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The fracture toughness measured on sensitized 304 stainless steel in simulated reactor water
Nuclear Engineering and Design 93 (1986) 95-106 North-Holland, Amsterdam
THE FRACTURE TOUGHNESS MEASURED STEEL IN SIMULATED REACTOR WATER Nobuya NAKAJIMA,
95
ON SENSITIZED
Seishi S H I M A *, H a j i m e N A K A J I M A
304 S T A I N L E S S
and Tatsuo KONDO
Department of Fuels and Materials Research, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-11, Japan Received 18 December 1985
The environmental conditions chemically equivalent to BWR primary water, e.g. 288°C, 0.2 ppm 02 and/or 98°C, air-saturated, were found to influence considerably the in-water fracture toughness values of furnace-sensitized Type 304 stainless steel. Notched compact tension and three point bend specimens sampled from two heats of standard materials (0.06% C) showed significant reduction in dJ/da values reflecting consistently the effects of loading rate, temperature, dissolved oxygen concentration and degree of sensitization. In particular the crack enhancement with lowering the loading rate was significant. The effect became apparent with dJ/dt at and below 1 x 10 -1 kgf.mm/mmZ/min (1.6 x 10 J/m2/s) in the typical BWR environment. Based on the results, it is suggested that a critical consideration is needed on the significance of such an environmental effect in the LWR structural safety evaluation, in particular that the probability of instable fracture at the "rings" of sensitized material near welded joints is subject to reviewing.
1. Introduction
Materials for the primary system piping of the nuclear power plants are generally required to have sufficient ductility and toughness, while flaws either existing potentially or to be generated during service can cause unexpected failures of the pressure boundary structures through various environment-enhanced degradations. Typical mechanisms of such flaw generation and growth are the corrosion-assisted processes, namely the stress corrosion cracking and the corrosion fatigue, where quasi-brittle failure grows into ductile ligament through a time-dependent slow process. In contrast to such a slow, environment-assisted subcritical crack growth, consideration of the response of material has generally been made without taking into account the effect of chemical environment. If such a consideration is justified one may suppose the occurrence of ductile tearing behavior for the ductile Type 304 stainless steel with a growing subcritical crack when exposed to large transient loading. In fact, the application of the non-linear elastic plastic fracture mechanics approach to the analy* Visiting Research Fellow from Toshiba Corporation. Present address: Nuclear Energy Group, 8 Shinsugita-cho, lsogo-ku, Yokohama 235, Japan.
sis of the fracture behavior of the pipes of ductile material such as Type 304 stainless steel have been made with increasing success [1]. Practically, the tearing instability analysis has been widely applied at present in evaluating flaws in the L W R piping structures [2]. For such purpose, the so-called J-resistance curves are taken from the bend-beam or the compact tension type specimens of appropriate size. Traditionally, testing with parameters such as the J-integral has been conducted either in air environment at room temperature or at higher temperatures under some specified loading conditions so that the values intrinsic to the test materials are extracted without environmental disturbance. In contrast, from the view point of life prediction of a structure, the evaluation of practical fracture resistance of a material requires to take into account any factors that may enhance crack initiation and growth so that reasonable safety evaluations can be made based on such information. It has been already widely recognized that the intergranular stress corrosion cracking (IGSCC) that has been found in the weld heat affected zone (HAZ) of the piping of BWR plants is sensitive to the existence of small surface flaws, tensile stress, oxygenated water in contact with the flaws and the extent of the sensitization of the material. In view of the test methodology, many of the stress
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Table 1 Chemical
composition
and plate thickness of the test materials
C
Si
Mn
P
S
Ni
Cr
MO
N
A) SUS 304
.066
.55
1.04
,027
.005
8fl6
18.39
.09
.015
25 rnrnt
@J SUS
.06
.51
0.95
.033
.009
9.45
18.55
.Ol
-016
40 mmt
Material
304
Plate thickness
(wt O/o) transients during start-up and those during power generating conditions. The use of lower temperature. 98°C. water has an additional basis of choice because it provides a convenient reference basis with easier access and operation of the tests relative to those under high temperature and pressure conditions.
corrosion cracking tests under simulated reactor conditions can be roughly classified in two categories. i.e. those under constant load with either notched or smooth specimens, and those under monotonic slow strain rate condition (SSRT) with smooth bar specimens. In the present study, the influence of water environment on the fracture toughness under dynamic loading condition is examined under the premise that the plant components can be exposed to such a condition in the event of various kinds of transients. For such purpose, the fracture toughness tests with typical sensitized piping material under a variety of loading rates are conducted with specimens immersed in an aqueous environment. The environments employed are deionized water either at 98°C or 288°C simulating respectively the load
2. Experimental 2.1. Material Two heats of Type 304 stainless steel. with the chemical composition and the thickness shown in table 1, were used as the starting materials. These materials
- 20 “C
Thermocode
.-. u-4
d
S
-2O'C
PureWater
I
-
Present
Water
@; @ Fig. 1. Schematic
layout of 98°C water loop system.
;
Va Ive
(Opened
Pump
Loop)
, 0M ,
0;
; Motor Water
Sampling
97
N. Nakajima et al. / Fracture toughness of sensitized 304 stainless steel
were heat-treated at 650°C for 2 h to produce a furnace sensitized condition, which were the standard reference state of the materials for the present study. In addition to the above, different degrees of sensitization were given also to one of the starting materials by heating at 650°C for 0.5, 1.0 and 4.0 h in the furnace. The temperature of 650°C for sensitization was taken as a somewhat accelerating condition relative to those employed in simulating the welding followed by the so-called low temperature sensitization (LTS) [3] effect. The conditions chosen are considered to give a moderate conservatism to the evaluation. 2.2. Test apparatus
Two different test systems were used for low and high temperature environments. The equipment for the fracture toughness test with the oxygenated water environment at 98°C is shown schematically in fig. 1. The apparatus consists of the units for loading, recirculating water loop and data recording. The loading system is
__
Cross Head
--
Load Cell
Trctnsducer ( A ',
similar to the ordinary constant extension rate testing (CERT or SSRT) machine except its drive mechanism being capable of producing the cross head displacement speed down to 5 × 10 -5 m m / m i n , which is much slower than that employed in the conventional machines. The machine is also capable of performing constant load tests. The loading in the 98°C test was of the three point bending method, a schematic drawing of the assembly is shown in fig. 2. The measurement of the displacement is made by the two differential transformers which are set parallel to the measuring points (see A and B in fig. 2). The actual displacement was determined by the arithmetic mean of the two values from the detectors A and B so that possible error due to deflection of the center axis can be minimized. To maintain the water chemistry in the cell at the specified condition, the water appropriately prepared was supplied continuously with the refreshing rate of about 5 l / h at 98°C under air-saturated condition. Cooling jackets were set up the upper and lower positions of the pull rod to keep the temperature from the influence of the undefined diffusion of heat from the cell containing 98°C water. For high temperature tests, a recirculating autoclave system with a sealed-in loading section was used. The load was applied using a electromagnetic servo-hydraulic (MTS) testing machine, and its maximum load capacity was 7000 kgf (6.9 × 104 N). The displacement
Cooling 3ocket Transducer (B)
Bellow., Gloss Cell (Upper) O - rin( Bending Apporotus Pure Water Le~ Rolle
100
Insulator WQter
a) Three point bend specimen ( 3PB )
Gloss Cell (Lower) Wot~ Exhaust Volv
Test Piece
Bending Apparatus
SIG. 20%
62.50
I Fig. 2. Load assembly for three point bend test in pure water at 98°C.
