealing was large, but failed to predict the shift for the ones where it was small. The best correlation was obtained at the 28 J/3.15 J energy levels and, which is important, the degree of recovery measured with the KLST specimen was in each case lower than or equal to that measured with the ISO-V specimen. In general, the degrees of recovery at 68 J and 3.1 J showed a large variation between the values measured with different specimens.
6. Discussion
The relationship measured in this work between the transition temperatures of the ISO-V and the KLST type Charpy specimens at 41 J / 1.9 J and 68 J / 3.1 J is nearly equal to that reported elsewhere [2], supporting thus the validity of the 65oC temperature difference measured [2] for a large number of steels. On the other hand, the current result confirms that the formula (2) is valid also for the KLST specimen, according to which the transition temperature with this
specimen is 38oC higher as compared to that of the ISO-V specimen at the
35 J/cm2 energy level, i.e. when the absorbed energy is normalised in relation to the ligament area. The energy criteria would thus be, for example, 28 J and 3.15 J as it has been assumed in this work.
It has been shown (for a 15X2MFA steel) that lateral expansion criteria 0.9 mm / 0.3 mm are also consistent with the proposed energy criteria, i.e. the transition temperature difference is expected to be 65o C. However, the absorbed energy normalised in relation to the ligament area corresponds to a same lateral expansion for both specimens, which has been shown (for this steel) as well [3]. The discussed USE-based energy criteria (without lateral expansion) resulted in a standard deviation (18oC) that is equal or somewhat larger than that published elsewhere [2], i.e. around 15oC. According to the rather high standard deviation obtained in this work and the deviation from the linear 1:1 relationship observed with the USE-based criteria suggest that an improved (general) correspondence could be achieved, most likely with steels having a high transition temperature (T68 J > 70
o
C), by applying the criteria based on the ligament area, for example the value 35 J/cm2 used in this work. However, it is not feasible to definitely
address which one of the criteria discussed gives a better (general) correlation without including more different types of (structural) steels in the correlation. The relationship of the KLST and ISO-V transition temperatures has a slope close to 1:1 in both cases, but especially the 28 J/3.15 J correlation cannot be improved by estimating also the slope (Figs. 2 and 3).
Although the correlation of the transition temperatures between the different specimens could be described satisfactorily by employing the criteria based on the mean (and constant) ratio of USEs, the linear relationship of the USEs is not evident, but the dependence is following more likely the applied power law (5) than a linear relationship. The verification of this dependence is on the basis of only these measurements difficult because there was a quite large scatter in the USE values. One obvious reason for this scatter is that the USEs have been determined as an average of only a few test results. It follows also that the estimated parameters should be considered as first approximations rather than final values. By perceiving this fact the correspondence of the calculated and measured USEs can be regarded as moderate.
The deviations of the measured irradiation shifts in the transition temperatures is of major importance since the range of variation was large and the KLST specimens showed mostly lower shifts than the ISO-V specimens. For the 15X2MFA steels the shifts measured at the 68 J/3.1 J energy levels differed more than those measured at the other two levels, which can be explained by the low USEs of the irradiated weld materials. The ratios USE/68 J (ISO-V) and
USE/3.1 J (KLST) were in both irradiated welds in the range 0.8−0.9. As the
reason for the rest of the deviation in the irradiation shifts shown by the ISO-V and KLST specimens remain obscure, it should be recommended to add at least
30oC to any transition temperature shift measured with the KLST specimen,
independently of the criteria, to ensure a conservative estimate for the ISO-V specimen. This ”safety margin” also equals the 2*standard deviation measured for both correlations and hence, it can be regarded as a minimum margin for a transition temperature shift evaluation. Further, to avoid any excessive uncertainties, transition temperatures should not be measured at energy/lateral expansion levels exceeding approximately 75% USE. In general, the difference
KLST specimen tends to give shifts of up to 30−40oC lower than those measured with the ISO-V specimen.
7. Conclusions
In total 22 structural steels or steel conditions were included in the study. Steels with four basic compositions were studied in the irradiated condtion. The discussion is limited to various energy criteria proposed for determining Charpy transition temperatures from sub-size specimen data, excluding the lateral expansions. With these remarks the following general conclusions can be made from the results of this work.
1. The 3.1 J / 1.9 J / 0.3 mm transition temperatures of the KLST specimen are
approximately 65oC lower than the 68 J / 41 J / 0.9 mm transition
temperatures of the normal-size (ISO-V) Charpy specimen.
2. The 3.15 J transition temperature of the KLST specimen is approximately
38oC lower than the 28 J transition temperature of the ISO-V specimen, i.e. the difference is on the average equal to that measured with other types of impact specimens at the absorbed energy level normalised according to the ligament area, that is 35 J/cm2.
3. The USE of the ISO-V specimen can be estimated from the USE measured
with the KLST specimen by solving the formulas (4) and (5), where A and m have the values 188 J and 0.32, respectively. An iteration procedure can be used for solving E ISO-V.
4. Transition temperatures should not be measured at energy/lateral expansion
levels exceeding about 75% USE. The mean transition temperature shift (due to irradiation) measured with the KLST specimen should be raised by at least 30−40oC in evaluating the shift for the ISO-V specimen.
8. Acknowledgements
The reported work was part of the Finnish Research Programme on the
Structural Integrity of Nuclear Power Plants 1995−1998, sub-project RAVA1 on
Material Degradation in Reactor Environment. The work was performed in 1996−1997.
Co-operation with Rainer Rantala and Matti Ojanen from the Radiation and Nuclear Safety Authority (STUK), Ralf Ahlstrand and Jyrki Kohopää from IVO Ltd. together with many other colleagues is gratefully acknowledged.
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