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The effects of cold working and pre-irradiation heat treatment on void formation in neutron-irradiated type 316 stainless steel
Journal of Nuclear Materials 57 (1975) 103-118 0 North-Holland Publishing Company
THE EFFECTS OF COLD WORKING AND PRE-IRRADIATION HEAT TREATMENT ON VOID FORMATION IN NEUTRON-IRRADIATED TYPE 316 STAINLESS STEEL1 H.R. BRAGER Westinghouse Hanford Company, Richland,
Washington, USA
Received 29 October 1974
The effect of fast-neutron irradiation on void formation in Type 316 stainless steel having undergone specific thermalmechanical treatments was investigated by transmission electron microscopy. The study showed that, for irradiation at the three lower temperatures (420,475 and 580°C): (1) the void volume decreased with increasing cold work; (2) the reduction in swelling was due to a decrease in both void-number density and void size; (3) the decrease in void size with increasing cold-work level was enhanced at higher irradiation temperatures; (4) cold working from 0 to 10% decreased the voidnumber density, and void volume, more than in the range from 10 to 20%; (5) void formation in the 20% cw steel which had been heat treated 100 h at 650°C before irradiation was similar to that of the solution-treated steel. The,temperature dependence, of swelling of the cold-worked material was different from that of the solution-treated steel. Irradiation at 650°C resulted in a larger void volume in the cold-worked material than for irradiation at 475 or 580”C:The effects of cold work and irradiation temperature on void growth are consistent with the predictions of a diffusion-controlled model. L’effet de l’irradiation par des neutrons rapides sur la formation des vides dans un acier inoxydable du type 316 ayant subi des traitements thermo-mkcaniques sp&ifiques a ktk &udie par microscopic klectronique par transmission. L’ktude a montrk que par irradiation aux trois tempkratures inf&ie’ures (420,475, et 580°C): (1) le volume des vides diminue avec l’icrouissage croissant; (2) la reduction du gonflement est due B une diminution a la fois de la densite du nombre et de la taille des vides; (3) la diminution de la taille des vides avec l’dcrouissage croissant est encore accrue par une elevation de la temperature d’irradiation; (4) un ecrouissage de 0 a 10% diminuait la densite du nombre de vides et le volume des vides davantage qu’un ecrouissage compris entre 10 et 20%; (5) la formation des vides dans l’acier a ecroui de 20%, trait& thermiquement 100 h a 650°C avant irradiation etait similaire a celle observee darts l’acier soumis i un traitement de mise en solution. La relation entre gonflement et temperature de l’acier kcroui etait differente de celle de l’acier soumis a un traitement de mise en solution. L’irradiation a 650°C provoquait un accroissement du volume des vides dans l’acier ecroui plus 616~6 que par irradiation h 475 ou 580°C. Les effects de l’ecrouissage et de la temperature d’irradiation sur la croissance des vides sont en accord avec les previsions d’un modele control& par la diffusion. Der Bestrahlungseinfluss schnelJer Neutronen auf die Porenbildung wurde in rostfreiem Stahl 316, der einer spezitischen thermisch-mechanischen Behandlung unterworfen war, transmissionselektronenmikroskopisch untersucht. Die Bestrahlung bei den drei niedrigeren Temperaturen 420,475 und 580°C hat ergeben: (1) Das Porenvolumen nimmt mit zunehmender Kaltverformung ab. (2) Die Verringerung des Schwellens beruht auf einer Abnahme der Dichte und der Griisse der Poren. (3) Die Abnahme der Porengrosse mit zunehmender Kaltverformung ist starker bei hijheren Bestrahlungstemperaturen. (4) Porendichte und Porenvolumen werden durch die Kaltverformung zwischen 0 and 10% starker verringert als zwischen 10 und 20%. (5) Die Porenbildung im 20% kaltverformten Stahl, der vor der Bestrahlung 100 h bei 650°C warmebehandelt worden war, ist der im liistmgsgeghliihten Stahl Bhnhch. Die Temperaturabhiingigkeit des Schwellens des kaltverformten und liisungsgegliihten Stahls ist verschieden. Die Bestrahlung des kaltverformten Stahls hat bei 650°C ein griisseres Porenvolumen als bei 475 oder 580°C zur Folge. Der Einfluss der Kaltverformung und Bestrahlungstemperatur auf das Porenwachsturn ist mit den Voraussagen hinsichtlich eines diffusionsbestimmten Modells konsistent.
’ This paper is based on work performed by Hanford Engineering Development Laboratory, Richland, Washington, operated by Westinghouse Hanford Company, a subsidiary of Westinghouse Electric Corporation, under US Atomic Energy Commission Contract AT(45-l)-2170.
H.R. BragerfEffects
104
of cold
working and heat treatment
1. Introduction Major effort has been directed toward development of the Liquid Metal Fast Breeder Reactor (LMFBR) for commercial generation of electric power. One significant materials phenomenon discovered by these programs was that of irradiation-induced swelling in structural materials operating at normal fast-reactor temperatures. Considerable research has been directed toward determining the extent of swelling that would occur in the LMFBR reference material for cladding and core components: 20% cw Type 3 16 stainless steel. However, reactor components having the same ‘cold work’ level or reduction of area are formed by various manufacturing processes which impart different mechanical properties and microstructural features. To date, the studies on irradiation-induced metal swelling have been limited, for the most part, to solution-treated steel or to material having a single level of cold work. No comprehensive information exists regarding the effects of cold work and irradiation temperature on metal swelling due to neutron irradiation. The objectives of this study are to determine the effect that cold work and irradiation temperatures have on void formation in Type 3 16 stainless steel and to ascertain the influence which pre-irradiation heattreatment has on the swelling resistance of the 20% cw steel.
2. Experimental
procedure
Specimens of Type 3 16 stainless steel were examTable 1 Reactor location and dosimetry information
ined by transmission electron microscopy after stressfree irradiation in the Experimental Breeder Reactor II (EBR-II). These specimens were prepared with cold-work levels of 0, 10, 15,20 or 25% (reduction of area by cold-swaging) after solution-annealing 15 min at 1070°C. Some samples of 20% cw steel were also aged for 100 h at 650°C prior to irradiation. Sets of specimens were irradiated in sodium-filled subcapsules located within the active fuel region of EBR-II Row 2. The high thermal conductivity of the sodium insured that the materials in each subcapsule were irradiated at essentially the same temperature. The subcapsule irradiation temperatures were calculated by taking into account the y heat generated in each capsule and the thermal resistance of the gas gap between the capsule and the reactor sodium-cooled outer subassembly wall. The core location and dosimetry information [l] for each set of specimens examined by electron microscopy is shown in table 1. The small variation in neutron flux and energy spectrum was ignored in subsequent interpretation of the data. The chemical analysis of the steel used in the study was: C: 0.060 wt%; Mn: 1.72 wt%; Cr: 17.30 wt%; Ni: 13.30 wt%; MO: 2.33 wt%; Cu: 0.065 wt%; Si: 0.40 wt%; P: 0.012 wt%; N,: 0.048 wt%; B: 0.05 wt%; S: 0.007 wt%; balance: Fe. Transmission electron microscopy samples were examined in a Philips 200 electron microscope operating at 100 kV and equipped with a double-tilt specimen holder in a goniometer stage. The specimenthinning techniques have been previously described [2]. Void diameters, including the black fringe, were
for specimens.
Specimen location during irradiation
Neutron fluence
R (cm) a)
Z (cm) b,
(@t (total) 10” n/cm**)
@t (E > 0.1 MeV) (lo** n/cm*)
17.7
-8.5 1 -7.11 +0.95 +3.05 +5.33 +6.60
2.7 2.8 2.8 2.8 2.7 2.7
2.3 2.4 2.4 2.4 2.3 2.3
Spectrum averaged neutron energy E (MeV)
Displacements per atom (dpa)
0.76 0.77 0.79 0.79 0.78 0.77
11.4 11.6 12.1 11.8 11.6 11.3
a) R denotes the radial position relative to the centerline of the reactor. b)Z denotes the position below (-) or above (+) the reactor midplane. The fueled region of EBR-II extend from -17cm?Z?+l-7cm.
