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The microstructure of the 1.4914 MANET martensitic steel before and after irradiation with 590 MeV protons
Journal ot Nuclear Materials 191 194 ( ! t~q2) 8911-895 North-! Iolland -
mk
The microstructure of the 1.4914 MANET martensitic steel before and after irradiation with 590 MeV protons D. Gavillet, P. Marmy and M. Victoria Paul &herrer hz~t:tutc. C!!-5232 ~lhgt't~ PSL Sw:tzerland
Ophcal and transmission electron microscope observations, together w~th SEM (scanmng electron m~eroseope) and ASTEM (analytical ",cmnmg transmission electron microscope) mlcroanalysls have been performed m samples of the DIN 1 4914 martens~hc ,,ted (MANET castl, both befi~re and after irradiation with 590 MeV protons to doses up to 1 dpa at temperatures between 363 and 7d3 K The chemical composnlon of the different carbide geometries have been obtained. No ,,ubstanthd modlficanon of the carbide ,tad preopnate structure is observed after either deformation under fatlgt,.- or after ~rrad~atlon to 1 dpa at 703 K No bubbles have been observed in a speomen ~rrad~ated to 117 dpa, containing 87 appm He
1. Introduction Fcrrittc-martensmc steels show an improved behavtour over that of austenittc steels with respect to vo~{l swelimg, irradiation creep and hehum embnttlemerit. Within th:s cla~s of steels, the D I N 1.4914 ( M A N E T ) steel [1] has been ~tudied as a reference material m the European Fusion Program As part of the same program, the steel has been mechanically tested before and after ~rradmtton w~th medium energy protons [2,3]. The structure resulting from the usual normahzing and tempering treatment of this type of steel consists of elongated martenslte laths with d high dislocation density, distributed "~,lthln the prior austemte grams. Carbide precipitation ts mostly in the form of M2~Cc, located mtergranularly at prior austemte and lath boundaries and an MX phase (mostly NbC) distributed m the lath structure Some M~C type carbides have also been observed m these steels for some heat treatment condmons [1,4-6]. In the present paper, observatmns that confirm this mtttai mxcrostructure are reported, together with the observed modificatton~ mduced by both deformation and irradiation with 590 MeV protons. Hehum ts produced by protons of th~s energy simultaneously with the displacement damage The hehum production rate m this steel has been measured to be 131) a p p m / d p a
2. Experimental procedure The exact composmon of the steel ~s given m table 1. The material was homogemzed for 2 h at 1223 K, austemzcd for 30 mm at 1348 K then air cooled and finally tempered for 2 h at 11123 K.
Substze tensile and fatigue specimens were irradiated in the P I R E X II facthty [7] with 590 M e V protons at temperatures between 363 and 703 K and doses from 0.111 up to I dpa. Post trradmtton tensile and fatigue tests were performed on the irradiated specimens. The results of these tests and the comparison w~th the properties of the nonirradiated steel are presented m a separate paper ,n these Proceedings [3]. Specimens were cut from the nonirradiated and irradiated material for surface observation (fight microsccpe and SEM), mlcrostructure observation and analysis in T E M (transmission electron mmroscopy) The carbide composition has been deter..mmed by X-ray mlcroana!ysts ( E D X ) and the carbide size dlstr:button by T E M observation on carbon extraction replicas.
