CLOSING OF CIRCUMFERENTIAL WELDS OF THICK STEELS

The accomplishment of the circumferential closed welds in thick-walled cylindrical shells encounters specific difficulties connected with a setting-up of the conditions at the weld closing, as there exists a danger of the defect initiation at this region. The E.O. Paton Welding Institute has developed the technique and technology of EBW of up 150 mm thick steels, which provides the high quality of the welded joints not only in the linear part of the welds, but, what is especially important, in the region of their overlapping and crater run - off. Figure 1 shows the longitudinal and transvers macrosections of the weld closing region in EBW of 150 mm thick steel. During development of the circumferential weld closing technique the new special procedures were used to prevent the defect formation due to decrease in the effect of perturbation factors on the weld formation and increase in the welding pool stability.

Fig.1. The longitudinal and transverse macrosections of the weld closen region in electron beam welding of 150 mm thick steel.

 BIREFRINGENCE SCANNING

Application of the birefringence scanning of beam (1) became an additional means for the weld shape control (Figure 2).

Fig. 2. Scheme of electron beam scanning a) conventional scanning with one deflection system, b) birefringence scanning (with use of two deflection systems), A-scanning amplitude.

Scanning is performed by two electromagnetic deflection systems MN and PQ located one after another along the gun axis . By changing the ratio between the currents in the deflection systems it is possible to control the position of the deflection centre, namely: to immerse into the welding pool depth, to scan in height with respect to the workpiece (2), to change the scanning amplitude and, due to this, to control the weld shape.

 EBW PROCESS CONTROL

Taking into consideration the complexity in EBW process control at the region of the welding conditions setting- up, a software and hardware were developed by using a processor being compatible with IBM-PC/AT.

When programming the EBW conditions at the fade-out region, the operator divides all the length of the region into separate areas, sets the coordinates of points of the beginning and end of the regions into the program, indicates the currents of a beam, focusing lens, welding speed and other condition parameters. In the process of welding the parameters are displayed on the monitor in the form of graphs in a real time (Figure 3).