H.R. Brager~Effe~~sof cold warning and heat rreatme~t
measured on positive prints using a calibrated-recticie optical magnifier. Five different void diameters are reported for each specimen. In addition to the minimum, mean and maximum values, the diameter which correspond to the value for which 5% of the voids were smaller or larger, respectively, are also reported. These values provide a range of void sizes which is not greatly influenced by the number of voids measured. The foil thicknesses were obtained from two or three stereoscopic measurements on each area analyzed. The void volume was computed taking into account the surface-intersection probability for each voidsize increment at the measured foil thickness. 3. Results 3. I, Effect of cold work Typical transmission micrographs of the solutiontreated, 10% cw and 20% cw specimens are shown for each irradiation temperature in figs. 1-4. The voidformation data for the specimens investigated are summarized in table 2. Since the neutron fluence for all the specimens varied only from 2.3-2.4 X 1O22 n/cm2 (E > 0.1 MeV), the only experimental variables which would significantly influence void formation
105
are feral-mechanics treatment of the steel and irradiation temperature. The void-volume data for the solution-treated, 10 and 20% cw specimens are shown as open symbols in fig. 5 as a function of irradiation temperature. The principal features of the data shown in fig. 5 are that, first, cold working decreases swelling, and second, the temperature dependence of swelling is related to the cold-work level. The reduction in swelling in the steel due to cold working agrees with previous experimental data [3,4]. The present results show that 10% cold work induces a much greater suppression of void formation than does a further increase in the cold-work level from 10 to 20%. This non-linearity of void nucleation with cold-work level apparently is due to the very strong relationship shown between defect supersaturation and void-nucleation rate [S-7]. At 6.SO°C, the magnitude of swelling of the solution-treated and 10% cw specimens was small but larger than that of the 20% cw material. In these three specimens, void formation appeared to be nearly homogeneously distributed throughout the matrix. This reduction in swelling resistance of the 10% cw material is probably due to a lower thermal stability of its dislocation network, which has greater long-range stresses than exist in the 20% cw condition.
Fig. 1. The effect of cold work on void formation in Type 316 stainlesssteel irradiatedat 420°C to 2.3 x 10” n/cm2 (E > 0.1 MeV). (a) Solution-treated, (b) 10% cw, (c) 20% cw.
106
H.R. BragerlEffects
of cold working
Fig. 2. The effect of cold work on void formation
(E > 0.1 MeV). (a) Solution-treated,
and heat treatment
in Type 316 stainless steel irradiated at 475°C to 2.3 (b) 10% cw, (c) 20% cw.
The void volume in the solution-treated steel continually decreases with increasing temperature while in the cold-worked steel signi~cant void formation occurs only at the lower- and higher-temper-
X
10" n/cm2
ature regions investigated. The lower-temperature region of void formation may occur by the same mechanism as proposed for solution-treated steels: vacancy supersaturation due to stress-biased net
Fig. 3. The effect of cold work on void formation in Type 316 stainless steel irradiated at 580°C to 2.4 X 10z2 n/cm2 f.E> 0.1MeV). (a) Solution-treatea, (b) 10% cw, (c) 20% cw.
Fig. 4. The effect of cold work on void formation in Type 316 stainless steel irradiated at 650°C to 2.4 X 10” n/cm2 (E > 0.1 MeV). (a) Solution-treated, (b) 10% cw, (c) 20% cw. 1.6
I
I
1.4
1.2
1.0 3 @ 8
0.8
5 ,’ 5, 0.6
0.4
10% 0.2
% 20% 0
C.W.
\ i.w\
I
I
400
450
IRRADIATION
Fig. 5. The effect of irradiation temperature, 3 16 stainless steel.
550
500
TEMPERATURE
cold work and preirradiation
600
,^_
(?,
ageing 100 h at 650°C on the swelling of 20% cw Type
650 k60
0% cw 10% cw 20% cw 20% cw + aged
37-6A 376F 37-6G 37-6D
2.4
2.4
2.3
2.3
Neutron fhence +r (10Z2 n/cm3)
90 112 101 112
202 179 101 123 112 123 124 123
270 179 101 191
134 146 123 157
124 112 123 123 112 134
89 89 112 67 89 101 112 134 101 101
5% Min.
Min.
Void diameter
212(391) c, 280(400) ‘) 185(310) c, 359(476) ‘)
471 517 314 651
730 290 258 449
415 281 235 348
280 236 236 236 225 292
5% Max. _~
~~ ~
~.
data.
503 235 192 32014231 b,
244 210 177 255
207 172 180 180 163 217
Mean
and immersion-density
a. Heterogeneons Void formation b. Value in f ] denotes non-typical value c. () denotes voids only; number preceeding () is for voids + bubbles
-
580 +40
O%CW lO%cw 20% cw 20% cw + aged
36-6A 36-6F 36-6G 36dD a)
475 525
0% cw 10% cw 20% cw 20% cw + aged
420 *lo
34-6A 34-6F 34-6G 35-6D
___I
had. temp. (“C)
O%cw 10% cw 15% cw 20% cw 25% cw 20% cw + aged
_
TMT
4CK3A 41-6F 41-6H 41-6G 41-61 40-6D
-.
IlO.
Sample
Table 2 Summary of transmission electron-microscopy
674 596 438 865
708 290 269 505
460 348 314 449
348 281 269 314 303 337
Max.
635(153) ‘) 196( 94) ‘) 437(59) c, 284( 170)‘)
147 3 17 57
857 283 492 385
449 820 443 1210 220 682
Number of voids measured
p
0.71(0.20) 0.43(0.23) 0.85(0.13) O-24(0.15)
0.38 0.008 0.07 0.14[0.51]
8.2 0.6 1.0 5.8
19
5.5 6.1 2.0
26 11
‘) ‘) c, ‘)
b)
$1014 voids/cm3)
Void density
0.09(0.08) 0.09(0.09) 0.04(0.02) 0.1 l(O.11)
0.30
0.80 0.032 0.032 0.60
1.48 0.33 0.19 0.21 0.052 1.15
‘) c) ‘) c)
b,
Void volume A V/V0 (%)
~.~
-0.21 +0.08 +O.lO +0.04
-0.36 +O.Ol +0.03 -0.10
-0.46 + 0.04 +0.06 a.10 +O.lO -0.23
(%)
change A P/PO
Density
H.R. 3mgerfEffe~ts
of cofd wor~nga~d
109
heat treatment
200
240 f
s 180 G - 16D
220 0s 6 200
l
\
180
.
1.0
IA A
B
1.2 1.0 2
0.8
3=-0.8 0.4 0.2 0.0 500 400 03 ‘308 200
T,
560°C
‘----A-* D.8_S 0.4 ;i 0 0.2 g *
- 1.0 - 0.8 “i;;
C
_ u - 0.8 3
0.0 -!xD
O------Y
- 0.4 =z .
-.0.2 f l
-, 0.0
-•
O? 5 -
0:d l-•
+0.c
0
1D.
20
CM0 WORK, %
0
10 CO10WORK, %
20
Fig. 6. The effect of irradiation temperature and cold-work level on void formation in type 316 stainless steel @t- 2.3X 1O22 n/cm2 (E > 0.1 MeV).
attraction of interstitials to dislocations [8,9]. The increasing irradiation temperature decreases the vacancy supersaturation in a thermally stable dislocation network, suppr~~ing void nucleation. At high temperatures, near 600°C the dislocation structure becomes thermally unstable. The resultant decrease in defect sink density thereby promotes void
formation until thermal-equilibrium vacancy concentrations dominate the behavior at still higher temperatures. The microscopy data for void diameter, number density and void volume are plotted as a function of cold-work level for the four irradiation temperatures in figs. 6a-d. The curves show a distinct difference
110
H.R. 3r~~erl~~ff~crs oj.cold worlcing and heat treatment
SOLUTION lIz/
Fig. 7. The effect of irradiation temperature
and pre-irradiation microstructure
TREATED
20% CW [27%CW AT 395°C)
on the void-size distribution in Type 316 stainless
St&
between the three lower-temperature data sets and the hi-tem~rature (65O’C) data. For the lower temperatures, the void parameters all decreases in magnitude with increasing cold-work level. The large reduction in swelling for 10% cw is associated with large decreases in both the void size and number density. At 650°C, where both, voids and helium bubbles, are formed, the void-formation data for the solution-treated and 10% cw data are nearly identical,
while the inhibition of swelling still persists at the 20% cw level. The relationship between void diameter and coldwork level changes significantly with increasing irradiation temperature. This behavior is more clearly shown in fig. 7. This figure includes the void data for solution-treated and 27% cw specimens from the same steel used in the present study but irradiated in a different sub-assembly at a lower temperature
H.R. BragerfEffects of cold working and heat treatment
Fig. 8. Frank-loop and dislocation-microstructure dependence on irradiation temperature in 20% cw Type 316 stainless steel. The micrographs are of material, irradiated to a neutron fluence of 2.3-2.4 x 102’ n/cm2 (E > 0.1 MeV), oriented with the 110 direction nearly parallel to the electron beam. a, b, and c are weak-beam dark-field micrographs; d is bright-field. (a) T = 420°C (b) T = 475”C, (c) T = 58O”C, (d) T = 650°C. Magnification approx. 67 500 X.
