1974 Volume 43 Issue 12 Pages 1183-1193
In the previous paper. it was fully clarified that the electron beam welding process where the metal is inserted at the joint to be welded, that is, "Insert-type" electron beam welding process has a significant effect on "Uranami Welding", that is, root pass welding with smooth back bead and that the metallurgical and mechanical properties of dissimilar metal joints of 2 1/4Cr-IMo steel and austenitic stainless steel can be remarkably improved by the "Insert-type" electron beam welding.
This paper discusses the problems involved when the "Insert-type" electron beam welding is employed for "Uranami Welding" of boiler tube, especially, those factors controlling be external and internal weld bead on the overlapped section between start and stop ends of the bead in circumferential welding and results of investigations on the joint characteristics of the welded section.
Conclusions obtained may be summarised as follows;
1) Defects noted at the weld bead of the overlapped section are concavities at the external weld bead; those found at the internal weld bead are concavities, excessive reinforcements and incomplete penetrations.
2) Both the Ib' value, which is an assumed value of beam current being determined geometrically by the increase rate of beam current in starting a weld, that is, upslope rate, and the decrease rate of beam current in finishing a weld, that is, downslope rate and the downslope rate influence the defects at the internal weld bead of the overlapped section. Incomplete penetration appears invariably when the Ib' value is low, and an excessive reinforcement is apt too ccur when the Ib' value is too high. These defects do not ocur if the Ib'value is within a proper range. This proper Ib', value increases with an increase in wall thickness. Excessive reinforcement and concavity are apt to appear with an increase of downslope rate, even if the Ib' value lies in the proper range.
3) Only the downslope rate influences singnificantly the extenal weld bead appearance of the overlapped section. 7.5mA/cm may be defined as the critical downslope rate regardless of the wall thickness of pipe used. Therefore, at the external weld bead of the overlapped section, the concavity defect appears intensively when the downslope rate is higher than the critical value.
4) A starting point of downslope does not influence the internal weld bead appearance if the Ib' value is proper.
5) Metallurigcal and mechanical proerties of the welded joint are quite satisfactory.