3. Experimental results The observation of the specimen surface shows the presence of inclusions of a few microns m size, w~th a very high content in sthcon The number density of these inclusions is very low. The observed mtcrostructurc of the umrradiated M A N E T steel has already been presented in a previous paper [2]. It consists of a fully t e m p e r e d martensltlc structure with elongated subgrams (laths) separated by an ordered array of high dislocation density Elongated carbides of various sizes (20 to 500 nm) were observed at the lath and grain boundaries (fig la). Most of the carbides in the lath are spherical with an extended size distribution, from 20 nm to more than I I~m diameter. The number density of the large carb~des is very small The same type of structure ts observed on the etched surface and on the extraction
0022-3115/92/$05 (10 ~ 1992 - Elsevier Scmnce Pubhshers B V All rights reserved
D GariUet et al / The mtcrostrucmre of the MANET steel
89l
Table 1 MANET steel (DIN 1 4914) composition (in wt%, rest is iron) C
Cr
NI
Mo
V
Nb
SI
Mn
B
N
0.13
10 6
0 87
0.77
0.22
0.16
0.37
0.82
0.1)085
0:92
replicas (fig. lb). The lath boundaries can be recognized from the distrlbutmn of the elongated carbide. The carbide size dlstrlbut~on of the nonirradiated, n o n d e f o r m e d steel has been obtained by measurements on extraction replicas. A n eqmvalent diameter WdS calculated, which corresponds to the d m m e t e r of a
~ "~, " r l ~
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.
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circular contrast having an identical area as the one of the considered carbide. T h e equivalent diamete~ d~.~tribution of the carbide structure is presented in fig. 2. E D X analysis and S A D (selected area diffraction) or mierod~ffraction have b e e n performed on a n u m b e r of carbides in order to determine their composition
.l.....
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,
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,
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Fig l. TEM observation of the carbide structure m the MANET steel before ]rradlatlon and deformation (a) thin specimen, (b) extraction replica
D. Gar'dlet et al / The mtcmstructure o f the MANET steel
892
B) 533 Carbtdea Measured
A) 1665 Carbides Measured
2O
20
|,0 e 0
h,.... 0
50 |O0 150 200 260 Equivalent Diameter In nm
C) 739 Carbides Measured
|'
0
0
L
,SO IO0 1SO 2OO 2S0 EqulvmlentDlametor It,
D) 2786 Carbides Measured
20.
2o
_c15
-o, 0
K . ....
50 100 150 200 Equivalent Diameter In m
J:illiilll ,
250
Ill,,
O
50 100 150 200 250 Equivalent Dllln,eter In run
Fig 2 Carbide equivalent diameter d]stnbutton observed m the MANET steel (a) basic structure, (b) after fatigue deformation at 723 K, (c) after lrradaatlon at 700 K and 0 7 dpa, (d) after irradiation at 640 K, 0.6 dpa and fatigue deformation at 623 K (5100 cycles, Act = 0.6%). Table 2 EDX analysis of carbides m MANET steel Size [nm]
Shape
Concentration [%] Cr
Fe
Mo
N]
200x 75 85 × 40 180×85 170x 75 104 × 45 180×55 72×56 135 x 62 95 X 62 72x40
elhphcal elhpttcal faceted faceted elhptlcal faceted elhpticai elhptlcal elhptlcal elhptlcal
59 0 63 1 63 2 63 9 64.3 64 6 64 6 65 0 65 1 65.3
31 1 30 4 29 6 30 2 29.7 27.8 27.7 28 1 28 0 27 2
83 5.1 54 47 48 58 56 57 5.4 61
0.7 0.6 07 07 0.7 08 09 05 05 0.5
09 08 11 06 06 1.1 12 0.8 0.9 09
0.0 0.0 0.0 00 00 00 0.0 0.0 00 00
30 20 20 30 68 50 700
spherical spherical sphencal spherical sphencal sphencal sphencal
64.5 0.0 00 00 0.0 00 00
28 5 00 0.0 00 00 00 00
56 0.0 0.0 0.0 0.0 0.0 00
06 0.0 00 00 0.0 0.0 0.0
08 18 5 10.8 11.0 60 32 15
00 81 5 89 2 89 0 94.0 97.9 98.5
V
Nb
893
1), Gat'dlet et al. / 7he microstructure of the MANETsteel
and structure, Two carbide geometries have been observed m this steel: - Elongated carbides: They have an fcc structure w~th a large lattice parameter (around 1.06 nm). Their composition (not taking into account the carbon) is on average 64%Cr, 28%Fe, 5.5%Mo, 0.7%Ni and l%V. Details of the analysis are given in table 2. - Spherical carbides: The large (larger than 50 nm) have an fcc structure with a lattice parameter around 0 44 nm, with a Nb content higher than 98% and a vanadium trace. The small spherical particles (20 to 50 nm) can have two comp(~.~itions. They are either C r / F e carbides with the same composition as the elongated ones, or a N b - V carbide, where fi clearly
appears that the vanadium content of the carbides decreases with their size, see table 2. One of the smallest Nb carbides analysed (20 nm diameter) has a V content of 18.5%. The deformation in a fatigue or tensile test at temperatures between 393 and 730 K does not modify the carbide structure, distribution or composttion. Even when the lath structure is destroyed by the deformation and replaced by a dislocation cell structure [2,8], the same carbide size and spatial distribution are observed on the extractmon replicas (fig. 2b). The martensitic lath structure seems unaffected by Irradiation at all irradiation temperatures (360-700 K) and doses up to 1 dpa. No clear recovery has been
............. '
7
.......
