Short description
(English)
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A programme of work is proposed to examine the applicability of the LICON methodology to a low alloy ferritic 1%Chromium-Molybdenum-Vanadium steel and to ascertain if refinements are required to the procedure for this class of material, in particular when used to predict remaining life. The LICON methodology was originally developed using experimental creep crack incubation (CCI) results from high temperature fracture mechanics tests on three advanced 9%Chromium steels, including P91. The CCI characteristics determined in this project for the low alloy steel will be compared with those existing for the P91 steel.
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Partners and International Organizations
(English)
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BE, CH, CZ, DE, DK, ES, FI, FR, IT, LT, NL, PL, PT, SE, SK, UK
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Abstract
(English)
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The main objective of the EMPA COST 538 CH4 project was to examine the applicability of the LICON multi-axial assessment procedure [1] to a low alloy creep resistant ferritic steel (1%CrMoV), and to ascertain if refinements were required to the methodology for this class of material, in particular when used to predict remaining life. The LICON procedure was originally developed and validated with material properties determined for a number of advanced 9%Cr martensitic steels, including Grades 91, 92 and E911 [1,2]. The LICON approach was devised in the late 1990s to predict the long term creep rupture behaviour of new generation steels (including their welded joints) from the results of relatively short duration multi-axial specimen tests. The methodology relies on the acceleration of creep damage development under multi-axial loading conditions to enable extended extrapolation of rupture strength into the long time fracture regime. The approach provides similarity with the loading conditions experienced in real structures and enables a more accurate evaluation of the future in-service performance of welded components for which no long term service experience exists [3]. The CH4 Final Report covers the results of an investigation into the effectiveness of the LICON methodology when applied to a low alloy creep resistant 1%CrMoV steel. The project has highlighted the quantity of information required for this type of evaluation, i.e. - An awareness of the relevant creep damage mechanism regimes - The results of uniaxial and multi-axial experimental testing - The results of numerical analysis (for CT testpiece and target component/structure) - Identification of the effective stress - A knowledge of the steady-state creep triaxiality factor - A knowledge of the multi-axial rupture criteria obeyed by the material In the COST 538 toolbox, the LICON approach offers most promise for the life assessment of high temperature structures containing welds. The applicability of the LICON approach to a 1%CrMoV low alloy ferritic steel was investigated in the present study because of the availability of a heat of this material which was already well characterised in terms of its material pedigree and long duration uniaxial creep properties at 550°C. More importantly, the creep damage mechanism regimes for this steel were established for this test temperature. Constant load creep crack incubation (CCI) tests were performed using fully instrumented 25mm thick CT testpieces with side-grooves. Load point displacement was recorded using a clip-on type mechanical extensometer attached to testpiece integral knife edges, while creep crack development was continuously monitored using electrical DCPD instrumentation. Four tests were continued to the attainment of ~5mm creep crack extension in durations up to 5,000h. It was confirmed that the long time intergranular cracking mechanism exhibited by the 1%CrMoV steel had successfully been generated in the relatively short duration multi-axial CCI tests. These test results provided the multi-axial parameters necessary for the LICON analysis. In order to undertake a LICON assessment, it is necessary to characterise the stress state under steady state creep conditions of the CCI testpiece geometry and the critical feature of the component/ structure under evaluation. This was carried out in the CH4 project using a 3-dimensional non-linear elasto-plastic-creep finite element analysis procedure involving a comprehensive sensitivity check on mesh configuration. Finite element analysis was employed to determine steady state creep effective stresses and triaxiality factors. An important requirement for application of the approach is a knowledge of the multi-axial rupture criterion obeyed in the critical mechanism regime. The appropriate value of this parameter was established. It is demonstrated that the LICON methodology can be effectively applied to the low alloy creep resistant ferritic steel, but that it is not simply a case of performing a series of relatively short duration CCI tests. Additional information including uniaxial creep strain data, uniaxial and multi-axial rupture time properties and the results of advanced finite element analysis is also required. Nevertheless, using this approach, the 100,000h creep rupture behaviour of a new metallurgically active alloy can be reliably estimated within 3 years of development. References 1. Mendes Martins, V. and Holdsworth, S.R. The LICON methodology for predicting the long term service behaviour of new steels. Materials at High Temperature, 19(2), 99-104 (2002). 2. Auerkari, P., Holdsworth, S.R., Rantala, J.H., Hurst, R.C., Coussement, C. and Hack, R. Predicting long term creep behaviour using the LICON methodology. In: Proc. 3rd Conf. on Advances in Material Technology for Fossil Power Plants, Univ. Wales Swansea, April, 329-339 (2001). 3. Holdsworth, S.R. and Mazza, E. Exploring the applicability of the LICON methodology for a 1%CrMoV steel. Materials at High Temperatures, 25(4), 267-276 (2008).
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