safe temperature for shrinkage

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Ok, first post here folks!First of all I would like to thank the members for posting the info on combatting welding shrinkage. (my little bag-o-horrors)I had to weld up an 8" x 3/16" C channel and add a length of plate to the open side, thus making it a 8x2 box tubing rather than an open C.I sheared the filler plate, tacked in in with 1" tacks every foot, making sure that it never got hot... or even warm enough to be uncomfortable to the touch.No warpage...I then welded short randomly placed welds on the channel/plate seam. Maximum of four 6" welds at a time, then moved over to the second beam, letting the first beam cool.At no time could you not run your hand on the beam due to heat soak.Used 2 rods (1/8" 7014) into four randomly placed 6" welds, switch over to the second beam, then kill duty cycle time (50%). repeat... endlessly...It cooled over night and warped 6"-8" over a 15' length. I managed to straighten the warp by rosebud and wet rag method, but I didn't allow it to get red (or dull red) hot due to fears about changing the beams strength. My heat gun topped out at 888*f but it was not visibly red yet nor did it burn the paint. (charred, but not burned)Straight now, but what is the strength issue here?What methode other than pre-stressing in the opposite direction could I have used to prevent this?What temperature will cause structural problems in mild steel?Thx, JimNuthin' says its a "guy's place" quite like having a car hanging from your ceiling.
Reply:you could have clamped it down with a shim under the back about as much asyou would think it will warp the tack and weld.and to ruin the steel you got to get it real hot like yellow hot.now I'm shore i Will be correctedChuckASME Pressure Vessel welder
Reply:Originally Posted by SniffyOk, first post here folks!First of all I would like to thank the members for posting the info on combatting welding shrinkage. (my little bag-o-horrors)I had to weld up an 8" x 3/16" C channel and add a length of plate to the open side, thus making it a 8x2 box tubing rather than an open C.I sheared the filler plate, tacked in in with 1" tacks every foot, making sure that it never got hot... or even warm enough to be uncomfortable to the touch.No warpage...I then welded short randomly placed welds on the channel/plate seam. Maximum of four 6" welds at a time, then moved over to the second beam, letting the first beam cool.At no time could you not run your hand on the beam due to heat soak.Used 2 rods (1/8" 7014) into four randomly placed 6" welds, switch over to the second beam, then kill duty cycle time (50%). repeat... endlessly...It cooled over night and warped 6"-8" over a 15' length. I managed to straighten the warp by rosebud and wet rag method, but I didn't allow it to get red (or dull red) hot due to fears about changing the beams strength. My heat gun topped out at 888*f but it was not visibly red yet nor did it burn the paint. (charred, but not burned)Straight now, but what is the strength issue here?What methode other than pre-stressing in the opposite direction could I have used to prevent this?What temperature will cause structural problems in mild steel?Thx, Jim
Reply:Yer lucky ya didn't make a horse shoe!Pre stress is the only way I could think to do it.  I doubt you hurt hot rolled steel by getting it hot.  Its mild steel, so nothing to loose.  It was never heat treated.DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:Originally Posted by dave powelson1-What methode other than pre-stressing in the opposite direction could I have used to prevent this?      Other than the fact that doing the above--works to minimize distortion--why did you not do this?2-It's not clear if you plated the entire length of the channel, with 15' of plate, or the middle....or what?3-Are you saying you ended up with complete welds along the channel flanges in 6" segments?
Reply:Originally Posted by SniffyThe C channel was plated along its entire length essentially making it an 8"x2" box tube. It had to be welded along its entire length to effectively seal it from outside contamination from mud packing in and causing it to rust out (what happened to the old piece). Two reasons for the lack of pre-stressing. I could clamp it down, but getting to the back side to shim it would have been difficult. The second was that I couldn't find and info on pre stressing amount formulas. I figured that going slow and cold would produce less problems than over bending it and not being able to straighten it.  Six of one, a dozen of another... I've really learned a lot about getting myself OUT of this pickle by heat-n-cool but I would like to know for the next time how to avoid this to start with! Is there a rule of thumb for pre stressing a C channel?(ie; 1/2" bend per foot?) and would this be done after the tacking but before the lateral welds? If so can you then dispense with the short welds and do a long weld on either side? :dizzy:
Reply:http://books.google.com/books?id=UT5...BT9E#PPA128,M1Hotrodder posted this a while back.  It seems to be a good resource book.Last edited by tapwelder; 05-28-2008 at 08:34 AM.
Reply:Perfect sketch! I think that the trouble of removing the beam to properly pre-stress it would have been easier than leaving it in the location and fixing the problem after the fact.Thanks a lot. I'll source that book tonightABEbooks has it for $27Nuthin' says its a "guy's place" quite like having a car hanging from your ceiling.
Reply:Originally Posted by SniffyPerfect sketch! I think that the trouble of removing the beam to properly pre-stress it would have been easier than leaving it in the location and fixing the problem after the fact.Thanks a lot. I'll source that book tonightABEbooks has it for $27
Reply:why not go to the steel shop and buy a peice of rhs the right size?