EFFECT OF PRE-HEAT TREATMENT ON HYDROGEN CONCENTRATION BEHAVIOR OF y - GROOVED WELD JOINT BASED ON A COUPLED ANALYSIS OF HEAT TRANSFER - THERMAL STRESS - HYDROGEN DIFFUSION

摘要

Hydrogen induced cracking occurs at the welded position of the structure due to concentration of hydrogen during cooling process of welding. In order to prevent the hydrogen induced cracking, Pre-Heat Treatment (PHT) is conducted. However, since PHT takes high cost, it is important to find out the suitable PHT condition based on computational mechanics. One of authors has been proposed α multiplication method which magnifies the hydrogen driving term in the diffusion equation to find out detailed behaviors of hydrogen concentration around a local stress field. In this study, in order to clarify the effect of PHT on hydrogen diffusion and concentration behaviors, a coupled analysis of heat transfer - thermal stress - hydrogen diffusion combining with α multiplication method was conducted for the model of y-grooved weld joint under various PHT conditions. This analytical method is as follows. At first, heat transfer analysis was conducted by finite difference method (FDM). And, temperature at each grid obtained by heat transfer analysis was interpolated to each node for thermal stress analysis by the finite element method (FEM). Then, thermal stress was calculated for each node using the interpolated temperature. After that, thermal stress obtained by this analysis was interpolated to each grid point for analysis of hydrogen diffusion by FDM. Using the interpolated thermal stress, stress driven hydrogen diffusion analysis was performed. By conducting sequentially these calculations mentioned above, hydrogen diffusion and concentration behaviors during cooling process were analyzed. The temperature of weld metal was 1500°C. And at initial state, hydrogen was introduced in weld metal. Thermal stress analysis was conducted under plane strain condition. As a result, hydrogen diffusion and concentration behaviour at weld joint during cooling process was found to be typical at the site of maximum hydrostatic stress and to be affected not only the gradient of hydrostatic stress but also the gradient of diffusion coefficient induced by temperature distribution.