(395°C) [lo]. The mean diameter and void-size distribution found in the 27% cw material irradiated at 395°C are practically identical to those observed in the corresponding solution-treated steel. At 42O”C, and progressively more so at high temperature, the void-diameter distribution in the 20% cw steel is narrower than that observed in the solution-treated material. At 65O”C, in addition to the large voids, a high concentration of small helium bubbles is observed in both the solution-annealed and the cw materials. In summary, the present data show that cold-working reduces swelling by reducing both void size and number density, and that the predominant effect is dependent upon the irradiation temperature. Previously, UK investigators stated that cold work reduced swelling by decreasing the void size [ 111 while US observations were to the contrary, that the major influence was a reduction of voidnumber density [lo, 121. In addition to the voids, the radiation-induced dislocation substructures were also investigated. Typical micrographs showing dislocations and Frank loops in the 20% cw steel are seen in fig. 8. Weakbeam dark-field micrographs are used to illustrate
the structure of the material irradiated at the lower temperatures. Irradiation at 420°C fig. 8a, results in an appreciable reduction in the dislocation density from over 2 X 101’ to about 3 X lOlo cm/cm3 and in the formation of a high concentration of Frank loops, approximately 1015 loops/cm3. The solution-treated steel irradiated at 420°C contains a high density of Frank loops similar to those formed in the coldworked steels. A low dislocation density (
112
Fig. 9. Pre-irradiation microstructure 10% cw, (d) 20% cw.
H.R. Bragerlfiffccts
of cold working and heat treatment
Type 316 stainless steel (a) Solution-treated,
3.2. Effect of p$e-i~adiu~o~ heat treatment of 20% cw steel In examining the effect of the ageing treatment on swelling stability, it is appropriate to consider the microstructures of ~irradiated Type 3 16 stainless steel with various treatments (0, 10,20% cw or 20% cw + aged]. The 10% cw ‘as-fabricated’ structure, fig. 9c, cons&es primarily of a farily high dislocation concentration in the form of a complex network. A poorly defined ceil structure is evident in addition to a number of small dislocation segments (dislocation debris). The 20% cw structure,,fig. 9d, has a higher dislocation density and a somewhat better-defined cell structure than that of the 10% SWsteel. In addition, a number of microtwins and stacking faults are present; their concentration is dependent upon the orientation of each grain relative to the cold-working direction. The 20% cw steel which had been aged for 100 h at 650°C still has a high dislocation density, fig. 9b. The 65O’C heat treatment resulted in some disiocation motion with removal of most dislocation segments and production of a lower-energy dislocation network. However, the gross features of the aged structure are certainly much more similar to those of the 10% cw material than to those of the solution-treated steel. As shown in table 2, the swelling is substantially greater in the aged material than in the 10% cw steel. Therefore, there appear to be
(b) 20% cw + 100 h anneal at - 650°C Cc)
other factors besides dislocation density which influence swelling resistance. The deterioration of the swelling resistance of cold-worked steel due to preirradiation ageing might be due to the formation of an ‘atmosphere’ around the dislocations. The ‘atmosphere’ of solute atoms should decrease the stress field of the dislocation thereby reducing its effectiveness as a defect sink. Fig. 10 shows typical micrographs illustrating void formation in the aged samples irradiated at the four different temperatures. Comparison of these micrographs with those shown in figs. l-4 shows that the void formation in the aged sample is more to that of the solution-treated steel than to that of the 20% cw material. The closed symbols in fig. 5 show the void volume of the aged specimens as a function of irradiation temperature. The variation in swelling behavior of the aged specimen irradiated at 58O”C, fig. 5, is due to heterogeneous void formation. Most of the steel, including regions having many planar defects (stacking faults, microtwins and deformation bands) which were heavily decorated with precipitates, contained voids whose volume was similar to that of the 20% cw steel. Localized regions of this (20% cw t aged) steel contained many voids, with a void volume comparable to that of solution-treated steel. These regions of high void volume were associated with a few of the planar defects at which substantial precipitation had
H.R. Brager/Effects of cold working and heat treatment,
113
ia)T = 420°C
icf T
= 580 *C
Fig. 10. Void formation as a function of irradiated temperature 650°C before irradiation.
(d) T = 650 % in 20% cw Type 316 stainless steel that had been aged 100 h at
114
H.R. BragerlEffects
of cold working and heat treatment
occurred. The reason for this heterogeneous void formation is not clear. One factor which migh influence void formation would be the effect of localized stresses maintained during irradiation at these highly strained regions. Wolfer et al. [ 141 have shown that the net attraction or bias of vacancies and interstitials to voids and loops is dependent upon the hydrostatic and deviatoric stresses, respectively. These stresses can influence the incubation time for void nucleation and the swelling rate. Another factor which might locally influence void formation would be the effect of matrix chemical variation caused by extensive precipitation. Bates [ 151 showed that small differences in minor concentrations of alloying elements
have a significant influence on swelling of neutron-irradiated Type 3 16 stainless steel. Except for the heterogeneous void formation in the aged specimen irradiated at 58O”C, the void volume in the 20% cw steel is less than one fifth that of either the solution-treated or the aged material. Pre-irradiation ageing of the 20% cw steel for 100 h at 650°C effectively removes the beneficial influence of cold-work.
4. Discussion The data of the present study show that variations in both cold work and irradiation temperature influ-
K-
\
\
COLD
WORK
(%) [X]
Fig. 11. Swelling data of cold-worked steel, normalized using the solution-treated eq. 1.
information,
compared with curves based on
H.R. BragerfEffects
ence the magnitude of swelling in neutron-irradiated Type 3 16 steel. Various methods of cold-working austenitic steel with the same reduction in area procedure materials which have significantly different mechanical properties and microstructural features [16]. The different modes of cold working reactor components (cladding, wire wrap, duct, etc.) as well as minor variations in the cold-work level could influence the amount of metal swelling. In order to provide some design basis for estimating the effects of cold work on swelling, a functional relationship between swelling, irradiation temperature and cold-work level was derived from the experimental data: A!‘(x)/F
= [Av(O)/V]
X exp [(0.90 - 2.5 X 10e3 T)x] ,
115
of cold working and heat treatment.
