Years ago an associate brought a bicycle over to my house and asked if I could weld the set post part of the frame where there was a crack. I brushed it and washed it down with acetone and took a shot at it with my little flux core welder. It didn't touch it. No matter what I tried it looked like it had been some weld spatter on it, but I got no penetration at all. It wasn't thick tube. I hit it with as much heat as the little welder would put out. It wasn't a duty cycle issue. That welder would always give me about an inch of good weld on anything I'ld used it on before. Now I admit really knew nothing about welding back then (18-19 years ago) and I don't know much more now, but it baffled me. I am asking purely for speculative reasons now. That person and their broken bicycle have long since moved on, but what do you think the reason was?
It looked like a cheap noname Chinese bicycle which back then probably meant soft high lead steel, but I suppose it could have been Chromally tube. If I recall it had mediocre looking welds at all the joins. I recall thinking my weld wouldn't look to horrible next to the factory welds. Well it wouldn't if I could have made any welds. LOL.