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Thread: Pitfalls of flanged weld seams

  1. #1
    Registered Member MP&C's Avatar
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    Pitfalls of flanged weld seams

    Ok, you guys have heard me talk incessantly about the downside of using flanged seams in sheet metal repair, discussing the ghost lines that can and will occur with these type of repairs. Today I got a short video to show the effect. This car was neat as a pin, with extensive rust repair performed, in basically the same locations as the 55 we're working on. The only flaw I could find in the paint was a line in the finish where the rear tailgate repair patch was seamed. I asked the owner if he had used flanged seams in the repair, and he said yes. He did offer for me to take pictures that others may learn from it..

    Butt welds people!!!!!!!


    https://www.youtube.com/watch?v=OGhFEfVqxb0


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    Robert



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    Registered Member NickP's Avatar
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    dang it................................................ ................................................sa d

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    Registered Member WagonCrazy's Avatar
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    Good eye Robert.
    1957 Nomad- LS1/T56 on C4 chassis
    1959 Fleetside Apache 1/2 ton, shortbed, big window, 327ci.

  4. #4
    Registered Member chevynut's Avatar
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    Wow that's a shame. I agree that you shouldn't ever use flanged seams and butt welds are the only way to go. However, you said this, and I disagree:

    ".......shows how the differing expansion rates of one layer vs. two layers eventually results in a ghost line exactly where the repair took place."

    The expansion of metal doesn't depend on the thickness, assuming it's the same temperature throughout. Since steel is a good conductor of heat, I doubt the temperature varies measurably from the top sheet to the bottom sheet. I'm betting the line is a result of the different expansion rate of the filler material versus the steel. In fact, it may be due to shrinkage of the filler. Does the line show regardless of temperature? If so, it's probably due to shrinkage of the filler in the seam. I'll bet a leaded seam wouldn't show that line.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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  5. #5
    Registered Member chevynut's Avatar
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    Also, I wanted to comment on something that we discussed years ago on the other site. A guy had a trunk that was sandblasted and there were dips at every location where you could access the sheetmetal from the bottom. I mentioned that it was due to the sand stretching just one side of the metal, and guys said that couldn't happen because the metal was too thin. Of course it can happen. I think Robert proved that was the cause with some experimentation.

    I actually was a mild victim of this. I blasted my Nomad along edges and jambs with 80 mesh silica sand after stripping the paint. I blasted the roof brace and barely hit the roof sheetmetal with the sand. Same with the hood....I blasted the rear brace from the bottom and hit the upper sheetmetal a little, even though I was trying to be careful not to do so. After priming and blocking, I noticed I had a dip along each brace. On the roof it only took another coat of primer to fill, but I actually had to do some bodywork to get the hood bent back up. It was down 1/32" or so....I didn't measure. I should have put a sheetmetal shield between the braces and outer sheetmetal before blasting. This can happen on fenders too....probably did to me.

    A couple of weeks ago I had a rusty sheetmetal cover panel that was oilcanning. I put it into my glass bead blaster and blasted it clean, then I kept blasting the edges of the cover until it wouldn't oilcan any more. It was clear to me that the metal was stretching from the glass beads hitting it. I put a flat piece of sheetmetal in the blaster and blasted just one side.....it caused it to curve. When I blasted the other side it straightened out. And that was with glass beads that are almost like powder....imagine what sand would do.

    So this shows that you CAN stretch just one side of a piece of sheetmetal and cause it to warp. Also, if you know what you're doing and have access you could probably straighten it back out by blasting the other side. If I'd known that at the time, I probably would have just blasted a line on my roof and top of the hood.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  6. #6
    Registered Member MP&C's Avatar
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    Quote Originally Posted by chevynut View Post
    Wow that's a shame. I agree that you shouldn't ever use flanged seams and butt welds are the only way to go. However, you said this, and I disagree:

    ".......shows how the differing expansion rates of one layer vs. two layers eventually results in a ghost line exactly where the repair took place."

    The expansion of metal doesn't depend on the thickness, assuming it's the same temperature throughout. Since steel is a good conductor of heat, I doubt the temperature varies measurably from the top sheet to the bottom sheet. I'm betting the line is a result of the different expansion rate of the filler material versus the steel. In fact, it may be due to shrinkage of the filler. Does the line show regardless of temperature? If so, it's probably due to shrinkage of the filler in the seam. I'll bet a leaded seam wouldn't show that line.
    Therein lies the problem, it doesn't remain the same temperature throughout.. To better illustrate, let's use an extreme example. Take a 1' square of 18 ga sheet metal and a 1' square of 1/2" plate out of your air conditioned shop and place them in direct sunlight. Do they both increase in temperature equally, or does the thin piece warm up more quickly? Does it not also cool off more quickly when the sunlight has gone for the day?

