grinding welds

Discussion in 'Metal Boat Building' started by alanrockwood, Feb 18, 2011.

  1. MikeJohns
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    MikeJohns Senior Member

    Heavy handed poor practices of any description in metal fabrication will cause problems and poor grinding techniques along with poor welding techniques will have deleterious results. But for example, we don’t say that because poor welding causes failure that you shouldn’t weld. The same holds with grinding.

    From a metallurgical perspective grinding improves weld geometry and consequently reduces fatigue failure. Significant to this thread the heat from grinding can actually have a beneficial effect on a weld similar to peening. Other methods of flattening a weld such as Plasma dressing are also beneficial to weld metallurgy.
     
  2. welder/fitter
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    welder/fitter Senior Member

    "Ad Hoc", I think that I was getting more than a bit confused.:rolleyes:
    I know better than to be comparing coupons to the hull of a boat, in this regard, and you've aptly illustrated why I shouldn't have.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    I think it may have something to do with:

    :eek::eek::eek:
     
  4. welder/fitter
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    welder/fitter Senior Member

    ...As well, I was being somewhat facetious when suggesting that I'd grind my hull welds. During a lay-off some years ago, I went to another of my union's client-companies, for a short spell. I arrived as a welder, but they asked me to do a "fitter test" &, subsequently asked me to fit/fabricate for them, instead, as their shop foreman & senior fabricator was on vacation in portugal for 6 months. the company
    http://www3.telus.net/unr3al/EARLS/index.html
    built container spreaders and required that their welders grind 100% of the welds! When a buddy of mine who was welding there told me that they had to do this, I was astonished! Yes, there were specific areas where associated components required that there be flush surfaces, but there are many external welds that didn't need grinding.

    Longer story made short, they asked me to stay permanently but they paid less than the drydock and the labour costs waste & anal approach to construction was too much for me to want to stick around(wasn't even allowed to write NFG on things!).

    Hopefully, they followed my advice to build a jig for their twin-twenties, as each unit had to be assembled "free-form". Obviously, I still haven't come to terms with their dislike for beautiful welds! But, as they never explained their reasoning, I'm curious as to whether there are paint adhesion issues on the welded areas? I've never noted issues with this in marine vessel construction/repair, but my knowledge of fairing compounds, primers & paints is almost nil. How about it? Is adhesion significantly better with a flush, smooth surface?
    Mike
     

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  5. welder/fitter
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    welder/fitter Senior Member

    lol, yeah, I'm about a month(and a year) behind in my plan to buy a boat & head to The Philippines, via South Pacific, so sleep is a necessary evil these days! Unfortunately, it means missing out on some of the great discussions, here, but such is life!
     
  6. NZ_Shipwright
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    NZ_Shipwright Junior Member

    Hi, I / we give all welds a "tickle up" with the grinder. Run over bead lightly to knock the edges ,etc. off. Once you blast and paint it looks good and it is sound.
    Regards
    NZ_Shipwright
     
  7. Poida
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    Poida Senior Member

    Mike Johns Wrote:
    Heavy handed poor practices of any description in metal fabrication will cause problems and poor grinding techniques along with poor welding techniques will have deleterious results. But for example, we don’t say that because poor welding causes failure that you shouldn’t weld. The same holds with grinding.

    From a metallurgical perspective grinding improves weld geometry and consequently reduces fatigue failure. Significant to this thread the heat from grinding can actually have a beneficial effect on a weld similar to peening. Other methods of flattening a weld such as Plasma dressing are also beneficial to weld metallurgy.

    Mike hit the nail on the head but didn't explain why.
    Steel gets its strength from its flexibilty. Some large ships can flex 1 metre over their length. Bridges also flex and are supported by rollers to take up the flexion.

    This why we stitch weld, it allows the steel to flex between the welds. When you weld you must be sure not to take away this flexibility.