b) Compact tension specimen (1T-CT)
Fig. 3. Specimen geometry (unit : mm).
98
N. Nakajima et aZ / Fracture toughness of sensitized 304 stainless steel
of the compact tension specimen in autoclave at 288°C under load was measured with a MTS water-immersible displacement gauge. The water, which was distilled and then deionized, was held in a 50-1itre tank. The exact water chemistry specified in terms of dissolved oxygen content and electrical conductivity was conditioned at room temperature by bubbling the mixture of appropriate oxygen content through water in the holding tank. The test section was situated inside a 5-1itre autoclave at 288°C with the compact tension specimen located at the central position. The oxygen content, pH and conductivity of the water were monitored continuously at the spots located between the holding tank and the pressurizing pump. The refreshing rate of the water was maintained at 10 l / h .
J-integral values were determined from the load (P) versus displacement (D) plots. The areas (A) under the P - D curves were calculated with numerical integration method. The value of J for the three point bend specimens was calculated using the formula originally developed by Bucci et al. [4]. The expression of the formula is given as 2A J=-BT
(1)
For the compact tension specimens, calculation was performed using the formula proposed by Merkle and Corten [5], which had been recommended by ASTM. Namely 2A
J=~l~
2P6 + a 2 Bb '
(2)
2.3. Test procedure
Fig. 3 shows the dimension of the three point bend specimen and the compact tension specimen used in the present study. The three point bend specimens were tested at 98°C under air-saturated condition while the compact tension specimens were tested at 288°C with 0.2 ppm 02 and 8 ppm O 2. The test procedure for the fracture toughness test is outlined below (see fig. 4): (1) Prior to the test, the specimens were fatigue-precracked to a crack length of a / W = 0.5, where " a " and " W " denote crack length and specimen width respectively. (2) Cleaning was made by reagent grade acetone in an ultrasonic cleaning device. (3) Mounting the specimen into the equipment, it was preloaded to about 10 kgf (98 N) before the system was brought to the test temperature. (4) Fracture toughness test was started with a slow and constant displacement rate. (5) The load-time and displacement-time relations were monitored during the test, which provided the basic mechanical data required for later analyses. (6) The test was terminated as the pre-determined displacement value was reached by extension of the crack. (7) After the test, the specimen was opened through cyclic loading to grow the crack by fatigue for surface fracture examinations. (8) All specimens so processed were examined with an optical microscope to determine the initial crack length (a0) and the crack extension (/~a) at the surface. The process was repeated following the steps (1) through (8) to obtain the J-resistance curves based on the multi-specimens method.
where A= B= b=
area under the load versus displacement curve, thickness of the specimen, remaining ligament of the specimen, coefficients developed by Merkle and Corten O~1 , Og2 to account for the tension component in the compact tension specimen, p~ final load value and, 8= final load point displacement. After the fracture toughness tests at 98°C and 288°C, detailed scanning electron microscopic (SEM) examination was performed on the crack-opened specimens. =
3. Results and discussion 3.1. Effect o f displacement rate
Plots of J versus crack extension (Aa) for different cross head speeds ( b ) are presented in fig. 5. Displacement rates in the r a n g e 5 x 1 0 -5 t o 5 x 1 0 1 m m / m i n were employed. At higher displacement rates, the sensitized material showed a behavior similar to the annealed material tested in air environment at the same temperature. In water at 98°C, the fracture toughness of the sensitized material decreased as the rate of displacement was lowered. For instance, the decrease in the value of J was by a factor in excess of 3. The value of J1c, obtained as the intersection of linear regression line and the blunting line, decreased with lowering the displacement rate. The Jk value in water was almost diminished at the lowest displacement rate employed in the present tests. At the same time the slope of the J-resistance plot was decreased by a factor of 2. The same behavior was
N. Nakajirna et aL / Fracture toughness of sensitized 304 stainless steel
99
observed in simulated boiling water reactor environment at 288°C as will be described later.
i
3.2. Effect of degree of sensitization
0
From the fractographic feature, the observed decrease in fracture resistance in the water environment was judged to have been caused by the effect common with that prevailing in the IGSCC. The effect of degree of sensitization on the J-resistance curves can be seen in fig. 6. Though the data were somewhat scattered, a similar behavior was observed in case of the materials with lower degree of sensitization at the displacement rate of 1 × 10 3 m m / m i n . While the value of J decreased as the degree of sensitization was increased at the same displacement rate, e.g. a reduction by the factor of 3 was recognized between the maximum and minimum cases in the present study. It was noted that the value of Jlc and the slope of the J-resistance plot decreased as the degree of sensitization was increased. Based on the results of this study, consideration of the fracture problems in BWR piping with inner surface cracks is suggested to recognize the importance of the thermal history of the piping material. Especially the possible effect of long time aging of weld heat affected zones of LWRs at operating temperature, known as the
Iko"~ co n s ~
constant
d
i" 0
Start at 98* or 288°C
[
Stop
" Time
Fig. 4. Experimental procedure of J fracture toughness test in simulated reactor water.
160
I
SUS 304 Steel of 98°C. 3PB Specimen['
650*(; b (mm/min)
As Rec. x2hrs
A
I !
12o
i I
NE ~:
/
~.