~
i .
2~.]~
~.
:%
',
,
/ Fig 3. TEM observation of the dislocation network and carbide structure of the MANET steel after ~rradlation at 640 K, 0 6 dpa and fatigue deformation at 623 K (5100 cycles, Aet = 0 6%) (a) Thin specimen (b) Extraction replica
894
D Gacdlet et al, / Tl:e mzcrostructure of the MANETsteel
observed (fig 3a) Also, the carb,de structure is not modified after irradiation for all doses and temperatures. No new phases have been observed m the steel structure. The analysis performed on the extraction replicas shows no modification in the carbide shape, size distribution or average composition after irradiation. Representative examples of the carbide structure and size distnbuUon are shown in figs. 3b and 2c, respcctwely. The deformation of the irradmted specimen produces the same effect on the dislocation structure as that observed in the nonirradiated [2] and does not modify the carbide structure (fig. 2d) In addition, no hehum bubbles have been observed in TEM m any disk specimens, even for the one ,rradiated at 700 K to 0.7 dpa (91 appm He).
4. Discussion As indicated above, the microstructure observed in the nomrradiated, nondeformed MANET stee; is comparable to that observed previously and is s~,nilar to that found in most of the 9-21Cr, IMo steels studied
[11. From the mtcroanalysis performed on the carbide structure it can be deduced that the elongated and small spherical particles containing a high amount of Cr and Fe are of the M,3C 6 type and that the spherical ones with high Nb content are MX type precipitate'~. The large spherical particles are NbC carbide undissolved during the austemzation process. But in the small ones, M can either be Nb or V and X could be C or N It was not possible to reach a defimtive conclus,on only by the structural analysis and no microanalytical techn:ques were available to make the difference Similar observations have been made by other authors in stmdar steels [1,4,16]. Due to the high tempering temperature, the M2C phase has not been retained in the structure. The influence of 14 MeV neutron ,rradiatlon at different temperatures (20 to 823 K) and low doses (max,mum: 4 × 10 ,2 n/m-', 0.009 dpa) on the C r - M o steels has been reviewed by Kohyama et al [8] At high temperatures, the major effects are a small recovery of the dislocation structure and some rod,cation of enhanced carbide precipltat,on. The production of a high density of small defect clusters is postulated to explain the changes in mechanical behav,or. The observed results of 590 McV proton trradmtion to relatively low doses (I dpa) are comparable to those after neutron ,rradmtion. The features of the microstructure (dislocation network and carb,de d~stribution) in the MANET steel are not substantmlly modified and the max,mum proton dose obtained in the present work ,s not yet high enough to induce visible phase modiflcat,ons
Helium bubbles have been observed to form in the 1.4914 steel irradmted in a mixed neutron spectrum at 863 K. The helium content after irradiation was about 90 appm and the bubbles were observed on lath and grain boundaries as well as in the matrix [9]. It has also been shown [11,12] that m martensitic steels containing Nb, W and Ni, where Ni was added to enable helium productie,n between 80 and 400 appm under this type of spectra, very small bubbles are observed in the material after irradiation at 573 K and 38 dpa, while voids are formed in irradiations at 673 K and above. In another study, bubbles were observed after neutron irradiation at 673 K and 36 dpa with a helium content of only 30 appm [16] In the MANET steel implanted with energetic (104 MeV) alphas, bubbles are observed in samples implanted with 340 appm He at 573 K [13]. In the san,c material, bubbles have been observed after implantation of 190 appm He at 873 K [14] No helium bubbles have been observed in the proton irradiated specimen in spite of a measured He concentration of 130 a p p m / d p a . The specimen actually observed had a helium concentration of 91 appm (0.7 dpa), irradiated at 700 K From the results reviewed above, it is clear that the bubble formation in martens'de steels is a complex phenomena involving the He concentration, the displacement dose, the temperature and the microstructure From the preliminary results presented here we can just conclude that the displacement d o s e / H e content (linked in this type of irradiation) is still too low to produce visible bubbles. Although plastic deformation produces the same mod,fication of the dislocation network as that observed m the nonirradmted material [3,15], the mechanical behavior is very different. An important radiation hardening is measured r3]. We expect then that a high densRy of submicroscopic defects of very small size (lower than 10 nm) is produced in the material by the ,rradiation [3,10,15] This hypothesis is m agreement with the analys,s of the material behavior after 14 MeV neutron irradiat,on [8]. New analytical measurements and TEM observations are now underway in order to try to demonstrate the presence of the very small defects in the proton irradmted MANET steel
5. Conclusions The mlcrostructure ol the MANET martensitic steel (DIN 1.4914) has been studied by analytical electron microscopy before and after irradiation. The important features of the microstructure are a thin martenslte lath structure with elongated M23C6 carb,de at the lath boundaries and NbC or MX spherical particles in the lath.
D Gat'dlet et al / The mtcrostru~ture of the MANETstet,I
The carbide structure ts stable under tensile and fatigue deformation at temperatures up to 730 K, and the martensRe laths are replaced by a cellular dislocation structure after deformation. The structure was not modified by irradiatmn with 590 M e V proton to a dose of 1 dpa at temperatures between 360 and 730 K. In o r d e r to explain the mechamcal behavior of the material, the producuon of a high density of small defect is postulated. In specimens with 87 appm helium produced by proton irradmt~on of the M A N E T steel at 700 K, no bubble formation was observed.
Acknowledgement The authors would like to thank Ph. Buffat from the Ecole Polytechmque F6d6rale de Lausanne (EPFL). Switzerland, R. Wessiken from the Eidgen6ssische Hochschule in Ztirich ( E T H Z ) , Switzerland and A. Horsewell from R I S O Nattonal Laboratory m Denmark for their help with their respective A S T M systems.
References [1] K Q. Bagley, E A. Little, V Levy, A. Alamos, K Ehrhch, K Andde~k,: a,~d A. Calza Bin1, Nucl Energy 27 0988) 295 [2] P. Marmy, Y Ruan and M. Victoria, J Nucl Mater. 179-181 (1991) 697
895
[3] P Marmy and M. Victoria, in these Proceedings (ICFRM-5), J Nucl Mater. 191-194 (1992) 862. [4] E.A. Lmle and L.P. Stoter, ASTM-STP 782 (1982) 207 [5] J.M Vttek and R L Klueh, Met. Trans AI4 (1983) 1047 [6] R.G. Faulkner, L Schhfer, G.J Adetunjt and E.A. Little, J Nucl Mater. 155-157 (1988) 612 [7] P. Marmy, M. Daum, D. Gdvillet, S. Green, F. Hegedus, S. Proenneeke, U. Rohrer, U Stsefel and M. Vletona, Nucl. lnstr and Meth. B47 (1989) 37 [8] A Kohyama, K Hamada, K. Asakura and H. Matsm, ASTM STP 1046 (1990) 404 [9] C WasslleW, K. Herschbach, E. Materna-Morris and K. Ehrhch, Ferritlc Alloys for use m Nuclear Energy Technologies, eds J.W Davis and D J Michel, The Metallurgical Soc of ,~un. Inst. of Mining, Metallurgical and Petroleum Engineers, Warrendale, PA (1984) pp 607614. [i0] P. Marmy and M Victoria, Proc. 9th Int. Conf on the Strength of Metals and Alloys, ICSMAg, eds. D G. Brandon, R Chalm and A. Rosen (1991) p 841 [11] P J Mazlasz, R.L. Klueh and J M Vitek, J Nucl Mater 141-143 (1986) 929. [12] PJ. Maziasz and R.L Klueh, ASTM-STP 1046 (1989) p 35 [13] A Moeslang and D. Preininger, J Nucl Mater 155-157 (1988) 1064. [14] U. Stamm and H. Schroeder. tbld. p. 1059; see also U. Stamm, Benchte der KfA, Jfihch no 2225 (1988) [15] P Marmy, Th~se de Doctorat, Ecole Polytechmque F6d6rale de Lausanne (1991). [16] J M Vlttek and R L Klueh, J Nucl Mater 122 & 123 (1984) 254.
