No announcement yet.

Flame Straightening a Weldment? (Pictures!)

  • Filter
  • Time
  • Show
Clear All
new posts

  • Flame Straightening a Weldment? (Pictures!)

    Put some time into the railing today.. this is a stainless steel railing:
    1/2" solid stainless rounds fitted into 1-1/8" x 1/8" wall stainless round
    tube posts. All the positions were drilled out in the mill.. the rounds
    fit snuggly into the posts.

    Since this whole thing is long and thin, I was quite careful about
    heat distortion from welding -- at least I thought I was.

    All tig welded. Very small 1/8" beads.. ok maybe 3/16" in some places
    where the angles got tight. Made sure to walk around the whole
    thing welding on 'opposite' sides. First tacked the whole thing, square
    it up, and then went in and alternated my welds.

    All welded, the thing looked great (very little distortion).

    This thing is going to have a brushed finish. Pickle the welds, then
    3M rotary brush.

    I made the mistake of grinding these kind of welds down before. Its
    not easy on stainless to get smooth continuous transitions. So this
    time I decided the do a "wash pass" with the tig welder, to even and
    smooth the beads out.

    Although the heat was much higher, I didn't think distortion would be
    bad since everything was welded in place.

    I was wrong.

    The washing looks great.. nice and smooth. Stainless is nice that way.
    The solid rounds have nice 1/4" fillets now where they mate with the

    But it bent like a noodle. I'm only really interested in the first post -- i've
    gotten 1" deflection over a 36" length. I think the small solid rounds will
    move back into place fine if I can get the warp out. See images below.

    I've never tried my hand at flame straightening, but I'm considering
    giving it a shot here. Thought I could heat the outsides and maybe it'll
    settle back into place? Is the outside the right side to do?

    I could make a jig to mechanically straighten this thing.. but I'm worried
    about getting a hard bend or kink.

    Any thoughts greatly appreciated.

    Sorry if pics are grainy.. flash wasn't working great on stainless.
    If anyone is interested I'll post my results.


  • #2
    You'll be amazed at how easy it is. Using your TIG or OA torch, heat the large tube 180* from each weld. The reasoning is that as you welded, the tubing tried to get longer on the welded side due to thermal expansion. The other side of the tube wasn't as hot, and therefore kept the tube from getting longer. As the weld cooled, the metal contracted, pulling the tube over. All you need to do is reverse the process. As you heat the opposite side, the bow will get a little worse, but as it cools again, it will straighten right out.

    Hope that helps,


    • #3
      I used to straighten SS boat shafts and a torch and wet rag did wonders but be very careful. heat and then cold water draws the metal.
      It's only ink and paper


      • #4
        Thanks fellas. Is there any particular order I should do this in? Ie start
        from one end and work my way across? Start in the middle and work
        my way out? Does it make a difference?

        I guess I'm not sure how sensitive its going to be. Do I only get one shot?
        I don't want to ruin the tubing either. ("burn" the stainless.. if thats possible)

        Kerry.. TIG or OA -- I was thinking OA, large soft flame.. but thats quite
        a difference -- pinpoint heat from TIG and huge OA flame.. how much of
        the tubing am I heating? and how hot? red?

        Carld-- wet rag makes reinforces my paranoia -- can you elaborate?

        Thanks. (I have visions of having to cut this tube out and rewelding!)


        • #5
          When I've done this in the past, I just did a bit of welding on the other side
          of the tube, directly opposite the problematic welds... this caused shrinkage and pulled
          the tube over. I'd then use a flapper wheel to make the welds disappear. If you don't want
          to get it that hot, you'll need to clamp it to something heavy and straight, and then heat
          the same spots until the metal relaxes (yields). Since your welds are quite localized,
          you should be able to make the bends disappear. It's harder when the structure is more

          Note that clamping tubes to something straight when they will be welded on one side
          only helps a lot in keeping this from happening in the first place.

          If you're worried about the repair process, weld on one side of some
          scrap to cause the problem to occur, and then try getting rid of it.

          - Bart
          Bart Smaalders


          • #6
            I'd use a AO rig with a small tip... you want precise control over the heat in the metal which should be above 900 but below 1050... heat crayons would be nice if you have them. I'd start just behind the second weld down heat about an inch and use a wet rag to cool it quick. Depending on how the metal moved will determine where you heat it next. But start at the highest part of the crest.
            Wow... where did the time go. I could of swore I was only out there for an hour.


            • #7
              You can straighten bent shafts the same way.. or tube rollers from a paper mill.. or...

              heat one side, apply water, it draws it up shrinking it.

              A "shrinking disc" (like a pot lid on a grinder) used in sheetmetal shaping does the same thing.. you heat the high spots, then spray mist water bottle on it.. and they shrink down.. peen the low spots up with a spoon.. soon.. instead of a fender full of bondo you have a smooth and flat "all metal" fender..
              Excuse me, I farted.


              • #8
                Might I suggest getting some scrap tube of about the same diamiter and playing with that? Idealy a long peice so the warpage is easy to spot (Can warp the hell out of the first 1~2' without ruining the whole pipe)

                Also it seems your messurement in the picture are off?
                shows the big tube as 1 1/8" and the small tubes as 1 1/2"?