~
;"
V//
/
~/
•
5/10
A
•
5/,oool
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e
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•
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Air
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80
_
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_
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0.5
1.o Crack
extension
t5 AO.
2.o
(mm)
Fig. 5. Effect of displacement rate on E- R curves of sensitized 304 stainless steel in pure water at 98°C.
N. Nakajima et aL / Fracture toughness of sensitized 304 stainless steel
100
160
I
r
SUS 304 steel at 98"C, 3PB Specimen
120 -
-
Heat treat ment
I I
--•:--
[) = 0 001 ram/rain
"~O--
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re(:.
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650°Cx1.0hr 650°Cx 2.0 hrs 650°Cx 4.0hrs
0
---
-
!-o
•
"
40
0
0.5 Crack
1.0 extension
Aa
1.5 (ram)
2,0
Fig. 6. Effect of degree of sensitization on J - R curves of 304 stainless steel in p u r e water at 9 8 0 ( ".
140| 1201"~ ~-~ / N
I
!
S U 5 3 0 4 Steel at 288'C 1T-CT(20% S.G.)
I D.O.(ppm) 0.2 8
,oo t
t
~+o.
•
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o0 0.01
.o+
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1.0 CrQck extension
.5
2.0
Am, ( r a m )
Fig. 7. The J - R curves of sensitized 304 stainless stell in s i m u l a t e d reactor w a t e r at 288°C.
N. Nakajima et al. / Fracturetoughness of sensitized 304 stainless steel where E = o0 =
low temperature sensitization [3], must be subjected to critical consideration.
Young's modulus, flow stress and d J / d a = slope of the J-resistance plot. Now, the value of d J / d a is considered as the basic parameter in evaluating the fracture instability. All the data of d J / d a in the present study are shown in fig. 8. The observed d J / d a values of the sensitized Type 304 stainless steel in the water environment were varied significantly with lowering the displacement rate. The annealed materials and the moderately sensitized materials exhibited higher d J / d a values than the highly sensitized ones. The lowest values of d J / d a appeared in the case of test in water with 8 ppm 02 at 288°C, where the d J / d a was decreased by a factor of 5. Similar trend of the d J / d a versus displacement rate relationship was recognized in both environments employed in the present study, i.e. water environments at 98°C air-saturated and at 288°C with 0.2 ppm 02.
3.3. Effect of temperature In order to study the effect of temperature/oxygen combinations on the crack growth, the fracture toughness tests were carried out in 0.2 ppm O 2 and 8 ppm 02 at 288°C. The test results on the sensitized material are shown in fig. 7. From the limited number of data points, it is already clear that the value of Jic and the slope of the J-resistance plot at 288°C display a trend very similar to that observed in water at 98°C. The J-value decreased to substantially low values when the tests were conducted in water containing about 8 ppm O 2 at 288°C. For instance, the J value for the lowest displacement rate was 70 to 80% less than that for the highest displacement rate. These results indicate that the practical fracture toughness in the environment is strongly affected by the oxygen content in water at 288°C.
3.5. Fractographic observation Fracture surfaces of the specimens tested were examined by the SEM. Fig. 9 shows typical SEM views of the crack surfaces for cases under different crack extension conditions in water at 98°C. The ductile fracture with transgranular cracking mode is observed in the surface of specimen (A), the quasi-brittle fracture with intergranular elements is seen in the specimen (B). Fig. 10 shows the SEM micrographs of the fracture surfaces
3.4. Comparison of d J / d a From the present results the tearing modulus [6] was evaluated. The tearing modulus is defined as E dJ 02 d a '
T-
(3)
140 120 --O ~100 E
--
E
f
~
.
.
.
.
.
.
E 80
E ," I ~198"c
O 40
A /
20
l
O,O:.D i s s o l v e d o*ygen 010=
10-3
I
S U S 3 0 4 Steel J 288'c _ D.O.
/
I
~ o I ( • I ~ I I
101
e
(p m) I ( I I 0 I
I
10-z Displacement rate,(mmlmJn)
Meat
treatment Asreceiv~ 650"CXOlShr 6SO'Cxl.0hr 650"Cx2.0hr
_
650"C x 4.0 hr 10-1
Fig. 8. The summary of dJ/da versus displacement rate in simulated reactor water.
100
102
N. Nakajima et al. / Fracture toughness of sensitized 304 stainless steel
1 i
i
¢-
d~ tD ,zl
tD
,¢ o
E ts3 0 0 C)
Y~
I!
to) tt)
k~
103
N. N a k a j i m a et al. / Fracture toughness o f sensitized 304 stainless steel
8
d
H
A
\
.=_
\ d
II
11
I
k
7~ e..
7~
/I /
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J
I
n
l
°t"~
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\ e~ 0
r~
104
N. Nakajirna et aL / Fracture toughness of sensitized 304 stainless .steel
obtained in water at 98°C at various extension rates. The ductile fracture mode was recognized for the case tested at the displacement rate of 5 x 10 -1 m m / m i n , while brittle mode was the case at lower displacement rates. In the specimen tested at 5 × 10 3 m m / m i n , the fracture surface was seen to be of the mode of mixed transgranular and intergranular cracking. The feature of the fracture surfaces of the specimens tested in water at 288°C is shown in fig. 11. The specimen tested at 5 × 10 1 m m / m i n , in water with 0.2 ppm 02 showed a ductile fracture surface, while the specimen tested at 1 × 10 .3 m m / m i n in water with 8 ppm O 2 showed brittle fracture mode. Those results obtained suggest clearly that the tendency of IGSCC increases as the displacement rate is lowered in the simulated boiling water reactor environment. In other words, the fracture mode similar to those generally observed in inert environment can occur in water at 98°C and 288°C only when the displacement rates are sufficiently high to prevent the occurrence of I G S C C mode fracture.
The crack enhancement in water environment observed in the form of reduced fracture toughness is, therefore, judged to be due to the "stress corrosion cracking" effect. The enhancement occurs in two ways. For the condition J > Jlc, the crack extension at low displacement rates in water finds easier paths through IGSCC. As the lowering of Jlc value indicates the onset of crack extension is also affected significantly by the water environment. Here the term of J~c~, is conveniently given to the Jlc obtained in water at low displacement rates. 3.6. Crack growth rate In order to see more clearly the crack extension enhancement by the environment, an acceleration factor is defined as [da/dt]water/[da/dt]air. Using such a factor, the results of multi-specimen fracture toughness test are plotted as shown in fig. 12. The acceleration factor increased by a factor of 2.5 as the displacement rate was lowered to the lowest level within the range tested.
