Evaluation of creep damage in a MnCr austenitic steel

Scripta

METALLURGICA

Vol. 23, Printed

pp. 65-70, 1989 in the U.S.A.

Pergamon Press plc All rights reserved

E V A L U A T I O N OF CREEP DAMAGE IN A Mn-Cr A U S T E N I T I C STEEL

Matera

R.

C o m m i s s i o n of the E u r o p e a n C o m m u n i t i e s Joint R e s e a r c h Centre,

Ispra E s t a b l i s h m e n t

2 1 0 2 0 Ispra, Italy (Received (Revised

At high temperature,

metals and alloys

coalescence

oY microdefects

been widely

investigated

under

at the grain

(i),

both

August October

19, 19,

stress

1988) 1988)

fail as a result

boundary.

The m e c h a n i s m s

for the round type o f defect

of nucleation,

of the (voids),

three

growth

processes

which occurs

and have

at h i g h

temperature and low stress, and for the wedge shaped type, which occurs at lower t e m p e r a t u r e and higher stress. The problem o f m e a s u r i n g w h a t is c o m m o n l y r e f e r r e d to as creep damage has also been a d d r e s s e d by many authors. M e t h o d s based on the v a r i a t i o n

of a physical

property,

like density

(2,9),

small

angle n e u t r o n s c a t t e r i n g (3,4), electrical r e s i s t i v i t y (5), sound velocity (6) or methods b a s e d on m e t a l l o g r a p h i c techniques (7) have been p r o p o s e d in the literature. A feature common to all these methods is that the e v a l u a t i o n of the creep damage is p e r f o r m e d on that part of the crept s p e c i m e n w h i c h has not failed.

As the creep

damage

is

not

necessarily

evenly

distributed

the

value thus m e a s u r e d is e x p e c t e d to be less than or equal to critical creep damage for failure. Moreover, when the m e t h o d is based on the variation of a physical property, it requires a substantial

amount

of m a t e r i a l

and

i n t e g r a t e d n a t u r e o f the measure.

is not able to give point

by point

It is thus d i f f i c u l t to e x t r a p o l a t e

information

the value

due

of creep

to

the

damage

at the location of fracture, which is the only datum of real interest. The s e n s i t i v i t y and the a c c u r a c y o f m e a s u r i n g creep damage vary from method to m e t h o d and can be very high, p r o v i d e d that the m a t e r i a l does not u n d e r g o structural changes during the creep test. This

is, however,

precipitation austenitic

in

lattice

seldom the case austenitic to

shrink

for c o m m e r c i a l

stainless and

the

materials.

steel o c c u r r i n g density

to

in

increase

A

typical

the thus

creep

example

is

regime.

It

counterbalancing

d e c r e a s e due to the cavity formation. In these cases it is g e n e r a l l y between cavity f o r m a t i o n and o t h e r extraneous effects.

difficult

A =

c

i

-

carbide

causes the

the

density

to d i s c r i m i n a t e

The m e t h o d p r o p o s e d here starts from the d e f i n i t i o n o f creep damage due to Kachanov

w

the

(8]

r (I)

--

A

w h e r e ~ is the creep damage, A and A, respectively, are the actual r e s i s t i n g area and the apc r p a r e n t a r e a as m e a s u r e d by the e x t e r n a l d i m e n s i o n s o f the sample. In Kachanov's model the material is e x p e c t e d to fail w h e n ~ r e a c h e s unity. c It is a c o m m o n observation, however, t h a t the f r a c t u r e surface o f a creep specimen, even a f t e r long creep duration, does not feature a c o m p l e t e l y i n t e r g r a n u l a r type of fracture mode. It is t h e r e f o r e i n f e r r e d that the final r u p t u r e occurs before the creep damage, as defined in Kachan o v ' s model, r e a c h e s unity. R e c a l l i n g the d e f i n i t i o n o f the r e d u c t i o n in area, if follows that

0036-9748/89 Copyright (c) 1 9 8 9

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66

CREEP

IN

A Mn-Cr

AUSTENITIC

A r (i - Z

we = i - A o where A

is the original cross-sectional o a r e a due to the c r e e p deformation. As

the

test

sisting

proceeds,

area A

both

creep

, will decrease

STEEL

Vol.

23,

No.

