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Advanced narrow gap welding technology for thick walled components

Conventional submerged arc welding (SAW) of V and U-groove welds (edge angles up to 10 deg) has proven itself over the years and was the most frequently used welding procedure for thick walled nuclear primary components. With constantly increasing wall thicknesses the narrow-gap SAW (NGW) with parallel weld edges gradually replaced the open angle weld shapes and became the standard welding technology for circumferential joints.

Hardly any design modifications have been necessary when introducing narrow-gap SAW to replace conventional SAW, because only the edge angle needed to be reduced, without having to change the weld-root buildup.

This approach is particularly useful in case when quality requirements allow modifications of edge angles without needing expensive and time consuming additional WPQT.

Experience with conventional SAW processes (edge angles up to 10 deg) indicates that very precise bead build-up on the seam edges is essential for attaining in the base metal of an uniform HAZ with reduced coarse-grain fractions. 

This is made easier when welding at a vertical edge without wire (electrode) deflection. This basic condition of the narrow-gap SAW, combined with a suitable weld pool geometry and heat input can completely and systematically eliminate all coarse-grain fractions by the reheating effects of subsequent weld seams.

A disadvantage of NGW is the more difficult repair of weld defects during welding (e.g. side wall lack of fusion), especially when they occur in deeper regions of the gap. For such cases, a special repair concept had to be developed and qualified, consisting of a preparing step followed by the re-welding of the affected place.

For the NGW of the mentioned MnMoNi reactor steel standard welding conditions have been developed.

Two weld beads per layer are used for thicknesses up to 450 mm, and three weld beads per layer for thicknesses  exeeding 450 mm (Figs 1 and 2).

Three weld beads per layer have been necessary for example for welding a 670 mm thick joint made between the dome section and the flange at the closure head of the hitherto largest reactor pressure vessel - "Atucha 2".

The macrograph made from the WPQT of this application can be seen in     Fig. 2a. In Fig. 2b is reproduced the closure-head design and the weld-joint configuration. 

The weld root - detail "A" -  has been placed in a pocket provided additionally by shape welding and machined down along the broken curve after heat treatment.

All parts of the "Atucha 2" RPV were forged in Japan.





 Fig. 1


Fig. 2a


Fig. 2b

Fig. 3 shows an overall view of the welding installation during the welding of this application. The location of the milling unit developed for accompanying repairs is indicated by a green arrow. 

On the same platform were installed:

- The narrow-gap welding unit,

- the automatic height control,

- the auxiliary equipment for flux supply,

- vacuum flux recovery,

- slag break-down,

- slag removal by suction, and 

- the online television monitoring. 

Some of this auxiliary equipment was developed and supplied by the client especially for performing this circumferential weld.

Fig. 3

The essential narrow gap welding equipment shown in Fig. 4 was developed by GHH, and was installed on a welding boom. The wire straightener comprising several modules is seen on the top. This modules are infinitely variable, thus enabling a precise alignment of the wire. 

Immediately below there is a weaving unit designed such that the welding torch positions left-right (for two-bead welds) or left right center (for three-bead welds) are obtained through a slight rotation (with a catch)  crosswise to the direction of welding.

The welding head rolling along the weld edges is kept in position by means of adjustable spring tension.

The welding head, itself, comprises an energized and a de-energized part. The welding wire is guided at the de-energized part without changing the distance to the weld edge. The energized part is also kept pressed to the welding wire by spring tension. This compensates the wear of the contact jaw and brings about a satisfactory current transfer.

Fig. 4

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