Austenitic stainless steels with improved resistance to radiation-induced swelling

Scripta

METALLURGICA

Vol. I0, pp. 303~308, 1 9 7 6 P r i n t e d in the U n i t e d S t a t e s

Pergamon

Press,

Inc

AUSTENITIC STAINLESS STEELS WITH IMPROVED RESISTANCE TO RADIATION-INDUCED SWELLING* E. E. Bloom, J. O. Stiegler, A. F. Rowcllffe, and J. M. Leltnaker Metals and Ceramics Division Oak Ridge National Laboratory Oak Ridge, Tennessee 57850 ( R e c e i v e d N o v e m b e r 28, 1975) ( R e v i s e d F e b r u a r y 19. 1976) Introduction Shortly after the discovery of irradiatlon-lnduced void formatlon by Cawthorne and Fulton (i) in 1967 it became apparent that the phenomenon was sensitive to alloy composition. Pure metals such as aluminum and nickel exhibit void formation at much lower fluences than complex alloys such as stainless steels (2,3). Within a class of alloys such as the 300 series of austenltlc stainless steels (nominally 12% Ni, 17% Cr, bal Fe) the observed swelling and mlcrostructural changes are found to be very sensitive to composition (4,5). More recently it was found that the Irradlatlon-lnduced swelling of commercial type 316 stainless steel is much lower than that observed in the alloy base composition; that is, a pure Fe--17% Cr--12% Ni--2.5% Mo alloy swells 20 to 50 times as much as the commercial alloy, establishing a collective influence of elements such as C, N, S, Mn, and Si in retarding both void nucleation and growth, and thus reducing swelling (6). Experiments by Bates (7) and an analysis of the behavior of different heats of stainless steel (8) show that silicon is a particularly potent element in reducing swelling, at least for neutron fluences up to about 3 x 1022 n cm -2 (>0.i MeV) or about 15 dpa (displacements per atom). Other studies have shown that titanium has a significant influence on the postirradiation mechanical behavior of an alloy having the nominal composition of type 316 stainless steel (8). Lastly, a heat of type 316 stainless steel containing relatively high silicon (0.9%) as well as other minor alloying elements such as Ti, V, W, and Zr was irradiated to neutron fluences up to about 6.6 x 1022 n cm -2 (>0.i MeV) in the annealed condition and exhibited much lower swelling (9) than expected on the basis of published empirical swelling equations (I0) for an alloy having about 0.4% Sl. Consideration of this evidence in toto prompted the preparation of a type 316 alloy designated LSI, which contained additions of Si and Ti and smaller quantities of V, W, Zr, and Cu at levels greater than those normally present in vacuum-melted type 316 stainless steel. A second alloy, designated NY, was also prepared with the chromium level reduced to 12 wt % to lessen the tendency for slgma-phase formation and with reduced levels of V, W, Zr, and Cu. A further set of four alloys was prepared in which these minor elements were added singly to the N7 alloy. All these alloys were bombarded with nickel ions at elevated temperatures to simulate fast neutron damage, and their swelling behavior was compared on the basis of surface step height measurements. Experimental The experimental alloy LSI, having the nominal composition of type 316 stainless steel but containing about 1% Si, 0.15% Ti, and small additions of Cu, V, W, and Zr was produced by melting the high-purity alloy constituents under an argon cover gas. The alloy was drop

*Research sponsored by the Energy Research and Development Administration under contract with Union Carbide Corporation.

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cast into a water-cooled c o p p e r mold and subsequently rolled to sheet. Commercial type 316, titanium-modified 316, and high purity 316 (P7) were rolled from bar stock. All alloys received a homogenization anneal at 1150 to 1200°C in vacuum and were finished by cold rolling 50% and annealing 15 min at 1050ec to recrystallize except LSI, which received 1 hr at I050°C. A portion of the commercial 316 stainless steel was rolled to give 20% reduction in thickness in order to examine the effect of cold working on swelling. The swelling behavior of the alloys under nickel-lon bombardment was compared. The results of the neutron irradiation of a hlgh-purlty steel similar to P7, which showed considerable swelling, have been reported previously (6). Chemical analysis of these alloys is shown in Table 1. Included in this table are the compositions of beats DO and 02, for which some neutron data are available and are considered in the discussion. TABLE 1 Compositions

