Underwater Cutting and Welding

This blueprint combines electricity, a diver and an explosive gas. Each is manageable; the combination demands a plan.

1. Electric shock. Use DC only, never AC — AC causes muscle tetany, so a diver cannot let go. Use a knife switch on the surface that is CLOSED only while the diver calls for current and opened the instant they stop.
2. Hydrogen and oxygen accumulation. Never cut or weld upward into an enclosed space, under an overhang, or inside a compartment where gas can collect. Vent the space first.
3. Decompression. The work is at depth and takes time; the diver is on a schedule. Read the Haldane blueprint in this batch before planning any of it.
4. Insulated gloves, dry suit where possible, and never a bare hand on the electrode holder.

The gas is the hazard people underestimate — the arc splits water continuously, and hydrogen collecting in a closed-topped space has caused fatal explosions in salvage work for a century.

Understand why the arc does not simply go out, because the mechanism explains every technique that follows.

1. Strike an arc under water in a shallow tank, observing from a safe distance behind a shield.
2. Film it and look at the region immediately around the arc.
3. Note the gas rising continuously from the work.

The arc vaporises water instantly and burns inside a bubble of its own steam and dissociated gas, so it is not actually in contact with liquid water at all. The bubble is continuously created and continuously collapsing, which is why the arc is noisier and less stable than in air.

The gas rising is not steam alone. The arc dissociates water into hydrogen and oxygen — the arc is running a small electrolysis cell as a side effect, and that is where the explosion hazard comes from.

Cutting underwater does not use the arc's heat to remove metal. It uses the arc to start a chemical reaction.

1. Set up a tubular cutting electrode with an oxygen line running through its centre.
2. Strike the arc to bring the steel to bright heat at one spot.
3. Open the oxygen valve and observe what happens.
4. Cut a plate and examine the kerf and the slag.

Once steel is hot enough, oxygen makes it BURN — and the reaction is strongly exothermic, so it supplies most of the energy for the cut itself. The arc is only the match; the oxygen jet does the work and simultaneously blows the molten oxide out of the kerf.

That is why oxy-arc cuts thick steel underwater far faster than any purely electrical process could. It is also why it only works on steel: aluminium and stainless form refractory oxides that do not burn away cleanly.

A weld made underwater is a compromise, and it is worth knowing exactly which one.

1. Make a test weld in air and another underwater in the same steel.
2. Section both, polish and etch them, and compare the grain structure near the fusion line.
3. Bend-test coupons from each to failure.

The wet weld cools far faster — water is an aggressive quench — producing a hard, brittle martensitic zone, and it picks up hydrogen from the dissociated water, which causes cracking days after the work is finished. Fast cooling plus dissolved hydrogen is the classic recipe for hydrogen embrittlement.

So wet welding is used for what it is good at: temporary repair, salvage patches, and structures where a brittle weld is acceptable because it is holding rather than carrying.

Where a real structural weld is needed, the answer is to remove the water: a habitat is sealed around the joint and the water blown out with gas, so the welder works dry at ambient pressure. The weld is then nearly as good as a surface weld, at enormous cost in setup.