Microstructural design of PCA austenitic stainless steel for improved resistance to helium embrittlement under HFIR irradiation

Journal of Nuclear Materials 122 & 123 North-Holland, Amsterdam

305

(1984)305-310

MICROSTRUCTURAL OESlGN OF PCA AUSTENITIC EMBRITTLEMENT UNDER HFIR IRRADIATION* P. J. MAZIASZ Metals

STAINLESS

STEEL FOR IMPROVED RESISTANCE

TO HELIUM

and D. N. BRASKI

and Ceramics

Division,

Oak Ridge National Laboratory,

Oak Ridge, TN

37831

Several variants of Prime Candidate Alloy (PCA) with different preirradiation ther~l-~chanical treatments were irradiated in HFIR and were evaluated for embrittlement resistance via disk-bend Comparison tests were made on two heats of 20%"cold-worked type 316 stainless tensile testing. None of the alloys were brittle after irradiation at 300 to 400°C to -44 dpa and helium steel. levels of 3000 to -3600 at. ppm. However, all were quite brittle after similar exposure at Embrittle~nt varied with alloy and pretreatment for irradiation to 44 dpa at 500°C and 600°C. Better relative embrittlement resistance among PCA variants was found in to 22 dpa at 600°C. atlovs which contained prior grain boundary MC carbide particles that remained stable under irradiation.

1.

resistance

INTRODUCTION Titanium

or niobium modifications

to improve the helium embrittlement of austenitic

stainless

resistance

steels under neutron

In 1967, Rowcliffe et al.1 and

irradiation.l-3 Martin

are known

and Weir2 conjectured

such resistance

that the reason for

was interfacial

helium bubble

trapping

by g.b. MC carbides.

Kesternich

Rotha&'+

recently

embrittlement

resistance

demonstrated

resulting

from helium trapping

fine nmtrix MC particles

and

at

in helium pre~njected

and creep tested DIN 1.4970 (15 wt % Ni-15 Cr0.1 C-l.3 Mo-0.3

Ti stainless

Helium e~rittle~nt

resistance

higher

helium generation

fusion

(12-15 at. ppm He/dpa)

concern

for first-wall

optimized

distributions

(MC) carbide,

embrittlement vice.5

intended

2.

is an important

microstructurat

resistance

An important

EXPERIMENTAL Steel compositions

designations treatment

to evaluate

of grain boundary tita-

in fusion reactor ser-

of phase stability

under

The present work is

relative

and descriptions

in Table I.

Standard

3-mm-diam

from 0.254-mmthick

sheet stock.

at 300, 400, 500, and 6OOY 10.5 to -44 dpa.a

Four

in HFIR

to fluences

pro-

After irradiation,

disks were bend tested at their irra-

diation temperature. testing

in this

disks were

disks of each of these were irradiated

ducing

The

of the PCA pre-

variants are given elsewhere

publication.8 punched

appear

Details of the disk bend

and calculation

be found elsewhere.a-1l microscopy

of bend ductilities Scanning

(SEM) and optical

can

electron

stereomicroscopy

were used to examine cracks or fractures

on

and produced, with

feature was incorporation

irradiation.s*T

bend testing of various speci-

in HFIR at the high helium gen-

rates of 20 to 80 at. ppm/dpa.

Table

intended for better helium

of our understanding neutron

for

lifetime at higher telrper-

variants of PCA were designed

eration

selected

at the

rates expected

In 1981, preirradiation

atures.

nium

steel).

through

mens irradiated

embrittlement

PCAb ref. 316 N-lot 316

I.

Alloy CorrQositionsa (wt %) Cr

Ni

Si

14.0 17.3 16.5

16.2 12.4 13.5

z 0:5

(North-Holland Physics Publishing Division)

0.05 0.05 0.05

P 0.01 0.03 0.01

al.Gl.8 Mn, 2.3-2.5 MO, bal Fe. b0.24 Ti.

"Research sponsored by the Office of Fusion Energy, U.S. Department W-7405-eng-26 with the Union Carbide Corporation.

0022-3 11S/84/$03.00 0 Elsevier Science Publishers B.V.

C

of Energy, under contract

P.J. Maziasz, D.N. Braski / PCA aastenitic stainless steei

306 tested disks.