)

1

(2)

c

a r e a o f the t e s t

section and Z

is the

reduction

in

c

damage

and

according

creep

deformation

will

increase.

The

actual

re-

to the relation:

r A and the stress the u l t i m a t e

= A

r

o

(i - Z

) (I - ~ ) c

c

(3)

a c t i n g on it, F /A , w h e r e F is the a p p l i e d load, Will i n c r e a s e until it r e a c h e s cr o f the material, o*. ~t this point, p l a s t i c i n s t a b i l i t y will occur g i v i n g

strength

arise to l o c a l i z e d creep d a m a g e

~

n e c k i n g a n d to a fast f r a c t u r e

at the o n s e t o f rupture.

process,

freezing

the a c c u m u l a t e d

value o f the

Thus:

c F

c

--

A It is a s s u m e d occurring sured

h e r e t h a t the s t r e n g t h

during

the

creep

process,

in a s h o r t t i m e t e s t at the

variation

could

be

incorporated

o f the

change

creep

loss of load b e a r i n g ification in a r e a

Z

reduction

o f its u l t i m a t e which

occurs

t in a r e a Z

t i o n is r e a s o n a b l e tribute

capacity

tensile occurs

fast

in

the

the r e l a t i v e

Therefore,

ultimate should

damage"

boundary

the

process

period

can

o f creep

changes

tensile

as

not

"creep

contribution

"creep

microstructural

this

expression

A second assumption

fracture

during

true

due to the g r a i n

strength.

the

the

However,

sense

between

properties.

in spite o f the

to

temperature.

of a material

during

which

equal

in a b r o a d e r

of mechanical

(4)

o f the material, remains

this will not a l l o w us to d i s c r i m i n a t e and

: o*

r

stress

be

the

damage"

o f the c a v i t y is

defined

separation

neglected

damage

with

formation as

reduction

respect

accumulation.

This

to

o

(i

+ ~

uts

the

assump-

m e x c e e d s 20-30%, as the p r e s e n c e o f the creep m i c r o c a v i t i e s will c a m o n g the m a n y l i g a m e n t s e x i s t i n g b e t w e e n a d j a c e n t cavities. Therefore: o* =

the

a n d to the m o d -

~hen

the n e c k i n g

its

although

here

is t h a t the a d d i t i o n a l be

mea-

case,

dis-

)

(5)

u

F c

(1 - ~ ) ( i

A o

where rial

- z)

=

Gut s

(I

+ eu)

(6)

c

a is the e n g i n e e r i n g u l t i m a t e t e n s i l e strength, ~ the u n i f o r m e l o n g a t i o n o f the uts u (in a fast test) at the test t e m p e r a t u r e and Z the r e d u c t i o n in area m e a s u r e d after

fracture.

By s i m p l e

substitution

matecreep

we obtain: o o c

:

i

- - -

o is the i n i t i a l a p p l i e d stress. o By t h i s m e t h o d it is p o s s i b l e to e v a l u a t e

i

Gut s

i-+

i ~

i -

z

(7)

u

where

basis o f the r e s u l t s fracture)

o f the creep

test

a n d o f the s h o r t t e r m m e c h a n i c a l

The m e t h o d was a p p l i e d

to the

whose

g i v e n in

composition

is

results

the

itself

of

T a b l e I.

creep

damage

(initial

properties

at

applied

the

location

stress

of

fracture

and reduction

on

in a r e a

the at

o f the material.

stress-to-rupture

tests o f an

The m a t e r i a l

supplied

was

austenitic

Mn-Cr steel

as 30 mm t h i c k

hot-rolled

Vol.

23,

plate,

No.

1

in the

average

CREEP

solution

intercept

properties

for F u s i o n

Reactor

at long c o o l i n g

annealed

method

mechanical

was,

First

times

specimen

respectively.

Wall

of ~

during

by means

and o f E uts

creases

Blanket

which and

average

grain

were

Shield

67

size,

obtained

is c o n s i d e r e d

as

components

tests were p e r f o r m e d heads.