Crack growth direction ,,
lOOpm I
a) 13= 0.5 m m / m i n in O. 2 ppm 02 at 288"C Fig. 11. SEM photographs of fracture surface.
1O0 um
b) 0=0.001 m m l m i n i n 8 ppm 02 at 288"C
N. Nakajima et al. / Fracture toughness of sensitized 304 stainless steel
105
SLJS504 Steel of 98"C, 3PB Specimen • 650"C x 2hrs o As Rec.
3.0
i
o~
2.0
i
1.0
I
0
i 10-4
| 0 -~
10-2
[)
t0 -t
fO0
{mm/min )
Fig. 12. Relationship between the acceleration factor, [da/dt]water/[da/dt]air and the displacement rate at 98°C. The time dependent nature of the results as observed in the present study must be taken into account. In evaluating the fracture toughness of SCC-sensitive materials in general, it should be pointed out that the range of deformation rate in service environment is one
10o
I SUS 304 Steel
at 9O*C, 3PB Specimen
101 E
% E E 10-2
10-4
10-3 d Dldt, (ram I rain )
10-2
Fig. 13. Correlation between dJ/dt and dD/dt (displacement rate).
of the most important factors in assessing the cracking behavior. Since the values of displacement rate indicated are based on the experimental convenience for laboratory test, more generic expression may be appropriate. For such a reason the results are recognized in the relationship between J ( d J / d t ) and D ( d D / d t ) . The consequence at 98°C is shown in fig. 13. The trend of J and b fell on the straight line. F r o m fig. 8 and fig. 13, it can be stated that the effect of water environment must be taken into account in evaluating the fracture toughness for cases where d J / d t is below 1 × 10 - i kgf. m m / m m 2 / m i n (1.6 × 10 j / m 2 / s ) . The geometry of cracks in actual BWR piping may generally take the form of circumferential surface flaws. In evaluating the fracture resistance of such piping, a serious consideration of the following factors will be required; they are, possible variation in water chemistry and its state at the crack tip [7], the time dependent aspect of loading mode [8], temperature [9], and thermal history of the material [10]. Surface flaws in the piping, in contrast to those tested in specimens, may be under a more complex stress distribution. Behavior of surface cracks in pipings may be affected by these factors. From the results in this study, it is suggested that more critical consideration is necessary in the evaluation of the integrity and the safety of welded stainless steel piping in BWRs where the effect of water environment can by no means be ignored. This aspect is believed to be particularly important in evaluation of
106
N. Nakajima et al. / Fracture toughness of sensitized 304 stainless steel
the instability fracture of the pipings at the portions of the near-weld joints.
4. Summary The effect of fracture resistance of Type 304 stainless steel in simulated reactor water was examined with special attention focused on the loading rate. (1) The value of Jlc and slope of the J-resistance plot are strongly affected by the factors such as displacement rate, temperature, degree of sensitization a n d oxygen content in water. For the cases of d J / d a below 1 x 10-1 k g f . m m / m m Z / m i n (1.6 x 10 j / m 2 / s ) , the effect of water e n v i r o n m e n t a n d the nature of time-dependent p h e n o m e n a must be taken into serious account in evaluating possible instable fracture of the structure to be considered. (2) The e n h a n c e m e n t of cracking was caused by the participation of intergranular stress corrosion cracking. (3) The observed p h e n o m e n a need to be taken into account in the consideration of the fracture probability evaluation of nuclear reactor pipings.
Acknowledgment The authors wish to express their appreciation for the advices given by Professor H. Takahashi and T. Shoji of Tohoku University and Drs. S. Miyazono and T. Uga of J a p a n Atomic Energy Research Institute given during the course of this study. T h a n k s are also due to Mr. E. Ebine who assisted the fracture toughness tests, and to Mr. M. Kikuchi who conducted the SEM observation.
References [1] G.M. Wilkowski, J. Ahmad, A. Zahoor, C.W. Marschall, D. Brock, I.S. Abou-Sayed and M.F. Kanninen, Fracture of circumferentially cracked Type 304 stainless steel pipes under dynamic loading, in: Elastic-Plastic Fracture, (ASTM STP 803, ASTM, 1983) pp. II-331-II-350. [2] C.H. Popelar, J. Pan and M.F. Kanninen, A tearing instability analysis for strain-hardening Material, in: Fracture Mechanics (ASTM STP 833, ASTM, 1984) pp. 699-720. [3] W.G. Morris and A.M. Ritter, Microstructural investigations of low temperature sensitization in 304 stainless steel, Proc. Seminar on Countermeasures for Pipe Cracking in BWRs, EPRI WS-79-174, Vol. 2 (1980). [4] R.J. Bucci, P.C. Paris, J.D. Landes and J.R. Rice, J integral estimation procedures, in: Fracture Toughness (ASTM STP 514, ASTM, 1972) pp. 40-69. [5] J.G. Merkle and H.T. Corten, A J integral analysis for the compact specimen, considering axial force as well as bending effects, Trans. of ASME, J. of Pressure Vessel Technol., Series J, 96, No. 4 (1974) pp. 286-292. [6] P.C. Paris, H. Tada, A. Zahoor and H. Ernst, The theory of instability of the tearing mode of elastic-plastic crack growth, in: Elastic-Plastic Fracture (ASTM STP 668, ASTM, 1979) pp. 5-36. [7] P.L. Andresen and F.P. Ford, Technical considerations for startup deaeration procedures to minimize stress corrosion cracking of 304 stainless steel piping, Proc. Seminar on Countermeasures for Pipe Cracking in BWRs, EPRI WS-79-174, Vol. 1 (1980). [8] R.J. Eiber, W.A. Maxey, A.R. Dully and T.J. Atterbury. Investigation of the initiation and extent of ductile pipe rupture, Battelle Columbus Laboratories, BMI-1908 (June 1971). [9] W.H. Bamford and A.J. Bush, Fracture behavior of stainless steel, in: Elastic-Plastic Fracture (ASTM STP 668, ASTM, 1979) pp. 553-577. [10] R.L. Fullman, Predictability of low temperature sensitization in stainless steels, Proc. Seminar on Countermeasures for Pipe Cracking in BWRs, EPRI WS-79-174 Vot. 2 (1980).