Transmission

electron

microscopy

(TEM) was used to observe

grain boundary

structures

irradiated,

untested 3.

in identically

micro-

but

disks.6

Figure 2

(ref.-heat) as functions

of irra-

(opened symbols - no cracks;

exhibited

judged better or worse.

better behavior at higher

and fluences than the other

The te~erature tilities

(47 disks) are shown in Fig. 1.

cracking).

for fracture

specimens.

(equal to test temperature)

The data symbols also indicate fracture

intergranular

of specimens

temperatures

and fluence for the various alloy and pretreatment variants

better or worse.

In general, PCA-Bl and -82 as well as CW 316

Disk bend testing

temperature

judged relatively

behavior

Disk bend ductilities diation

behavior

shows SEM of obvious examples

RESULTS ANTI DISCUSSION 3.1

into general trend bands indicating

behavior

indicated

dependence

virtually no embrittlement

due to irradiations 400°C [Fig. l(a,b)f.

filled symbols -

The data are separated

optical

of the bend duc-

of -22 and 44 dpa at 300 to Consistent

stereomicroscopy

with this,

indicated considerable

FIGURE 1 Plots of bend ductility as functions of irradiation temperature at (a) -22 dpa and (b) 44 dpa and as functions of fluence at (c) 500°C and (d) 6OOY for the alloys indicated after irradiation in HFIR. Note that the helium content (due to higher nickel content) is higher in PCA (-3600 at. ppm) than the CW 316s (-3000 at. ppm) after exposure to -44 dpa. Scatter bands indicate relatively better or worse behavior as judged from both fracture behavior and bend ductility.

P.J. Maziasz, D.N. Braski / PCA austenitic

307

stainless steel

-44 d~a.*3600at.~Prn

FIGURE 2 SEM fractography at high and low magnifications of HFIR-irradiated and bend tested (Ttest = Tirr) disks and that failed (i.e., CW 316 N-lot designated worse) and passed (i.e., PCA-Bl designated Irradiation conditions are indicated. better) for comparison.

cupping

of the disks to conform to the punch

during testing

and SEM showed no cracking.

500 to 6OO"C, embrittlement increased

temperature

among the specimens.

(postirradiation At

increased with

and fluence, PCA-Bl

(solution annealed

plus 8 h at 800°C plus 8 h at 900°C) and -82 annealed

cold-work

plus 2 h at 75O'C) consistently

better ductility than PCA-A3 annealed),

plus 8 h at 800°C plus 25%-

and embrittlement

(25%-cold worked),

particularly for instance,

-Al (solution

PCA-Bl in Fig. 2).

have similar behavior;

At 500°C and

PCA-Bl, and -82 all

this is also the case for

6OO'C at both -22 dpa (with multiple

appears

This is

true at 500°C after -44 dpa (see

-22 dpa, CW 316 (ref.-heat),

testing)

show

resistance

or CW 316s at 500 to 600°C.

and -44 dpa.

specimen

The CW 316 (ref-heat)

to be less prone to embrittlement

than other

diated in HFIR.12 The heat-to-heat

but varied

(solution

tensile testing)

heats of type 316 (primarily the DO-heat)

variation ferences

embrittlement

may be due to residual elemental (i.e., P content).

production

irra-

Furthermore,

in CW 316 (ref.-heat)

difhelium

is considerally

less than PCA in HFIR due to a 22 to 25% lower nickel content

(see Table I).

The fluence dependence

of bend ductilities

shows that all of the alloys eventually etibrittled at 600°C. tlement

resistance

However, continued

was achieved

-82 at 500°C [Fig. l(c,d)]. embrittlement

occurred

are embrit-

in PCA-Bl and

At 6OO"C, severe

in PCA-Al after only

-11 dpa and -550 at. ppm He.

At -22 dpa, PCA-Al,

-A3, and CW 316 (N-lot) are all similarly

embrit-

tled at 600°C and grow only slightly worse as fluence

increases

[see Fig. l(d)].

