Report on creep and LCF-Data on ODS-Materials for GEN IV-Reactors A Hobt 1 , A. Klenk 1 M. Serrano 2 , R. Hernandez 2 1 MPA University of Stuttgart 2 CIEMAT, Technoligy Department Abstract For the next generation of high temperature nuclear reactors new structural materials are necessary to meet the requirements of the loading and sufficient life time. Therefore several Oxide-Dispersion-Strengthened (ODS) ferritic-martensitic materials have been development. Within the GetMat-project there have been testes different materials, differing in chromium content, under creep, fatigue and creep-fatigue loading at temperatures between 600°C and 750°C. The materials were made from sheets and rods. The work was then continued in the MaTiSSE-project for tube materials. This contribution deals with the evaluation of the uniaxial creep tests for rod material with specimens extracted in different orientations and plate material. Especially for structural materials the properties and material behaviour in different orientations and different products are essential to know within the life-time assessment and the evaluation of the operational loadings in terms of stress and strain field analysis. This can be achieved by providing appropriate material laws for an optimized description of the material behaviour.
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Report on creep and LCF-Data on ODS-Materials for GEN IV-Reactors
A Hobt1, A. Klenk
1
M. Serrano2, R. Hernandez
2
1 MPA University of Stuttgart
2 CIEMAT, Technoligy Department
Abstract
For the next generation of high temperature nuclear reactors new structural materials
are necessary to meet the requirements of the loading and sufficient life time.
Therefore several Oxide-Dispersion-Strengthened (ODS) ferritic-martensitic materials
have been development. Within the GetMat-project there have been testes different
materials, differing in chromium content, under creep, fatigue and creep-fatigue
loading at temperatures between 600°C and 750°C. The materials were made from
sheets and rods. The work was then continued in the MaTiSSE-project for tube
materials. This contribution deals with the evaluation of the uniaxial creep tests for
rod material with specimens extracted in different orientations and plate material.
Especially for structural materials the properties and material behaviour in different
orientations and different products are essential to know within the life-time
assessment and the evaluation of the operational loadings in terms of stress and
strain field analysis. This can be achieved by providing appropriate material laws for
an optimized description of the material behaviour.
Introduction
In order to keep up the competitiveness of the European nuclear power stations in the long run,
innovative solutions must be found, guaranteeing a more efficient and at the same time safe
operation of the power plants. The increase in efficiency normally goes along with an increase in the
operating temperature and pressure. At the same time the power generation is on the step to a new
demand on the operation due to a rising portion of renewable energies forcing a flexible operation of
the power plants. To meet the demand, new materials must be developed with increased strength.
Beside the material development, the behaviour of the materials under operating conditions must be
described using adequate material models.
For this purpose the GETMAT project was launched to address the cross-cutting aspects of classical
ferritic-martensitic materials and the new Oxide-Dispersion-Strengthened(ODS) materials for core
and primary circuit components. This envelopes the issues of qualification of the materials by
mechanical testing, investigation of joining and fabrications techniques and the development of
modelling techniques based on a fundamental understanding of the material. The present
publication deals with the mechanical characterisation of ODS alloys with different chromium
contents and the link to the microstructure which enables an interpretation of the material
behaviour.
Material characterisation and specimen fabrication
Two ferritic ODS alloys and one martensitic ODS alloy are characterised. A 14Cr ODS extruded bar,
supplied by CEA (J27 heat) and a 12Cr ODS and 9Cr ODS plates supplied by Kobelco in the framework
of the GETMAT 7FWP project [2]. The 14Cr ODS bar was produced by CEA by mechanical alloying f a
master alloy of composition (wt. %) 13.98Cr, 1.03W, 0.39Ti, 0.29Mn, 0.32Si, 0.17Ni with 0.3 wt%
Y2O3 under hydrogen atmosphere in a vertical attritor. Subsequently, the material was hot extruded
in the form of bar at 1100 ºC and annealed for 1.5 h at 1050 ºC [3]. By this process, there are
generated two regions with different mechanical properties and also material properties, which can
be clearly distinguished, see Figure 1a). The 12Cr ODS plate were produced with a pre alloyed metal
powder of composition (wt. %) 11.59Cr, 1.87W, 0.22Ti and 0.1Si was prepared by argon gas
atomisation method. The powders were mechanically alloyed in dry type attrition ball mill with 0.23
wt. % Y2O3 and subsequently extruded at 1150°C, hot forged at 1150°C and annealed at 1150°C for
1h. Finally, the forged plates were cold rolled with 40 % reduction and annealed to re-crystallisation
at 1200°C for 1 hour [4], see Figure 1b). The 9Cr ODS plate was produced my mechanical alloying T91
powders with 0.3 wt. % Y2O3. These powders were extruded at 1160ºC, hot forging at 1150ºC and
heat treatment at 780ºC for 1 hour and air cooled afterwards. Finally a cold working up to 40%
reduction were performed and heat treated for normalization at 1050ºC for 1 hour and tempered at
750ºC for 1 hour, [5].
