Transient and Residual Stresses in Multi-Pass Welded Butt Joints of 15cm Thick Plate

Abstract

When thick plates are connected, it is usual to apply a multi-pass welding process. Although there have been many studies reported on weld cracks in multi-pass welds, it is very difficult to find information concerning detailed mechanical behavior of multi-pass welded joints in the entire course of welding. The main reason can be attributed to difficulties encountered in both theoretical analysis and experimentation.
Since one of the authors has developed an analytical method of thermal elastic-plastic problems solving based on the finite element method, the difficulties in theoretical analysis were fundamentally resolved and the application of this method has provided useful information on the mechanical behaviors of weld joints of many types.
In this paper, an investigation was carried out into welding stresses and strains induced in multipass welds in a research model (total 167 passes). The theoretical analysis was performed by the developed method, in which changes in the mechanical properties due to thermal cycles can be accounted with consideration of metallugical changes. An experiment was also conducted to be compared with the theoretical analysis.
From the results of the investigation, the following important information was obtained.
(1) The residual stresses predicted by the theoretical analysis coincide well with those obtained by the experiment. This implies that the analytical method applied is reliable even for this kind of very complicated problem, and can be expected to provide information which would not be readily obtained by experiment.
(2) The highest transient value of σ_x always appears below the surface of the latest layer, where it is comparatively small.
(3) In the distribution of residual stress, σ_x, the location of the highest stress is several layers below the finishing bead in the midsection, and is about 20 mm away from the toe of the bead on the surface.
(4) Delayed cracking will initiate below the finishing bead and propagate to the surface.
This new information is very useful to discuss the conventional construction procedure of structures of thick plates, such as pressure vessels, including prevention of weld cracking, meaning of intermediate annealing, etc.