Water works

Welding high-strength steels is never a simple task. But welding underwater, or ‘wet welding’, makes the job even more difficult.

Defence support specialist DML, based at the Royal Navy’s Devonport dockyard, has now succeeded in wetwelding two high-strength steel grades used in submarines, and believes the technique can be extended to other underwater structures such as pipelines and oil rigs.

But wet welding isn’t just welding underwater — it’s manual arc welding without anything to protect the metal, such as a hyperbaric chamber.

DML has been working on the problem because of the need to decommission nuclear submarines. Wet welding techniques allow this to be done with the submarine in the water, removing the need for dry-docking.

To keep the submarines afloat during the procedure, blanking plates have to be welded on to the aperture of the subs’ ballast tanks to seal them, and that means that high-strength, low-alloy steels have to be welded together.

There are two particular grades of steel used for these applications — Q1N and QT35 — which although extremely tough, and with high-tensile and yield strength, are vulnerable to hydrogen-induced cracking.

The high temperature conditions of welding split water molecules into their constituent hydrogen and oxygen, and the structure of the steel allows hydrogen to penetrate the metal — the higher the temperature, the faster the migration.

Once inside the structure, the hydrogen can become trapped and cause cracks.

‘Welding materials such as these is much more challenging than normal steels,’ said DML’s welding design manager Barry Richards.

‘The chemical composition of the consumable is far more complex, and control of the arc is critical — and highly demanding — especially when the work is being undertaken in diving kit, overhead, in depths of about seven metres.’

DML’s first breakthrough came late last year, with the development of a specialised underwater welding electrode with a high nickel content.

Using this created a weld with an austenitic structure — a very ductile and strong form of steel which is not vulnerable to hydrogen cracking. This, along with two coatings — one to stabilise and control the arc, the other to prevent electrochemical corrosion — allowed Q1N to be fillet welded to a strength which matched the structural integrity of structures above the water.

QT35 is even more awkward than Q1N, however, due to its high level of phosphorus and sulphur. These do not form part of the metal’s solid solution structure and leave grain boundaries where the hydrogen can penetrate, making it even more vulnerable to hydrogen cracking, said Richards.

The technique has now been fully qualified for both grades, according to the American National Standards Institute and American Welding Society specifications for underwater welding, and has also been approved and verified by Lloyd’s Register.

‘This means that we can weld attachments without having to dock down the submarine, which represents significant cost-savings,’ said Richards. The technology can also be developed for other industries.

With high-strength steels used in the oil and gas industry, such as oil rigs, floating production platforms, terminals, jetties and pipework, the sea is the limit.