Just joined? Please introduce yourself.
Page 1 of 4 123 ... LastLast
Results 1 to 10 of 36

Thread: Pitfalls of flanged weld seams

Hybrid View

Previous Post Previous Post   Next Post Next Post
  1. #1
    Registered Member MP&C's Avatar
    Join Date
    Dec 2013

    Member #:2017
    Location
    Southern MD
    Posts
    1,290

    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


    .
    Robert



    MP&C Shop Projects-Metalshaping Tutorials


    Instagram @ mccartney_paint_and_custom


    .

  2. #2
    Registered Member NickP's Avatar
    Join Date
    Mar 2012

    Member #:1653
    Location
    De Queen, AR
    Posts
    4,157
    dang it................................................ ................................................sa d

  3. #3
    Registered Member WagonCrazy's Avatar
    Join Date
    Mar 2012

    Member #:530
    Location
    Santa Clarita, CA
    Posts
    1,793
    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
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    10,835
    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

  5. #5
    Registered Member BamaNomad's Avatar
    Join Date
    Nov 2016

    Member #:3217
    Location
    Rocket City, USA (Huntsville, AL area)
    Posts
    3,774
    I need to do some research on this, but off the top of my head, I'm not sure I agree with CN's statement below...??

    "Guys have speculated about it being due to the hardness of the MIG weld. Hardness should not affect thermal expansion."

    ?? I think checking the thermal expansion coefficients of various hardness steels would supply an answer, but I don't have a manual handy...?

  6. #6
    Registered Member
    Join Date
    Apr 2012

    Member #:571
    Posts
    4,671
    From what I know, if it's carbon steel of any hardness (carbon content could vary), thermal expansion will be the same for all.

    Stainless steels can have different thermal expansion rates, but they will only vary in a fairly small range, unless it's a specialty alloy with small thermal expansion rate (and that may not even be "stainless steel" per se).

    Aluminum will have greater expansion rate, and the various alloys will vary, but again not by much.

    Again, I don't think thermal expansion rate is what's happening when you have a "ghost line", it's primarily joint stiffness, and maybe the amount of metal in a given location - I.e., a lap joint has roughly twice the metal as the surrounding panel. Also a lap joint is 4 times stiffer than a panel, or a butt welded panel.

    Remember too, that an adhesive bonded lap joint will also exhibit "ghost lines". So it's not just welded joints either.

  7. #7
    Registered Member BamaNomad's Avatar
    Join Date
    Nov 2016

    Member #:3217
    Location
    Rocket City, USA (Huntsville, AL area)
    Posts
    3,774
    Those articles were not clear (to me) as to whether they were referencing properties of the 'raw steel' used to make the panels, OR strength characteristics achieved AFTER forming (and then various treatments of the formed part) ??

  8. #8
    Registered Member chevynut's Avatar
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    10,835
    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


    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 MP&C's Avatar
    Join Date
    Dec 2013

    Member #:2017
    Location
    Southern MD
    Posts
    1,290
    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



    MP&C Shop Projects-Metalshaping Tutorials


    Instagram @ mccartney_paint_and_custom


    .

  10. #10
    Registered Member
    Join Date
    Apr 2012

    Member #:571
    Posts
    4,671
    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.

Page 1 of 4 123 ... LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •