Good run, but come apart?

[W.olly]

I have been welding for a while now, years, but i have never come across this, i am welding 25mmx10mmx1mm box section to make a frame, i am unsure of the the of steel, t is mild though as it is rusting.

Now i do spot welds which just dont hold to both surface and also a run which looks good but when i put it under pressure it comes apart and leaves one surface looking as if it hasnt been touched except for the heat marks.

Am i setting up wrong or could it be the steel, even my welding could be going all wrong

I will get a pic asap.

please help as i have a big job on for this steel soon.

 

[mike 109444]

Coating on material (oil, mill scale, rust) if newish then ask yourself why is it not ALL rusty? cos there is a coating? Could be a coating on inside that is bubbling out onto your welds.

 

[oilcooled :)]

Turn up the plant

 

[W.olly]

Turn up the plant

Well i popped back out and i did must that, weird thing is it worked, but i had to wack it up a fair bit to get it to weld properly as in the past, now it has done the job well untill i ran out of dam wire, oh well of shopping tomoz i hope.

Now what has puzzled me is the power i had to go upto to get the weld good, near twice the power towards when i last used the rig a few months ago.

 

[SiPMerlin150]

Well i popped back out and i did must that, weird thing is it worked, but i had to wack it up a fair bit to get it to weld properly as in the past, now it has done the job well untill i ran out of dam wire, oh well of shopping tomoz i hope.

Now what has puzzled me is the power i had to go upto to get the weld good, near twice the power towards when i last used the rig a few months ago.

Mig tacks and short beads are notorious for not penentrating until the parent metal has heated up adequately. For spots and tacks you will need to crank the heat right up to get the heat into it.

A flap disc to remove the paint, rust and mill scale to reveal a shiny smooth surface is the upmost important step in preparing for a joint. Hope this helps!

 

[spaf4]

I'd forget trying to tack it up using MIG. Instead use 2.5 rods.

Turn the juice up slightly and keep the tacks narrow (move faster than you would when laying down normal beads). Make your tacks about an inch long then when you weld over them with MIG you'll barely notice where it was tacked.