mk
The microstructure of the 1.4914 MANET martensitic steel before and after irradiation with 590 MeV protons D. Gavillet, P. Marmy and M. Victoria Paul &herrer hz~t:tutc. C!!-5232 ~lhgt't~ PSL Sw:tzerland
Ophcal and transmission electron microscope observations, together w~th SEM (scanmng electron m~eroseope) and ASTEM (analytical ",cmnmg transmission electron microscope) mlcroanalysls have been performed m samples of the DIN 1 4914 martens~hc ,,ted (MANET castl, both befi~re and after irradiation with 590 MeV protons to doses up to 1 dpa at temperatures between 363 and 7d3 K The chemical composnlon of the different carbide geometries have been obtained. No ,,ubstanthd modlficanon of the carbide ,tad preopnate structure is observed after either deformation under fatlgt,.- or after ~rrad~atlon to 1 dpa at 703 K No bubbles have been observed in a speomen ~rrad~ated to 117 dpa, containing 87 appm He
1. Introduction Fcrrittc-martensmc steels show an improved behavtour over that of austenittc steels with respect to vo~{l swelimg, irradiation creep and hehum embnttlemerit. Within th:s cla~s of steels, the D I N 1.4914 ( M A N E T ) steel [1] has been ~tudied as a reference material m the European Fusion Program As part of the same program, the steel has been mechanically tested before and after ~rradmtton w~th medium energy protons [2,3]. The structure resulting from the usual normahzing and tempering treatment of this type of steel consists of elongated martenslte laths with d high dislocation density, distributed "~,lthln the prior austemte grams. Carbide precipitation ts mostly in the form of M2~Cc, located mtergranularly at prior austemte and lath boundaries and an MX phase (mostly NbC) distributed m the lath structure Some M~C type carbides have also been observed m these steels for some heat treatment condmons [1,4-6]. In the present paper, observatmns that confirm this mtttai mxcrostructure are reported, together with the observed modificatton~ mduced by both deformation and irradiation with 590 MeV protons. Hehum ts produced by protons of th~s energy simultaneously with the displacement damage The hehum production rate m this steel has been measured to be 131) a p p m / d p a
2. Experimental procedure The exact composmon of the steel ~s given m table 1. The material was homogemzed for 2 h at 1223 K, austemzcd for 30 mm at 1348 K then air cooled and finally tempered for 2 h at 11123 K.
Substze tensile and fatigue specimens were irradiated in the P I R E X II facthty [7] with 590 M e V protons at temperatures between 363 and 703 K and doses from 0.111 up to I dpa. Post trradmtton tensile and fatigue tests were performed on the irradiated specimens. The results of these tests and the comparison w~th the properties of the nonirradiated steel are presented m a separate paper ,n these Proceedings [3]. Specimens were cut from the nonirradiated and irradiated material for surface observation (fight microsccpe and SEM), mlcrostructure observation and analysis in T E M (transmission electron mmroscopy) The carbide composition has been deter..mmed by X-ray mlcroana!ysts ( E D X ) and the carbide size dlstr:button by T E M observation on carbon extraction replicas.
