I have to admit that I haven't seen this problem before, so here is some general information that might prompt you (onsite) to identify the source of the problem.

A few items to check:
-    What gas is being used? Generally, 100% Argon is used with GTAW of Alloy20 (20Cb3)
-    Eliminate any contamination from grinders, i.e., get new grinding wheels (Aluminum Oxide not Silicon Carbide).
-    Use a large cup to dispense the gas over the area.
-       Use an Argon Purge on the backside of the weld.
-    Consider using E320LR  (Low Residuals).
-    Cleanliness is important, do not use crayons to mark.

Here is a link to the Technical Data BLUE SHEET from Allegheny Ludlum Corporation  Pittsburgh, PA for Alloy 20 http://www.alleghenyludlum.com/ludlum/pages/products/pdf/al20.pdf

Thank you for your reply.  We have been doing everything you have mentioned above except using the low residual filler material.  I think that the low residual filler material helps with the hot cracking situations.  I have also already read the Blue Sheet on this material and I could not find anything pertaining to this.  I only notice this on small diameter thin sections of pipe not on the thicker sections which leads me to believe it is caused by heat but still not sure.  Thank you very much for your suggestions.

Here is a few other ideas. You have me stumped. I have used the alloy many times, from small diameter sch 5 pipe up through 12" sch 120 pipe without problems except for the occasional crater crack.

- Nickel Oxide is black in color. Could the welder be using too much Gas and creating turbulence, hence pulling oxygen into the weld pool?  
- Does the Black material react to Nitric acid? (Check for Iron)

- Have you carefully ground out the defect. Is it all on the surface is dispersed throughout?

- If you can get a sample of the black material is it magnetic?

- Take a sample of the black material and put in in a small vial add household ammonia (small amount), shake, doe the ammonia change color? what color? If it turns blue (Royal Blue)it is probally a copper compound.

I wish I had a better answer. It concerns me that it is forming in the weld puddle.  

Rich

If the "trash" is visible only after cooling, It may be excessive heat input.

If it forms while welding I'd think it is foreign material.

3/4" Schedule 10 would be pretty tough to weld and control heat input.

Have a good day

G Austin

I realy appreciate your help guys!  Rich I will try your ideas they are great!  I will check the gas flow on the tig torch it should not be anymore than about thirty or so but I will check it out.  G.Austin I can see it in the puddle as I am welding so I am still stumped.  Everything is extremely clean, I even sand my filler wire, and your right it is extremely hard to control heat input on the very small diameters.  It is not a major problem just a little aggravating, I am a bit of a perfectionist. If you all can think of anything else let me know.  Once again thank you all very much for your time and knowledge.

Sincerely
Walt

Do you wipe off the wire after sanding. The silica could be a cause.

G Austin

Try using a different heat number of filler and if that doesn't change the condition, change the heat number of the pipe.

Seldom
"There's no such thing as a welding problem, just welding puzzles of assorted sizes!"

Oops, I forgot to mention this in my previous reply as a possibility.  

Your description of "black" specks could be construed as a chrome oxide that exhibits itself on the puddle surface in the P4x's when using too high of heat input.  I've observed & experienced this phenomenon and have been actually able to move the oxide around on the molten puddle similar to a jumble of sticks with the tip of my filler rod.  Though when the puddle is solidified, I personally can't describe the condition as exactly "black specks" but I could understand why somebody might describe them as such.

Seldom
"There's no such thing as a welding problem, just welding puzzles of assorted sizes!"