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General stainless question

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  • General stainless question

    This is just a general question

    When the metal melts on the back from the penatration, if its grey does that mean its to hot ? Or does the back side color not really mean a lot ?

    Also when I lay a nice bead down it will be shiney but when it cools down it kind of turns dull, and actually if you look in the right light you can see blotches on the beads. Almost like a brown color.

    Hard to explain really, but anyone have any ideas on what this could be from ?

  • #2
    The bead on the back will always be black or dark gray, unles you back purge. You can get away with tight fitup and a chill block clamped tight sometimes. The bead color on top should be silver but its very hard to do that all the time. Other colors can range from straw to blue. Anything else it's too hot and you now have a joint that could rust. As far as the brown it seems like a gas problem. You should be using the Biggest gas lens you can get for stainless, and dont move the torch until the postflow has stopped.


    • #3
      I'm using a gas lense, and a number 8 cup.

      I also have a large gas lense, should I use that ?


      • #4
        In regard to metal melting on back side of weld

        Hi 90blackcrx! I have been stainless production welding for a couple of years now. I'll offer up what I have learned so far.

        If you could please tell us the grade of S.S. i.e. 316L 304L etc. and your application it would be helpful.

        If you could provide what filler rod you have, because this will have an effect on your weld characteristics as well.

        From what I have read so far it sounds like you are welding on gage thickness sheet or tubing 14,16,etc. Unless you are welding a single bevel 1/4" plate or thicker butt joint, it would require a lot of heat to melt the back side of the material.

        O.K. having said that lets assume for now that its thin gage material. Joebass is absolutely correct in advising you to use a backing purge or a cooling plate, preferably copper or brass, 1/4" thick by 1 1/2" wide, by whatever length your seam is.

        You may want to consider flux paste. This works just about as well as a gas purge, depending on your application you might have to remove the melted flux.

        The problem with excessive penetration without shieldlng is crystalization of the parent metal. This is also referred to as "grapes" for the black lumps that protrude through the back of the joint.

        "Grapes" cause porosity in the weld. This should be avoided in situations where maximum strength in your welds are a requirement.

        If "grapes" are forming on the back of the joint, you are overheating the metal and it needs shielding.

        Sometimes it will collapse inward because you are melting the S.S. with no shielding.

        So having said all that, if you can still see your origional seam line with no grape or distortion, the grey or black color isn't that bad, you simply didn't get 100% penetration. If you can reach it, back grind the seam with an angle grinder and a thin grinding wheel and weld the other side.

        In regard to weld color. Thin gage S.S. is difficult to keep from turning brown or black. At work when I have a 14 gage butt joint I use a large gas lens with a #12 cup and about 25-30 cfm of 100% argon and a gas purge when ever it is practical, otherwise I use flux paste.

        This keeps color in the weld (blue amber) for me, and my ASME inspector approves.

        You mentioned blotches, this could be caused by impurities on the S.S. try cleaning the weld area with a S.S. wire brush and mineral spirits or acetone. (just be careful not to start any fires! )

        Well, I hope this helps somehow, if I am wrong about anything it was not my intent to mislead. I'm just recalling what the old ones at the shop have passed on to me.


        • #5
          Thanks, I'm using 308 filler , and welding 11 gauge 304 stainless. I think the problem was the tiny gap that was in the metal in the one spot, I forgot all about the spot.

          Can you tell me a little about the flux paste, really interested.


          • #6
            The flux paste is called Solar Flux type B Its a powder and you mix it with ethanol or rubbing alcohol till its like wet mud then brush it along the back of the seam.

            When it dries, weld away, only now make sure that you have enough heat to melt through.

            It looks pretty good when you clean it off. I use a variable speed angle grinder with a S.S. wire wheel for that.

            Solar Flux has a website with some info


            • #7
              i have and use solar flux, it works well. I would rather backpurge though. It's the best way to get a nice clean joint. Depending on what your doing either has advantages. When recently welding thin stainless it seemed to interfere with my ability to get the 2 pieces to flow together but working on repairing some air tubes on a set of stainless headers it worked great. One more tool in the arsenal imho.
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