of Stainless

Steels Compared

in Irradiation

Swelling

Content, wE Z Alloy Cr

Ni

Mo

SI

Mn

Ti

A1

Nb

V

V a r i a t i o n s o f T~pe 316 S t a i n l e s s

Commercial Ti-modlfied P7 (pure) Beat DO LS1 Heat 02

17.6 17.5 17.0 18.0 16.8 16.9

14.4 14.0 16.7 13.0 13.5 11.2

2.5 2.5 2.5 2.6 1.9 2.46

0.36 0.10 0.I0 0.80 0.87 0.92

1.67 1.41 0.03 1.9 2.0 2.02

0.01 0.29 0.01 0.05 0.15 0.01

0.005 0.05 0.02 0.03 0.02 0.02 <0.001 0.10 0.02 0.02 0.04 <0.02 <0.01

Co

Zr

W

Cu

C

N

Steel

0.02 0.02 0.03 0.03 0.01 0.02

0.01 0.03 0.02 0.03 0.06 0.02 0.001 0.03 0.01 0.I0 0.05 0.05 0.05 0.25 0.045 <0.02 <0.05 0.03 0.07

0.018 0.004 0.002 0.05 0.002

E x p e r i m e n t a l A l l o y s with Lower Chromium H7

10.9

15.4

1.9

0.87

1.76

0.22

<0.001

0.07

K7 L7 M7 N7

11.8 11.8 12.2 11.8

14.3 15.1 15.6 14.4

2.1 2.0 2.1 2.0

0.85 0.87 0.86 0.90

1.80 1.84 1.90 1.86

0.23 0.23 0.23 0.20

<0.001 <0.001 0.07 <0.001

<0.001 <0.001 <0.001 <0.001

0,07

0.018

0.04

0.15 0.055 0.07 0.045

0.02 0.07 0.024 0.008

0.03 0.02 0.04 0.04

In a second set of experiments an alloy series with compositions similar to type 316 stainless steel but with a lower chromium content (12 wt %) was prepared. Nominal additions of 1% Si and 0.25% Ti were made together with small separate additions of Cu, V, W, and Zr. The analyses of these alloys are also shown in Table i. All these alloys except heats DO and 02 were bombarded at elevated temperatures with nickel ions, and their swelling behavior was assessed from the measurement of the step height between masked and unmasked regions of a polished surface by use of a technique pioneered by Johnston et al. (Ii). In our experiments i0 to 15 rectangular specimens were bombarded simultaneously. Figure I shows a specimen holder after irradiation. The boundary between the masked and unmasked regions is clearly visible. Before irradiation the specimens were carefully polished on successively finer abrasives, finishing with 0.5-pm diamond. Helium was then injected at a level of about 8 or 16 at. ppm at room temperature with a 2ttCm203 source. Details of this technique have been presented by C o g h l a n e t al. (12). Bombardments were carried out using the ORNL 6-MV Van de Graaff using 4-MeV Ni 2÷ ions at a flux of about 6 x 10 12 ions cm- 2 s e c - .1 The pressure at the specimen was maintained at about i0- 7 torr (13 ~Pa), and temperatures were measured by six thermocouples pressed against the back of the specimen holder. In a separate experiment, thermocouples were spot-welded onto the surface to be bombarded and the surface temperatures measured as a function of beam current at various furnace settings. The ion beam profile was continuously monitored, and a signal from the beam profile monitor was integrated over the period of each bombardment. Between bombardments, this signal was calibrated against the beam current measured in a deep Faraday cup substituted for the specimen assembly. The measured variation in beam intensity over the bombarded areas was ±15%. The number of atomic displacements (dpa) created in the peak damage

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FIG. i Portion of Multlspecimen Holder After lon Bombardment. The lighter appearing band is the region that was covered by the mask during ion irradiation. The individual samples are 0.020 by 0.125 In. (0.51 x 3.18 mm). region was calculated by use of the EDEP-I code of Manning and Mueller (13) and a secondary displacement model (14) using values of 40 eV for the effective threshold energy E d and 0.8 for the constant 8. Results Flgure 2 shows the swelling-induced surface step height as a function of Ion dose for the type 316 alloys shown in Table i at a bombardment temperature of 635 ± 15°C with 8 ppm preinjected helium. The amount of swelling occurring in the peak damage region may be estimated wlth Johnston's empirically determined relationship (5) for a similar alloy and experimental conditions. He observed that 1% swelling at the peak damage region produced a surface step of approximately 60 A. The exactness of thls relationship for the alloys and conditions used here is being investigated. The data for all alloys considered in Fig. 2 may be represented as an initial low-swelllng incubation period followed by a higher linear swelling rate. The hlgh-purlty steel P7 exhibited a very short incubation period followed by a hlgh swelling rate (0.43% per dpa), ultimately reaching 120% swelling at 300 dpa. owns.- ow~ 7 s - , o r ~ PEAK DISPLACEMENT DOSE (dpo) 5O I

