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Weld Strength (for critical applications)

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  • Weld Strength (for critical applications)

    So I've been welding quite a bit and at some point want to make a roll cage for my offroad vehicle. Weld strength will be critical because it will be there for the safety of me and possibly someone else. So far I have used a couple things to judge weld strength. Aside from destroying an object on purpose, the major one I have found is the discoloration of the metal on the opposite side of a piece of steel when welded.

    Someone correct me if I am wrong on judging my welds this way but I haven't had to weld on a critical component yet. Is there a better way to tell if weld strength is good without destroying an object?

  • #2
    I'm far from a professional, but that is one of the criteria I use to grade my welds. I always look to run the back of the weld area cherry red. But I rarely deal with more than 3/8" with my MIG. Of course, I also look at the weld itself, nice and flat, not "cold". I am certain you will get some excellent advice and better descriptions on this from some of the more experienced folks on here. Best of luck with your cage, let us know how things turn out.
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    • #3
      Good question. I'm a total amateur but it seems like destructive testing would be best, at least until you became confidently consistent in your procedure and quality.

      I'm curious to see what the experts recommend.
      Last edited by Krunch; 02-10-2009, 08:56 AM.


      • #4
        There is a vague question if ever there was one... not knowing how you are fitting up the joint leave me near to entirely in the dark.
        1. are the joined pieces of the same thickness?
        2. how thick is the material?
        3 are you beveling the weld edges?
        4 are you using a backing plate?
        5 what are you using for filler?
        6 what process and proceedure are you using?

        as a general rule, a weld inspected with 5X magnification, that you can assure has FULLpenetration, and is otherwise free of slag inclusions , undercut, and other weld defects can be considered to be 100% efficient.

        so it will be as strong as the base material and the design allows for. in short failure is no more likely to happen in the weld, then in the underlaying material

        though if you are using a filler rated lower then the base material that goes right out the window!!!


        • #5
          I'll try to tackle this one.

          Generally in the proffessional world of welding whenever a new material, job or weld detail is laid out a process goes into qualifying it.

          First:you find out what your base metal and filler metals are, and if they are compatable.

          Second: sample welds are produced, at which point machine settings are recorded and the welds are destructively examined to find defects.

          If no defects are found and the weld is good
          third: The parameters are set in stone the welder is certified and all welding is (theoretically) done under identical parameters to insure the proper recipe for a proper weld is followed.

          Fourth: inspectors and NDT specialists come in to check or assure that the proper recipe was followed and good results were obtained

          Now for the home or hobbyist it's impractical to go through all the steps due to expense, training and the availability of special tools and equipment.

          If I were in your situation, I would definatley mock up some material and perform destructive tests while recording your parameters, something that can be done cheap and easy. Furthermore I would track down information on the material your using and try to find information on complete joint penetration welds, I.E welds that go through the material 100% with good fusion. These are your bread and butter for critical welds

          Visual inspection of welds can find obvious defects such as underfilled craters, concave welds, porosity etc. but many things cannot be caught in such a manner.

          using the colour of the surrounding metal, also known as tempering colours is not a relieable indicator because many things such as joint configuration, material type etc will affect these colours. In the stainless steel world often times people will look for the "pretty" multi colored weld beads as a sign of a good weld. while excessive oxidation of stainless steel will produce a nasty bluish black and an unsound weld, the less colour you have the better.

          In clean ground mild steel some surface oxidation will occur in the area around the weld although the colors aren't nearly as bright or pretty. Those roughly indicate the amount of heat put into the material. But I'll emphasize they "roughly" indicate the amount of heat put into the material. Finally, depending on a lot of factors you could be trying to go for more or less heat input so it's hard to set a criteria for what is desirable and what is not.

          Alternatively it is fairly cheap and easy to use liquid penetrant tests, to check for cracks, holes and porosity in welds. Most larger welding suppliers will carry LP test cans, sometimes it's called "spot checker". Comes in aersol cans that you shoot at the weld where it will penetrate cracks then leak out. It can cheaplly catch many things that the naked eye cannot, but it won't be able to determine if you're getting good fusion or complete joint penetration.

          Hope that helps
          In case of welding emergency
          use a bolt


          • #6
            oh I forgot to mention, I've worked on making crash cages, car frames, bicycle frames and several other hobby projects over the years.

            I'm gonna take a reasonable guess and say that you will be using thin walled tubular steel or the likes.

            In which case fitup and joint preparation are a bigger part of getting a good weld then the actual welding. Also things like backing rings, can really help reduce the difficulty and chance to blow through on thin wall material, while still getting good penetration.
            In case of welding emergency
            use a bolt


            • #7
              I'm not trying to post a smart azz answer here, but I think that you will know when you are comfortable welding up your roll cage. If you are having to ask questions (which is good) you are probably not ready quite yet. Get some material like you plan to use and practice welding it.

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