Welding against water

By bluenose Date 03-09-2008 00:08

Note: I posted this in the "General" area and have reposted here as I think it is a better fit.

The topic has come up over the years about performing welding with water on the other side of the weld. For instance, welding on the underwater section of a ship hull, the outside of a water tank or even "padding" a boiler tube with the boiler full of water.
My thinking is that the water would provide a heat sink that would cause severe quenching of the weld and cause cracking.
Welding 2 1/4 chrome superheater tubes containing water came up as a question recently. The welding on the outside of the tube was used to restore the thickness of a worn area. My opinion was that the min preheat of 300 F was impossible to maintain with 70 F water inside the tube.
If the tube was welded under these conditions what kind of damage would be expected? Cracking?

By makeithot (***)

Date 03-11-2008 16:15

I do not know how you would accomplish this in regards to tube welding but when doing weld repairs on a ship a coffer dam is set over the weld area on the outside of the ship (This requires a crew of divers) this then protects the weld area from quenching and cracking. This is dependent on accesabilty from the inside of the ship, They may choose to weld in the wet or build a habitat depending on which class they are trying to pass and weld all in the dry from outside the vessel ( again this requires a crew of divers,but this crew needs to know how to weld) all welding is done inside the habitat allowed to cool then the habitat is flooded and removed. This technique produces class-A repairs and may or may not require back gaugeing from the inside depending on material thickness etc and weather you were able to fit backing strips ceramic or other wise.

By 357max

Date 03-11-2008 16:33

Its quite simple to wrap induction heating wraps to bring the pipe to temperature and hold that temperature.

By 803056

Date 03-12-2008 12:41 Edited 03-12-2008 12:48

I would expect any metal with a high CE, that are not austenitic, or heat treatable to be difficult to weld with water or any other quenchant on the opposite side of any material that requires substantial preheat.

Any means of preheating will only boil the water on the opposite side of the weld and be replaced by cooler water. Convection is going to create a flow of water that will continually cool the area being heated.

Pipelines that allow for hot taps, steel used for ship hulls, etc. usually have relatively low CE or are relatively thin and don't usually require preheat. When that isn't the case, the welding involved is typically a temporary repair. In wet welding, austenitic filler metals are often used with carbon steels to accommodate any hydrogen which has higher solubility in face centered cubic crystalline structures than in body centered cubic structures.

Wet welding has inherent problems that are well recognized by industries that use the technique.

best regards - Al