8OOO - -

ID0 ]

150 I

ZOO I

250 I

Y

(] HIGH PURiTy 31G -

o

COMMERCIIL

•

Ti MODIFIED 316 2D%COLD WORKED 3~6

300 I../ t20

3~8

6DO0

tD0

FIG. 2

~ooo

so

Dose Dependence of Swelling at 635°C wlth 8 ppm Prelnjected Helium. uJ

20 ~I ° tODD

O

-

D

5

t0

t5 ,onl/c,. z

20

25 (zlO t6)

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The commercial 316, which contains higher levels of Si, Mn, C, and N compared with alloy P7 showed an increased incubation period and a reduction in linear swelllng rate to 0.35% per dpa. Cold-working the same commercial type 316 stainless steel further increased the incubation period and lowered the swelling rate slightly. A ~dlfied type 316 stalnless steel containing very low sillcon and an addition of 0.29Z Ti exhibited a swelllng rate of 0.24Z per dpa. The alloy LS1 exhibited a greatlY reduced swelling rate (0.06% per dpa) compared with the other alloys, although the incubation period was about the same as the others. Figure 3 shows the temperature dependence of swelling for these alloys in the range 600 to 715°C again with 8 ppm preinJected h~!_lum. The dat a for the annealed co,-,~rclal type 316 and tltanlum-~Rodlfled type 316 are represented by a single llne. The data points at 610, 665, and 715°C were taken from bombardments conducted to doses in the range 280 to 300 dpa. The values for 635°C are linear extrapolations of the data shown in Fig. 2. The important point about Fig. 3 is that alloy LS1 exhibited low swelllng relative to type 316 stainless steel over the entire t ~ e r a t u r e range investigated. The bombardments at 635°C were repeated to doses of 1.6 x 1017 ions cm-2 with the prelnJection of 16 at ppm He. The linear swelllng rate was increased by factors of 1.5 and 2.0 for the type 316 and titanium modified type 316 steels, respectively. The LSI alloy and the cold worked 316 steel followed essentlally the same swe11Ing behavior observed previously with 8 ppm preinJected hellum.

t20

• • •

Ti MOO4FN[h 318 2 0 % CQLO WORKED 31 LSI

FIG. 3 3 Temperature Dependence of Swelling ~roduced by Bombardment with 2.4 x 10 I? Ni ions cm-2 (Peak Dose 280 dpa). Samples contained 8 ppm prelnJected hellum.

m

°

o 600

I 640 600 TEMPF.R&TUR((e(;)