3. Experimental results The observation of the specimen surface shows the presence of inclusions of a few microns m size, w~th a very high content in sthcon The number density of these inclusions is very low. The observed mtcrostructurc of the umrradiated M A N E T steel has already been presented in a previous paper [2]. It consists of a fully t e m p e r e d martensltlc structure with elongated subgrams (laths) separated by an ordered array of high dislocation density Elongated carbides of various sizes (20 to 500 nm) were observed at the lath and grain boundaries (fig la). Most of the carbides in the lath are spherical with an extended size distribution, from 20 nm to more than I I~m diameter. The number density of the large carb~des is very small The same type of structure ts observed on the etched surface and on the extraction
0022-3115/92/$05 (10 ~ 1992 - Elsevier Scmnce Pubhshers B V All rights reserved
D GariUet et al / The mtcrostrucmre of the MANET steel
89l
Table 1 MANET steel (DIN 1 4914) composition (in wt%, rest is iron) C
Cr
NI
Mo
V
Nb
SI
Mn
B
N
0.13
10 6
0 87
0.77
0.22
0.16
0.37
0.82
0.1)085
0:92
replicas (fig. lb). The lath boundaries can be recognized from the distrlbutmn of the elongated carbide. The carbide size dlstrlbut~on of the nonirradiated, n o n d e f o r m e d steel has been obtained by measurements on extraction replicas. A n eqmvalent diameter WdS calculated, which corresponds to the d m m e t e r of a
~ "~, " r l ~
° ,"
•
%
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t~,¢..~l~,Ls'$x
~.'.~
,
~
~:,"
.
.
.~,~_~:~'~. , ~ , . ~"£~,,
-
~
circular contrast having an identical area as the one of the considered carbide. T h e equivalent diamete~ d~.~tribution of the carbide structure is presented in fig. 2. E D X analysis and S A D (selected area diffraction) or mierod~ffraction have b e e n performed on a n u m b e r of carbides in order to determine their composition
.l.....
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.
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-
.,-,..,,
•
~ "/ • . _ , ~ .,; ~
~-~..
.,;
-~.
"
:-
-'
,-
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~ 4."g.
. ' ~
,
•
-', ~
,
~
,.-
~
"
.
a',i~" ~ : I t ~',~, :41m,.~
Fig l. TEM observation of the carbide structure m the MANET steel before ]rradlatlon and deformation (a) thin specimen, (b) extraction replica
D. Gar'dlet et al / The mtcmstructure o f the MANET steel
892
B) 533 Carbtdea Measured
A) 1665 Carbides Measured
2O
20
|,0 e 0
h,.... 0
50 |O0 150 200 260 Equivalent Diameter In nm
C) 739 Carbides Measured
|'
0
0
L
,SO IO0 1SO 2OO 2S0 EqulvmlentDlametor It,
D) 2786 Carbides Measured
20.
2o
_c15
-o, 0
K . ....