These disks

308

P.J. Maziasz, D.N. Braski

I PCA austenitic stainless steel

show very little cupping and very large, opened

irradiated

intergranular

embrittlement

cracks

via SEM [see CW 316 (N-lot)

in Fig. 21, as well as clearly

defined and sharp

drops in their load versus displacement By comparison,

PCA-Bl, -B2, and CW 316 (ref.-

heat) show less embrittlement by higher ductilities cracking

curves.lO

500°C.

retained, 3.2

strongly

compared

Grain boundary untested

structure

influence

also

embrittlement.

(MC) prior to irradiation.

These precipitates

were produced by aging for

8 h at 800°C after solution annealing.5 fabricated

from irra-

All alloys irradiated

in

in bubble and

at boundaries

g.b. MC precipitate

irradiation

the medium-coarse

itation developed

The as-

microstructure

can be seen for PCA-Bl in Fig. 4(a,c).

TEM was obtained

disks.

However, the variation of among the alloys irradiated

g.b. precipitates

to the other alloys.

microstructural

are mot-e

In this work, only PCA-Bl and -B2 contained

is a sign that some ductility was

especially

Grain boundary observations

diated,

precipitate

curve) even after -44 dpa at

Their ability to resist rapid intergran-

ular cracking

level.

HFIR indicated that differences

in SEM (see

Fig. 2) and they exhibit no sharp load drop (load-displacement

indicate that

and g.b. bubble formation

embrittlement

PCA-Bl and -82 cracks

These observations

embrittlement

damage

prior to catastrophic

than the other disks.

fluenceslz

of

below about 65O'C to very high

the result of helium buildup than displacement

at 22 dpa judged

show only small intergranular

in EBR-II show no evidence

Durino

g.b. MC precip-

via pretreatments

in PCA-Bl

at 300 to 400°C showed similar tiny bubbles.

and -82 remained

However,

up to 44 dpa at 300 to 500°C and up to -22 dpa

at 500 and 6OO"C, both grain boundary

bubble and precipitation significantly variations. helium

daries,

varied

Figure 3 shows that increased

generation

increases

development

with alloy and pretreatment

under neutron

bubble nucleation

for CW 316 (DO-heat)

to 550°C in EBR-II and HFIR.

irradiation

at the grain bounirradiated

at 525

Similar steels

at 600°C.

stable and virtually unchanged

The latter case is shown in Fig. 4(b,d).

By contrast,

fine MC developed

in PCA-C (25%-cold-worked dissolved

via pretreatments

plus 2 h at 75O'C)

after only 11 dpa at 600°C.

Neither

PCA-Al nor -A3 develop any g.b. MC under similar irradiations. fairly

The g.b. MC stability was also

independent

of matrix

void formation.

FIGURE 3 Grain boundary bubbles in 20%-cold-worked 316 (DO-heat) irradiated at 525 to 550°C in (a) EBR-II to 36 dpa and -22 at. ppm He and (b) HFIR to 17.8 dpa and 1020 at. ppm He.

At

309

P.J. Maziasz, D.N. Braski / PCA austenitic stainless steel

a

FIGURE 4 A comparison of grain boundary MC in PCA-Bl as fabricated (a) and (c), and after HFIR irradiation 600°C (b) and (d) to demonstrate its stability under irradiation.

600°C and 22 dpa PCA-Bl develops swelling whereas

and radiation-induced PCA-B2 develops

enhanced

thermal

diation.

mrch less swelling

precipitation

However,

PCA-Bl and -82 does dissolve 6OO"C,

possibly

occurs

due to differential

adjacent

(MC) under irra-

MC in both

after -44 dpa at

due to g.b. migration

grains.

and

both retain their g.b. MC

The medium-coarse

distributions.

ence at 600°C.

high void

precipitation,

which

void swelling

grain boundaries

CW 316 (ref.-heat)

have fairly uniform and

stable dispersions

of medium-coarse

in

MsC parti-

clesla after -10 dpa in HFIR at 400 to 550°C. This observation resistance

again relates embrittlement

to g.b. precipitate

stability

for

these heats of CW 316. 3.3

in

When the g.b. MC was stable,

By contrast,

at

Bend test - microstructural and summary

This work emphasizes

correlation

that embrittlement

re-

it caused rmch finer g.b. helium bubble distri-

sistance was anticipated

butions

due to trapping

the stability

daries,

particularly

without

MC (cf PCA-Al and -82 in Fig. 5).