The fabrication process leads to inhomogeneous grain size distribution comparing the longitudinal
and transversal direction. The microstructure of the 14Cr ODS bar show an elongated-grained
structure parallel to the extrusion direction, see Figure 3a.
Figure 1: Product forms from which the specimens have been extracted
a) 14Cr b) 14Cr c) 12Cr / 9Cr
The mean gran size in the transverse orientation is 432 nm length and 415 nm width, while the size in
the longitudinal orientation is 826 nm length and 496 nm width. A preferential crystallographic
orientation of the grains along <110>// to the extrusion direction is observed. A detailed
microstructure description of the 14Cr ODS bar can be found in [6]. Regarding the microstructure of
the 12Cr ODS plate, in all the sections studied it is possible to distinguish a bimodal grain size
distribution, existing small grains with sizes usually lower than ten micron and strange bent shapes
and some big and elongated grains along the extrusion direction which sizes are bigger than tens of
micron up to two hundred micron, see Figure 3b. This could be an indication of an incomplete
recrystallization on the fabrication process. EBSD maps show that the smallest grains seem to have
an orientation along <110> in the ED, while larger grains have not got a preferential orientation. No
detailed microstructure of the 9Cr ODS were performed but information can be found in [7].
Figure 2: Microstructure in the as-received state
a)
14
Cr
b)
12
Cr
Results from conducted tests and first derivation of material laws
Within the GETMAT project the 9-14 %Cr-ODS materials have been analysed by basic experiments,
comprising, both in air atmosphere and Argon atmosphere at high temperatures:
• Tensile tests at different strain rates
• Creep tests
• Fatigue tests
Based on these tests it is possible to describe the material behaviour with macroscopic material
models which then can be used in the analyses of components under high temperature operation.
For the design of the creep tests and the fatigue tests, plastic flow behaviour of the materials was
derived by tensile tests. To quantify the influence of the loading speed, the strain rate of the tensile
tests was changed for the 9Cr ODS and the 14Cr ODS materials. Due to the high temperature of
750°C, all tests at MPA Stuttgart on the 14Cr ODS material were conducted in Argon atmosphere at
ambient pressure. The tests on the other two material were conducted in ambient air.
The basic material characterisation was done by tensile tests. At MPA Stuttgart, the specimens were
all extracted in longitudinal direction. The testing temperature was the main temperature also for
the further investigations, see Table 1, together with the main results. At CIEMAT, also specimens in
transversal direction have been analysed. At CIEMAT also tensile tests were performed, using with
dog-bone specimens (gage section 15x3x2 mm) mechanised in longitudinal (L) and transverse (T)
orientation for the 14Cr ODS and 12Cr ODS steel, defined as the direction of the loading during the
test, for the bar and plate. Tensile tests were performed at 22°C, 400°C, 600°C and 700°C a
displacement rate of 0.1 mm/min (corresponding to a strain rate of 1x10-4
/s) in a servo-hydraulic
MTS testing machine. Total elongation and reduction in area measurements were performed on the
broken specimens. A summary is given in Table 2.
Table 1: Summary of conducted tensile tests, MPA Stuttgart
Specimen Material Temp / °C Strain rate / 1/s UTS / MPa Strain at failure /%
2AB4
14Cr ODS 750
10-4 315 1,8
2AB1 2·10-5 315 1,34
2AB2 2·10-5 306 1,31
2AB3 10-6 283 0,54
X2C/2D 12Cr ODS 650 10-5 402 13,8
9CB2
9Cr ODS 600
10-3 592 37
9CA2 10-4 488 35,5
9CB1 10-5 394 27,9
9CA1 10-6 354 16,4
Table 2: Summary of conducted tensile tests, CIEMAT
Specimen Material Temp / °C Strain rate / 1/s UTS / MPa Strain at failure /%