Abstract
Welding is an important process used during the construction and maintenance of nuclear reactor components. Welding results in residual stresses, distortions and microstructural changes in the joined components, which can have significant and deleterious effects on their in-service performance. It is thus crucial for engineers to effectively predict these effects.Ferritic steels undergo solid-state phase transformations (SSPT) during heating and cooling, thus making welding simulation challenging. The strains associated with SSPTs can also cause transformation-induced plasticity. The significance of transformation plasticity for single-pass, autogenous welding of a thick component is the subject of this paper.Electron beam (EB) welding was the technique chosen to weld 30-mm thick ferritic steel plates using a single pass. The welded plates were instrumented with thermocouple arrays, to capture the far-field and near-field thermal transients on the top and bottom surfaces during welding and the cooling down process. Welding distortions were subsequently measured using laser profilometry. Distributions of the developed residual stresses were measured using the neutron diffraction (ND) method.Numerical finite element analysis (FEA) was used to simulate the welding process. After calibrating the thermal solution using thermocouple data, mechanical analysis was conducted using three different approaches: (i) taking account of anisothermal SSPT kinetics with transformation plasticity; (ii) taking account of anisothermal SSPT kinetics without transformation plasticity; and (iii) assuming isothermal SSPT kinetics. The predicted residual stresses and structural distortions are compared to the experimental data, thus assessing the influence of different SSPT phenomena.
Highlights
SA508 is a ferritic, low alloy steel commonly used in nuclear components with SA508 Gr.3 being mainly used in Pressurized Water Reactors (PWR)
When only isothermal solid-state phase transformation (SSPT) kinetics are implicitly considered (Case 3), the tensile weld residual stresses (WRS) from the heat affected zone (HAZ) extends into the fusion zone, such that these regions are indistinguishable from each other
The distribution appears to be smoother in the SSPT+Transformation Plasticity (TP) case, whereas the variation of longitudinal stresses is more intense in the case of SSPT-TP model, as stresses rapidly vary from highly negative to highly positive over a narrow region of the fusion zone and HAZ
Summary
SA508 is a ferritic, low alloy steel commonly used in nuclear components with SA508 Gr. being mainly used in Pressurized Water Reactors (PWR). The welding methods currently being used to weld these components are based on arc welding processes such as manual metal arc (MMA) welding, submerged arc welding (SAW) and gas tungsten arc welding (GTAW). Traditional arc welding, even in the case of narrow groove (NG) arc welding, involves multiple passes as well as the addition of filler metal. Electron beam (EB) welding, on the other hand, is autogenous and can be a single pass process. EB welding performed under vacuum is called Reduced Pressure Electron Beam (RPEB) welding, which has the advantage of no oxidation (Duffy (2014)). A single pass process can lead to significantly higher productivity, since multiple days of welding could be replaced by a several-hour long procedure
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