Small angle neutron scattering study of the damage induced by creep deformation in AISI 304 stainless steel

Small angle neutron scattering study of the damage induced by creep deformation in AISI 304 stainless steel


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Commission of the European Communities
nuclear science and technology
IN AISI 304 STAINLESS STEEL Commission of the European Communities
nuclear science and technology
A. BOEUF V) - R. COPPOLA (ï) - R. MATERA (ï) - P. PULITI (2)
(') Physics Division, J.R.C. Ispra (Italy)
(2) Facoltà di Ingegneria, Università di Ancona (Italia)
Directorate-General for Science, Research and Development
Joint Research Centre - Ispra Establishment - Italy
1982 EUR 7821 EN Published by the
Information Market and Innovation
Bâtiment Jean Monnet
Neither the Commission of the European Communities nor any person
acting on behalf of the Commission is responsible for the use which might
be made of the following information
1 ECSC-EEG-EAEC Brussels · Luxembourg 1982 CONTENTS
1. Introduction 1
2. Experimental methods and results 3
2.1. Material characterization
2.2. Creep tests and aging treatments
2.3. Metallography 4
2.3.1. Optical and scanning electron
3. Transmission electron microscopy 6
3.1. Results on aged samples
3.2.s for creeps 9
3.2.1. Previous data
3.2.2. Present data
4. Carbide composition determination 11
4.1. Introduction 1
4.2. Results and discussion
5. Density measurements3
6. Neutron small angle scattering measurements 14
6.1. Description of experimental techniques
6.2. Results5
6.2.1. Data elaboration procedure 1
6.2.2. Aged samples 16
6.2.3. Creep-tested specimens 22
7. Conclusions 2
Tables 30
Figures6 - 1 -
The material of structural components operating in the creep
range undergoes microstructural changes throughout its useful
life time, due to the combined effect of temperature and stress.
According to chemistry of the alloy, to its previous thermo-
mechanical treatment and to the particular loading conditions to
which it is subjected, these microstructural modifications can
present different features, but their effect is, in most of the
cases, to adversely affect the load bearing capability of the struc­
In the austenitic stainless steels, a class of materials largely used
in the nuclear power plants, creep induces essentially two new
microstructural effects: the nucleation and growth of grain boun­
dary cavities and the precipitation of carbides. Carbides preci­
pitate as a consequence of pure thermal treatment, whereas for
the occurrence of voids the presence of stress is necessary.
It should be noted, however, that the kinetics of precipitation and
even the precipitate crystal structure are changed by the intro­
duction of dislocations due to straining during creep.
The aim of the present study is a quantitative characterization
of the creep damage occurring in a stainless steel of the type
X 6 C rNi 8 11 (AISI 304). The experimental techniques used are
optical and electron microscopy, microprobe analysis and small
angle neutron scattering. This latter technique will be described
in more detail, as it is more and more applied to the general
problem of damage in last few years.
In general it is believed that neutron scattering techniques are
used in a successful way only in connection with fundamental re­
search involving the structure and the dynamics, both at an
atomic level, of condensed matter. In the last few years, however,
the interest of neutron scattering, in particular of SANS in inves­
tigations of technological interest, has grown continuously. The
Argonne National Laboratory has commissioned an investigation
concerning the opportunities offered by the neutron scattering
techniques in research and testing of industrial interest . Further­
more the FIAT Research Centre, in collaboration with the CAMEN
Centre at Pisa has installed since 1973 a facility for SANS, which
was applied to a variety of practical applications which are re­
viewed in ref. 5 and which will be mentioned below.
The interest of SANS in industrial research and, in particular, in
non-destructive testing, is a consequence of the high sensitivity - 2 -
of this technique in the characterization of degradation effects
in materials, which are due to "long­range order" inhomogeneities,
such as precipitates,microvoids, dislocations. Furthermore, an
advantage of neutrons is given by the low absorption cross section
of this radiation which enable large penetration distances in in­
vestigated materials, namely of the order of a few centimetres.
Finally, it must be emphasized that the SANS technique resolution
is situated between the Bragg neutron scattering and the neutron
radiography resolution. In fact, SANS allows the observation of
objects having dimensions between 10 A and 104 A. Further details
on the SANS technique can be found in ref s. 6, 7, 8.
Several examples of the application of SANS can be found in litera­
ture. Walther et al. ' investigated the influence of thermal treat­
ments on carbon steel, the evolution of size of precipitates (inter­
metallic NÌ3(A1, Ti) compounds) in the Inconel 750 Ni­superalloys,
the creep and fatigue in the Incoloy 800 steel, the degradation due
to the aging process of aircraft turbine blades of Inconel 700 Ni
super­alloy and the ferritic phase formation in welded steel
AISI 304. They concluded that the applications of neutron small­
angle scattering are already convenient on an economic basis and
in future increasing interest should be shown in such techniques in
order to eliminate the ignorance part of the so­called safety factors.
