Prediction of Residual Stresses after Local Post-Weld Heat Treatment

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1 GROUP SPONSORED PROJECT OUTLINE PR July 2013 Summary Residual stresses caused by fabrication or repair welding can affect the resistance to fracture or corrosion damage of a thick component. Post-weld heat-treatment (PWHT) is usually recommended after manufacturing. If the component is too large to be fully heat-treated, codes for pressure vessels or piping allow the welded joints to be locally heat-treated, and give recommendation on the heat treatment procedures to minimise the residual stresses due to the temperature gradient applied during PWHT. In some cases, these recommendations cannot be applied, most often because the geometry of the component is not addressed by the codes of practice. In this case, finite element analysis (FEA) can be used for prediction of the residual stresses after PWHT, but predictions need to be validated. TWI proposes to perform finite element analyses and experimental measurements on a number of structural specimens in order to define a suitable procedure for modelling PWHT, and provide documented benchmark cases. These results will be available to the project s sponsors to verify their own models, so that they can predict with more confidence the residual stress to be used in the assessments of their components.

2 Project Concept Welding thick walled components generates residual stresses which can contribute to brittle fracture or corrosion cracking. Residual stresses can be reduced by heat treatment of the whole component in a furnace. However, when the component is too large to be furnace heat-treated, local heat treatment of a circumferential band (for a cylindrical geometry) or a circular patch (for a spherical cap) is allowed. This may be applied at circumferential butt welds or at branch or nozzle connections. International codes give recommendations regarding the size of the heat treated zone in order to avoid generating excessive residual stresses due to the temperature gradients applied during the heat treatment. There is demand from industry to reduce the size of the heated zone, either to avoid overlapping other connections or heat sensitive attachments, or to reduce power consumption. However, a heated zone of reduced size will increase the thermal gradients and hence may generate unacceptable residual stresses during cooling. Heat transfer and thermal stress FEA may be used for the prediction of the residual stress. The results may be used in fatigue fracture or corrosion assessment of the welded structure. However the reliability of the results is dependent on the methodology and the input data. In this project, TWI will carry out FEA and experiments to provide, for a selected number of geometries and materials, a set of data (temperature and residual stresses) for the prediction and validation of each model. The description of the methodology, recommendations, and experimental results will provide sufficient information for benchmark analyses, and give confidence in the analysis of components where the geometry does not allow the recommendations from local PWHT standards to be applied directly. Objectives The objectives of this project are: Manufacture a number of mock-ups for the measurement of temperature and residual stresses during and after welding and PWHT. Gather a set of material properties for the structures investigated, in particular for modelling stress relaxation at high temperature. Perform heat transfer and thermal stress analyses for prediction of the residual stress after PWHT, and compare the predictions with the experimental results. Perform sensitivity studies in order to determine the effect of data such as a certain material property or initial residual stress on the reliability of the residual stress prediction. Produce a set of data to be used as benchmarks for the validation of PWHT predictions for the geometries investigated. Benefits Reduced energy use by post-weld heat-treating a lower volume of material. Capability to use local PWHT where geometry or attachments do not allow the use of heated bands of standard dimensions. Validated technique available for the project s sponsors to perform analyses for residual stress prediction after PWHT. Improved safety owing to better accuracy of FEA residual stress predictions used in fracture and corrosion assessments. Reduction in maintenance costs by use of local PWHT to mitigate residual stress in repair welds.

3 Approach FEA can be used to optimise PWHT by predicting residual stresses whose magnitude may be compared to the acceptable value obtained from an ECA. In an earlier study by TWI (Bastid, 2003), FEA was used to predict the residual stresses after local PWHT of a branch-pipe connection. Elastic and elastic-visco-plastic models were analysed assuming various heated zone geometries: constant width band, variable width band, circular patch or oval patch. Predicted residual stresses were compared with criteria from standards or the literature. This work demonstrated that, although the patch heat treatment is not standardised, a significant reduction of residual stress could be obtained (to about 50% of the room temperature yield stress). However this result has not been validated by measurements of residual stress performed on a mock-up of the geometry investigated. The American code AWS.D10.10 (1999) allows the use of finite element analysis for the calculation of residual stress after local PWHT, on the condition that the analysis is well documented. The main objective of the proposed project is to design a procedure and to build a database of validated models that can be used to give confidence in the numerical predictions. Five different models will be investigated and validated. The geometry and material of each model will be agreed with the group of sponsors. It is suggested that three to four geometries would be defined, and two to three materials would be chosen. The same material would be used for all the geometries, and one or two other materials would be used for a single geometry. The mock-ups will be manufactured at TWI. The temperature will be measured near the joint during welding and residual stress will be measured by centre hole drilling (CHD) after welding. These data will then be available for validation of the preliminary models where the residual stress field from welding is predicted before local PWHT is modelled. The models of the welding stage will be analysed using materials data from the literature. Tensile strain-stress curves at high temperature will be produced if necessary. Input data will be calibrated in order to obtain good agreement between predicted and measured temperature and residual stress. Since the project will address PWHT rather than welding, perfect agreement will not be necessary at this stage of the project. However these analyses may be refined if the final stress field after local PWHT is predicted to be significantly dependent on the initial stress field after welding. Models of local PWHT will be analysed in order to predict the residual stress obtained using full PWHT, local PWHT with standard heated band geometry, and local PWHT with a number of non-standard heated band geometries for each case. For each mock-up geometry, non-standard heated bands will be investigated and predicted residual stresses will be compared with measured values. Prediction of residual stress relief depends on the relaxation behaviour of the material, when held at high temperature during the time recommended for the PWHT. In the earlier project conducted by TWI, it was found that material data for modelling relaxation are scarce. Since relaxation is modelled as creep at constant strain, creep data should be sufficient to describe the relaxation behaviour. However, creep data are not available at PWHT temperatures because materials are usually not considered for continuous use under load at such high temperature. Extrapolating data obtained at lower temperature may be a source of error because the creep mechanisms activated may vary with temperature. Relaxation tests at PWHT temperature will therefore be performed in order to accurately model the stress relief. Sensitivity studies will be performed in order to advise on the data required to predict residual stresses with sufficient accuracy. References: P. Bastid: Local post-weld-heat-treatment of branch connections, TWI Report No /02/ , AWS D10.10: Recommended practices for local heating of welds in piping and tubing, ANSI, 1999 Copyright TWI Ltd 2013

4 Deliverables Methodology for modelling local PWHT where recommendations from standards cannot be applied. Documented set of benchmark cases for prediction of stress relief by local PWHT. Evidence to support the use of FEA for modelling local PWHT for presentation to regulatory bodies and standards committees. Price and Duration The overall estimated price for the work is 600,000 (excluding VAT), which requires 40,000 per year for 3 years from each of the 5 Sponsors. The Project will commence (with a reduced scope of work) as soon as 3 Sponsors have committed funding. Further Information For further information on how a Group Sponsored Project (GSP) runs please visit: GSP Co-ordinator: Tracey Stocks gsp@twi.co.uk Project Leader: Philippe Bastid/Ruth Sanderson Philippe.bastid@twi.co.uk ruth.sanderson@twi.co.uk

5 Copyright TWI Ltd 2013