Welding a steel hull

Discussion in 'Metal Boat Building' started by Wynand N, Jun 23, 2008.

  1. Brent Swain
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    Brent Swain Member

    Depends on how far back the flux has fallen off. If it has been bumped well back, an inch of drag is not going to do it.
    Dragging along the weld is a good option, tho.
    Brent
     
  2. shugabear
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    shugabear jr member

    hi
    im not trying to get off the subject yet i put a post i the question thread and know one has answered it.

    this is it
    i have looked all over the forum and cant seem to find what i do large vessel repair if you look up Bender Shipbuilding and Repair you will see what im talking about i work on military and commercial vessels could some one please point me it the right direction? thank you and sorry for the interuption.
     
  3. lumpy bumpy
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    lumpy bumpy Junior Member

    Why would the flux fall off !
     
  4. Brent Swain
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    Brent Swain Member

    When you are fitting in close awkward quarters , things get banged around a bit and flux gets knocked off. Only an armchair welder who has never welded will never have experienced flux knocked off the end of a rod.
    If dragging weakens a weld , then how could the weld itself be 100% strength when welding does a lot more to the metal than simply dragging?
    For stitch welding the stringers to the hull, dragging along the weld is not an option. Existing welds are too short for that.
    Brent
     
  5. lumpy bumpy
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    lumpy bumpy Junior Member

    Brent id hate to see the condition of the confined spaces after you,ve been in . There must be arc strikes all over the place . Maybe i was a bit vague when describing welding without leaving arc marks.
    Heres a simplified example , say you wanted to weld along a 6 inch rule .You strike an arc at the 3inch mark , move the rod back to zero then continue welding till you reach the 6 inch mark , thus as you go you are welding over your original arc strike . You do not drag ya rod over your welded area .
    If ya still not sure just ask any armchair welder and he,ll keep you right , good luck .
     
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  6. Brent Swain
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    Brent Swain Member

    3 inches is not always enough to burn back in to the flux.
    As was pointed out, arc strikes are a non issue in the size of boats we are dealing with. Sorta like worrying about the sky falling. They have zero failure rate.
    Do you have your asteroid hit insurance.
    Brent
     
  7. lumpy bumpy
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    lumpy bumpy Junior Member

    You can lead a horse to water but a pencil must be lead .
     
  8. Mastadon
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    Mastadon Junior Member

    You can burn a bare rod down to its flux by striking in the weld zone as far ahead of your intended start point as needed. The key is that you have reached the flux and are holding a tight arc once you reach your start point, and to build and carry a hot puddle as you lay in your pass. (Maybe not a good technique for less experience welder but I learned this from the best and have used it on the most stringent pressure codes without fail).

    In my experience, as long as material is properly preheated (250-300F for mild steel) stray arcs are not a big deal. On steel cool enough to touch for more than a couple seconds, I have seen cracks develop the very next day even though the stray arc had been touched up with a grinder. When a stray arc occurs -no matter how brief- the parent metal in contact is (even just a tiny bit) is increased to welding temps (<5000F). The resulting contraction can lead to hairline cracks that can compromise the surrounding material.
     
  9. Brent Swain
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    Brent Swain Member

    That which has been weakened bends more easily than that which hasn't . if it doesn't bend more easily, then it hasn't beeen weakened. I've never seen the point of arc strikes bend more easily than the surrounding metal in over 40 years of bending metal.
    An arc strike is too short a duration to affect the metal in any significant way, and certainly too short to heat it enough to make any difference. I've never heard of metal failing in the middle of a plate o smal craft. It simply doesn't hapen..
    Brent.
     
  10. Mastadon
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    Mastadon Junior Member

    From my welding inspector manual.

    "Stray Arcing: The damage on the parent material resulting from the accidental striking of an arc away from the weld. A small volume of base material is melted when the arc is struck. This molten pool is quenched due to the rapid diffusion of heat through the plate. This may lead to the formation of a crater that lends itself to cracking, or change in grain structure by creating a martensitic or brittle grain structure in the area of the arc strike. These discontinuities may lead to extensive cracking in service. Cause - operator error."

    Its your workmanship.
     
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  11. rwatson
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    rwatson Senior Member

    That seems to be the crux of the 'arc strike' discussion - change in grain structure, not necessarily weaker in the sense of being easier to deform.