0)
where A v(x)/ V = swelling for x% cold work and T = irradiation temperature (“C) (=4OO”C < T<550°C). For the present, this relationship is limit&d to fluences of 2-3 X 1O22 n/cm2 (I?> 0.1 MeV) and to irradiation temperatures from about 400 to 550°C. At temperatures much above about 55O”C, a second swelling range has been shown to exist in cold-worked steel [17] and this trend is consistent with the 650°C data set of the present study. The four low-temperature sets of data showing the effect of cold work on swelling are compared with the predictions of eq. (1) shown in fig. 11. Although the curves of eq. (1) fit all the microscopy information, only the 420°C data set provides sufficient information to accurately estimate the influence of cold work on swelling. The equation reflects the experimental observation that the relative’effectiveness of cold work in suppressing swelling increases with temperature. Recently published [ 181 data showing the effect of cold-work level on swelling for Type 3 16 stainless steel are also plotted on fig. 11. These data were for pressurized tubes (150 MN/m2) irradiated at 460°C in oxide-fuel reactor to a peak fluence of 7.5 X 1O22 n/cm2 (total), which should be equivalent to about 6.5 X 1O22 n/cm2 (E > 0.1 MeV) [19]. Despite the differences in calculating irradiation temperature, in alloy chemistry and in manufacturing techniques, the data agree quite well with those presented here. Previously, it was shown that at 395°C the mean void sizes in-the solution-treated and 27% cold-worked
material were the same [lo]. The present results indicate that the effect of cold work on void growth is quite temperature dependent. For irradiation et 42O”C, the void size decreased with increasing coldwork level. At higher temperatures (475 and 580°C) cold work had an even greater influence on void growth. This dependence of void-growth rate on cold work and irradiation temperature can be shown to be consistent with a simplified swelling model considered by other authors [20,21]. The steady-state concentration of point defects produced in an irradiated material is controlled by a dynamic balance between production, diffusion to sinks and recombination. Using a set of coupled steady-state equations for vacancy and interstitial concentrations, C, and C,, respectively: dC,/dt=O=G-RWSS,LS,V, dC,/dt
=O=G-RVI-S&-S&
(2)
where G = defect production rate; R = recombination coefficient; V, I = D,C,, DIC1; D, C = diffusivity and concentration of point defects; S, = a geometric factor describing the tendency for a void to capture (or emit) a vacancy or an interstitial; S,, SL = geometric factors describing the tendency for a dislocation to capture (or emit) a vacancy or an interstitial, respectively, and a void-growth rate equation for a void centrally located in a sphere of material with the voidgrowth rate dependent upon the diffusion of point defects through the matrix: (r/Q) dr/dt = V - I ,
(3)
where r = void radius, a = atomic volume, t = time, and assuming no trapping of defects or diffusion to sinks other than’voids and dislocations, it has been shown [22,23] that d(r2)/dt
= 2(S-;> - SD) Sl Gf/(S,
g 2~ !YlGf/S ,
+ SV) (S;> + S,) (4)
where E = net increased attraction of interstitial to dislocations compared to the vacancy-dislocation interaction, E = (S;>- SD)/@, + Sv), S = total sink density Z (SD + SV) or (S6 + Sv), f = fraction of vacancies, or interstitials, arriving at voids, or dislocations, respectively, f = (SD + S,) V/G = (Sb t S,) I/G, and
H.R. BragerlE’fjFects
116
of cold working and heat treatment
using eq. (2) it is readily shown that f=@/2
{(1+4/9)%
d(r’)/dt
1,
(5)
where 4 = (S,, + SV) (S;, t S,)/
RG rS2/RG
(6)
s 2e R (G/R);
,
(8)
or that the void-growth rate should be independent of sink density, which is in agreement with the essentially identical void-size distributions observed in the solutiontreated and the 27% cw specimens irradiated at 395°C [IO]. At high temperatures where D, is large, 4/4~ < 1,
and d(r2)/dt R=6[Cl/d2DV,
(7)
based on the relationships: recombination rate = v C, C, t Q [24] and D, = $ d2 [25]; 5 = number of sites neighboring a vacancy for which annihilation?is certain if the site is occupied by an interstitial and d = jump distance. Eq. (4) can be used to indicate trends for the irradiation-temperature dependence of void growth in cold-worked steel. Using eqs. (4-7), it can be readily shown that at lower temperatures where D, in eq. (7) is small (i.e. for 4/G 9 1):
E 2e 52 G/S,
(9)
and the void-growth rate would be inversely proportional to the total concentration of defect sinks. This result is quite consistent with the higher-temperature data shown in fig. 6 and with the result that the effect of cold work on void size is dependent upon the irradiation temperature. Fig. 12 provides a more quantitative relationship showing the normalized void size as a function of sink density for different irradiation temperatures. The void diameter was normalized to the void size corresponding to that found in material having a low sink density typical of solution-treated
I.0
0.9
0.7 0
Fig. 12. The effect
of sink density
and irradiation
temperature
on void growth
based on diffusion-controlled
model.
H.R. BragerjEffects of cold working and heat treatment
steel , x 1Og cm/cm3;
(10) where r(0) = void size in material having a low sink density (x log cm/cm3) and for typical values: G z 1017 displacements/cm3 . set, t z 12, d 2 2 .5 A > D,
Z
e-30 OOO/RT cm2/sec.
Fig. 12 show the increased influence that dislocation density, or defect sink concentration, has on inhibiting void growth at higher irradiation temperatures. The results of the diffusion-controlled void growth model are consistent with the trends of the data for irradiation temperatures up to 580°C and compare favorably with the result shown in fig. 11. The observation that pre-irradiation ageing of the 20% cold-worked steel for 100 h at 650°C causes the steel to swell an amount similar to that of solutiontreated material has direct implications to reactor fuel management. If at any time during the early life of a fuel subassembly, the cladding or duct materials are exposed to excessively high temperatures, the swelling behavior of these materials should be assumed to follow that of the solution-treated steel. Any conceivable quality-control tests which subject the full subassembly to such temperature conditions prior to irradiation are particularly inappropriate, since the beneficial aspects of cold working may be lost or significantly reduced by procedures of this sort.
5. Conclusions Increasing the cold-work level decreased the swelling in Type 316 stainless steel irradiated at 420,475 and 580°C. The reduction in swelling was due to a decrease in both the void-number density and void size. The decrease in void size with increasing coldwork levels was enhanced at higher irradiation temperatures. Cold working from 0 to 10% more effectively suppresses void-number density and swelling than by increasing the cw level from 10 to 2%. Void formation in the 20% cw steel which had been heat treated 100 h at 650°C before irradiation was in general very similar to that of the solution-treated steel. While swelling of the solution-treated steel decreased with temperature, irradiation at 650°C resulted in a larger void volume in the cold-worked material than
117
for irradiation at 475 or 580°C. Swelling of the materials irradiated at 650°C in the solution-treated, the 10% cw plus aged conditions was similar and slightly greater than that of the 20% cw steel.
Acknowledgements The author is particularly indebted to Mrs. H. Brady for preparation of the electron-microscopy specimens and for assistance in TEM specimen examination and data analysis, to Mr. R. TerQs for radiometallurgical assistance and to Mrs. C.L. Berwick for typing the manuscript. Helpful discussions with members of the Materials Engineering Department technical staff, espcially Drs. J.J. Laidler and J.L. Straalsund, are gratefully acknowledged.
References [l] Fast Reactor Materials Dosimetry Center, HEDL, Richland, Wash. private communication, Dec. 1973. (21 H.R..Brager and J.L. Straalsund, J. Nucl. Mater. 45 (1973) 134. (31 J.J. Holmes and J.L. Straalsund, Trans. ANS 13 (1970) 559. [4] J.I. Bramman, C. Cawthorne, E.J. Fulton and W.D.J. Sinclair, Voids Formed by Irradiation of Reactor Materials (BNES, Reading University, March 25-26, 1971) pp. 27-33. [5] J.L. Katz and H. Weidersich, J. Chem. Phys. 55 (1971) 1414. [6] K.D. Russell, Scripta Met. 6 (1972) 209. [7] H. Weidersich and J.L. Katz, Nucl. Met. 18 (1973) 530. [8] S.D. Harkness, J.A. Tesk and Che-Yu Li, Nucl. App.‘and Tech. 9 (1970) 24. [9] R. Bullough, B.L. Eyre and R.C. Perrin, Nucl. Appl. and Tech. 9 (1970) 346. (lo] J.L. Straalsund, H.R. Brager and J.J. Holmes, in: Radiation-Induced Voids in Metals, ed. J.W. Corbett and L.C. Ianniello (USAEC Symposium Series 26, Albany, 1971) pp. 142-154. [ll] C. Cawthome, E.J. Fulton, J.I. Bramman, G.A.V. Linekar and R.M. Sharpe, in: Voids Formed by Irradiation of Reactor Materials, ed. S.F. Pugh, J.F. Loretto and D.I.R. Norris, BNES European Conf., Reading (1971) pp. pp. 35 -43. [12] J.O. Stiegler and E.E. Bloom, J. Nucl. Mater. 41 (1971) 341. [ 131 H.R. Brager and J.L. Straalsund, J. Nucl. Mater. 47 (1973) 105.
118
H.R. BragerlEffects
of cold working and heat treatment
[ 141 W.G. Wolfer et al., 7th ASTM Internat. Symp. on Radia-
tion Effects on Structural Materials, Gatlinburg, Tenn., June 11-13, 1974, to be published. [ 151 J.F. Bates, 7th ASTM Intern. Symp. on Radiation Effects on Structural Materials, Gatlinburg, Tenn., June 11-13, 1974, to be published. [ 161 T.R. Padden, K.C. Thomas and E.J. Tarby, Trans. ANS 15 (1972) 771. [17] H.R. Brager and J.L. Straalsund, J. Nucl. Mater. 46 (1973 103. [ 181 J.M. Dupouy et al., Colloque sur le Combustible et les Elements Combustibles pour les Reacteurs Rapides, %ruxelles, July 2-6, 1973.
[ 191 W.N. McElroy, HEDL, Richland, Wash. private communication, Febr. 1973. [20] H. Wiedersich, Rad. Effects 12 (1972) 111. [21] S.D. Harkness and Che-Yu Li, Met. Trans. 2 (1971) 1457. [22] W.G. Wolfer and J.L. Straalsund, Scripta Met. 7 (1973) 161. [ 231 J.L. Straalsund, J. Nucl. Mater. 5 1 (1974) 302. [24] A.C. Damask and G.J. Dienes, Point Defects in Metals (Gordon and Breach, N.Y., 1963) p. 84. [25] P.G. Shewmon, Transformations in Metals (McGraw-Hill, N.Y., 1969) p. 38.
