For technical purposes you can use 18 karat solders to join 22k.
Of course it is more apparent when the seam is not hidden, e.g.
on the back of the shank when sizing. You are allowed a certian
amount of solder in your jewelry according to the US stamping
laws, and it doesn’t need to be the same karat as the piece you
are making. In other words, you can use 14K on an 18 karat
piece as long as the amount of solder used is within the
tolerance allowed by law. The amount of solder you are allowed
in your jewelry will vary if you want to stamp your piece with a
P (18KP), denoting the piece is plumb (75% for 18K and 58.33%
for 14K, some jewelers stamp thier pieces 585 when creating
pieces using a higher percentage of gold). For exact tolerences
of solder allowed contact the Federal Trade Commission for a
copy of the stamping law. I believe you can also get a copy of
the most recent guidelines from the Jewelers Vigilance