    It sounds like in very high stressed, vibrating or loaded structures, this could be a problem. Say ( and I am making this up as I go here) an arc strike near a through hull that was close to the engine mount. If you take the boat to Alaska where you encounter relatively warm water, and the through hull was just above the waterline - the temperature differential caused a crack to start. Further operation from the vibration of the engine 'worked' the crack to the through hull ...... this is the supposed scenario.

    It has happened in some situations, which is why all the text books warn against the problem, but in real life steel yacht world, you just don't hear about it being a problem. I dont believe for a moment that this is some kind of 'confidentiality clause' is hiding the problem, my 'gut feel' is that the average steel yacht is way over engineered for its purpose, and the rigours of day to day operation will destroy the hull long before any small 'grain structure' problems get a chance to be a worry. Check with the insurance companies - I dont think they do a lot of checking on welder credentials for the average home built boat.

    If you were building some bit of machinery that had to hold in high pressure liquids while it was bolted to a vibrating pump, out in the desert where the temperature varied greatly - worth thinking about.

    And - what happens to the 'grain structure' where the welding is done? - as Brent has pointed out, (several times with no answer). The temperatures there, and close to the edge of the welding work are going to have their grain structures messed about a lot more than the odd arc strike.

    I dont think its enough to play 'textbook hero' - you actually have to provide proven examples of problems, and I would be amazed if anyone could find arc strike problems playing any part in steel yacht hull failures.
     
  12. Mastadon
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    Mastadon Junior Member

    Brent may very well be right in his assertion that stray arc strikes do not affect a steel hulls structural integrity. I dont know, I am not an engineer and I do not conduct forensic analysis of damaged boats.

    My post is in response to his talking out the *** in saying: "An arc strike is too short a duration to affect the metal in any significant way, and certainly too short to heat it enough to make any difference." This is simply not true.

    As for the HAZ (heat affected zone) created during welding; the thermal conductivity is lowered in relation to the amount of heat introduced through welding process. As the conductivity is lowered, the thermal diffusivity from the HAZ is lowered. This means your weld area will cool at a lower rate allowing time for the grain structure to normalize. If your weld were to be quenched by rapid thermal diffusion the grain growth would be halted, trapping carbon atoms within the crystalline structure thereby creating what is called martensite.

    A grain structure with an excess of martensite is very hard and brittle. It is much more likely to fracture in the event of high impact.
     
  13. rwatson
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    rwatson Senior Member

    Doesnt make a lot of sense to my small mind.

    You are saying if I run a bead of three inches on a plate, it will cool slowly and not go brittle.

    If I do a quick accidental 'zap' a few inches away on the same plate, it will make a dangerous brittle area - because somehow it 'cools' differently?

    If I were to hold my bare hand behind the three inch bead, I would burn my fingers. If I had my hand behind the 'zap', it would barely register as warm.

    yet, according to your information, the barely warm 'zapped' area becomes structurally unsound.

    Just doesnt seem plausable - but I would love to get more hard evidence.
     
  14. Mastadon
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    Mastadon Junior Member

    As long as your three inch weld has introduced enough heat so that the thermal conductivity of the steel will not dissipate that heat below the critical temperature required for the formation of a lamellar structure in a time shorter than is required for the lamellar structure to form.

    Yes, if the steel around the location of your 'zap' is at a temperature low enough for the thermal conductivity to dissipate the heat from the 'zap' below the critical temperature required for the formation of a lamellar structure in a time shorter than is required for the lamellar structure to form.

    If you don't want to take my word for it (or can't understand my garbled prose) there are plenty of great resources on the web where you can learn. Here is a great place to start: http://www.esabna.com/EUWeb/OXY_handbook/589oxy9_1.htm
     

  15. rwatson
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    rwatson Senior Member

    Yes, I have done lots of lookup on the net, but not found that site. Its interesting reading. eg:

    "Quenching. This is defined as the rapid cooling of steel from an elevated temperature, usually by immersion in oil or water. The purpose of quenching is always to harden the steel; usually that implies the transformation of the steel to the martensitic structure previously described. Quenching is usually followed by tempering."

    But an arc strike doesnt usually get "quenched". It dissipates heat exactly like a proper bead.

    If there was any chance of short welding bursts creating brittle sections, then all the 'tacks' used to hold a hull together before welding up would be a potential hazards.

    I just dont see how you can create a brittle area on a plate unless you really tried hard eg "quenched" it!

    I will keep looking further, but real world boat building practice just doesnt seem to support the fear that arc strikes are a real hazard.
     
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