You won't get anywhere near the claimed amperage output of those cheap migs, they are for mot bodge welding not for fabrication work. Just because a weld looks ok doesn't mean it has penetrated the steel, it's more likely it's a cold weld just sat on the top.
You have to take into consideration the plate and tube thickness when welding, not just a bit of 3mm plate etc, as two pieces of 3mm surprise surprise becomes 6mm when welding so those cheap n cheerful migs are not up to it.
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