I

.I ?20

The a l l o y s h a v i n g t h e low-chromium c o m p o s i t i o n s g i v e n i n T a b l e 1 a l o n g w i t h s p e c i m e n s o f LS1 a n d P7 w e r e mounted t o g e t h e r , p r e i n J e c t e d w i t h 8 ppm He, and bombarded a t 640 and 710eC t o p e a k d i s p l a c e m e n t d o s e s o f 218 and 196 d p a , r e s p e c t i v e l y . The s t e p h e i g h t m e a s u r e m e n t s a r e shown i n T a b l e 2. The d a t a p o i n t s a t 218 dpa i n F i g . 2 f o r P7 ( t h e h i g h - p u r i t y t y p e 316) a n d LS1 were t a k e n f r o m t h i s e x p e r i m e n t . At 640eC t h e s t e p h e i g h t m e a s u r e m e n t s f o r a l l o y N7 w i t h no a d d i t i o n s and t h e a l l o y s w i t h s i n g l e a d d i t i o n s o f Z r , W, Cu, and V a l l f e l l w i t h i n t h e e x p e r i m e n t a l s c a t t e r o f m e a s u r e m e n t s on a s i n g l e a l l o y ; t h a t i s , no e f f e c t o f a d d i n g these:elements to the base alloy could be detected. The s t e p h e i g h t m e a s u r e m e n t s f o r LS1, which contains all these elements together, also fell within this range. At 715eC, s w e l l i n g c o u l d b a r e l y b e d e t e c t e d i n a n y o f t h e a l l o y s e x c e p t t h e h i g h - p u r i t y s t e e l P7. Discuss Ion Relatively little s w e l l i n g o c c u r r e d (9) i n a c ~ e r c i a l a i r - m e l t e d t y p e 316 s t a i n l e s s s t e e l ( h e a t DO) c o n t a i n i n g i n c r e a s e d l e v e l s o f T i , S i , V, W, Z r , and Cu u n d e r n e u t r o n irradiation i n t h e r a n g e 450 t o 660°C and a t f l u e n c e s up t o 8 x 1022 n cm- 2 (>O.1 MeV). I t h a s now b e e n d e m o n s t r a t e d t h a t a l a b o r a t o r y - m a d e a l l o ~ o f s i m i l a r c o m p o s i t i o n (LS1) e x h i b i t s low s w e l l i n g o v e r t h e t e m p e r a t u r e r a n g e 600 t o 710°C a t h i g h l e v e l s o f i o n d - m - g e . The p e a k s w e l l i n g t e m p e r a t u r e f o r t h i s a l l o y i s a t a b o u t 635°C, compared w i t h 675°C f o r a c o m m e r c i a l t y p e 316 s t e e l bombarded u n d e r i d e n t i c a l c o n d i t i o n s . A f t e r a d i s p l a c e m e n t d o s e o f 280 d p a , t h e s w e l l i n g o b s e r v e d i n LS1 a t t h e p e a k t e m p e r a t u r e i s o n e - s i x t h t h a t o b s e r v e d i n a

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TABLE 2 Comparison of the Step Height Measurements on Alloys Having Reduced Chromium Contents with LSI and P7

Step Height, a A Alloy

640°C, 218 dpa

710°C, 196 dpa

LSI

530

260

N7

550

<200

H7

450

<300

K7

610

<200

L7

527

<200

M7 P7

590

<200

4800

6800

astep height measurements after nlckel-lon bombardment to

about 200 dpa peak dose with 8 ppm p r e i n J e c t e d helium. commercial type 316 at its peak swelling temperature. Thls reduction in swelling appears to be the result of a reduction in the swelling rate, rather than a delay in the onset of swelling. Doubling the amount of preinJected helium has Very little effect on subsequent swelling behavior of LS1. The swelling behavior of alloy N7 closely follows that of LSI, indicating that low swelling behavior is not confined to a single composition and may possibly be induced in a range of base compositions. The levels of titanium and sillcon in N7 were high, 0.2 and 0.7 wt %, respectively, whereas the levels of the minor elements Zr, W, Cu, and V were in the range normally encountered in commercial vacuum-melted type 316 stainless steel. Increasing the levels of these elements separately in alloys H7, K7, L7, and M7 had no measurable effect on swelling. Thus in this base composition the presence or absence of low levels of this latter group of elements in unimportant, and low swelling is associated primarily with the additions of titanium and silicon. The same is probably true for alloy LSI Evidence of the influence of titanium and silicon on void swelling in 300 series stainless steels may also be found in recent neutron irradiation data. Bates (7) has shown that additions of silicon inhibit swelling in type 316 stainless steel a t neutron fluences up to 3 x 1022 n cm -2 (>0.1 MeV) and at temperatures of 400 to 550°C. This improvement, however, is not necessarily maintained at higher fluences. Some neutron swelling data for two heats of type 316 are shown in Table 3, and the alloy compositions are shown in Table 1. The swelling observed in heat 02 at 6 x 1022 n cm -2 is similar to that observed in type 316 stainless steels wlth normal levels of sillcon (i.e., ~0.4 wt %) and is 3 times that observed in heat DO, which contained silicon, titanium, and other minor alloying elements. It has also been observed (8) that the addition of 0.2 wt % Ti to a type 316 steel containing about 0.4 wt % Si changed the temperature dependence of swelling at 7 x 1022 n cm -2, with the net result of a maximum reduction in swelling of a factor of 2 to 3 over a narrow temperature range around 500°C. Thus, neutron data indicate that, although some modification in swelling behavior occurs when silicon and titanium are added separately, a much larger reduction in swelling occurs over a wide temperature range when they are present together at increased levels, as in heat DO. The effects of silicon and titanium on swelling have recently been further demonstrated by Johnston et el. (15), who found that under ion bombardment the swelling of a high-purity Fe-15% Cr-20% Ni alloy was strongly suppressed by the separate additions of Si, Ti, Zr, or Nb, and that a combination of SI and TI was far more effective than the single addition of either of these two elements.