THE FRACTURE TOUGHNESS MEASURED STEEL IN SIMULATED REACTOR WATER Nobuya NAKAJIMA,
95
ON SENSITIZED
Seishi S H I M A *, H a j i m e N A K A J I M A
304 S T A I N L E S S
and Tatsuo KONDO
Department of Fuels and Materials Research, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-11, Japan Received 18 December 1985
The environmental conditions chemically equivalent to BWR primary water, e.g. 288°C, 0.2 ppm 02 and/or 98°C, air-saturated, were found to influence considerably the in-water fracture toughness values of furnace-sensitized Type 304 stainless steel. Notched compact tension and three point bend specimens sampled from two heats of standard materials (0.06% C) showed significant reduction in dJ/da values reflecting consistently the effects of loading rate, temperature, dissolved oxygen concentration and degree of sensitization. In particular the crack enhancement with lowering the loading rate was significant. The effect became apparent with dJ/dt at and below 1 x 10 -1 kgf.mm/mmZ/min (1.6 x 10 J/m2/s) in the typical BWR environment. Based on the results, it is suggested that a critical consideration is needed on the significance of such an environmental effect in the LWR structural safety evaluation, in particular that the probability of instable fracture at the "rings" of sensitized material near welded joints is subject to reviewing.
1. Introduction
Materials for the primary system piping of the nuclear power plants are generally required to have sufficient ductility and toughness, while flaws either existing potentially or to be generated during service can cause unexpected failures of the pressure boundary structures through various environment-enhanced degradations. Typical mechanisms of such flaw generation and growth are the corrosion-assisted processes, namely the stress corrosion cracking and the corrosion fatigue, where quasi-brittle failure grows into ductile ligament through a time-dependent slow process. In contrast to such a slow, environment-assisted subcritical crack growth, consideration of the response of material has generally been made without taking into account the effect of chemical environment. If such a consideration is justified one may suppose the occurrence of ductile tearing behavior for the ductile Type 304 stainless steel with a growing subcritical crack when exposed to large transient loading. In fact, the application of the non-linear elastic plastic fracture mechanics approach to the analy* Visiting Research Fellow from Toshiba Corporation. Present address: Nuclear Energy Group, 8 Shinsugita-cho, lsogo-ku, Yokohama 235, Japan.
sis of the fracture behavior of the pipes of ductile material such as Type 304 stainless steel have been made with increasing success [1]. Practically, the tearing instability analysis has been widely applied at present in evaluating flaws in the L W R piping structures [2]. For such purpose, the so-called J-resistance curves are taken from the bend-beam or the compact tension type specimens of appropriate size. Traditionally, testing with parameters such as the J-integral has been conducted either in air environment at room temperature or at higher temperatures under some specified loading conditions so that the values intrinsic to the test materials are extracted without environmental disturbance. In contrast, from the view point of life prediction of a structure, the evaluation of practical fracture resistance of a material requires to take into account any factors that may enhance crack initiation and growth so that reasonable safety evaluations can be made based on such information. It has been already widely recognized that the intergranular stress corrosion cracking (IGSCC) that has been found in the weld heat affected zone (HAZ) of the piping of BWR plants is sensitive to the existence of small surface flaws, tensile stress, oxygenated water in contact with the flaws and the extent of the sensitization of the material. In view of the test methodology, many of the stress
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Table 1 Chemical
composition
and plate thickness of the test materials
C
Si
Mn
P
S
Ni
Cr
MO
N
A) SUS 304
.066
.55
1.04
,027
.005
8fl6
18.39
.09
.015
25 rnrnt
@J SUS
.06
.51
0.95
.033
.009
9.45
18.55
.Ol
-016
40 mmt
Material
304
Plate thickness
(wt O/o) transients during start-up and those during power generating conditions. The use of lower temperature. 98°C. water has an additional basis of choice because it provides a convenient reference basis with easier access and operation of the tests relative to those under high temperature and pressure conditions.
corrosion cracking tests under simulated reactor conditions can be roughly classified in two categories. i.e. those under constant load with either notched or smooth specimens, and those under monotonic slow strain rate condition (SSRT) with smooth bar specimens. In the present study, the influence of water environment on the fracture toughness under dynamic loading condition is examined under the premise that the plant components can be exposed to such a condition in the event of various kinds of transients. For such purpose, the fracture toughness tests with typical sensitized piping material under a variety of loading rates are conducted with specimens immersed in an aqueous environment. The environments employed are deionized water either at 98°C or 288°C simulating respectively the load
2. Experimental 2.1. Material Two heats of Type 304 stainless steel. with the chemical composition and the thickness shown in table 1, were used as the starting materials. These materials
- 20 “C
Thermocode
.-. u-4
d
S
-2O'C
PureWater
I
-
Present
Water
@; @ Fig. 1. Schematic
layout of 98°C water loop system.
;
Va Ive
(Opened
Pump
Loop)
, 0M ,
0;
; Motor Water
Sampling
97
N. Nakajima et al. / Fracture toughness of sensitized 304 stainless steel
were heat-treated at 650°C for 2 h to produce a furnace sensitized condition, which were the standard reference state of the materials for the present study. In addition to the above, different degrees of sensitization were given also to one of the starting materials by heating at 650°C for 0.5, 1.0 and 4.0 h in the furnace. The temperature of 650°C for sensitization was taken as a somewhat accelerating condition relative to those employed in simulating the welding followed by the so-called low temperature sensitization (LTS) [3] effect. The conditions chosen are considered to give a moderate conservatism to the evaluation. 2.2. Test apparatus
Two different test systems were used for low and high temperature environments. The equipment for the fracture toughness test with the oxygenated water environment at 98°C is shown schematically in fig. 1. The apparatus consists of the units for loading, recirculating water loop and data recording. The loading system is
__
Cross Head
--
Load Cell
Trctnsducer ( A ',
similar to the ordinary constant extension rate testing (CERT or SSRT) machine except its drive mechanism being capable of producing the cross head displacement speed down to 5 × 10 -5 m m / m i n , which is much slower than that employed in the conventional machines. The machine is also capable of performing constant load tests. The loading in the 98°C test was of the three point bending method, a schematic drawing of the assembly is shown in fig. 2. The measurement of the displacement is made by the two differential transformers which are set parallel to the measuring points (see A and B in fig. 2). The actual displacement was determined by the arithmetic mean of the two values from the detectors A and B so that possible error due to deflection of the center axis can be minimized. To maintain the water chemistry in the cell at the specified condition, the water appropriately prepared was supplied continuously with the refreshing rate of about 5 l / h at 98°C under air-saturated condition. Cooling jackets were set up the upper and lower positions of the pull rod to keep the temperature from the influence of the undefined diffusion of heat from the cell containing 98°C water. For high temperature tests, a recirculating autoclave system with a sealed-in loading section was used. The load was applied using a electromagnetic servo-hydraulic (MTS) testing machine, and its maximum load capacity was 7000 kgf (6.9 × 104 N). The displacement
Cooling 3ocket Transducer (B)
Bellow., Gloss Cell (Upper) O - rin( Bending Apporotus Pure Water Le~ Rolle
100
Insulator WQter
a) Three point bend specimen ( 3PB )
Gloss Cell (Lower) Wot~ Exhaust Volv
Test Piece
Bending Apparatus
SIG. 20%
62.50
I Fig. 2. Load assembly for three point bend test in pure water at 98°C.
