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Structural performance of multi-metal component

Final Report Summary - MULTIMETAL (Structural performance of multi-metal component)

Executive Summary:

Existing standards for fracture resistance testing like ASTM E1820-13, ISO 12135:2002 and engineering procedures or schemes like DNV for the estimation of J-integral values are intended for specimens made of homogeneous material. There are no such standards or procedures for multi-metallic components or specimens that are made up of different material sections joined together via a weld. Thus in early 2012 a group of 11 European organisations started the research project MULTIMETAL, which has the aim to fill the above gaps. The objectives of MULTIMETAL are:
• Development of a standard for fracture resistance testing of multi-metallic specimens and
• Development of harmonized procedures for dissimilar metal welds (DMWs) brittle and ductile integrity assessment.

The underlying aim of MULTIMETAL was to provide recommendations for a good practice approach for the integrity assessment of DMWs as part of overall integrity analyses and leak-before-break (LBB) procedures. The project duration was three years and it was funded by the European Commission (EC) within its 7th Framework Program. MULTIMETAL had seven work packages (WPs). The core activities of the project took place in WP3, which contained the manufacturing of mock-ups and all the experimental tests, i.e. material characterisation tests, fracture tests and residual stress measurements. The main task in WP4 was performance of a benchmark exercise on the J-integral value estimation of different multi-metallic specimens that were used in the fracture tests in WP3. The aim of the materials testing program was to develop a procedure for measuring fracture toughness in DMWs. The project promotes the development of a common understanding for structural integrity assessment of DMWs in existing and future NPPs in EU member states. It provides the technical basis for the development of harmonised European codes and standards for multi-metal components, which are currently non-existing.
Finally a training course and exchange program for young scientists based on outcomes and experience gained from MULTIMETAL was organised.

Project Context and Objectives:
The first objective of this project is to gather relevant information from field experience: typical locations of DMWs in Western as well as Eastern LWRs will be identified and their characteristics considered, as well as applicable assessment methods. Modelling of ductile failure processes will be used as an innovative technique to augment current numerical methods for structural integrity assessment of DMWs, taking account of ageing related phenomena and realistic stress distributions in the weld area and be supported by a comprehensive material test program.
One objective of the project is to develop a procedure for measuring fracture toughness in DMWs. Overall the project serves to promote common understanding of structural integrity assessment of DMWs in existing and future NPPs of EU member states. This will be the technical basis towards the development of harmonised European codes and standards for multi-metal components, which is currently not available.
The underlying aim of the project is to provide recommendations for a good practice approach for the integrity assessment of dissimilar metal welds as part of overall integrity analyses and leak-before-break (LBB) procedures.

Project Results:
Main conclusions of the project are the following:
• All DMW design variants show high resistance to crack growth under the condition of investigation.
• The MULTIMETAL project has confirmed that in Dissimilar Material Welds, the most critical zone in terms of fracture lies in a narrow band around the weld-Low Alloy Steel interface.
• The characterization of local tensile properties is a key issue for analyzing the toughness tests as well as the test on mock-ups. The MULTIMETAL project has proposed new performing procedures for tensile testing.
• The project recommends the use of CT specimens (subsized if necessary) for toughness characterization of DMWs. In case of SEN(BB) specimens, rotation correction should be applied.
• One of the main issues is the determination of the blunting line in fracture toughness testing
• Residual stress fields in tough DMWs such as those made in NI base filler material may be disregarded in the analysis.
• The determination of residual stress fields through measurements or computations is delicate and requires cross comparisons.
• The prediction of ductile tearing requires using local approaches developed for ductile tearing.
• It is recommended to use ASTM 1820 to assess fracture toughness of DMW, location of the crack must be at the fusion line (+/-0 mm) between ferritic heat affected zone and Ni-base alloy for Ni-based narrow gap DMW, but may lead to an underestimation of the toughness initiation value.
• For CT-specimen, the use of eta factor from ASTM 1820 is acceptable compared to numerical investigation of the test.
• For other specimen types 3D numerical assessment should be performed.
• The size of the specimen required to assess fracture toughness must be checked since plastic collapse can occur in too small specimen.
• The blunting line should be constructed using the weakest material (from the tensile point of view), here using the Ni-based alloy. The slope of the blunting line is driven by the factor M, large M factor leads to low fracture toughness value. M = 2 and M = 4 were taken into account during the project. The M factor should be selected to follow the slope obtained during testing.
• For tough DMW components (Ni-variant with Ni-based alloy) the integrity assessment can be limited in a penalizing way to a limit load analysis. It is recommended to use existing codification ASME XI, KTA, R6.
• The residual stresses are in general erased by the plasticity which appears in the specimen and therefore do not impact the final result of tearing initiation.
• For the analysis of crack initiation, the concept of toughness and the parameter J0.2 remains relevant for DMW as shown by the ADIMEW or STYLE projects.
• For testing using SEN(B) specimen, compliance given in ASTM 1820 underestimates the crack growth in the specimen during tearing.
• 3D numerical assessment is needed for verification of eta and gamma for configuration which were not investigated during the project.

Recommendations for future work
• Improve the guidelines for fracture toughness testing
• Complete the recommendations for Eta factor estimates
• Development of approximate but simple formulae for assessing the cold work in DMWs.
• Develop guidelines for the application of local approaches of ductile tearing
• Develop an exemption criterion for not considering residual stress in the fracture analysis of DMWs on the basis of the resistance to ductile tearing and the expected level of residual stress acting on the crack.
• Further work is needed to assess the validity of the J-concept for tough material (Jlimit), as well as the transferability from specimen to larger component. Possible differences in the fracture behaviour of small specimens and large scale mock-ups should be investigated. Experimental benchmarks should be performed using scanning electron microscopy (SEM) on ductile tearing fracture surfaces to establish a link between ductile fracture toughness properties obtained from small scale specimens and fractures from large scale tests. The validity of the blunting line should be investigated as well by detailed assessment of the stretch zone, fine numerical analyses, combined to SEM observation of the stretch zones, shall provide conclusive results.
• SEN(T) specimen are promising due to a constraint level more similar to the condition of welds in piping, however the numerical assessment show large scatter which was not expected, and this should be investigated in details.
• Scatter in the final results have been discussed within a qualitative way, however the propagation of uncertainties within the fracture toughness testing as well as in the numerical assessment has not been treated and a quantitative assessment should be performed.

A more detailed presentation of the results can be found in the attached Final Report.

Potential Impact:
The project makes possible better mastering of material characterization and has potential impact on safety demonstrations. More generally the project will contribute to safe operation of existing nuclear power plants and to maintaining high safety standards due to more accurate assessment methods resulting in increased margin of safety. MULTIMETAL will reinforce the competitiveness of the European nuclear industry by the achievement of innovative research results in the field of maintenance of nuclear power plants and by the participation of leading industrial companies in this field. The project may have outstanding impact on national and international research activities concerning the use of advanced tools for structural integrity assessments and fracture mechanic testing. It is expected that the project will be actively involved in specific European networks, which are fed by national and international research activities. An important part of the project is to give students and young researchers the opportunity to work on challenging problems side by side with their more experienced colleagues. Taking this into account the project will generate a remarkable added-value in maintaining and improvement of collaborative research activities in the field of nuclear energy in the EU.

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