The

as

measured

during

the

a possible because

by

the

of

its

lineal

evaluation

substitute

in t e n s i o n u n d e r c o n s t a n t

specimen

gauge

length

was in the range 773-973

of an optical

and

K and

to w i t h i n + 2°K.

microscope

are those r e p o r t e d

o f the a n a l y s i s

lower

of

of

the

AISI 316

radioactivity

equipped

load on smooth cylin-

diameter

the

were

test

The r e d u c t i o n

with a p r e c i s i o n

50 and

environment

i0 mm,

was

air.

in area at failure

X-Y.

stage.

The

values

in (9).

in a p a r a b o l i c should

occur

are shown

is p l o t t e d way

The e x t r a p o l a t i o n

damage

results

The

the test was c o n t r o l l e d

i the creep damage

ture.

The

70 pm.

STEEL

u

The r e s u l t s Fig.

AUSTENITIC

condition.

The test t e m p e r a t u r e

measured

was

Mn-Cr

(9,10).

w i t h threaded

The t e m p e r a t u r e

A

o f this m a t e r i a l

Creep and s t r e s s - t o - r u p t u r e drical

IN

with

to w

and

the

c

in the f o l l o w i n g

against

a

increasing

o initial

gives

each

+0

fracture

for

should

figures

, the initial

for the five test temperature.

applied

applied

stress.

stress

temperature

the

and

a

with

values

o intragranular

be e n t i r e l y

The

creep

damage

decreasing above

with

a

In de-

tempera-

which

no creep

dimpled

fracture

mode. Fig.

2 indicates

fracture

modes;

tions c o n f i r m men

crept

17,469.6

the r e g i o n s the

diamond

the results

at

500°C

in the points

stress-temperature represent

of the damage

under

h. The c a l c u l a t e d

an

initial

creep

damage

the

analysis.

Fig.

applied is 50%,

space c o r r e s p o n d i n g

creep

test

matrix.

3 shows

stress a value

of

the

The

to the two d i f f e r e n t

fractographic

fracture

260 MPa.

surface

The

in good a g r e e m e n t

observa-

of

a speci-

time-to-rupture with

was

the f r a e t o g r a p h i c

appearance. Fig. The

4 shows

the f r a c t o g r a p h y

time-to-rupture

with no e v i d e n c e

strongly

creasing In Fig. damage

a specimen As

predicted

by Fig.

650°C under an a p p l i e d 2 the

fracture

mode

is

stress

of 330 MPa.

completely

ductile

crack formation.

seem to s u g g e s t

temperature

crept at

that b o t h the n u c l e a t i o n

dependent;

the

former

increases

and g r o w t h while

the

of grain b o u n d a r y latter

decreases

defects with

in-

temperature. 5 the creep damage

appears

to a p p r o a c h

1 0 0 % at infinite

o f d a m a g e e.g. 500 to 700°C.

20%,

~he basis

time.

is r e a c h e d

with

intergranular

increasing

layer,

the log of the time-to-rupture.

which

is

The i n f l u e n c e

at 500°C after

longer,

the

lower

of t e m p e r a t u r e

with t e m p e r a t u r e

temperature.

In

w h i c h is r e s p o n s i b l e

this

for the g r a i n

in an

tends c The same amount

carbides

boundary

incubation

These results

of carbides

steel,

creep

w

of 4 in creep damage g o i n g

÷ 0 results

c to less than one h o u r at 700°C.

kinetics

A measurable

temperature,

to less than one hour at 700°C.

of a factor

to ~

the

is remarkable.

lO00 h as c o m p a r e d

is an increase

o f these curves

hours at 500°C

o f the a c c e l e r a t e d

against

period

of lO00 h there

The e x t r a p o l a t i o n

from a few h u n d r e d boundaries

is p l o t t e d

after an i n c u b a t i o n

For a t i m e - t o - r u p t u r e

on

of

0.2 h.

of i n t e r g r a n u l a r

These o b s e r v a t i o n s are

was

time

can be e x p l a i n e d

precipitation tend

to

from

ranging

at the g r a i n a

continuous

and to the c o l l e a g u e s

of the Ma-

separation

form (I0).

Acknowledgments The a u t h o r w i s h to express terial S c i e n c e

Division

his g r a t i t u d e

for the h e l p f u l

to Prof.

G. Bernasconi

discussions

and to Mr.

G. M a t h e y

and Mr.

H. Weir for the

experiments. References I. R.W.

