·Table of Contents
·Materials Characterization and testing
Assessment of Steels Ageing by Measuring the Seebeck and Thomson Effects (STEAM)L. Debarberis, B. Acosta, M. Beers, C. McGirl and F. Sevini
Commission of the European Communities, Joint Research Centre
Institute For Advanced Materials, IAM,
Postbus 2, 1755 ZG Petten, The Netherlands
Tel: +31-224-565435, Fax: +31-224-561432
At the Institute for Advanced Materials (IAM) of the EC-JRC, and within the framework of the AMES programme (Ageing Materials Evaluation and Studies), a number of projects in the fields of ageing of materials (both thermal and irradiation induced) are carried out. Also the recovery of the mechanical properties of the material using annealing is investigated. The experimental studies on ageing of materials have as a principal aim the research on the mechanisms of ageing and annealing of RPV steels, and the development of new non-destructive techniques, such as STEAM to evaluate the stages of ageing of the materials.
Materials are ageing due to many different causes; e.g. thermal treatments, thermal ageing, stress ageing, corrosion, and irradiation embrittlement. At EC-JRC-IAM, Petten, experimental studies are carried out for both thermal and irradiation embrittlement. The reliability of the NDT techniques, such as the thermal-electric power measurement, is carefully studied for its capability to distinguish different stages of ageing of steels. The main tests to determine the mechanical behaviour of materials are the tensile, Charpy and fatigue tests, which are destructive. The possibility of using non-destructive tests would facilitate the surveillance of the materials that form the reactor vessel, on the one hand making possible in-situ inspection, and on the other by benefiting those surveillance programmes that have an insufficient amount of material available.
The use of Non Destructive Techniques, NDT, for assessing material ageing is a matter of strong interest for the EC-JRC-IAM and further investigation, assessment and qualification work is needed. Within the frame of the AMES-NDT concerted action lead by JRC-IAM, preparatory work, including a state of the art report on NDT techniques existing worldwide, is carried out. Demonstration tests are performed by using various NDT techniques; including magnetic, electric, ultrasonic and techniques based on atomic and nuclear physics phenomena.
The JRC-IAM has developed prototype equipment called STEAM (Seebeck and Thomson Effects on Aged Material). This is based on the measurement of the thermoelectric power, in order to evaluate the ageing degrees of ferritic steels. The purpose is to correlate the results of the Charpy tests, hardness and STEAM measurements with the change in the mechanical properties when ageing and annealing. In this way the aim is to reach the development of the STEAM technology for deployment in aged materials evaluations.
This paper gives an overview of the EC-JRC-IAM experimental work in the field on materials ageing and the use of thermoelectric power measurement as NDT technique.
Nuclear energy represents a significant proportion of the Community's total energy production and is likely to do so in the future. However, nuclear fission can play this role only to the extent that, in the entire Community as well as in neighboring countries, a maximum level of safety is achieved for all stages of the nuclear cycle.
As nuclear power plants age, material ageing of key reactor components becomes a crucial consideration for continued safe plan operation. Decisions regarding the verification of design plant lifetime and potential license renewal periods involve a determination of the metal condition.
Materials are ageing due to many different causes; e.g. thermal treatments, thermal ageing, stress ageing, corrosion, irradiation embrittlement, etc. The different causes, and their synergism, are acting on the materials in many different ways, e.g. vacancies generation and dislocations, migration of low melting temperature metals to the grain boundary, matrix damage, etc. Each of these causes, even though by mean of different mechanism, is responsible for the embrittlement of the material making it more prone to rupture. The European Network AMES (Ageing Materials Evaluation and Studies), managed by the Institute for Advanced Materials of the Joint Research Centre of the European Commission (EC-JRC-IAM), is specially focusing on all the above mentioned ageing issues.