50 100 150 200 Equivalent Diameter In m
J:illiilll ,
250
Ill,,
O
50 100 150 200 250 Equivalent Dllln,eter In run
Fig 2 Carbide equivalent diameter d]stnbutton observed m the MANET steel (a) basic structure, (b) after fatigue deformation at 723 K, (c) after lrradaatlon at 700 K and 0 7 dpa, (d) after irradiation at 640 K, 0.6 dpa and fatigue deformation at 623 K (5100 cycles, Act = 0.6%). Table 2 EDX analysis of carbides m MANET steel Size [nm]
Shape
Concentration [%] Cr
Fe
Mo
N]
200x 75 85 × 40 180×85 170x 75 104 × 45 180×55 72×56 135 x 62 95 X 62 72x40
elhphcal elhpttcal faceted faceted elhptlcal faceted elhpticai elhptlcal elhptlcal elhptlcal
59 0 63 1 63 2 63 9 64.3 64 6 64 6 65 0 65 1 65.3
31 1 30 4 29 6 30 2 29.7 27.8 27.7 28 1 28 0 27 2
83 5.1 54 47 48 58 56 57 5.4 61
0.7 0.6 07 07 0.7 08 09 05 05 0.5
09 08 11 06 06 1.1 12 0.8 0.9 09
0.0 0.0 0.0 00 00 00 0.0 0.0 00 00
30 20 20 30 68 50 700
spherical spherical sphencal spherical sphencal sphencal sphencal
64.5 0.0 00 00 0.0 00 00
28 5 00 0.0 00 00 00 00
56 0.0 0.0 0.0 0.0 0.0 00
06 0.0 00 00 0.0 0.0 0.0
08 18 5 10.8 11.0 60 32 15
00 81 5 89 2 89 0 94.0 97.9 98.5
V
Nb
893
1), Gat'dlet et al. / 7he microstructure of the MANETsteel
and structure, Two carbide geometries have been observed m this steel: - Elongated carbides: They have an fcc structure w~th a large lattice parameter (around 1.06 nm). Their composition (not taking into account the carbon) is on average 64%Cr, 28%Fe, 5.5%Mo, 0.7%Ni and l%V. Details of the analysis are given in table 2. - Spherical carbides: The large (larger than 50 nm) have an fcc structure with a lattice parameter around 0 44 nm, with a Nb content higher than 98% and a vanadium trace. The small spherical particles (20 to 50 nm) can have two comp(~.~itions. They are either C r / F e carbides with the same composition as the elongated ones, or a N b - V carbide, where fi clearly
appears that the vanadium content of the carbides decreases with their size, see table 2. One of the smallest Nb carbides analysed (20 nm diameter) has a V content of 18.5%. The deformation in a fatigue or tensile test at temperatures between 393 and 730 K does not modify the carbide structure, distribution or composttion. Even when the lath structure is destroyed by the deformation and replaced by a dislocation cell structure [2,8], the same carbide size and spatial distribution are observed on the extractmon replicas (fig. 2b). The martensitic lath structure seems unaffected by Irradiation at all irradiation temperatures (360-700 K) and doses up to 1 dpa. No clear recovery has been
............. '
7
.......
~
i .
2~.]~
~.
:%
',
,
/ Fig 3. TEM observation of the dislocation network and carbide structure of the MANET steel after ~rradlation at 640 K, 0 6 dpa and fatigue deformation at 623 K (5100 cycles, Aet = 0 6%) (a) Thin specimen (b) Extraction replica
894
D Gacdlet et al, / Tl:e mzcrostructure of the MANETsteel
observed (fig 3a) Also, the carb,de structure is not modified after irradiation for all doses and temperatures. No new phases have been observed m the steel structure. The analysis performed on the extraction replicas shows no modification in the carbide shape, size distribution or average composition after irradiation. Representative examples of the carbide structure and size distnbuUon are shown in figs. 3b and 2c, respcctwely. The deformation of the irradmted specimen produces the same effect on the dislocation structure as that observed in the nonirradiated [2] and does not modify the carbide structure (fig. 2d) In addition, no hehum bubbles have been observed in TEM m any disk specimens, even for the one ,rradiated at 700 K to 0.7 dpa (91 appm He).
