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http://www.gullco.com/products/katbak/katbak.htm
Are you using the 1G20-RD backing? On the site it says...
"These tiles are for similar uses as the 1G6-RD but for thicker plates up to 15mm thick",
and you stated you have joints up to 100mm thick and the 38mm plate that you tried has centerline cracked.
Maybe you need to try the triangular backing as suggested earlier and adjust your groove angles to allow for the correct width to depth ratio.
Here is what the site had to say about the triangular backing.....
"All 1G33 tiles are suitable for K or X preps where the bevel angles are 45, 60 or 90 degrees. Also suitable for welding with M.I.G./M.A.G. solid wire, metal cored or flux cored wires. They are a substitute for round tiles where the benefit of the 1G33 range of tiles is a "full-face" contact with the joint preparation... reducing the risk of burn-through as may occur with the round tiles where you have a single point contact of the tile with the work piece. 1G33-90 also used when reverse side of a single bevel, single V butt joint where the reverse angle will always be 90°."
John Wright