b) Compact tension specimen (1T-CT)
Fig. 3. Specimen geometry (unit : mm).
98
N. Nakajima et aZ / Fracture toughness of sensitized 304 stainless steel
of the compact tension specimen in autoclave at 288°C under load was measured with a MTS water-immersible displacement gauge. The water, which was distilled and then deionized, was held in a 50-1itre tank. The exact water chemistry specified in terms of dissolved oxygen content and electrical conductivity was conditioned at room temperature by bubbling the mixture of appropriate oxygen content through water in the holding tank. The test section was situated inside a 5-1itre autoclave at 288°C with the compact tension specimen located at the central position. The oxygen content, pH and conductivity of the water were monitored continuously at the spots located between the holding tank and the pressurizing pump. The refreshing rate of the water was maintained at 10 l / h .
J-integral values were determined from the load (P) versus displacement (D) plots. The areas (A) under the P - D curves were calculated with numerical integration method. The value of J for the three point bend specimens was calculated using the formula originally developed by Bucci et al. [4]. The expression of the formula is given as 2A J=-BT
(1)
For the compact tension specimens, calculation was performed using the formula proposed by Merkle and Corten [5], which had been recommended by ASTM. Namely 2A
J=~l~
2P6 + a 2 Bb '
(2)
2.3. Test procedure
Fig. 3 shows the dimension of the three point bend specimen and the compact tension specimen used in the present study. The three point bend specimens were tested at 98°C under air-saturated condition while the compact tension specimens were tested at 288°C with 0.2 ppm 02 and 8 ppm O 2. The test procedure for the fracture toughness test is outlined below (see fig. 4): (1) Prior to the test, the specimens were fatigue-precracked to a crack length of a / W = 0.5, where " a " and " W " denote crack length and specimen width respectively. (2) Cleaning was made by reagent grade acetone in an ultrasonic cleaning device. (3) Mounting the specimen into the equipment, it was preloaded to about 10 kgf (98 N) before the system was brought to the test temperature. (4) Fracture toughness test was started with a slow and constant displacement rate. (5) The load-time and displacement-time relations were monitored during the test, which provided the basic mechanical data required for later analyses. (6) The test was terminated as the pre-determined displacement value was reached by extension of the crack. (7) After the test, the specimen was opened through cyclic loading to grow the crack by fatigue for surface fracture examinations. (8) All specimens so processed were examined with an optical microscope to determine the initial crack length (a0) and the crack extension (/~a) at the surface. The process was repeated following the steps (1) through (8) to obtain the J-resistance curves based on the multi-specimens method.
where A= B= b=
area under the load versus displacement curve, thickness of the specimen, remaining ligament of the specimen, coefficients developed by Merkle and Corten O~1 , Og2 to account for the tension component in the compact tension specimen, p~ final load value and, 8= final load point displacement. After the fracture toughness tests at 98°C and 288°C, detailed scanning electron microscopic (SEM) examination was performed on the crack-opened specimens. =
3. Results and discussion 3.1. Effect o f displacement rate
Plots of J versus crack extension (Aa) for different cross head speeds ( b ) are presented in fig. 5. Displacement rates in the r a n g e 5 x 1 0 -5 t o 5 x 1 0 1 m m / m i n were employed. At higher displacement rates, the sensitized material showed a behavior similar to the annealed material tested in air environment at the same temperature. In water at 98°C, the fracture toughness of the sensitized material decreased as the rate of displacement was lowered. For instance, the decrease in the value of J was by a factor in excess of 3. The value of J1c, obtained as the intersection of linear regression line and the blunting line, decreased with lowering the displacement rate. The Jk value in water was almost diminished at the lowest displacement rate employed in the present tests. At the same time the slope of the J-resistance plot was decreased by a factor of 2. The same behavior was
N. Nakajirna et aL / Fracture toughness of sensitized 304 stainless steel
99
observed in simulated boiling water reactor environment at 288°C as will be described later.
i
3.2. Effect of degree of sensitization
0
From the fractographic feature, the observed decrease in fracture resistance in the water environment was judged to have been caused by the effect common with that prevailing in the IGSCC. The effect of degree of sensitization on the J-resistance curves can be seen in fig. 6. Though the data were somewhat scattered, a similar behavior was observed in case of the materials with lower degree of sensitization at the displacement rate of 1 × 10 3 m m / m i n . While the value of J decreased as the degree of sensitization was increased at the same displacement rate, e.g. a reduction by the factor of 3 was recognized between the maximum and minimum cases in the present study. It was noted that the value of Jlc and the slope of the J-resistance plot decreased as the degree of sensitization was increased. Based on the results of this study, consideration of the fracture problems in BWR piping with inner surface cracks is suggested to recognize the importance of the thermal history of the piping material. Especially the possible effect of long time aging of weld heat affected zones of LWRs at operating temperature, known as the
Iko"~ co n s ~
constant
d
i" 0
Start at 98* or 288°C
[
Stop
" Time
Fig. 4. Experimental procedure of J fracture toughness test in simulated reactor water.
160
I
SUS 304 Steel of 98°C. 3PB Specimen['
650*(; b (mm/min)
As Rec. x2hrs
A
I !
12o
i I
NE ~:
/
~.
~
;"
V//
/
~/
•
5/10
A
•
5/,oool
0
e
[3
•
Z/IO00"!~ Water ~/tO00 [
E
Air
51'oooo)
¢=.
Blunting Line j.=4CYf.~ a ~ , Y
80
_
/
/
[
_
I o
0.5
1.o Crack
extension
t5 AO.