THE EFFECTS OF COLD WORKING AND PRE-IRRADIATION HEAT TREATMENT ON VOID FORMATION IN NEUTRON-IRRADIATED TYPE 316 STAINLESS STEEL1 H.R. BRAGER Westinghouse Hanford Company, Richland,
Washington, USA
Received 29 October 1974
The effect of fast-neutron irradiation on void formation in Type 316 stainless steel having undergone specific thermalmechanical treatments was investigated by transmission electron microscopy. The study showed that, for irradiation at the three lower temperatures (420,475 and 580°C): (1) the void volume decreased with increasing cold work; (2) the reduction in swelling was due to a decrease in both void-number density and void size; (3) the decrease in void size with increasing cold-work level was enhanced at higher irradiation temperatures; (4) cold working from 0 to 10% decreased the voidnumber density, and void volume, more than in the range from 10 to 20%; (5) void formation in the 20% cw steel which had been heat treated 100 h at 650°C before irradiation was similar to that of the solution-treated steel. The,temperature dependence, of swelling of the cold-worked material was different from that of the solution-treated steel. Irradiation at 650°C resulted in a larger void volume in the cold-worked material than for irradiation at 475 or 580”C:The effects of cold work and irradiation temperature on void growth are consistent with the predictions of a diffusion-controlled model. L’effet de l’irradiation par des neutrons rapides sur la formation des vides dans un acier inoxydable du type 316 ayant subi des traitements thermo-mkcaniques sp&ifiques a ktk &udie par microscopic klectronique par transmission. L’ktude a montrk que par irradiation aux trois tempkratures inf&ie’ures (420,475, et 580°C): (1) le volume des vides diminue avec l’icrouissage croissant; (2) la reduction du gonflement est due B une diminution a la fois de la densite du nombre et de la taille des vides; (3) la diminution de la taille des vides avec l’dcrouissage croissant est encore accrue par une elevation de la temperature d’irradiation; (4) un ecrouissage de 0 a 10% diminuait la densite du nombre de vides et le volume des vides davantage qu’un ecrouissage compris entre 10 et 20%; (5) la formation des vides dans l’acier a ecroui de 20%, trait& thermiquement 100 h a 650°C avant irradiation etait similaire a celle observee darts l’acier soumis i un traitement de mise en solution. La relation entre gonflement et temperature de l’acier kcroui etait differente de celle de l’acier soumis a un traitement de mise en solution. L’irradiation a 650°C provoquait un accroissement du volume des vides dans l’acier ecroui plus 616~6 que par irradiation h 475 ou 580°C. Les effects de l’ecrouissage et de la temperature d’irradiation sur la croissance des vides sont en accord avec les previsions d’un modele control& par la diffusion. Der Bestrahlungseinfluss schnelJer Neutronen auf die Porenbildung wurde in rostfreiem Stahl 316, der einer spezitischen thermisch-mechanischen Behandlung unterworfen war, transmissionselektronenmikroskopisch untersucht. Die Bestrahlung bei den drei niedrigeren Temperaturen 420,475 und 580°C hat ergeben: (1) Das Porenvolumen nimmt mit zunehmender Kaltverformung ab. (2) Die Verringerung des Schwellens beruht auf einer Abnahme der Dichte und der Griisse der Poren. (3) Die Abnahme der Porengrosse mit zunehmender Kaltverformung ist starker bei hijheren Bestrahlungstemperaturen. (4) Porendichte und Porenvolumen werden durch die Kaltverformung zwischen 0 and 10% starker verringert als zwischen 10 und 20%. (5) Die Porenbildung im 20% kaltverformten Stahl, der vor der Bestrahlung 100 h bei 650°C warmebehandelt worden war, ist der im liistmgsgeghliihten Stahl Bhnhch. Die Temperaturabhiingigkeit des Schwellens des kaltverformten und liisungsgegliihten Stahls ist verschieden. Die Bestrahlung des kaltverformten Stahls hat bei 650°C ein griisseres Porenvolumen als bei 475 oder 580°C zur Folge. Der Einfluss der Kaltverformung und Bestrahlungstemperatur auf das Porenwachsturn ist mit den Voraussagen hinsichtlich eines diffusionsbestimmten Modells konsistent.
’ This paper is based on work performed by Hanford Engineering Development Laboratory, Richland, Washington, operated by Westinghouse Hanford Company, a subsidiary of Westinghouse Electric Corporation, under US Atomic Energy Commission Contract AT(45-l)-2170.
H.R. BragerfEffects
104
of cold
working and heat treatment
1. Introduction Major effort has been directed toward development of the Liquid Metal Fast Breeder Reactor (LMFBR) for commercial generation of electric power. One significant materials phenomenon discovered by these programs was that of irradiation-induced swelling in structural materials operating at normal fast-reactor temperatures. Considerable research has been directed toward determining the extent of swelling that would occur in the LMFBR reference material for cladding and core components: 20% cw Type 3 16 stainless steel. However, reactor components having the same ‘cold work’ level or reduction of area are formed by various manufacturing processes which impart different mechanical properties and microstructural features. To date, the studies on irradiation-induced metal swelling have been limited, for the most part, to solution-treated steel or to material having a single level of cold work. No comprehensive information exists regarding the effects of cold work and irradiation temperature on metal swelling due to neutron irradiation. The objectives of this study are to determine the effect that cold work and irradiation temperatures have on void formation in Type 3 16 stainless steel and to ascertain the influence which pre-irradiation heattreatment has on the swelling resistance of the 20% cw steel.
2. Experimental
procedure
Specimens of Type 3 16 stainless steel were examTable 1 Reactor location and dosimetry information
ined by transmission electron microscopy after stressfree irradiation in the Experimental Breeder Reactor II (EBR-II). These specimens were prepared with cold-work levels of 0, 10, 15,20 or 25% (reduction of area by cold-swaging) after solution-annealing 15 min at 1070°C. Some samples of 20% cw steel were also aged for 100 h at 650°C prior to irradiation. Sets of specimens were irradiated in sodium-filled subcapsules located within the active fuel region of EBR-II Row 2. The high thermal conductivity of the sodium insured that the materials in each subcapsule were irradiated at essentially the same temperature. The subcapsule irradiation temperatures were calculated by taking into account the y heat generated in each capsule and the thermal resistance of the gas gap between the capsule and the reactor sodium-cooled outer subassembly wall. The core location and dosimetry information [l] for each set of specimens examined by electron microscopy is shown in table 1. The small variation in neutron flux and energy spectrum was ignored in subsequent interpretation of the data. The chemical analysis of the steel used in the study was: C: 0.060 wt%; Mn: 1.72 wt%; Cr: 17.30 wt%; Ni: 13.30 wt%; MO: 2.33 wt%; Cu: 0.065 wt%; Si: 0.40 wt%; P: 0.012 wt%; N,: 0.048 wt%; B: 0.05 wt%; S: 0.007 wt%; balance: Fe. Transmission electron microscopy samples were examined in a Philips 200 electron microscope operating at 100 kV and equipped with a double-tilt specimen holder in a goniometer stage. The specimenthinning techniques have been previously described [2]. Void diameters, including the black fringe, were
for specimens.
Specimen location during irradiation
Neutron fluence
R (cm) a)
Z (cm) b,
(@t (total) 10” n/cm**)
@t (E > 0.1 MeV) (lo** n/cm*)
17.7
-8.5 1 -7.11 +0.95 +3.05 +5.33 +6.60
2.7 2.8 2.8 2.8 2.7 2.7
2.3 2.4 2.4 2.4 2.3 2.3
Spectrum averaged neutron energy E (MeV)
Displacements per atom (dpa)
0.76 0.77 0.79 0.79 0.78 0.77
11.4 11.6 12.1 11.8 11.6 11.3
a) R denotes the radial position relative to the centerline of the reactor. b)Z denotes the position below (-) or above (+) the reactor midplane. The fueled region of EBR-II extend from -17cm?Z?+l-7cm.
