The impact of transformation plasticity on the electron beam welding of thick-section ferritic steel components

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.