PCA-Bl and -82, despite their higher helium con-

are almost unresolvable

tent, also show consistently

Bubbles ticles

at the interphase

boun-

when compared to g.b.'s

at these MC par-

In comparison to PCA-Bl or -82, CW 316 (Nlot) developed tation

bubbles and little g.b. precipi-

at 5OO"C, and bubble and MsC precipitate

structures

that coarsened

and beneficial

considerably

with flu-

bubble refinement

of g.b. MC in the PCA alloys at 500 to 600°C.

ment resistance

at lower fluence.

and did correlate with

N-lot.

better embrittle-

than the CW 316s, especially

The CW 316 (ref.-heat)

from better stability

may also benefit

of g.b. MsC.

and below, embrittlement

At 400°C

does not appear to be

a problem for any of these alloys.

Precipitate-

310

P.J. Maziasz, D.N. Braski / PCA austenitic

stainless steel

FIGURE 5 A comparison of grain boundary bubble structures in (a) PCA-Al with no MC and (b) PCA-BE irradiated HFIR at 600°C to -22 dpa (-1760 at. ppm He) with prior grain boundary MC. free g.b.'s covered with many bubbles consistently

appear quite brittle and prone to cracking

for all alloys at 500 and 600°C. boundary

MC, and hence embrittlement

does not naturally

mechanical

with

by appropriate

treatments

g.b. migration

thermal

before irradiation.

the MC instability

appears to correlate

under irradiation,

g.b. MC

stability

should be improved with more swelling

resistant

grains.

structure,

Therefore,

resistance

with the g.b. MC structures PCA-B3 is produced material working 4.

a new PCA micro-

PCA-B3, has been developed

the void swelling

REFERENCES 1.

resistance,

develop under irradiation,

but nust be produced

Because

Stable grain

of PCA-A3

2. 3. 4. 5. 6.

to combine (ref. 8)

of PCA-Bl and -BZ.

by aging solution-annealed

7. 8.

(PCA-Al) for 8 h at 800°C and then cold 25%.

SUMMARY

9.

The bend test results suggest qualitatively the possibility

that embrittlement

ished by suitable microstructure.

These results wust be supported relevant

to verify the embrittlemnt

mchanical resistance

qualify these alloy conditions tor applications.

can be dimin-

design and control of the

by more engineering

testing and to

for fusion reac-

in

10. 11. 12. 13.

A.F. Rowcliffe et al., Effects of Radiation on Structural Metals, ASTM-STP-426 (American Society for Testing and Materials, 1967), pp. 161-199. W.R. Martin and J.R. Weir, ibid., pp. 440-457. P.J. Maziasz and E.E. Bloom, Trans. Am. Nucl. Sot. 27 (1977) 268-269. W. Kesternich and J. Rothaut, J. Nucl. Mater. 103&104 (1981) 845-852. P.J. Maziasz and T.K. Roche, J. Nucl. Mater. 103&104 (1981) 797-802. E.H. Lee, P.J. Maziasz, and A.F. Rowcliffe, Conf. Proc. Phase Stability During Irradiation, eds. J.R. Holland, LIK. Mansur, and D.I. Potter (The Metallurqical Society of AIME, Warrendale, PA, 198lj, pp. 191-218. P.J. Maziasz, ADIP Quart. Prog. Rep. June 30, 1980, DOE/ER/0045/3, pp. 75-129. P.J. Maziasz and D.N. Braski, "Improved Swelling Resistance for PCA Austenitic Stainless Steel under HFIR Irradiation Through Microstructural Control," this volume. R.L. Klueh and D.N. Braski, "The Use of Nonstandard Subsized Specimens for Irradiation Testing," (presented at Symp. Use of Non-Standard Subsized Specimens in Irradiated Testing, Albuquerque, NM, Sept. 23, 1983; to be published by American Society for Testing and Materials). F.H. Huana. M. L. Hamilton, and C.L. Wire, Nucl. Technol. 57 (1982) 234-242. M. Manahan et al., same as ref. 9. A.F. Rowcliffe and M.L. Grossbeck, "Radiat$;dffects in Austenitic Steels," this

.

P.J. Maziasz, Oak Ridge National Laboratory, unpublished data, 198l-82.