H.R. Brager~Effe~~sof cold warning and heat rreatme~t
measured on positive prints using a calibrated-recticie optical magnifier. Five different void diameters are reported for each specimen. In addition to the minimum, mean and maximum values, the diameter which correspond to the value for which 5% of the voids were smaller or larger, respectively, are also reported. These values provide a range of void sizes which is not greatly influenced by the number of voids measured. The foil thicknesses were obtained from two or three stereoscopic measurements on each area analyzed. The void volume was computed taking into account the surface-intersection probability for each voidsize increment at the measured foil thickness. 3. Results 3. I, Effect of cold work Typical transmission micrographs of the solutiontreated, 10% cw and 20% cw specimens are shown for each irradiation temperature in figs. 1-4. The voidformation data for the specimens investigated are summarized in table 2. Since the neutron fluence for all the specimens varied only from 2.3-2.4 X 1O22 n/cm2 (E > 0.1 MeV), the only experimental variables which would significantly influence void formation
105
are feral-mechanics treatment of the steel and irradiation temperature. The void-volume data for the solution-treated, 10 and 20% cw specimens are shown as open symbols in fig. 5 as a function of irradiation temperature. The principal features of the data shown in fig. 5 are that, first, cold working decreases swelling, and second, the temperature dependence of swelling is related to the cold-work level. The reduction in swelling in the steel due to cold working agrees with previous experimental data [3,4]. The present results show that 10% cold work induces a much greater suppression of void formation than does a further increase in the cold-work level from 10 to 20%. This non-linearity of void nucleation with cold-work level apparently is due to the very strong relationship shown between defect supersaturation and void-nucleation rate [S-7]. At 6.SO°C, the magnitude of swelling of the solution-treated and 10% cw specimens was small but larger than that of the 20% cw material. In these three specimens, void formation appeared to be nearly homogeneously distributed throughout the matrix. This reduction in swelling resistance of the 10% cw material is probably due to a lower thermal stability of its dislocation network, which has greater long-range stresses than exist in the 20% cw condition.
Fig. 1. The effect of cold work on void formation in Type 316 stainlesssteel irradiatedat 420°C to 2.3 x 10” n/cm2 (E > 0.1 MeV). (a) Solution-treated, (b) 10% cw, (c) 20% cw.
106
H.R. BragerlEffects
of cold working
Fig. 2. The effect of cold work on void formation
(E > 0.1 MeV). (a) Solution-treated,
and heat treatment
in Type 316 stainless steel irradiated at 475°C to 2.3 (b) 10% cw, (c) 20% cw.
The void volume in the solution-treated steel continually decreases with increasing temperature while in the cold-worked steel signi~cant void formation occurs only at the lower- and higher-temper-
X
10" n/cm2
ature regions investigated. The lower-temperature region of void formation may occur by the same mechanism as proposed for solution-treated steels: vacancy supersaturation due to stress-biased net
Fig. 3. The effect of cold work on void formation in Type 316 stainless steel irradiated at 580°C to 2.4 X 10z2 n/cm2 f.E> 0.1MeV). (a) Solution-treatea, (b) 10% cw, (c) 20% cw.
Fig. 4. The effect of cold work on void formation in Type 316 stainless steel irradiated at 650°C to 2.4 X 10” n/cm2 (E > 0.1 MeV). (a) Solution-treated, (b) 10% cw, (c) 20% cw. 1.6
I
I
1.4
1.2
1.0 3 @ 8
0.8
5 ,’ 5, 0.6
0.4
10% 0.2
% 20% 0
C.W.
\ i.w\
I
I
400
450
IRRADIATION
Fig. 5. The effect of irradiation temperature, 3 16 stainless steel.
550
500
TEMPERATURE
cold work and preirradiation
600
,^_
(?,
ageing 100 h at 650°C on the swelling of 20% cw Type
650 k60
0% cw 10% cw 20% cw 20% cw + aged
37-6A 376F 37-6G 37-6D
2.4
2.4
2.3
2.3
Neutron fhence +r (10Z2 n/cm3)
90 112 101 112
202 179 101 123 112 123 124 123
270 179 101 191
134 146 123 157
124 112 123 123 112 134
89 89 112 67 89 101 112 134 101 101
5% Min.
Min.
Void diameter
212(391) c, 280(400) ‘) 185(310) c, 359(476) ‘)
471 517 314 651
730 290 258 449
415 281 235 348
280 236 236 236 225 292
5% Max. _~
~~ ~
~.
data.
503 235 192 32014231 b,
244 210 177 255
207 172 180 180 163 217
Mean
and immersion-density
a. Heterogeneons Void formation b. Value in f ] denotes non-typical value c. () denotes voids only; number preceeding () is for voids + bubbles
-
580 +40
O%CW lO%cw 20% cw 20% cw + aged
36-6A 36-6F 36-6G 36dD a)
475 525
0% cw 10% cw 20% cw 20% cw + aged
420 *lo
34-6A 34-6F 34-6G 35-6D
___I
had. temp. (“C)
O%cw 10% cw 15% cw 20% cw 25% cw 20% cw + aged
_
TMT
4CK3A 41-6F 41-6H 41-6G 41-61 40-6D
-.
IlO.
Sample
Table 2 Summary of transmission electron-microscopy
674 596 438 865
708 290 269 505
460 348 314 449
348 281 269 314 303 337
Max.
635(153) ‘) 196( 94) ‘) 437(59) c, 284( 170)‘)
147 3 17 57
857 283 492 385
449 820 443 1210 220 682
Number of voids measured
p
0.71(0.20) 0.43(0.23) 0.85(0.13) O-24(0.15)
0.38 0.008 0.07 0.14[0.51]
8.2 0.6 1.0 5.8
19
5.5 6.1 2.0
26 11
‘) ‘) c, ‘)
b)
$1014 voids/cm3)
Void density
0.09(0.08) 0.09(0.09) 0.04(0.02) 0.1 l(O.11)
0.30
0.80 0.032 0.032 0.60
1.48 0.33 0.19 0.21 0.052 1.15
‘) c) ‘) c)
b,
Void volume A V/V0 (%)
~.~
-0.21 +0.08 +O.lO +0.04
-0.36 +O.Ol +0.03 -0.10
-0.46 + 0.04 +0.06 a.10 +O.lO -0.23
(%)
change A P/PO
Density
H.R. 3mgerfEffe~ts
of cofd wor~nga~d
109
heat treatment
200
240 f
s 180 G - 16D
220 0s 6 200
l
\
180
.
1.0
IA A
B
1.2 1.0 2
0.8
3=-0.8 0.4 0.2 0.0 500 400 03 ‘308 200
T,
560°C
‘----A-* D.8_S 0.4 ;i 0 0.2 g *
- 1.0 - 0.8 “i;;
C
_ u - 0.8 3
0.0 -!xD
O------Y
- 0.4 =z .
-.0.2 f l
-, 0.0
-•
O? 5 -
0:d l-•
+0.c
0
1D.
20
CM0 WORK, %
0
10 CO10WORK, %
20
Fig. 6. The effect of irradiation temperature and cold-work level on void formation in type 316 stainless steel @t- 2.3X 1O22 n/cm2 (E > 0.1 MeV).
attraction of interstitials to dislocations [8,9]. The increasing irradiation temperature decreases the vacancy supersaturation in a thermally stable dislocation network, suppr~~ing void nucleation. At high temperatures, near 600°C the dislocation structure becomes thermally unstable. The resultant decrease in defect sink density thereby promotes void
formation until thermal-equilibrium vacancy concentrations dominate the behavior at still higher temperatures. The microscopy data for void diameter, number density and void volume are plotted as a function of cold-work level for the four irradiation temperatures in figs. 6a-d. The curves show a distinct difference
110
H.R. 3r~~erl~~ff~crs oj.cold worlcing and heat treatment
SOLUTION lIz/
Fig. 7. The effect of irradiation temperature
and pre-irradiation microstructure
TREATED
20% CW [27%CW AT 395°C)
on the void-size distribution in Type 316 stainless
St&
between the three lower-temperature data sets and the hi-tem~rature (65O’C) data. For the lower temperatures, the void parameters all decreases in magnitude with increasing cold-work level. The large reduction in swelling for 10% cw is associated with large decreases in both the void size and number density. At 650°C, where both, voids and helium bubbles, are formed, the void-formation data for the solution-treated and 10% cw data are nearly identical,
while the inhibition of swelling still persists at the 20% cw level. The relationship between void diameter and coldwork level changes significantly with increasing irradiation temperature. This behavior is more clearly shown in fig. 7. This figure includes the void data for solution-treated and 27% cw specimens from the same steel used in the present study but irradiated in a different sub-assembly at a lower temperature
H.R. BragerfEffects of cold working and heat treatment
Fig. 8. Frank-loop and dislocation-microstructure dependence on irradiation temperature in 20% cw Type 316 stainless steel. The micrographs are of material, irradiated to a neutron fluence of 2.3-2.4 x 102’ n/cm2 (E > 0.1 MeV), oriented with the 110 direction nearly parallel to the electron beam. a, b, and c are weak-beam dark-field micrographs; d is bright-field. (a) T = 420°C (b) T = 475”C, (c) T = 58O”C, (d) T = 650°C. Magnification approx. 67 500 X.
