Welding processes induce residual stresses and distortion in the welded joint and the connected components. For manufacturing purpose distortion is the main issue and up to now the problem is handled by post weld corrective actions. Welding residual stress fields are not considered at the design stage in French codes and standards. However, it is well known that residual stresses are likely to increase the risks of fatigue or corrosion and may cause failure in brittle materials. Ferritic parts of large components are post-weld heat treated; allowing disregarding the influence of residuals stresses thanks to their relief. Preventive measures, including mitigation by fine polishing are undertaken in corrosion sensitive zones. The influence of residual stresses on fatigue is more complex to analyze: in low cycle fatigue, residual stresses should be relieved or redistributed after few cycles with plastic straining, and for high cycle fatigue, residual stress effects are accounted for through a mean stress offsett. When considered, residual stress fields are often represented in a very crude manner by a membrane distribution of the most influent stress component through the thickness of the structure. In a less rough way, several codes or fitness-for-purpose guidelines (API [1], British standards [2]) propose residual stress profiles relative to several weld configurations. Nevertheless for a given case, the given profiles may differ significantly for several reasons: the degree of conservatism, the number of covered cases, the embedded margins accounting for uncertainties. Some ill-posed benchmark problems have shown that numerical simulation of residual stresses may deliver very scattered results. AREVA has therefore developed a methodology to validate welding simulations. The scope is limited to fusion welding. The simulations are based on a Thermo-Metallurgical Mechanical model in which the welding energy is represented by an equivalent heat source. This paper presents the actual state of development of this methodology which will be illustrated through 4 examples of residual fields in Dissimilar Metal Welds. Residual stress measurements have been performed for each of the four mock-ups by different techniques. Based on this important experimental and numerical campaign some actions of improvement of the validation methodology are finally listed.
For nuclear reactor applications, AREVA NP has to perform junctions between ferritic low alloy steel heavy section components and austenitic stainless steel piping systems. For Gas Tungsten Arc Welding (GTAW) of dissimilar metal weld (DMW) narrow gap, AREVA NP has developed special manufacturing procedures guaranteeing high quality standards and resistance in service. Since a decade, AREVA NP is developing the numerical simulation of welding to have a better understanding of involved physical phenomena and to predict residual stresses. In spite of the large thickness of Pressurized Water Reactor (PWR) components, the distortion issue may also be important. Narrow gap welding requires indeed a close control of the groove width. This paper presents numerical simulations performed by AREVA NP on 14″ narrow gap DMW mock-ups as part of a research project carried out internally. The simulations focus on the predictions of microstructure and residual stress distribution. The analysis simulates the main steps of the mock-up manufacturing procedure. Multi pass welding simulation reproduces the deposit of each bead by thermo-metallurgical and mechanical calculations. A special attention has been paid on the buttering of the ferritic side. Generally a post weld heat treatment (PWHT) is carried out after the buttering of the ferritic side in order to relieve residual stresses. For some repair operations, a PWHT is not feasible. Thus a temper bead process can be used. During this process, a large part of the previous heat affected zone is tempered to guarantee a limited hardness and to reduce the risk of cold cracking. The results in terms of microstructure and stress obtained with the two techniques are compared. With the temper bead process, the final level of hoop stresses in the heat affected zone (HAZ) of the buttering remains significant as stresses are not relieved by viscous effects implied during PWHT. Nevertheless the temper bead process has a positive effect on the material hardness as the proportion of tempered phase is higher. One of the objectives of this task is to compare the numerical results with measurements. This comparison is not only a validation of numerical simulation of welding but also a way to investigate the relevance of residual stress measurement by Deep Hole Drilling (DHD). Calculated stresses are globally in good agreement with measurements made by DHD. A comparison with axial shrinkage is also made for validation of the modelling methodology.
The present paper deals with the hybrid laser/MIG welding process, which allows to assembly high thickness steel sheets. The laser heat source added to the MIG torch improves the process productivity while respecting quality standard. The multi-pass welding simulation of a plate is presented in this paper and numerical results are compared to experimental measurements (temperature and stresses).
The present paper deals with the hybrid laser/MIG welding process, which allows to join high thickness steel sheets. The laser heat source added to the MIG torch improves the process productivity while respecting quality standard. The multi-pass welding simulation of a plate is presented in this paper and numerical results are compared to experimental measurements (temperature and stresses).
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