Cracks in welds - MIG, sheet metal

[Geofm]

I am working on the top cowl panel of a '69 Mustang Mach 1. Water got in under the sealant at the bottom of the windshield and ate right through both cowl panels. I cut out and replaced the rusted metal with a patch made of cold rolled 20-gauge sheet metal. I thoroughly cleaned the weld zone and secured the patch in place using Eastwood's panel clamps for butt welding. Using a 110-volt Lincoln SP135 Plus I welded the patch into the cowl OK - good penetration, then ground the weld bead with a dremel tool to blend with the surface. Now I see very tiny hairline cracks here and there right in the middle of the weld bead. Some areas are OK - good solid weld, others have these little cracks. Tried re-welding and re-grinding, same problem. Stress cracks from cooling? Does someone have any recommendations to solve the problem.
Thanks and much appreciated.

[scott79]

what kind of wire are you using? are you using gas or flux cored wire? what amps ? metal thickness?

[Geofm]

I'm using Lincoln Electric std .023" copper-clad steel wire, 25% Argon/75% CO2 gas, power setting is "D" (fine adjustment is possible and I've been varying the power setting between 1/2 way between "C" and "D", to right on "D". Wire speed varies between 3.5 and 4.
Sheet metal is 20-gauge cold-rolled steel.
Thanks
Geoff

[scott79]

sounds like your heat and wire speed is ok, dont know about the wire though, i use 70s6 mild steel. are you cooling the weld any? sometimes cooling too much will cause cracks. check the numbers on your spool of wire. what is your gas setting? i like 20-25cfh.

[Geofm]

I am using Lincoln Electric L-56 wire. Is there a difference between L-56 and 70s6 wire? I'm not blowing cold gas or air at the weld but my garage is a little cool right now (40F). I will use a propane torch and preheat the panel a little before welding and see what happens. More later.
Thanks

[Cmdrray]

Hi Geofm,
The only time I have had the problem you are describing is when I ground the weld to thin in one spot:o. Rewelding didn't help there either. One solution you might try as a last resort is to put a small backer strip of steel behind the area where you are having a weld problem and weld it in place as you reweld the area. I know we hate to put something like this in but if the backside is invisible to anyone when the car is finished it might solve your problem. LOL