Evans and B. Wilwhire,

Creep o f M e t a l s

and Alloys,

The I n s t i t u t e

o f Metals,

London

1985

68

CREEP

IN A M n - C r

AUSTENITIC

STEEL

Vol.

23,

No.

1

2. N.G. Needham, J.E. Wheatley and G.W. Greenwood, Acta Met., Vol. 23, (1975), 23. 3. R. Matera & F. Rustichelli, in Creep of Engineering Materials and Structures G. Bernasconi & G. Piatti Eds., Applied Science Publishers Ltd., London, (1979), 389-412. 4. R.J.R. Miller, S. Messoloras, J. Stewart and G. Kostorz, J. Appl. Crystallography, ii (1978), 583. 5. G.B. Thomas and H.R. Tipler, NPL Report DMA 74, NPK 7/05, September (1971). 6. R. Kompfner and C.F. Quate, Phys. Techn., (1977), 231. 7. B.F. Dyson and D. McLean, Met. Sci. J. Vol. 6, (1972), 220. 8. L.M. Kachanov, Izv. Akad. Mauk. SSSR. OTN. Tech. Nauk. 8 (1958), 26. 9. G. Piatti, S. Matteazzi and G. Petrone, Nucl. Eng. Design/Fusion 2 (1985), 351. i0. E. Ruedl, R. Matera and G. Valdr~, J. Nucl. Mat. 151 (1988), 238.

TABLE I Chemical Composition of the Austenitic Steel.

C

0.i0

Si

0.6

Mn

P

S

Ni

Cr

Mo

Cu

Ti

A1

B

N

Fe

17.2

0.02

0.01

0.i

i0.i

0.07

0.06

--

0.005

28 ppm

0.2

Bal.

100 90.

[] 500°C 0 550°C z~ 600°C + 650°C 0 700°C

0

80, ~J tg

E

70. 60.

¢0

50. O. ~J L

z~

40. 0 30.

+ +

20"

o

D 0

[]

z~

0

D

:1:

+

o

10" O" 0

I

l

I

l

I

I

l

I

I

I'

1O0

°'

°

I

I

l

l

l

|

i

200

i

i

l

i

l

i

i

l

I

'

'

l

l

300

•

I

I

•

I

|

~00

Initial applied s t r e s s ( M P a )

FIG. 1 Creep damage as a function of temperature and initial applied stress (MPa).

Vol.

23,

No.

1

CREEP

700-

IN

A

Mn-Cr

O O0 O

AUSTENITIC

STEEL

69

<> O ~

O O0 00 O O O xO O 0

Mixedoofracture ¢ 0 0 0 0

0 ~ 0 x Dimples

600 Dimples + cavitated 9rain boundaries 0 0 0 0

500' 0

\

0 O0

x

\

0 O0 0 x 100 200 300 ~00 Initial applied stress, MPA

I I l l U U l l U | I I ' I I

0 0

\

V ~ I V I I I I , , , I

J U ] I , , , I I

I l U l l , l , u u

FIG. 2 I d e n t i f i c a t i o n o f the region o f o c c u r r e n c e o f creep damage. The test matrix

i,u

500

is indicated

by the

diamonds.

Fig. 3 F r a c t u r e s u r f a c e o f a creep s p e c i m e n r u p t u r e d after 1 7 , 4 6 9 . 6 h at 500°C under an initial a p p l i e d stress o f 2160 MPa. The c a l c u l a t e d creep damage is 50%.

70

CREEP IN A Mn-Cr AUSTENITIC

STEEL

Vol.

23,

No.

1

FIG. 4 Fracture surface of a

creep

specimen

ruptured

after

0.2 h

at

650°C

under

an

initial

stress o f 330 MPa. The fracture mode is e n t i r e l y ductile.

100 I

=o-I

~,,~ ~..~;~o ~o~ ~oooc

/

~ " ,

.,~'1=~

V

------0--

60

500oc

, .o4 1o 0 011

. . . .

I

'

1.0

'

'

"

'

I

'

"

'

"

'

I

'

'

,

'

'

i

,

. . . .

I

. . . . .

I

10.0 100.0 1000.0 10000.0 100000.0 Rupture life (hours/

FIG. 5 C r e e p damage as a f u n c t i o n o f the time to r u p t u r e a n d temperature.

applied