EFFECT OF NEUTRON IRRADIATION ON VISCOPLASTIC BEHAVIOUR OF REACTOR PRESSURE VESSEL STEEL

HAL Id: jpa-00224799

https://hal.archives-ouvertes.fr/jpa-00224799

Submitted on 1 Jan 1985

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

EFFECT OF NEUTRON IRRADIATION ON VISCOPLASTIC BEHAVIOUR OF REACTOR

PRESSURE VESSEL STEEL

J. Buchar, Z. Bílek

To cite this version:

J. Buchar, Z. Bílek. EFFECT OF NEUTRON IRRADIATION ON VISCOPLASTIC BEHAVIOUR OF REACTOR PRESSURE VESSEL STEEL. Journal de Physique Colloques, 1985, 46 (C5), pp.C5- 517-C5-522. �10.1051/jphyscol:1985566�. �jpa-00224799�

E F F E C T OF NEUTRON I R R A D I A T I O N ON V I S C O P L A S T I C BEHAVIOUR OF REACTOR PRESSURE V E S S E L S T E E L

J. Buchar and 2 . ~ilek

J n s t i t u t e of PhysicaZ MetaZZurgy, CzechosZovak Academy o f Sciences, ZiSkova 22, 616 62 Bmzo, CzechosZovakia

Resume

-

Abstract

-

6f at 1 % strain, are analysed in terms of the theory of visco-

plasticity. The values of certain parameters of this theory are calculated.

I

-

The change of mechanical properties of reactor pressure vessel steels exposed to neutron irradiation is an area of extensive research. Much has been written in recent years about neutron embrittlement of struc-

tural steels. These reports have clearly demonstrated trends in em- brittlement with neutron exposure which involve a large increase in the brittle-ductile transition temperature. One of the ways how to establish this transition is based on the knowledge of the dependence of the flow stress 6f upon temperature T and strain rate E in nonirradiated and irradiated conditions /l/. This way enables one to interpret the radiation embrittlementin terns of the plastic defor- mation mechanism defined by T and &

.

Theories of the plastic behaviour of materials represent an attempt to describe macroscopic flow in terms of microscopic mecha- nisms based on dislocation movement. It is now generally recognised that there are three region of strain rate in which the behaviour has to be explaned in terms of different mechanisms /2/. In the case of irradiated materials, the question arises of what kind is the influ- ence of neutron irradiation on the mechanisms of plastic flow in the entire spectrum of strain rate and temperature. Literature reviews show that sufficient for irradiated materials tested at strain rate higher than ":'01 are locking.

The present paper. studies the influence of neutron irradiation on the dependence 6f(& ) in relatively broad spectrum of strain rate and temperature. In order to obtain a high strain rates the split Hopkinson pressure bar technique (HSPB) was used.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985566

C5-518 JOURNAL DE PHYSIQUE

I1

-

For the experiment a reactor pressure vessel steel with the chemical composition 0.13 C, 0.59 Mn, 0.16 Si, 0.023 P, 0.01 S, 1.99 Cr, 0.07 Ni,.0.61 MO, 0.15 V was chosen. Some specimens were prepared from the nonirradiated steel and irradiated by thermal and fast neutrons at

.v130° C 'n th li ht water reactor with thermal neutron exposure 0 =

= l m-'d:a fast neutron (E 70.5 MeV) exposure = 1.18~

X1023 m-2.

The two independent xper men a1 procedures were applied:

(a) the values of . i a ( 10-5, IO-~)S-~ were achived by 200 kN ZWICK electronic testing machine.

( b) HSPB procedure was used. P nny-shaped specimens, 1 .4x1 02' m in diameter and length 10 = 2x10-9 were loaded by stress pulse o (t) with a duration of .\I = 3x10-5s and amplitudes from 900

jo

Such amplitudes guarranted a strain rates of the order 10 'S

.

Experiments were run in three stages:

1. A control group of nonirradiated specimen was loaded 2. Specimen after irradiation were tested

3. Irradiated specimens, annealed at T = 350° C for various times ta = 16, 48, 72 and 168 hours, were tested.

Experiments were performed within the temperature range -196' C to 500' C. From the results, the dependence of 6f on 6 and T at 1%

strain, was obtained.

I11

-

Fig. 1 shows the influence of strain rate on flow stress 6f at different temperatures for nonirradiated steel. In the curves of Fig.

1 we can clearly distinguish two r gions of Tifferent behaviour:

(a) Strain rate values from 1 1 0-3 to .v 10- S- in which the flow stress is sensitive to temperature but relatively low sensitive to strain rate.

(b) Strain rates above this in which the flow stress is also marked- ly strain rate sensitive.

The detail analysis of obtained results leads to the following expre- asion for 6f(&) in the two regions:

region a

(2) 6f = 6B + a& region b

where 61 if stress associated with the smallest value of &l =

= 1.8~10'4~'

.

