Don't worry about it..
Everything in the differential/axle tubes is splash lubricated (or a sealed bearing). Small amounts of carbon flaking are just going to fall, sink in the rear lube and lay there for all eternity.. (or till you change your
oil .. if .. they even washed down to the center section) Even if the carbon got in a bearing/gear it would disintegrate and wash out with the
oil.. Weld those puppies and keep on gettin' it!
Dave