(395°C) [lo]. The mean diameter and void-size distribution found in the 27% cw material irradiated at 395°C are practically identical to those observed in the corresponding solution-treated steel. At 42O”C, and progressively more so at high temperature, the void-diameter distribution in the 20% cw steel is narrower than that observed in the solution-treated material. At 65O”C, in addition to the large voids, a high concentration of small helium bubbles is observed in both the solution-annealed and the cw materials. In summary, the present data show that cold-working reduces swelling by reducing both void size and number density, and that the predominant effect is dependent upon the irradiation temperature. Previously, UK investigators stated that cold work reduced swelling by decreasing the void size [ 111 while US observations were to the contrary, that the major influence was a reduction of voidnumber density [lo, 121. In addition to the voids, the radiation-induced dislocation substructures were also investigated. Typical micrographs showing dislocations and Frank loops in the 20% cw steel are seen in fig. 8. Weakbeam dark-field micrographs are used to illustrate
the structure of the material irradiated at the lower temperatures. Irradiation at 420°C fig. 8a, results in an appreciable reduction in the dislocation density from over 2 X 101’ to about 3 X lOlo cm/cm3 and in the formation of a high concentration of Frank loops, approximately 1015 loops/cm3. The solution-treated steel irradiated at 420°C contains a high density of Frank loops similar to those formed in the coldworked steels. A low dislocation density (
112
Fig. 9. Pre-irradiation microstructure 10% cw, (d) 20% cw.
H.R. Bragerlfiffccts
of cold working and heat treatment
Type 316 stainless steel (a) Solution-treated,
3.2. Effect of p$e-i~adiu~o~ heat treatment of 20% cw steel In examining the effect of the ageing treatment on swelling stability, it is appropriate to consider the microstructures of ~irradiated Type 3 16 stainless steel with various treatments (0, 10,20% cw or 20% cw + aged]. The 10% cw ‘as-fabricated’ structure, fig. 9c, cons&es primarily of a farily high dislocation concentration in the form of a complex network. A poorly defined ceil structure is evident in addition to a number of small dislocation segments (dislocation debris). The 20% cw structure,,fig. 9d, has a higher dislocation density and a somewhat better-defined cell structure than that of the 10% SWsteel. In addition, a number of microtwins and stacking faults are present; their concentration is dependent upon the orientation of each grain relative to the cold-working direction. The 20% cw steel which had been aged for 100 h at 650°C still has a high dislocation density, fig. 9b. The 65O’C heat treatment resulted in some disiocation motion with removal of most dislocation segments and production of a lower-energy dislocation network. However, the gross features of the aged structure are certainly much more similar to those of the 10% cw material than to those of the solution-treated steel. As shown in table 2, the swelling is substantially greater in the aged material than in the 10% cw steel. Therefore, there appear to be
(b) 20% cw + 100 h anneal at - 650°C Cc)
other factors besides dislocation density which influence swelling resistance. The deterioration of the swelling resistance of cold-worked steel due to preirradiation ageing might be due to the formation of an ‘atmosphere’ around the dislocations. The ‘atmosphere’ of solute atoms should decrease the stress field of the dislocation thereby reducing its effectiveness as a defect sink. Fig. 10 shows typical micrographs illustrating void formation in the aged samples irradiated at the four different temperatures. Comparison of these micrographs with those shown in figs. l-4 shows that the void formation in the aged sample is more to that of the solution-treated steel than to that of the 20% cw material. The closed symbols in fig. 5 show the void volume of the aged specimens as a function of irradiation temperature. The variation in swelling behavior of the aged specimen irradiated at 58O”C, fig. 5, is due to heterogeneous void formation. Most of the steel, including regions having many planar defects (stacking faults, microtwins and deformation bands) which were heavily decorated with precipitates, contained voids whose volume was similar to that of the 20% cw steel. Localized regions of this (20% cw t aged) steel contained many voids, with a void volume comparable to that of solution-treated steel. These regions of high void volume were associated with a few of the planar defects at which substantial precipitation had
H.R. Brager/Effects of cold working and heat treatment,
113
ia)T = 420°C
icf T
= 580 *C
Fig. 10. Void formation as a function of irradiated temperature 650°C before irradiation.
(d) T = 650 % in 20% cw Type 316 stainless steel that had been aged 100 h at
114
H.R. BragerlEffects
of cold working and heat treatment
occurred. The reason for this heterogeneous void formation is not clear. One factor which migh influence void formation would be the effect of localized stresses maintained during irradiation at these highly strained regions. Wolfer et al. [ 141 have shown that the net attraction or bias of vacancies and interstitials to voids and loops is dependent upon the hydrostatic and deviatoric stresses, respectively. These stresses can influence the incubation time for void nucleation and the swelling rate. Another factor which might locally influence void formation would be the effect of matrix chemical variation caused by extensive precipitation. Bates [ 151 showed that small differences in minor concentrations of alloying elements
have a significant influence on swelling of neutron-irradiated Type 3 16 stainless steel. Except for the heterogeneous void formation in the aged specimen irradiated at 58O”C, the void volume in the 20% cw steel is less than one fifth that of either the solution-treated or the aged material. Pre-irradiation ageing of the 20% cw steel for 100 h at 650°C effectively removes the beneficial influence of cold-work.
4. Discussion The data of the present study show that variations in both cold work and irradiation temperature influ-
K-
\
\
COLD
WORK
(%) [X]
Fig. 11. Swelling data of cold-worked steel, normalized using the solution-treated eq. 1.
information,
compared with curves based on
H.R. BragerfEffects
ence the magnitude of swelling in neutron-irradiated Type 3 16 steel. Various methods of cold-working austenitic steel with the same reduction in area procedure materials which have significantly different mechanical properties and microstructural features [16]. The different modes of cold working reactor components (cladding, wire wrap, duct, etc.) as well as minor variations in the cold-work level could influence the amount of metal swelling. In order to provide some design basis for estimating the effects of cold work on swelling, a functional relationship between swelling, irradiation temperature and cold-work level was derived from the experimental data: A!‘(x)/F
= [Av(O)/V]
X exp [(0.90 - 2.5 X 10e3 T)x] ,
115
of cold working and heat treatment.