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TABLE 3 Neutron Swelling Data for Type 316 Stainless Steel at 6 x 1022 n cm -2 (>0.i MeW)

SweLling, % AV/V0 Heat 500-535"C

600-660"C

DO

0.37

1.53

02

1.37

4.34

In summary, it has been demonstrated that a type 316 stainless steel with additions of silicon and titanium exhibits low swelling over the entire swelling temperature range under hlgh-dose nlckel-lon bombardment. Neutron irradiation data on commercial alloys (8,9) and ion data on hlgh-purlty alloys (15) also indicate that the most effective suppression of swelling is achieved with combinations of silicon and tltani~m. It is suggested that suppression of swelling by alloying with silicon and titanium may be effective over a range of nickel and chromium base composition levels and will provide the basis for the development of low-swelllng alloys that are technologically similar to type 316 stainless steel. Finally it should be noted that the influence of silicon and titanium on swelling is likely to depend strongly on the concentrations of other elements such as carbon, oxygen, and nitrogen and on the extent to which silicon and titanium partition to various phases. References i.

C. Cawthorne and E. J. Fulton, Nature 216, 575 (1967).

2.

B. Mastel and J. L. Brimhall, J. Nucl. Mater. 28, 118 (1968).

3.

J. O. Stiegler, "Void Formation in Neutron-Irradiated Metals," p. 292 in Radiation

Induced Voids in Metalsj USAEC Syrup. Set. 26, CONF-710601 (April 1972). 4.

T. Lauritzen, A. Withop, and U. Wolff, Nucl. Eng. Design 9, 265 (1969).

5.

W. G. Johnston, J. H. Rosolowski, A. M. Turkalo, and T. Lauritzen, J. Natl. Mater. 54, 24 (1974). Leltnaker, E. E. Bloom, and J. O. Stiegler, J. Nucl. Mater. 49, 57 (1973/74).

6.

J.M.

7.

H. R. Brager, "Irradiation Induced Swelling Variatlons~Resultlng fromComposltlonal Modifications of Type 316 Stainless Steel," Seventh ASTM Symposium on Effects of Radiation on Structural Materials, June 11--13, 1974, Gatllnburg, Tennessee, ASTM ~ec~al Technical Publication No. 570 (to be published).

8.

E. E. Bloom, J. M. Leltnaker, and J. O. Stiegler, Effect of Neutron Irradiation on the MicPost~otume and Properties of Titan£~n-Stubilized Tspe $18 Stainless Steel, USERDA Report, ORNL-TM-4731 (Janurary 1975). Accepted for publication in NMcZeal~ Technology.

9.

P. S. Sklad and E. E. Bloom, Oak Ridge National Laboratory, unpublished data.

I0.

H. R. Brager and J. L. Straalsund, '~)efect Development in Neutron Irradiated Type 316 Stainless Steel," J. N~l. Mater. 46, 134 (1973).

ii.

W. G. Johnston, J. H. Rosolowskl, A. M. Turkalo, and T. Lauritzen, J. Nuol. Mate~. 46, 273 (1973).

12.

W . A . Coghlan, N. H. Packan, and M. J. Saltmarsh, "A Method for Doping Irradiation Samples with Helium Using 2 ~ C m , " to be presented at AIME meeting, Cincinnati, Ohio, Nov. 10--13, 1975.

13.

I. Manning and G. P. Mueller, Computer Phy 8. Comm. 7, 85 (1974).

14.

D. G. Doran, J. R. Beeler, Jr., N. D. Dudley, and M. J. Fluss, Report of the Working Growp on Displacement Models and Procedures for Do~nage Calculations, USEEDA Report, NEDL-TME-73-76 (1973) • W. G. Johnston, T. Lauritzen, J. H. Rosolowski, and A. M. Turkalo, ASM Seminar on Radiation Damage, Cincinnati, November 1975 (to be published).

15.

4