    In the same fashion, it will take longer to heat up two layers of sheet metal over one single thickness. Differing heat rates, differing expansion rates, and ghost lines that show exactly where the flange seam is located. It does not happen overnight, it may take a year or two, but given sufficient heating and cooling cycles, the ghost lines will appear.
    Last edited by MP&C; 07-07-2017 at 07:53 PM.
    Robert



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  7. #7
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    Much if not all of the reason for a ghost line is that the lap joint is stiffer than the surrounding metal. It doesn't matter if the joint heats up at a different rate than the surrounding metal, it's just that the surrounding metal moves more than the stiff lap joint. Same for cold. It doesn't matter what rate it heats or cools.

    We can argue all day over the details, but it happens and that can't be denied.

    The other disadvantage of a lap joint is that because the joint is significantly stiffer, you can't straighten it after welding, It's even difficult to beat a high spot down.

  8. #8
    Registered Member chevynut's Avatar
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    Quote Originally Posted by MP&C View Post
    Therein lies the problem, it doesn't remain the same temperature throughout.. To better illustrate, let's use an extreme example. Take a 1' square of 18 ga sheet metal and a 1' square of 1/2" plate out of your air conditioned shop and place them in direct sunlight. Do they both increase in temperature equally, or does the thin piece warm up more quickly? Does it not also cool off more quickly when the sunlight has gone for the day?
    What you're saying is true. Thicker material has more thermal capacity, but it equalizes at some temperature pretty quickly. And it has the same expansion rate. If the seam only showed up during temperature transitions, the difference in thickness might make sense. But I'll bet you can see it in the middle of the day when the car has been in the sun and the temperature is stabilized. That tells me thermal capacity has nothing to do with it.

    In the same fashion, it will take longer to heat up two layers of sheet metal over one single thickness. Differing heat rates, differing expansion rates, and ghost lines that show exactly where the flange seam is located. It does not happen overnight, it may take a year or two, but given sufficient heating and cooling cycles, the ghost lines will appear.
    If it takes years to happen that proves that it's not related to metal thickness, as that never changes. What does change is the filler shrinks. If it was due to metal thickness you'd see it immediately.

    I also don't think the line has anything to do with the stiffness of the metal. When metal expands, nothing is going to stop it. It expands at the same rate whether it's hardened or annealed, and a small area like that line isn't going to bow differently just because it's stiffer due to thickness. You'd see it in a larger area of the panel. Same argument if it was thickness that was causing it.....you'd see it over the entire doubled up area.

    I maintain it's the filler that you're seeing. There's a narrow channel of filler that's applied on the lap joint and the thermal expansion of filler is at least 6 times that of the steel (polyester is 10X). That's a huge difference. If the lap seam was fully welded and ground, or filled with lead that has a thermal expansion closer to steel (but still 2x) and no shrinkage over time, I don't think you'd see that line.

    Is it time for an experiment? Make a few panels with lap seams and fill with one with lead and another with polyester filler. Or take a panel and roll a narrow bead in it and fill it with filler. Then paint the panels and put them in the sun.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  9. #9
    Registered Member chevynut's Avatar
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    Out of curiosity and since I can't do anything but sit around with my messed up leg I decided to do some research on this topic. There are literally hundreds of posts about ghost lines on dozens of boards. Guys are seeing ghosting over all sorts of welds, even butt welds ground smooth on both sides. Some guys have even metalworked the joint to perfection, used no filler, and still see the ghost lines. Nobody seems to know what the true root cause is but there are a lot of theories, some that make some sense and some that don't.

    Guys have speculated about it being due to the hardness of the MIG weld. Hardness should not affect thermal expansion. Some say it's due to filler shrinkage, or difference in thermal expansion rates, but that doesn't explain it all when no filler is used.

    Some guys have this problem and some don't. Seems like guys who use a lot of bondo or other filler don't see it as much. That's probably because they're covering it up but it's still happening under the filler.

    So I got to thinking about what else could cause it. It looks like the thermal expansion of cold-rolled steel may be slightly anisotropic, meaning it's different in different directions. It does happen in stainless steel and some other metals:

    https://www.google.com/search?q=expa...+rolled+steel+


    We all know that any rolled sheetmetal, steel or aluminum, has a "grain" direction as the crystal structure is altered from rolling. Aluminum should usually be bent across the grain because it can crack if bent with the grain in some situations. I don't think anyone has really studied the thermal expansion in sheet steel. So is it possible that when a patch is installed with the "grain" of the sheet non-parallel with the patched panel they expand at slightly different rates? That would clearly show a line at the weld in a highly polished surface. But it would show up immediately, not after years of temperature cycling.

    In the case of the tailgate Robert posted, it would be interesting to tear it apart and study it to see how it was bent, how it was welded, how it was filled, and if the grain of the patch is in the same direction as the grain of the rest of the tailgate. There has to be an answer somewhere, but it seems elusive. Now I'm worried about my welds.
    Last edited by chevynut; 07-08-2017 at 04:58 PM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  10. #10
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    in the early 80's i owned a van shop. we painted 10 per day . at the end of the year all that was left was 2 tone chevy duallys . there was a 3/4 inch tape strip . we removed it, da sanded with 80 grit primed and painted . outside the 3/4 stripe would still be visible. also seen this with many cars that had factory tape stripes. especially the firebird . those eagles would ghost back every time. this was lacquer then urethane clear.

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