Other applications of neutron small­angle scattering concern:
(i) determination of the distribution of voids in irradiated Al
(ii) evidence of truncated octahedral shape of voids in Al crystals
irradiated by fast neutrons11,
(iii) evidence of faceted voids in quenched NiAl single crystals ,
(iv) experiments on voids in fast neutron­irradiated Cu and ΑΙ ,
(v) investigation of voids in irradiated pressure steel14,
(vi) size and distance determination of the voids produced in a
neutron irradiated 304 stainless steel, taken directly from
an operating reactor1 ->,
(vii) observation of pores in high­temperature fatigued Cu ,
(viii) study of the temperature and stress dependence of the micro­
structure of Nimonic alloys1',
(ix) study of the growth and dissolution of metastable precipitates
in an Al­11. 8% Zn alloy18.
All these applications of SANS to the investigation of microscopic aspect of degeneration in several materials of technological
interest suggested to use this technique to investigate, with other
experimental techniques available at JRC-Ispra, the creep pheno­
menon in AISI 304 stainless steel.
Due to the contemporary presence of grain boundary cavities and
carbide precipitates, our study has been divided into two parts:
- study of aged samples, i. e. samples subjected to thermal treat­
ment and in which only precipitates are formed (namely MgßC^);
- creep samples, the characterization of which has been done by
taking into account the results obtained from aged samples, in
order to separate their contribution to the global damage of the
Section 2. 1 of this report deals with the general material charac­
terization. Section 2.2 gives details of the creep tests and of the
aging treatments. Section 2. 3 reports the results of m etallo-
graphic investigations, including optical microscopy, and scanning
electronmicroscopy. The identification of the carbide precipitates
was performed by transmission electron microscopy (Section 3)
and microprobe analysis (Section 4).
Section 5 covers the fractional density change measurements and,
finally, Section 6 concerns the SANS results referring both to
thermally aged samples and to creep specimens.
2. 1 Material Characterization
The steel was supplied by Thyessen Edelstattwerk AG as 2 5 mm
diameter bars, solution-treated. The composition is given in
Table I. The general micrographie aspect is shown in Fig. 1.
The average grain size, measured according to the intercept
method described in ref. 19 is of 80 pm both in the longitudinal
and cross sections. The short time mechanical properties as
measured on the same specimen utilized for the creep tests (Fig. 2)
in tension at room and test temperature, are given in Table II.
2. 2 Creep Tests and Aging Treatments
Creep and creep rupture tests were performed in ADAMEL TAC
creep machines at 550 and 600 C under purified dynamic argon
atmosphere and in air. - 4 -
Two types of creep specimens were used: a cylindrical 4 mm
diameter, 50 mm gauge length button head specimen (Fig. 2),
and a rectangular test specimen 1 mm thick, 8 mm large and
with a gauge length of 70 mm.
Details of experimental conditions and of the test results are
reported in Table III. Typical creep curves are shown in Figs.
3 and 4. Strain was recorded as a function of time by means of
two push-rods connected to the specimen heads, which transmit
the displacement to the lever arms of a Chevenard extensometer.
With this measuring arrangement it is possible to resolve strains
of at least 1 χ 10" .
Figs. 5 and 6 show, at temperatures of 550 and 600°C respectively,
the rupture lives and the time for the creep strain of 0. 5, 1, 3 and
5% as a function of the initial applied stress. The figure on each
experimental point identifies the specimen number as given in
Table III. The numbers in brackets have not been taken into account
for the interpolation of the curves. The scatter in the rupture lives
is quite large at the temperatures and stresses included in the test
range. This anomalous scatter can be attributed, besides to the
influence of an insufficient control of the test parameters (mainly
temperature and bending strain due to non-axiality of the load), to
the small diameter of the specimens which were machined from
the central part of the original bar, where a larger than average
content of impurities is likely to occur.
Thermal aging was performed on discs of 1 mm thickness, cut
directly from the original bar, at temperatures of 600 and 650°C
for 26, 48, 120, 288, 528, 1008, 1656, 1922 and 2328 hours.
2. 3 Metallography
2. 3. 1 Optical and Scanning Electron Microscopy
Figs. 7 to 11 show some of the crack types observed on the longi­
tudinal section of rupture specimens. Most of the cracks are inter-
granular but some cracks were observed along the twin boundaries,
especially on short-time creep specimens. Either type of crack
tends to open perpendicularly to the stress axis. Grain boundary
cracks were also observed on the free surface of the creep speci­
men. Internal cracks (referred to as voids hereafter) and external
cracks (referred to as cracks hereafter), were counted separately
on the longitudinal section of the entire creep specimen.
The number of the cracks as a function of the distance from the
fracture is shown in Fig. 11. It is observed that the crack number