Extrapolating Equation (1) to the higher and Equation (2) to the lower strain rates-respectively, both curves interset at point (62, 62). Introducing &2, 62, Equation (2) may be written as

The parameters 61 an in Equation (1) are both non decreasing function of temperature .T as it can be seen in Fig. 2. The tempera- ture dependence of 62, € 2 and & is given by

(4 6 2 = 8 - b f i

(5) 62 = c + d/T

(6) a = T/(al + blT)

where a,b,c,d,al ,bl are given in Tab. 1 and T is now the absolute temperature.

rested in studying the influence of neutron irradiation on the coeffi- cients introduced in the above Equations .( 1 )-(6).

The Fig. 3 the dependences 6 vs & at different temperatures are plotted. It can be seen that tf;e irradiation shifts the region a to higher strain rates.

The detail analysis of the experimental results leads to the conclusion that neutron irradiation does not affect the strain rate sensitivity in the region a

.

.

This irradiation increases the value of a i.e. strain rate sensiti- vity in the region b and & 2

.

.

The observed influence of neutron irradiation is a consequence of the complex structural changes caused by radiation deffects. The concentration of such deffects is determined by neutron flux, neutron enera, irradiation temperature and etc. /4/. For the study of the influence either various doses of irradiation or gradual anealing may be apLlied. The latter was used in this paper, similarly as in /5/.

In Fig. 4 the curves a C ta) are shown. It is obvious that a is a decreasing function of ta for all testing temperatures. T

.

decay rate of t%(ta) curve is a measure of the material recovery.

The rate of recovery is increasing function of testing temperature.

It can be seen that after 168 hours the complete recovery occurs at every testing.temperature. The same results may be obtaimed for 62(t,) and &z(ta).

IV

-

In terms of the thepry of viscoplasticity the dependence of flow stress 6f on T, E ,& and

5

(7) & =

3

, t

L s ~ / ~ , ( T , ~ - ¹¹

Equation (7) implies that strain rate is a function of dynamic overs- tress above the 61 which is assumed to be independent of 6

.

generally different from the value of 61 used in Equation (1). The strain rate 6 is given by the viscosity coefficient

d

.

Rearraging Equations (1) and (3) we obtain:

(8) k =

i l

-

(9 _E

-

₇

-

The neutron irradiation increases values of 61 and 62 by constant value A 6

*

.

remain unchanged.

It means that for the investigation of neutron irradiation the more results for lower strain rates are needed. Owing to the indepen-

dence of 6 /I% on the high strain rate behaviour of irradiated steels may

$

In view of generally accepted division of plastic flow mecha- nisms in the space (e,T) the Equation (8) should describes the ther- mally activated processes. The linear from of 6f ( & ) is often inter- preted in terms of viscous damping of dislocation motion /3/. As it was found in /7/ this linear dependence of 6* on & is more proba-

C5-520 JOURNAL DE PHYSIQUE

bly evidence of mixed mode of plastic flow.

V

-

i) In the strain rate from I O - ~ to 10~s'' the flow stress at 1%

strain of the tested steel is strongly temperature dependent in the temperature range -196O C to 500° C.

ii) In the same temperature range, the flow stress is sensitive to strain rate. This strain rate sensitivity may by described in terms of the theory of viscoplasticity. It was shown that neu- tron irradiation did not affect the form of Equations describing the observed strain rate dependence.

iii) The neutron irradiation produce the additional hardening which is temperature and strain rate independent at relatively low strain rates. For higher rates of strain the increase of strain rate sensitivity is temperature dependent.

V1

-

/l / Pecherski, R., Engineering Trans.

a

/2/ Rosenfield, A. R. and Hahn, G. T., Trans. ASM 2 (1966) 962.

/3/ Buchar, J., Bilek, Z. and DuSek, F., Effects of Radiation on Materials, Eleventh Conference, ASTM STP 782 (H.R.Brager and J.S.Perrin, Eds.) (1982) 550.

/4/ Ghoniem, N. M. and Cho, D. D., Phys. Stat. Sol. (a) (1979) 171.

/5/ Buchar, J. and Bilek, Z., Phys. Stat. Sol. (a) Q ( 1981 ) 259.

/6/ Perzyna, P., Proc. 2nd Inter. Conf. SMIRT, Berlin Vol. V, Part L, 1.

/7/ Clifton, R. J., Gilat, A. and Li, C. H., Material Behaviour un- der High Stress and Ultrahigh Rates. Plenum Press, New York 1983, 1 .

TABLE 1 Parameters of Equations (4)-(6)

NONIRRADIATED g 10

MATERIAL 140 1 720 4.7 0 59 0.32

IRRADIATED

MATERIAL l l 10 14.3 930 6.7 0.42 0.006

Fig. 1

-

0 NONIRRADIATED

oL-8

-200-100 0 100 200 300 LOO 500 600

-.

800 ^A I R R A D I A T E D

;;600 a I

U _W

-

400

V)

-

V) W (L

200 -

TEMPERATURE

- 1 2 , a"

U I - 1 1

W t- W

I 70 2

h

9

Fig. 2

-

.

JOURNAL DE PHYSIQUE

1600 I

IRRADIATED STEEL

I

Fig. 3

-

* I T ] - IRRADlATED STEEL a l i l - NONIRRADIATED STEEL

Fig. 4

-