I understand how weld surface preparation is done, but my question is why things like oil, paint, etc on the surface of the part ruin the welds.

is it a lack of conductivity? do metals with conductive coatings (aluminized steel, for example) need to have the coating ground off to make a good weld (health hazards aside), or do they just need to be cleaned?

EDIT: one thing I thought of is that coatings (whether Zinc or Aluminum) could go into solid solution during welding and change the mechanical properties of the base metal in the HAZ, so I guess that could be an argument for removing all surface conditions before welding.

I've googled and used wiki, as well as searching H-T.

Thanks in advance




Modified by tharptroy at 1:24 PM 10/3/2007

 

I'm by no mean a "welder" so don't take my word for it. I've got a small stick welder and used it about 10 times so other people will shed more light on this than I will.

I noticed the arc was harder to get with dirty/rusty/painted surface. So the conductivity is greatly reduced if you do not clean/grind/sand the parts you need to weld.

I'm also guessing, by common sense, that any impurity will contaminate the weld. The weld may start rusting early if the weld isn't clean.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote"> but my question is why things like oil, paint, etc on the surface of the part ruin the welds.</TD></TR></TABLE>
They melt and contaminate the weld pool.

 

If youve welded before you can clearly tell why a surface needs to be cleaned, a dirty,rusted, paint surface, will cause popping and and the welds will just be all sloppy