2.o
(mm)
Fig. 5. Effect of displacement rate on E- R curves of sensitized 304 stainless steel in pure water at 98°C.
N. Nakajima et aL / Fracture toughness of sensitized 304 stainless steel
100
160
I
r
SUS 304 steel at 98"C, 3PB Specimen
120 -
-
Heat treat ment
I I
--•:--
[) = 0 001 ram/rain
"~O--
aS
-- --
650°Cx0.Shr
re(:.
-- -
650°Cx1.0hr 650°Cx 2.0 hrs 650°Cx 4.0hrs
0
---
-
!-o
•
"
40
0
0.5 Crack
1.0 extension
Aa
1.5 (ram)
2,0
Fig. 6. Effect of degree of sensitization on J - R curves of 304 stainless steel in p u r e water at 9 8 0 ( ".
140| 1201"~ ~-~ / N
I
!
S U 5 3 0 4 Steel at 288'C 1T-CT(20% S.G.)
I D.O.(ppm) 0.2 8
,oo t
t
~+o.
•
Y-
__
__
~) (ram/rain
o0 0.01
.o+
O.o.~o++.o+o
40
20
0
0.5
1.0 CrQck extension
.5
2.0
Am, ( r a m )
Fig. 7. The J - R curves of sensitized 304 stainless stell in s i m u l a t e d reactor w a t e r at 288°C.
N. Nakajima et al. / Fracturetoughness of sensitized 304 stainless steel where E = o0 =
low temperature sensitization [3], must be subjected to critical consideration.
Young's modulus, flow stress and d J / d a = slope of the J-resistance plot. Now, the value of d J / d a is considered as the basic parameter in evaluating the fracture instability. All the data of d J / d a in the present study are shown in fig. 8. The observed d J / d a values of the sensitized Type 304 stainless steel in the water environment were varied significantly with lowering the displacement rate. The annealed materials and the moderately sensitized materials exhibited higher d J / d a values than the highly sensitized ones. The lowest values of d J / d a appeared in the case of test in water with 8 ppm 02 at 288°C, where the d J / d a was decreased by a factor of 5. Similar trend of the d J / d a versus displacement rate relationship was recognized in both environments employed in the present study, i.e. water environments at 98°C air-saturated and at 288°C with 0.2 ppm 02.
3.3. Effect of temperature In order to study the effect of temperature/oxygen combinations on the crack growth, the fracture toughness tests were carried out in 0.2 ppm O 2 and 8 ppm 02 at 288°C. The test results on the sensitized material are shown in fig. 7. From the limited number of data points, it is already clear that the value of Jic and the slope of the J-resistance plot at 288°C display a trend very similar to that observed in water at 98°C. The J-value decreased to substantially low values when the tests were conducted in water containing about 8 ppm O 2 at 288°C. For instance, the J value for the lowest displacement rate was 70 to 80% less than that for the highest displacement rate. These results indicate that the practical fracture toughness in the environment is strongly affected by the oxygen content in water at 288°C.
3.5. Fractographic observation Fracture surfaces of the specimens tested were examined by the SEM. Fig. 9 shows typical SEM views of the crack surfaces for cases under different crack extension conditions in water at 98°C. The ductile fracture with transgranular cracking mode is observed in the surface of specimen (A), the quasi-brittle fracture with intergranular elements is seen in the specimen (B). Fig. 10 shows the SEM micrographs of the fracture surfaces
3.4. Comparison of d J / d a From the present results the tearing modulus [6] was evaluated. The tearing modulus is defined as E dJ 02 d a '
T-
(3)
140 120 --O ~100 E
--
E
f
~
.
.
.
.
.
.
E 80
E ," I ~198"c
O 40
A /
20
l
O,O:.D i s s o l v e d o*ygen 010=
10-3
I
S U S 3 0 4 Steel J 288'c _ D.O.
/
I
~ o I ( • I ~ I I
101
e
(p m) I ( I I 0 I
I
10-z Displacement rate,(mmlmJn)
Meat
treatment Asreceiv~ 650"CXOlShr 6SO'Cxl.0hr 650"Cx2.0hr
_
650"C x 4.0 hr 10-1
Fig. 8. The summary of dJ/da versus displacement rate in simulated reactor water.
100
102
N. Nakajima et al. / Fracture toughness of sensitized 304 stainless steel
1 i
i
¢-
d~ tD ,zl
tD
,¢ o
E ts3 0 0 C)
Y~
I!
to) tt)
k~
103
N. N a k a j i m a et al. / Fracture toughness o f sensitized 304 stainless steel
8
d
H
A
\
.=_
\ d
II
11
I
k
7~ e..
7~
/I /
I
J
I
n
l
°t"~
I I
\
\ e~ 0
r~
104
N. Nakajirna et aL / Fracture toughness of sensitized 304 stainless .steel
obtained in water at 98°C at various extension rates. The ductile fracture mode was recognized for the case tested at the displacement rate of 5 x 10 -1 m m / m i n , while brittle mode was the case at lower displacement rates. In the specimen tested at 5 × 10 3 m m / m i n , the fracture surface was seen to be of the mode of mixed transgranular and intergranular cracking. The feature of the fracture surfaces of the specimens tested in water at 288°C is shown in fig. 11. The specimen tested at 5 × 10 1 m m / m i n , in water with 0.2 ppm 02 showed a ductile fracture surface, while the specimen tested at 1 × 10 .3 m m / m i n in water with 8 ppm O 2 showed brittle fracture mode. Those results obtained suggest clearly that the tendency of IGSCC increases as the displacement rate is lowered in the simulated boiling water reactor environment. In other words, the fracture mode similar to those generally observed in inert environment can occur in water at 98°C and 288°C only when the displacement rates are sufficiently high to prevent the occurrence of I G S C C mode fracture.
The crack enhancement in water environment observed in the form of reduced fracture toughness is, therefore, judged to be due to the "stress corrosion cracking" effect. The enhancement occurs in two ways. For the condition J > Jlc, the crack extension at low displacement rates in water finds easier paths through IGSCC. As the lowering of Jlc value indicates the onset of crack extension is also affected significantly by the water environment. Here the term of J~c~, is conveniently given to the Jlc obtained in water at low displacement rates. 3.6. Crack growth rate In order to see more clearly the crack extension enhancement by the environment, an acceleration factor is defined as [da/dt]water/[da/dt]air. Using such a factor, the results of multi-specimen fracture toughness test are plotted as shown in fig. 12. The acceleration factor increased by a factor of 2.5 as the displacement rate was lowered to the lowest level within the range tested.