4. Discussion As indicated above, the microstructure observed in the nomrradiated, nondeformed MANET stee; is comparable to that observed previously and is s~,nilar to that found in most of the 9-21Cr, IMo steels studied
[11. From the mtcroanalysis performed on the carbide structure it can be deduced that the elongated and small spherical particles containing a high amount of Cr and Fe are of the M,3C 6 type and that the spherical ones with high Nb content are MX type precipitate'~. The large spherical particles are NbC carbide undissolved during the austemzation process. But in the small ones, M can either be Nb or V and X could be C or N It was not possible to reach a defimtive conclus,on only by the structural analysis and no microanalytical techn:ques were available to make the difference Similar observations have been made by other authors in stmdar steels [1,4,16]. Due to the high tempering temperature, the M2C phase has not been retained in the structure. The influence of 14 MeV neutron ,rradiatlon at different temperatures (20 to 823 K) and low doses (max,mum: 4 × 10 ,2 n/m-', 0.009 dpa) on the C r - M o steels has been reviewed by Kohyama et al [8] At high temperatures, the major effects are a small recovery of the dislocation structure and some rod,cation of enhanced carbide precipltat,on. The production of a high density of small defect clusters is postulated to explain the changes in mechanical behav,or. The observed results of 590 McV proton trradmtion to relatively low doses (I dpa) are comparable to those after neutron ,rradmtion. The features of the microstructure (dislocation network and carb,de d~stribution) in the MANET steel are not substantmlly modified and the max,mum proton dose obtained in the present work ,s not yet high enough to induce visible phase modiflcat,ons
Helium bubbles have been observed to form in the 1.4914 steel irradmted in a mixed neutron spectrum at 863 K. The helium content after irradiation was about 90 appm and the bubbles were observed on lath and grain boundaries as well as in the matrix [9]. It has also been shown [11,12] that m martensitic steels containing Nb, W and Ni, where Ni was added to enable helium productie,n between 80 and 400 appm under this type of spectra, very small bubbles are observed in the material after irradiation at 573 K and 38 dpa, while voids are formed in irradiations at 673 K and above. In another study, bubbles were observed after neutron irradiation at 673 K and 36 dpa with a helium content of only 30 appm [16] In the MANET steel implanted with energetic (104 MeV) alphas, bubbles are observed in samples implanted with 340 appm He at 573 K [13]. In the san,c material, bubbles have been observed after implantation of 190 appm He at 873 K [14] No helium bubbles have been observed in the proton irradiated specimen in spite of a measured He concentration of 130 a p p m / d p a . The specimen actually observed had a helium concentration of 91 appm (0.7 dpa), irradiated at 700 K From the results reviewed above, it is clear that the bubble formation in martens'de steels is a complex phenomena involving the He concentration, the displacement dose, the temperature and the microstructure From the preliminary results presented here we can just conclude that the displacement d o s e / H e content (linked in this type of irradiation) is still too low to produce visible bubbles. Although plastic deformation produces the same mod,fication of the dislocation network as that observed m the nonirradmted material [3,15], the mechanical behavior is very different. An important radiation hardening is measured r3]. We expect then that a high densRy of submicroscopic defects of very small size (lower than 10 nm) is produced in the material by the ,rradiation [3,10,15] This hypothesis is m agreement with the analys,s of the material behavior after 14 MeV neutron irradiat,on [8]. New analytical measurements and TEM observations are now underway in order to try to demonstrate the presence of the very small defects in the proton irradmted MANET steel
5. Conclusions The mlcrostructure ol the MANET martensitic steel (DIN 1.4914) has been studied by analytical electron microscopy before and after irradiation. The important features of the microstructure are a thin martenslte lath structure with elongated M23C6 carb,de at the lath boundaries and NbC or MX spherical particles in the lath.
D Gat'dlet et al / The mtcrostru~ture of the MANETstet,I
The carbide structure ts stable under tensile and fatigue deformation at temperatures up to 730 K, and the martensRe laths are replaced by a cellular dislocation structure after deformation. The structure was not modified by irradiatmn with 590 M e V proton to a dose of 1 dpa at temperatures between 360 and 730 K. In o r d e r to explain the mechamcal behavior of the material, the producuon of a high density of small defect is postulated. In specimens with 87 appm helium produced by proton irradmt~on of the M A N E T steel at 700 K, no bubble formation was observed.
Acknowledgement The authors would like to thank Ph. Buffat from the Ecole Polytechmque F6d6rale de Lausanne (EPFL). Switzerland, R. Wessiken from the Eidgen6ssische Hochschule in Ztirich ( E T H Z ) , Switzerland and A. Horsewell from R I S O Nattonal Laboratory m Denmark for their help with their respective A S T M systems.
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