Special Metals Corp site has information on Monel alloys and welding.

The Nickel Development Institute or NiDI has a great selection of publications.
______________________________________________________

You absolutely need to remove all contamination (that is generally true for welding on high alloy materials). I would start with acetone. The middle picture appears to show the remnants of a fillet weld (is this the dissimilar joint?). The crevice below the root is going to be a potential big problem for cleaning. Just heating to a high temperature might leave a residue that is problematic for welding. Excavate deeper if necessary.
I'm not sure what the last picture is showing, other than non-metal.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

The first rule of welding is that the base metal must be clean.

Remove the existing weld that appears to be "unsound" until you have sound base metal on which to deposit "good" weld.

Depending on the chemistry and thickness of the carbon steel, you may have to preheat before welding.

Best regards - Al

It is not uncommon for cast materials, how deeper you dig (arc-air, grind, ...), the worse the material gets.
So I wouldn't focus too much to start on "sound" material.

Also, I doubt you're going to find a definitive answer on the internet. Better retain someone local who can take all secondary information into account.
"Carbon steel" can mean all kinds of things, especially in this case. Preheat is almost a certainty, how much exactly is hard to tell without further investigation.

I worked with an aircraft engine manufacturer that took the position the weld didn't have to be any better than the base metal casting with regards to soundness. Simply put, if a certain amount of porosity in the casting was acceptable, the same amount of porosity was acceptable in the weld. That being said, rarely is a crack, incomplete fusion, and other linear discontinuities with sharp end conditions permitted whether it is in the casting or the weld. The photograph depicts what is most likely a crack in the weld. That should be removed before attempting to reweld the overlay. It would be nice to know what the drawings call for and what the original casting looked like. There is too little information to provide anything other that general suggestions.

It appears they are attempting to weld over a cracked weld that has been contaminated with who knows what.

Best regards - Al

Quote (gtaw)

an aircraft engine manufacturer that took the position the weld didn't have to be any better than the base metal casting with regards to soundness.

By what great leap of insight did they come to this conclusion? Welds typically see far higher stresses than the bulk base material by dint of their being located at joints (duh!). Who in their right mind plans to put defects in welds?

Please supply all the serial numbers of aircraft made with this philosophy so that I can warn my flying friends and colleagues...

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

ironic,

I don't know if it is correct for connecting machine parts , but seems the steel design standards always suggest to match the weld strength to the metal's yield strength to preserve ductile behavior. But in no way a build-in defect shall be tolerated, if it is avoidable. Please comment, if I am wrong.

Who knows with regards to castings the decision was made by people way above my pay grade. However, there's no such thing as a perfect weld. Even if the weld is "defect" free, the weld itself is going to have different properties than the base metal being joined. Consider the chemistry is typically different, the grain size is different, and the mechanical properties are going to be different.

As for "built in defect", you might consider what the effect of the faying surface where two members meet has on the integrity of a fillet or the unfused root of a partial joint penetration groove weld. We allow the use of both types of weld details for static loads and try to avoid them where there are cyclic loads.

Best regards - Al

I am lost as to which part(s) of the cover the close-up shots are from.

The third shot, showing a rough machined radius with spalls revealing the parent steel material or just incomplete machining next to end of a circumferential crack really has me scratching my head as to its location.

Do you have the body of the pump? What is it made of, and are the pilot bores cut directly in the parent material?

Is it possible the "monel welding on registers" was done as a repair, with the weld material chosen to resist crevice corrosion or some such ?

I picture the pilots being for location, so not very structural, at all. In that case the task of the weld repair is to not provide crevices, although I'm guessing there have to be chamfers on pump body pilots, forming Crevice Las Vegas right beside the monel treated rings.

Thank you all for your response. We were able to machine welded surfaces, get to the clean surface and weld the areas.

I have another important question. After welding and after PWHT, We checked the hardness on welded and non-welded surfaces and we are getting 20 RC-30 RC values.

Is this acceptable hardness range? I am not sure why hardness was check on first place. Is there any industrial procedure or welding procedure that states acceptable hardness ranges for monel weld on carbon steel base material??

OK I never welded this material but is what I do know.

what are the engineering and quality requirements?
was there a authorize repair procedure from engineering?
was there a schedule from a weld engineer.
what are the part engineering heat treat hardness requirements?
you don't guess, and asking here for internet to tell you is foolish.
how do you heat treat a part and not know the requirements!

what is the part fit, form or function?
hopefully this is not safety issue? where it will endanger some ones life.

Be careful about your choice of cleaning agents on materials to be welded -


https://www.thefabricator.com/thewelder/article/arcwelding/cleaning-material-with-brake-cleaner-before-welding-a-risky-move-that-can-turn-deadly