Crack growth direction ,,
lOOpm I
a) 13= 0.5 m m / m i n in O. 2 ppm 02 at 288"C Fig. 11. SEM photographs of fracture surface.
1O0 um
b) 0=0.001 m m l m i n i n 8 ppm 02 at 288"C
N. Nakajima et al. / Fracture toughness of sensitized 304 stainless steel
105
SLJS504 Steel of 98"C, 3PB Specimen • 650"C x 2hrs o As Rec.
3.0
i
o~
2.0
i
1.0
I
0
i 10-4
| 0 -~
10-2
[)
t0 -t
fO0
{mm/min )
Fig. 12. Relationship between the acceleration factor, [da/dt]water/[da/dt]air and the displacement rate at 98°C. The time dependent nature of the results as observed in the present study must be taken into account. In evaluating the fracture toughness of SCC-sensitive materials in general, it should be pointed out that the range of deformation rate in service environment is one
10o
I SUS 304 Steel
at 9O*C, 3PB Specimen
101 E
% E E 10-2
10-4
10-3 d Dldt, (ram I rain )
10-2
Fig. 13. Correlation between dJ/dt and dD/dt (displacement rate).
of the most important factors in assessing the cracking behavior. Since the values of displacement rate indicated are based on the experimental convenience for laboratory test, more generic expression may be appropriate. For such a reason the results are recognized in the relationship between J ( d J / d t ) and D ( d D / d t ) . The consequence at 98°C is shown in fig. 13. The trend of J and b fell on the straight line. F r o m fig. 8 and fig. 13, it can be stated that the effect of water environment must be taken into account in evaluating the fracture toughness for cases where d J / d t is below 1 × 10 - i kgf. m m / m m 2 / m i n (1.6 × 10 j / m 2 / s ) . The geometry of cracks in actual BWR piping may generally take the form of circumferential surface flaws. In evaluating the fracture resistance of such piping, a serious consideration of the following factors will be required; they are, possible variation in water chemistry and its state at the crack tip [7], the time dependent aspect of loading mode [8], temperature [9], and thermal history of the material [10]. Surface flaws in the piping, in contrast to those tested in specimens, may be under a more complex stress distribution. Behavior of surface cracks in pipings may be affected by these factors. From the results in this study, it is suggested that more critical consideration is necessary in the evaluation of the integrity and the safety of welded stainless steel piping in BWRs where the effect of water environment can by no means be ignored. This aspect is believed to be particularly important in evaluation of
106
N. Nakajima et al. / Fracture toughness of sensitized 304 stainless steel
the instability fracture of the pipings at the portions of the near-weld joints.
4. Summary The effect of fracture resistance of Type 304 stainless steel in simulated reactor water was examined with special attention focused on the loading rate. (1) The value of Jlc and slope of the J-resistance plot are strongly affected by the factors such as displacement rate, temperature, degree of sensitization a n d oxygen content in water. For the cases of d J / d a below 1 x 10-1 k g f . m m / m m Z / m i n (1.6 x 10 j / m 2 / s ) , the effect of water e n v i r o n m e n t a n d the nature of time-dependent p h e n o m e n a must be taken into serious account in evaluating possible instable fracture of the structure to be considered. (2) The e n h a n c e m e n t of cracking was caused by the participation of intergranular stress corrosion cracking. (3) The observed p h e n o m e n a need to be taken into account in the consideration of the fracture probability evaluation of nuclear reactor pipings.
Acknowledgment The authors wish to express their appreciation for the advices given by Professor H. Takahashi and T. Shoji of Tohoku University and Drs. S. Miyazono and T. Uga of J a p a n Atomic Energy Research Institute given during the course of this study. T h a n k s are also due to Mr. E. Ebine who assisted the fracture toughness tests, and to Mr. M. Kikuchi who conducted the SEM observation.
References [1] G.M. Wilkowski, J. Ahmad, A. Zahoor, C.W. Marschall, D. Brock, I.S. Abou-Sayed and M.F. Kanninen, Fracture of circumferentially cracked Type 304 stainless steel pipes under dynamic loading, in: Elastic-Plastic Fracture, (ASTM STP 803, ASTM, 1983) pp. II-331-II-350. [2] C.H. Popelar, J. Pan and M.F. Kanninen, A tearing instability analysis for strain-hardening Material, in: Fracture Mechanics (ASTM STP 833, ASTM, 1984) pp. 699-720. [3] W.G. Morris and A.M. Ritter, Microstructural investigations of low temperature sensitization in 304 stainless steel, Proc. Seminar on Countermeasures for Pipe Cracking in BWRs, EPRI WS-79-174, Vol. 2 (1980). [4] R.J. Bucci, P.C. Paris, J.D. Landes and J.R. Rice, J integral estimation procedures, in: Fracture Toughness (ASTM STP 514, ASTM, 1972) pp. 40-69. [5] J.G. Merkle and H.T. Corten, A J integral analysis for the compact specimen, considering axial force as well as bending effects, Trans. of ASME, J. of Pressure Vessel Technol., Series J, 96, No. 4 (1974) pp. 286-292. [6] P.C. Paris, H. Tada, A. Zahoor and H. Ernst, The theory of instability of the tearing mode of elastic-plastic crack growth, in: Elastic-Plastic Fracture (ASTM STP 668, ASTM, 1979) pp. 5-36. [7] P.L. Andresen and F.P. Ford, Technical considerations for startup deaeration procedures to minimize stress corrosion cracking of 304 stainless steel piping, Proc. Seminar on Countermeasures for Pipe Cracking in BWRs, EPRI WS-79-174, Vol. 1 (1980). [8] R.J. Eiber, W.A. Maxey, A.R. Dully and T.J. Atterbury. Investigation of the initiation and extent of ductile pipe rupture, Battelle Columbus Laboratories, BMI-1908 (June 1971). [9] W.H. Bamford and A.J. Bush, Fracture behavior of stainless steel, in: Elastic-Plastic Fracture (ASTM STP 668, ASTM, 1979) pp. 553-577. [10] R.L. Fullman, Predictability of low temperature sensitization in stainless steels, Proc. Seminar on Countermeasures for Pipe Cracking in BWRs, EPRI WS-79-174 Vot. 2 (1980).