Within the frame of the European Network AMES a large number of experimental studies on ageing, both thermal and irradiation induced, and annealing are conducted. A new challenge is the use of Non-Destructive Techniques (NDT) to assess material ageing. A non-destructive determination of the embrittlement state would extend the usefulness of the surveillance material by reducing the material used for destructive studies and would benefit surveillance programs having an insufficient amount of available test material, and ultimately allowing tests to be performed directly on the component to evaluate. Such NDT capabilities would provide substantial early warning of component deterioration and enable utilities to optimize their operating and maintenance practices, resulting in reduced costs and increased asset utilization.
The AMES network set up and operated by the EC-JRC-IAM brings together the organizations in Europe, which have the main capabilities on RPV materials assessment and research, with the following objectives:
AMES is also promoting the integration of national programmes, the validation of techniques, the definition of European Standards and the validation and establishment of safe limits for mitigation measures. Most of these activities led already to widely distributed AMES reports .
Within the frame of the European Network AMES a large number of experimental studies on ageing, both thermal and irradiation induced, and annealing are conducted. Most of the activities are international projects involving several European Member States laboratories and the Nuclear Fission Safety Programme of the EU finances most of these activities as SCA competitive projects .
The following table represents a short summary of the most important ongoing activities and projects for which an important share of the work is carried out by EC-JRC-IAM .
|MADAM||Generation of a conversion table of material damage indexes for possible comparison of results coming from different test programs and real operating plants.|
|RESQUE||Validation of Cv-N samples re-constitution techniques for obtaining more experimental fracture toughness data limiting the amount of material used. Different welding and joining techniques are compared.|
|REFEREE||To assess the correlation between different fracture toughness properties of aged steels; Charpy impact versus dynamic and quasi-static toughness transition shifts measurements.|
|AMES DOSIMETRY||To harmonize dosimetry practices for ageing studies and to establish the dosimetry of AMES activities.|
|ENUKRA||Implementation of modern methods for fracture mechanics testing and transfer of western country knowledge and experience to Ukrainian engineer relating to RPV metal condition assessment.|
|SINTER||To propose and study safety related innovative nuclear reactor technology elements for present and future type of plants.|
|INTACT||Review the current research activities and the state of the art in the field of ageing of metallic components, civil engineering structures, motor operated valves, electrical equipment, data acquisition systems, cables, test installations, tools and reference laboratories.|
|IRLA||To transfer of western countries knowledge and experience, to Ukrainian and Russian engineers, related to reactor pressure vessels residual life assessment, and check validity of the usual approach in the surveillance programmes of the RPV.|
|AMES-NDT||To study the applicability of ND techniques to monitor material embrittlement|
|GRETE||To study and validate the applicability of ND techniques to monitor material embrittlement due to neutron exposure and thermal fatigue|
|PISA||To study and quantify the effect of Phosphorus on material embrittlement|
|FRAME||To validate Master Curve approach for FT analysis|
|COBRA||To directly measure VVER surveillance temperature for correct validation of surveillance programmes.|
A new challenge is the use of Non-Destructive Techniques (NDT) to assess material ageing; the AMES-NDT Concerted Action has been devoted to study these promising techniques.
The assessment of material ageing requires normally extensive destructive testing by using different kind of testing and samples. One example is the Charpy impact technique where a set of Charpy specimens is broken at variable temperature in order to draw a so-called Energy-Temperature curve.
The possibility to qualify Non Destructive Techniques, NDT, would make the studies on ageing and the relative assessment much easier and cheaper, saving a lot of money and resources in Europe. Furthermore, it would make it possible to assess on field various components that now-a-day can only be assessed either by calculation or by means of very expensive sample gaining techniques. To follow in service the structural integrity of such components could reduce the uncertainties about their mechanical behavior along transient periods. The increasing in safety for all the EU citizens justify the efforts play in role to develop NDT techniques. On the other hand, in Non-European countries like Japan, the US and Russia, the most recent investigations are oriented to the subject of NDT to assessment of materials. In such countries NDT studies are in a more advanced stage of development and the importance of the NDT techniques has been clearly demonstrated .
AMES-NDT has focused on list all known NDT techniques and their applications and limits for the assessment and monitoring materials ageing.
