Pinholes in certain circumstances?

[AthlonAJ]

Howdy all, been stick welding for years and just recently switched to a MIG. My setup is a Miller Challenger 172, using arg/co2 mix and .030 wire. The problem I've been having is that certain welds pinhole, mainly when endcapping something like tube or butt weld two pieces where I cannot access the backside for cleaning. You really don't see any problem until you do some grinding then it's obvious. Would this be from contaminates on the inside of these pieces heating up from welding causing the problem?

Welding in any other situation the welds are just fine, it's only when doing these endcaps that it shows up. I always clean the metal and endpiece, but it's not always possible to clean the other side. Any help would be greatly appreciated, get's old quick having to grind it all down and reweld.

[Mike Sherman]

You have pretty much diagnosed your problem already. Porosity is generally caused by contamination that has turned to a gas and is trying to escape the weld. Three things you can do here: Clean the inside first, try a preheat to destroy the contamination before welding, drill a small weep hole in the tube so the gas has another avenue of escape. Slower welding and a weld puddle that stays wet longer, will also give the gas more opportunity to escape.

[AthlonAJ]

Thanks! Probably just have to preheat it really good and burn it all off, sometimes it's in areas where are not accessible for a good cleaning.