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Holding capacity of welds vs. heat expansion? Someone explain the physics to me

Discussion in 'The Lounge' started by mountainexplorer, Jan 15, 2006.

  1. mountainexplorer

    mountainexplorer 1/2 ton status

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    I'm sure many of us have had wheel bearings go out and had the inner part of the bearing weld or seize itself onto a spindle... or had other circumstances where two pieces of metal have become fused together by heat expansion making them near impossible to separate.

    Then there are man-made welds (done by a welder of any sort) in which we intentionally join metal together and want it to hold.

    Now, take my car trailer. Any man made weld with a welder, no matter how good the person is at welding, will break and not hold.

    After not checking wheel bearings for 9 years on my trailer and having them in perfect shape last June, and re-packing them properly with new grease last June, one of them decided since it was finally properly maintenanced that it would fry itself 7 months after having new grease.

    Now, the inner part of the bearing has fused itself to the spindle by such strength that far surpasses any other weld on my trailer. In order to break it away from the spindle seems to require such force as no man made weld would ever handle.

    Why is this.

    End of my complaint against the laws of physics for today.
     
  2. 79k20350

    79k20350 3/4 ton status

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    cause you wan one to come apart and opposite of the other, its just the way it is!:D
     
  3. sled_dog

    sled_dog 1 ton status

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    the bearing is somewhat of a press fit, metal ground together, AND welded.

    Have you TRIED to break a weld on your trailer? If you can do so manually, GET RID OF YOUR DEATHTRAP. A weld should be THE strongest part of the metal. Maybe the area around it (heat affected zone) will be the weakest, but most metal failures when there is a good weld are the area AROUND the weld and not the weld itself. So I think your theory is a little flawed boss :wink1:
     
  4. kyser_soze

    kyser_soze 1/2 ton status

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    All welds must be stronger than the surrounding metal in order for them to hold. In fact the center of the fusion weld zone is a mixture of parent metal and electrode metal.

    This zone is cast metal with a microstructure reflecting the cooling rate of the weld.

    Fusion welds are prone to all of the problems associated with metal casting, such as porosity, inclusions, blowholes,cracks and shrinkage.

    The crystaline forms are quite complicated depending on the cooling of the weld.

    Since the welds are actually cast metal they are stonger then the surrounding metal.

    Metalurgy 101 is concluded for the evening.:wink1:
     
  5. ntsqd

    ntsqd 1/2 ton status

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    :haha: :haha: :haha: :haha:
    The ads at the bottom of this thread when I opened it up were for "Hermetic Welding Equip." and "Trailer Bearings."

    The strength of weld metal depends on the base metal's alloy, the filler rod's alloy, and what crystalline structure the cooling rate put the metal in. Some structures are really strong, but very brittle. Others are not as strong, but quite ductile.

    The strength of the Weld depends on the strength of the weld metal and the minimum cross sectional area.

    In the case of your trailer bearing, there is a lot of cross section to the 'weld.''
     
  6. muscle

    muscle 1/2 ton status

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    so is it best to allow the weld to cool slowly? ive seen alot of guys weld some part and then dunk it into a nice bucket of cold water. also i guess the proper size wire or rod and heat to enable the weld to be completed with less heat affected zone?
     
  7. Resurrection_Joe

    Resurrection_Joe 1 ton status

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    I've forgotten 90% of what I ever knew about welding, but I think quick cooling can lead to brittleness and warping
     
  8. ntsqd

    ntsqd 1/2 ton status

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    In steel dunking it in water quenches the metal and leaves it in an extremely brittle condition. Think brittle like a file. This opens up a whole other thread on heat treating, one which I'm not qualified to start.

    The simplest way to reduce the HAZ is to pre-heat. Just a couple hundred degrees makes a huge difference.
     
  9. SkulzNBonz

    SkulzNBonz 1/2 ton status

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    Usually, when a "weld" fails, it is not actually the weld, but the joint itself. If the design of the joint is flawed, or there are outside forces (vibration or torsional) acting on the joint, the joint fails, and most of the time the weld takes the blame. A thorough visual inspection will usually show that in fact the joint has failed. As for a wheel bearing, I dunno, but they are mother bears once they fry themselves to a spindle.

    Now get this tech outta here!!!!!!!!!!!!!

    John
     

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