Weld integrity

Discussion in 'Metal Boat Building' started by SilverMack, Sep 9, 2014.

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  1. SilverMack
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    SilverMack Junior Member

    How do you check your MIG welds for strength and non porosity? (Non destructive of course)
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    You can check for porosity with X-rays. But you can't check strength with any NDT. It needs physically testing.
     
  3. gonzo
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    gonzo Senior Member

    You can also use dye testing. It is not as thorough, but can give an indication of quality.
     
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  4. AndySGray
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    AndySGray Senior Member

    eyeball it.

    It is very rare to be able to produce a great looking weld which is not up to scratch.

    For important welds clean the weld area well - MIG will weld rusty and scaley metal quite well but if you clean the weld area, you can see the bluing at the edges (indicating the heat you put in) there's also much less to become impurities. The weld bead angle with the metal is also a good indicator, as is reverse of the weld.

    Don't mess about with cheap earth clamps - that a common cause of poor welds - on bigger work weld a bolt on and guarantee a good earth.

    Good gloves allow you to get comfortable and supported position which again improves the bead.


    Use scrap pieces to test the settings - MIGs work well over a variety of settings but get the sweet-spot and it feels like they do all the work for you.

    Then you can also cut the scrap weld in half to check the bead profile, commercially they file and sand the scrap/sample weld then etch with oxalic acid to highlight poor technique/settings.
     
  5. TANSL
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    TANSL Senior Member

    porosity: X-ray, magnetic particle, liquid penetrant.
    Strength: through a test specimen, which should be made according to certain standards, subjecting the specimen to various mechanical tests.
    scrap pieces to test the settings: only test whether the scrap piece is well welded or not.
     
  6. Wynand N
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    Wynand N Retired Steelboatbuilder

    Common fault with MIG, especially on fillet welds, are lack of fusion/penetration...

    You can have a weld that looks first class in all respects and with a tap of a hammer break the weld clean off. Rather err the power settings on the high side and all should be well.
     
  7. monomad
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    monomad Junior Member

    Silver Mark,
    What are you going to weld (Street - Aluminium ) inside or out side? What's your power supply going to be ? What size welder can you use ? What thickness of plate will you be welding? What have you welded before.
     
  8. TANSL
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    TANSL Senior Member

    Monomad, all they are all very interesting questions but knowing their answers will not help at all to answer questions from Mark Silver
     
  9. rwatson
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    rwatson Senior Member

    It will if he can suggest the tests that match the technology that the OP is using.

    There are many tests that suit Alu welding, that aren't all that useful for steel

    eg magnets

    there are also many things to test for depending on the type and size of welder
    eg "Common fault with MIG, especially on fillet welds, are lack of fusion/penetration"

    Monomad is going in the right direction.
     
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  10. Ad Hoc
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    Ad Hoc Naval Architect

    And this obtains the strength of the weld ...how?
     
  11. TANSL
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    TANSL Senior Member

    rwatson, there are many, many tests, that can be done. For example, the vision of the welder (diopters), could be checked.
    You know what is resilience ?. It is a very important factor when certain grades of metal are welded and resilience tests are also done.
    When, as the OP says, it is "non strength and porosity", I think there are not so many tests to perform, just strength and porosity.
    Monomad is going in the right direction, but not to answer the OP´s question.
    Cheers
     
  12. rwatson
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    rwatson Senior Member

    Through " the tests " ( the appropriate ones) Dear Lisa, Dear Lisa
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Again, which "tests" are these that provide the strength of the weld via NDT?
     
  14. jehardiman
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    jehardiman Senior Member

    rwatson, I think what AdHoc and TANSL are trying to say is that there is no non-destructive test that is presently available to tell you the ultimate strength of that weld...that one right there in the hull. There are many test that you can do to look for faults in the weld...but nothing that will give you the strength.

    Mack, as have been pointed out, there are many NDT methods that can be used you determine the adequate workmanship (and porosity is just one of the things we look for) of a weld in place. Other than load testing fittings, there really is no method to determine the strength of a hull weld in place. And even a load test only proves that the weld will support a given load, not its ultimate strength.

    What normally happens is that, in accordance with the construction requirements, each shipyard through it's metallurgy office and welding engineers develops a set of process sheets for welding. These sheets list the important things like base metals, thickness, filler material, wire size, amperage, per-heat, inter-pass temp, etc...the idea being that the weld will never be weaker than the base metal (there are exceptions though, which is why structurally important welds are engineered). All weldors and welding machines, guns, joint design and NDT inspectors which will be used in welding are then trained and tested on the sheets, with all test samples tested to destruction and undergo metallographic inspection for things like fusion, penetration and enbrittlement. Once you have a qualified process, weldor, and equipment all you do is inspect for workmanship...and poor workmanship (undercut, re-entrant angle, and fillet size) can easily halve the ultimate strength of a weld.

    FWIW, NDT is rather a misnomer, you are not really testing anything except the inspector. It is rather an inspection to give maximum reasonable assurance that the weld will meet minimum requirements.

    If you really want to see what this involves for a situation that requires maximum assurance look at a military fabrication standard.

    http://ntpdb.ddlomni.com/TechPubs/1689/MIL-STD-1689 revA.pdf

    For the commercial world, there is AWS design sheets and software out there that will do this.

    http://www.youtube.com/watch?v=82IwU_y_w88
     
    Last edited: Sep 14, 2014

  15. CloudDiver
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    CloudDiver Senior Member

    I see the OP never answered the question of what material he is welding, and thickness would help too.
    Knowing the process, MIG, the most common is mild steel but you absolutley can weld alumium with a spool gun.
    For mild steel with MIG, yes you can weld through scale and rust, MIG is very forgiving in that sense. If you want 99.9% assurance you are getting the best weld go ahead and remove scale and clean all other contaminates. The key is setting the correct amperage for the thickness material you are welding and having a good gas cloud. What equipment is the OP using? I have a Millermatic 211 auto-set MVP. I always weld thick material on 220V and run a test piece to check for penetration. Not as pretty as TIG but I get a fairly attractive bead that visually shows no porosity. I just welded a new set of 3/16" thick hangers onto 2 x 1 1/2" angle iron for my trailer suspension. I use my TIG on mild steel when I want an attractive bead, total control, no clean-up, and want to be positive the strength is there with little chance of contamination. TIG, of course, takes much longer to set up and run a bead. Also, stainless is always TIG.
    I don't have a spool gun for aluminum wire on my MIG. I have trained on and used them before, same general rules apply; make sure your amperage is set correctly, have good gas flow, and run a test piece. Production welding with aluminum spool guns is done by thousands of feet every day without the need for any testing other than visual QA. If your equipment is set up correctly the base metal will fail before the weld will shear.
     
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