Product Applications – Ship Building 1
KAT Oscillator Multi-Pass Welding Operations in Shipbuilding Case Study
Product Application File – Kat Welding Oscillation In Shipbuilding Environment
Large section of a ship being joined with Kat Oscillation.
Welding carried out from the outside using a Kat Oscillator and flux cored welding wire in the overhead (PE) position
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It has long been considered that the most productive and cost effective welding position within the shipbuilding industry is the flat position (PA). To satisfy this, great emphasis is put in at the design stage to maximize the use of this position. However. Even when adopting this philosophy, welding has to be carried out in a variety of other positions. Many shipyards have found major benefits when welding a specific section of the ship hull in the overhead position (PE).
Carbon steel to carbon steel link ups.
Large block sections of the ship are welded together in the block building strategy. On each section there are a large number of webs, girders and longitudinal connections to be made, in addition to the main hull connection. The bottom shell of the hill has traditionally been welded from the inside using a single V preparation, 60 inclusive angle 6mm gap feather edge with flat ceramic tiles on the outside to produce a single sided weld. This weld requires no back gouging treatment, and for a time was considered to be the most effective way if completing the connection. However, there were disadvantages such as:
- Welding access,
- Obstructions from webs, girders, etc
- Potentially higher defect levels on multi pass runs passing through access gaps.
The alternative was to weld the connection from the outside, where a free run with minimum restrictions was available. This required reversing the preparation discussed earlier and welding the bottom shell of the ship in the overhead position. To achieve this the following was carried out.
Joint rooted from the inside against a round ceramic tile
Welding in the overhead position using the Kat Oscillator and flux cored wire
Oscillator welding process in the operation joining 316LN stainless steel. This resulted in very high quality welds with minimum surface discontinuities. Furthermore, following the welding operation the Kat Oscillator track could be used for mounting an X-ray head for NDT inspection purposes
Also radiography results are consistently better thank the previous practice. These benefits could not have been achieved without the following key elements
- A reliable oscillation system, and
- A high integrity flux cored wire
To date the Kat Oscillating System has been found to be extremely reliable in the variety of positions and environments within the shipyard. Two flux cored wires are being used, both are of Japanese origin, one is copper coated seamless wire and the other is a more traditional seamed wire. Both wires are E71T-1 type and can be used in all welding positions. This practice has developed from extensive experience gained elsewhere in the shipyard on carbon and stainless steel.
Stainless steel to stainless steel corrugated bulkheads
To date, four chemicals carriers have been built. The cargo tanks are constructed from 316LN stainless steel, and are made up from a series of corrugated bulkheads. The weld profile has to be as smooth as possible to remove potential sites for cargo retention. The oscillator welding process was put into operation, and resulted in extremely high quality welds with minimum surface discontinuities. As the joint to be welded was fully tracked, welds were produced without any intermediate stop/starts which further reduced the potential for discontinuities. As in the case of mild steel, the stainless steel FCAW wire was of Japanese origin.
The amount of potential back gouging was reduced by welding against a ceramic backing tile. On plate thickness of approximately 34mm, a 2/3 to 1/3 preparation was used with a round ceramic tile placed in the groove. It was found that a larger gap was required for stainless steel than for carbon steel, due to contraction effects. A gap of 8mm was used with the root run being applied with the oscillator. When the round tile was removed a good concave profile was obtained, which only required wire brushing prior to second side welding.
The use of the Kat Oscillator within shipbuilding has been increased significantly, and at present, procedures are available for almost all general welding positions. This includes the horizontal position (PC), which in the past has been particularly problematic, especially when rooting onto a ceramic backing tile. A further area being considered is the use of the Oscillator track to double up as an aid to radiography inspection. The X-ray head could be mounted onto the carriage and moved into position for shooting as required.