0)
where A v(x)/ V = swelling for x% cold work and T = irradiation temperature (“C) (=4OO”C < T<550°C). For the present, this relationship is limit&d to fluences of 2-3 X 1O22 n/cm2 (I?> 0.1 MeV) and to irradiation temperatures from about 400 to 550°C. At temperatures much above about 55O”C, a second swelling range has been shown to exist in cold-worked steel [17] and this trend is consistent with the 650°C data set of the present study. The four low-temperature sets of data showing the effect of cold work on swelling are compared with the predictions of eq. (1) shown in fig. 11. Although the curves of eq. (1) fit all the microscopy information, only the 420°C data set provides sufficient information to accurately estimate the influence of cold work on swelling. The equation reflects the experimental observation that the relative’effectiveness of cold work in suppressing swelling increases with temperature. Recently published [ 181 data showing the effect of cold-work level on swelling for Type 3 16 stainless steel are also plotted on fig. 11. These data were for pressurized tubes (150 MN/m2) irradiated at 460°C in oxide-fuel reactor to a peak fluence of 7.5 X 1O22 n/cm2 (total), which should be equivalent to about 6.5 X 1O22 n/cm2 (E > 0.1 MeV) [19]. Despite the differences in calculating irradiation temperature, in alloy chemistry and in manufacturing techniques, the data agree quite well with those presented here. Previously, it was shown that at 395°C the mean void sizes in-the solution-treated and 27% cold-worked
material were the same [lo]. The present results indicate that the effect of cold work on void growth is quite temperature dependent. For irradiation et 42O”C, the void size decreased with increasing coldwork level. At higher temperatures (475 and 580°C) cold work had an even greater influence on void growth. This dependence of void-growth rate on cold work and irradiation temperature can be shown to be consistent with a simplified swelling model considered by other authors [20,21]. The steady-state concentration of point defects produced in an irradiated material is controlled by a dynamic balance between production, diffusion to sinks and recombination. Using a set of coupled steady-state equations for vacancy and interstitial concentrations, C, and C,, respectively: dC,/dt=O=G-RWSS,LS,V, dC,/dt
=O=G-RVI-S&-S&
(2)
where G = defect production rate; R = recombination coefficient; V, I = D,C,, DIC1; D, C = diffusivity and concentration of point defects; S, = a geometric factor describing the tendency for a void to capture (or emit) a vacancy or an interstitial; S,, SL = geometric factors describing the tendency for a dislocation to capture (or emit) a vacancy or an interstitial, respectively, and a void-growth rate equation for a void centrally located in a sphere of material with the voidgrowth rate dependent upon the diffusion of point defects through the matrix: (r/Q) dr/dt = V - I ,
(3)
where r = void radius, a = atomic volume, t = time, and assuming no trapping of defects or diffusion to sinks other than’voids and dislocations, it has been shown [22,23] that d(r2)/dt
= 2(S-;> - SD) Sl Gf/(S,
g 2~ !YlGf/S ,
+ SV) (S;> + S,) (4)
where E = net increased attraction of interstitial to dislocations compared to the vacancy-dislocation interaction, E = (S;>- SD)/@, + Sv), S = total sink density Z (SD + SV) or (S6 + Sv), f = fraction of vacancies, or interstitials, arriving at voids, or dislocations, respectively, f = (SD + S,) V/G = (Sb t S,) I/G, and
H.R. BragerlE’fjFects
116
of cold working and heat treatment
using eq. (2) it is readily shown that f=@/2
{(1+4/9)%
d(r’)/dt
1,
(5)
where 4 = (S,, + SV) (S;, t S,)/
RG rS2/RG
(6)
s 2e R (G/R);
,
(8)
or that the void-growth rate should be independent of sink density, which is in agreement with the essentially identical void-size distributions observed in the solutiontreated and the 27% cw specimens irradiated at 395°C [IO]. At high temperatures where D, is large, 4/4~ < 1,
and d(r2)/dt R=6[Cl/d2DV,
(7)
based on the relationships: recombination rate = v C, C, t Q [24] and D, = $ d2 [25]; 5 = number of sites neighboring a vacancy for which annihilation?is certain if the site is occupied by an interstitial and d = jump distance. Eq. (4) can be used to indicate trends for the irradiation-temperature dependence of void growth in cold-worked steel. Using eqs. (4-7), it can be readily shown that at lower temperatures where D, in eq. (7) is small (i.e. for 4/G 9 1):
E 2e 52 G/S,
(9)
and the void-growth rate would be inversely proportional to the total concentration of defect sinks. This result is quite consistent with the higher-temperature data shown in fig. 6 and with the result that the effect of cold work on void size is dependent upon the irradiation temperature. Fig. 12 provides a more quantitative relationship showing the normalized void size as a function of sink density for different irradiation temperatures. The void diameter was normalized to the void size corresponding to that found in material having a low sink density typical of solution-treated
I.0
0.9
0.7 0
Fig. 12. The effect
of sink density
and irradiation
temperature
on void growth
based on diffusion-controlled
model.
H.R. BragerjEffects of cold working and heat treatment
steel , x 1Og cm/cm3;
(10) where r(0) = void size in material having a low sink density (x log cm/cm3) and for typical values: G z 1017 displacements/cm3 . set, t z 12, d 2 2 .5 A > D,
Z
e-30 OOO/RT cm2/sec.
Fig. 12 show the increased influence that dislocation density, or defect sink concentration, has on inhibiting void growth at higher irradiation temperatures. The results of the diffusion-controlled void growth model are consistent with the trends of the data for irradiation temperatures up to 580°C and compare favorably with the result shown in fig. 11. The observation that pre-irradiation ageing of the 20% cold-worked steel for 100 h at 650°C causes the steel to swell an amount similar to that of solutiontreated material has direct implications to reactor fuel management. If at any time during the early life of a fuel subassembly, the cladding or duct materials are exposed to excessively high temperatures, the swelling behavior of these materials should be assumed to follow that of the solution-treated steel. Any conceivable quality-control tests which subject the full subassembly to such temperature conditions prior to irradiation are particularly inappropriate, since the beneficial aspects of cold working may be lost or significantly reduced by procedures of this sort.
5. Conclusions Increasing the cold-work level decreased the swelling in Type 316 stainless steel irradiated at 420,475 and 580°C. The reduction in swelling was due to a decrease in both the void-number density and void size. The decrease in void size with increasing coldwork levels was enhanced at higher irradiation temperatures. Cold working from 0 to 10% more effectively suppresses void-number density and swelling than by increasing the cw level from 10 to 2%. Void formation in the 20% cw steel which had been heat treated 100 h at 650°C before irradiation was in general very similar to that of the solution-treated steel. While swelling of the solution-treated steel decreased with temperature, irradiation at 650°C resulted in a larger void volume in the cold-worked material than
117
for irradiation at 475 or 580°C. Swelling of the materials irradiated at 650°C in the solution-treated, the 10% cw plus aged conditions was similar and slightly greater than that of the 20% cw steel.
Acknowledgements The author is particularly indebted to Mrs. H. Brady for preparation of the electron-microscopy specimens and for assistance in TEM specimen examination and data analysis, to Mr. R. TerQs for radiometallurgical assistance and to Mrs. C.L. Berwick for typing the manuscript. Helpful discussions with members of the Materials Engineering Department technical staff, espcially Drs. J.J. Laidler and J.L. Straalsund, are gratefully acknowledged.
References [l] Fast Reactor Materials Dosimetry Center, HEDL, Richland, Wash. private communication, Dec. 1973. (21 H.R..Brager and J.L. Straalsund, J. Nucl. Mater. 45 (1973) 134. (31 J.J. Holmes and J.L. Straalsund, Trans. ANS 13 (1970) 559. [4] J.I. Bramman, C. Cawthorne, E.J. Fulton and W.D.J. Sinclair, Voids Formed by Irradiation of Reactor Materials (BNES, Reading University, March 25-26, 1971) pp. 27-33. [5] J.L. Katz and H. Weidersich, J. Chem. Phys. 55 (1971) 1414. [6] K.D. Russell, Scripta Met. 6 (1972) 209. [7] H. Weidersich and J.L. Katz, Nucl. Met. 18 (1973) 530. [8] S.D. Harkness, J.A. Tesk and Che-Yu Li, Nucl. App.‘and Tech. 9 (1970) 24. [9] R. Bullough, B.L. Eyre and R.C. Perrin, Nucl. Appl. and Tech. 9 (1970) 346. (lo] J.L. Straalsund, H.R. Brager and J.J. Holmes, in: Radiation-Induced Voids in Metals, ed. J.W. Corbett and L.C. Ianniello (USAEC Symposium Series 26, Albany, 1971) pp. 142-154. [ll] C. Cawthome, E.J. Fulton, J.I. Bramman, G.A.V. Linekar and R.M. Sharpe, in: Voids Formed by Irradiation of Reactor Materials, ed. S.F. Pugh, J.F. Loretto and D.I.R. Norris, BNES European Conf., Reading (1971) pp. pp. 35 -43. [12] J.O. Stiegler and E.E. Bloom, J. Nucl. Mater. 41 (1971) 341. [ 131 H.R. Brager and J.L. Straalsund, J. Nucl. Mater. 47 (1973) 105.
118
H.R. BragerlEffects
of cold working and heat treatment
[ 141 W.G. Wolfer et al., 7th ASTM Internat. Symp. on Radia-
tion Effects on Structural Materials, Gatlinburg, Tenn., June 11-13, 1974, to be published. [ 151 J.F. Bates, 7th ASTM Intern. Symp. on Radiation Effects on Structural Materials, Gatlinburg, Tenn., June 11-13, 1974, to be published. [ 161 T.R. Padden, K.C. Thomas and E.J. Tarby, Trans. ANS 15 (1972) 771. [17] H.R. Brager and J.L. Straalsund, J. Nucl. Mater. 46 (1973 103. [ 181 J.M. Dupouy et al., Colloque sur le Combustible et les Elements Combustibles pour les Reacteurs Rapides, %ruxelles, July 2-6, 1973.
[ 191 W.N. McElroy, HEDL, Richland, Wash. private communication, Febr. 1973. [20] H. Wiedersich, Rad. Effects 12 (1972) 111. [21] S.D. Harkness and Che-Yu Li, Met. Trans. 2 (1971) 1457. [22] W.G. Wolfer and J.L. Straalsund, Scripta Met. 7 (1973) 161. [ 231 J.L. Straalsund, J. Nucl. Mater. 5 1 (1974) 302. [24] A.C. Damask and G.J. Dienes, Point Defects in Metals (Gordon and Breach, N.Y., 1963) p. 84. [25] P.G. Shewmon, Transformations in Metals (McGraw-Hill, N.Y., 1969) p. 38.