Welding White Cast Iron in Sulphuric Acid Service

[JonD71]

We have a submerged pump casing with a minor pin hole through a web section. The casing is submerged and in hot sulphuric acid service.

I have spoken with the manufacturer who is not keen to advise on the material propoerties other than it is made of a white cast iron. They have also advised that the material is "unweldable" which I find hard to believe.

As the pump casing is only four years old and the defect appears to be from a localised alloying concentration, as the remaining pump casing seems fine, I am keen to try a localised weld repair. Does anybody have any advise on suitable weld methods, consumables and welding parameter control etc.

 

[arunmrao]

My guess is that it is a high chrome iron containing about 27% Cr and 2.5-3% C. This is a popular alloy for such application. If you have access to a portable spectrometer you can get the chemistry.

This alloy is termed nonweldable. However, if you insist on trying to fill up this pinhole defect,you may try a high Ni welding rod. This will fill the pinhole but the properties will differ as you are filling with a soft material. There will be no fear of cracking etc.Alternately putties are available with different grades of filler metals which you can try. Hope this helps.

 

[PUMPDESIGNER]

I do no information on your pressure or temperature, but the following is an alternative method.

Drill and tap the hole to 1/8" pipe thread.
Install a threaded plug.
If temperature is too high then plastic is not suitable as a plug. I am not sure what alloy plug would be suitable, but 1/8" pipe plugs can be obtained in many alloys and one could easily be custom made from the correct alloy, even the identical white iron alloy.


PUMPDESIGNER

 

[PUMPDESIGNER]

Mindless me,
Does the pump manufacturer use or have 1/8" plugs or perhaps 1/4" plugs available in that identical alloy that perhaps they use in their manufacturing?

PUMPDESIGNER

 

[arunmrao]

It is very difficult to drill a hole in a white iron casting. hence this possibility is ruled out. In a white iron casting drill and tap operation is performed by casting ms inserts insitu. It is only in these regions drilling is performed in the rest of the region only facing is done on a slow speed lathe.

 

[macmil]

If indeed your pump case is a high chrome iron, it is not suitable for welding. While you may fill the hole, you may generate a crack in the heat affected zone. I would offer two possibilities; one is to use a trowelable epoxy that resists your hot sulphuric acid (if available ); the other is to drill the pinhole to a round and plug it with a bolt with washers inside and out and nut on the outside, assuming you have room, again made of appropriate material either plastic or metal and bedded in an epoxy. I have used both methods successfully but not in hot sulphuric service.

 

[RunnerMan]

Whatever you decide to do, it might be advisable to have the defect and surrounding area radiographed - this might just be the tip of the iceberg.