                Oh wait thats an arrow line not a 1 in the 1 1/2. doh.
                Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


                • #9
                  Flame/heat straightening

                  Flame or heat straightening is more an art than a science.

                  It requires a lot of skill and patience but when done correctly can make a huge difference to a distorted weldment, sheet, plate or tube.

                  To see a really good Boiler-Maker or Panel-Beater is a real eye-opener.

                  Here is an extract:

                  Shrinking with heat is another option.

                  Before heating a part, check the vehicle maker’s heating recommendations for the type of metal. The theory behind heat shrinking is that when metal is heated it expands. The surrounding cooler metal keeps the heat and metal contained, causing it to bulge up. As the metal cools it contracts and eliminates the stretched metal. Quick-cooling the metal causes it to contract more than cooling naturally and will eliminate more stretched metal.

                  To heat-shrink steel, heat an area about the size of a small coin to a dull red. An oxyacetylene torch, stud welder with a shrinking tip, or an induction heater all work well for this. The heated area will expand and rise up. Using a dolly on the backside, lightly hammer the raised area almost flat, either starting in the centre and circling out or starting at the edge and circling in. The area can then be allowed to cool naturally, or quick-cooled with compressed air or water. This process may need to be done in a number of closely adjacent areas until all of the stretched metal has been shrunk.

                  Another way to heat shrink is with a tool called a shrinking disc.

                  A shrinking disc is basically a metal disc that is placed on a high-speed grinder and run over the panel in the area that is stretched. The friction of the disc running over the metal will create heat. Since the disc will only ride on the high spots, which is typically the stretched metal, that is where the heat will be generated. The area is then quick-cooled, which causes the metal to shrink. Monitor the panel and continue this process until all of the stretched metal has been shrunk. It will typically require multiple passes with the shrinking disc to shrink all but the smallest of stretched spots.

                  Final Finishing

                  If an area has been over-shrunk it can be stretched slightly using the hammer-on dolly technique and light blows with the hammer. In fact it is sometimes actually somewhat beneficial to slightly over-shrink an area and then go back in with a hammer and dolly and plannish the entire area. Plannishing is using many light overlapping blows with the hammer, hammering on-dolly, to cause a very slight amount of even stretch to the panel. This can be done to every part of the area repaired and into the surrounding areas of the panel around the repair. Doing this helps to smooth out any minor irregularities and remove any stresses that have built up in the panel during the repair process, leaving a very stable and stress-free panel.

                  With the panel successfully metal straightened, a skim coat of finishing filler can be used to smooth out any hammer marks and small irregularities if necessary. Also be sure to repair any damage to backside coatings on the panel, and ensure that proper corrosion protection has been restored.


                  Often a panel that could be straightened is instead replaced. Many times, replacing is just the best option when all things are considered. However, with the proper understanding of the theory behind metal shaping, the right tools, refined metalworking skills, and patience, damage can often be successfully repaired. For some repairs this means less intrusion into the vehicle and less factory welds and corrosion protection disturbed.


                  Part 1 is here:


                  The archives in that web site are well worth a good look too.

                  Last edited by oldtiffie; 12-27-2009, 06:58 PM.


                  • #10
                    What you have done is add a red hot gusset (the weld bead) between the two parts. It cooled, shrunk and pulled the part. I think it will take force to bend the part. Heating alone can relieve residual stress but I don't see it bending the part.


                    • #11
                      Just to close the thread out -- I gave the flame straightening tricks a try
                      and it worked great. Have to admit it was a bit frightening putting giant
                      black spots on new stainless -- but the bend came right out:

                      The slight bend you see in this picture is from lens distortion on my
                      camera -- but the post is nice and straight.

                      First heated all the opposite sides. That took some out, but not all, then
                      I went in the in-between spots.. which took care of the rest.

                      Lots of acid and lots of buffing and its installed. (using a 6" buffing wheel
                      on a 4" angle grinder is a whole different story in itself!)

                      Thanks all for the tips -- this opens up a whole new world to me.



                      • #12
                        ps.. i'll be adding a wooden hand rail later. I have some straight dried
                        oak I'll be cutting into strips. I've got a woodworking dark-side.



                        • #13
                          Beautiful job Tony!

                          I haven't tried it, but there's a whole chapter on Flame Straightening/Bending in "Metalworking: Sink or Swim" -- the best metalworking tips book I've seen (much better than Machine Shop Trade Secrets, IMHO).

                          MSC carries it, so the 30% off / free shipping codes apply (I did it).
                          "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."


                          • #14
                            One little trick I have used in the past when doing something similar is prior to welding in the rods, and after you have the holes cut, reach inside the tube with some stick rod and weld a little bit on the inside of the tube opposite of where the rod will stick. This will tend to pre-warp in the reverse direction. This won't entirely solve the problem but it will make for a much simpler fix after.

                            I've built hundreds of "sight" columns for ammonia systems which use small sight glasses welded to small diameter pipe, same thing only different. Stainless is outrageous for warping.



                            • #15
                              I know there had to be some anxious moments during the construction, but what a beautiful end result.