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This page is about the problems you can have MIG welding over mill scale.
Notice I called it mig welding "over" mill scale and not "thru" mill scale.
thats exactly what can happen. If you do enough things wrong, you can
weld right over millscale without penetrating thru it at all.
Short circuit MIG is a great process when settings are good and hot and best practices are applied like a short stickout, appropriate gun angle and good techniques.
But its also possible to not even penetrate thru the mill scale if you use low settings, a long stickout, and bad technique.
This video shows what can happen with short circuit mig when too many things are done wrong.
When hot rolled steel plate is made, a thick oxide layer called mill scale forms on the surface.
There are varying degrees of mill scale too and thick hot rolled plate usually has a very thick layer of mill scale compared to other structural shapes like tubing.
Mill scale is made up of iron oxides and oxides are not desirable in a weld puddle.
During welding, mill scale hinders the flow of the liquid weld puddle.
In welding processes where there is flux, ( like stick welding and flux core processes) the flux really helps the fluidity of the puddle and also scavenges the oxides into the slag.
But short circuit mig relies on deoxidizers in the bare wire.
and since heat input with short circuit mig is lower than other modes, short circuit mig welding over mill scale often results in lack of fusion.
Cleaning all the mill scale before welding helps tremendously.
The video shows that even low mig settings work pretty well when all the mill scale is removed.
And recommended hot settings even worked pretty well without cleaning mill scale. ( but things work even better when you clean the mill scale)
Removing mill scale before welding with short circuit mig can make a world of difference.
Removing mill scale lessens the likelihood of lack of fusion.
Using a mig welding technique that keeps the arc in the leading edge of the puddle can really help with penetration.
use a short stickout for short circuit mig welding.
pushing the puddle works and so does pulling. but dont get carried away with gun angle.
you can test your penetration using a simple fillet weld break test or macro etch test, or both.
recommended settings from a mig chart are a good starting place and are close enough for flat and horizontal welding but are too hot for vertical uphill.
When in doubt, clean the mill scale.