In addition to activities to support the AMES European Network strategy, the EC-JRC-IAM is carrying out other projects in the field of thermal ageing, irradiation ageing and annealing of steels, and tests on correlation of material ageing with non-destructive methods like hardness and thermoelectric measurements .
Thermal Ageing and Annealing of Steels
Consists on the study of the influence of thermal ageing in the mechanical properties of ferritic steels by testing JRQ material. Material is thermally treated at a temperature range 300 to 900 ºC for different duration; the change in the mechanical properties of the material is monitored in one hand by Charpy impact test and in the other hand by NDT techniques such hardness and thermoelectric measurements.
The annealing studies described in this paper are carried out on embrittled JRQ steel by heating at 900 ° C 1 hour and quenching in water.
In order to understand the role of annealing parameters (temperature and time) annealing has been carried out as follows:
1. Parameter: temperature
2. Parameter: time
The technique is based on the Seebeck and Thomson effects that lead to a thermoelectric effect in metals . This thermoelectric tension is a characteristic of the material that may be described as its capacity, when coupled with another metal, to generate an electron flux when crossed by a thermal flux. The absolute tension of a pure metal may take values ranging from one to a few dozens of m V/° K depending on its temperature difference (D T), chemical composition and its atomic arrangement. All modifications of the crystalline lattice of a metal may modify its thermoelectric power, so thermoelectric measurements can be correlated to the change in mechanical properties of the aged material.
Promising non-destructive techniques are considered for application in order to assess material ageing, especially hardness and thermoelectric measurements as demonstrated by recent EDF studies ,.
Thermoelectric measurements, using a JRC-IAM developed equipment called STEAM, and hardness measurements were carried out on different steel samples. In particular the results described on this paper have been performed on samples coming from the annealing studies. Annealed specimens were non-destructively tested according to the Table 1:
|Table 1: Annealing Treatment|
As Table 1 shows, tests were performed in to sets of annealed samples. First group concerns to samples annealed at different temperatures and same duration (1 hour). STEAM and hardness measurements in such series are plotted in Figure 1. As can be seen, when annealing temperature increases the STEAM and hardness decrease in similar ratio. Regarding with annealing temperature is important to say that above 700 ºC grain boundary segregation saturates.
|Fig 1: Effect of Annealing Temperature on STEAM and Hardness Measurements|
Figure 2 shows the correlation between hardness and STEAM measurements for samples annealed at different temperatures. Figure demonstrates that STEAM measurements track the change in material properties when ageing and annealing, the STEAM values in m V/ ºC decrease when the material becomes less brittle which means that hardness by Hardness Vickers 10 test decrease.
|Fig 2: Hardness-STEAM, Annealing Temperature Influence|
|Fig 3 : Effect of Annealing Time on STEAM and Hardness Measurements|
Within the frame of the AMES programme, a number of projects dealing with ageing of materials, radiation and thermal ageing, and annealing have been performed or are ongoing at the Institute for Advanced Materials (IAM) of the JRC of the EC.
The experimental studies on ageing materials are focused on the understanding of ageing/annealing mechanisms in steels, and studying non-destructive techniques for assessing material ageing (AMES-NDT Project).
Within the frame of the AMES-NDT project, promising ND techniques like STEAM, and hardness are applied and correlation with ageing material characteristic are verified in specific cases. The result of ND testing, hardness and STEAM shows a consistent correlation and can follow the changes in material properties due to annealing temperature. The STEAM measurements are more sensitive to annealing time than the hardness test.
The AMES-NDT Concerted Action, have made use of non-irradiated, artificially embrittled steel obtained by extreme thermal treatments; i.e. water quenching of hot plates of JRQ steel; up to 700 ºC. More realistic studies need to be done on neutron induced embrittled steel. For this scope a new shared cost action project (GRETE) involving most of the European stakeholders has been prepared and has been recently approved for EC financing in the Nuclear Fission Safety Programme.
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