    Welded steel plates on the hull. Leaving the bead on a weld makes the joint more rigid. This is not what you want as hulls are subject to cyclic stress and the steel plates are flexing in and out continously. A rigid weld will prevent the plate from flexing at the joint and create fatigue along the side of the weld.

    Mild steel can't be heat treated, as it doesn't contain enough carbon that is why welding doesn't change its properties. Neither does grinding. Holding a grinder in one spot until the steel turns blue wont change its property but will expand the steel in that area causing stress.

    Grind welds carefully, and finish with a sander and don't reduce the thickness of the plate.

    My 2 cents worth
     
  8. MikeJohns
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    MikeJohns Senior Member

    Poida

    Steel gets it's strength from it's molecular bonds, not from its elasticity!
    Not only is steel strong but it's also tough, fatigue resistant, easily welded and STIFF. Its actually not very elastic at all.

    Every structure flexes be it a blade of grass or an oil tanker, how much depends on it’s design and the E value (Youngs modulus) of the material used to make it. The higher the E value the less elastic and the stiffer the material.

    For example:
    E for Nylon 4 GPa
    E for steel 200 GPa

    So if you made your ship out of Nylon it would flex so much it would be impracticable but the material is overly weak too and it would break apart.

    Steel Warships have often been fully welded for maximum strength. And often framing on deep sea trawlers ice breakers and tugs and all high stressed connections are fully welded.
    Far from wanting flexure around the chain welds you actually want it as stiff and as firmly attached as possible. The bugbear with more elastic materials is the resulting stress concentrations at connection points.

    Welds in highly stressed components are stress risers and fatigue inducers but the plating on a ship is thicker than and the stresses lower for this to become a concern. The problem stresses that can lead to failure are through poor material shrinkage allowance. When correctly welded plates do fail it’s nearly always in the adjacent heat affected zone and not in the weld itself which is thicker and stronger than the adjacent plate. All other things being equal.
     
  9. Poida
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    Poida Senior Member

    Thanks for your response Mike.
    The definition as I understand it of elasticity is not how far a material will stretch but its ability to return to its original length after stretching. Steel is very elastic as it does up to a point return to its original length after being stretched, why it is excellent for steel structures. If it didn't a bridge for example would continue sagging.

    Further to this Mike, please read my post again and count how many times I used the word elastic.

    Cheers
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Poida

    I think you’re getting a bit confused.

    That is correct. If a material returns to its former shape after a load is applied then removed, it is said to be “elastic”. Or put another way “capable of sustaining stress without permanent deformation”.

    That’s where your interpretation begins to fall down.

    All materials have a point of “elasticity”. Whenever engineers/designers use a material, the desired behaviour is simply that…once an applied load is removed, there is no permanent deformation.

    Steel, Aluminium, Wood, Copper, Composite…etc etc….every material has an “elastic limit”.

    The amount of elasticity also refers to how easy it bends under a certain load. Steel is NOT very elastic. Just look again at those figures posted by Mike. Nylon has an E of 4GPa and steel of 200GPa. If you apply a load to nylon, and the same load to steel, which will bend more??...easy the Nylon. Thus Nylon is more elastic than steel. They are both elastic materials, when used under normal conditions, but Nylon is far more flexible and this more elastic than steel..

    Why do you think the term is synonymous with “elastic bands”…they are easy to stretch and bend…

    This can be further explained by the attached graph, of steel and aluminium.

    stress-strain.jpg

    The vertical line from “A” intersect the stress/strain curve for ally at roughly 60MPa, for steel the same intersects at 200MPa. This means that for an extension of “A” the steel is more highly stressed than the aluminium.

    But, we can also look at this another way.

    For a stress of 200MPa in the steel, extend this line to get 200MPa in the aluminium. You can easily see that the aluminium has an extension far greater than the steel. It is a ratio of 3:1. Thus steel is not “very elastic” when you compare against aluminium nor Nylon.

    A designer designs inside the “elastic limit” of the material. So when using aluminium, a stress of 200MPa in the aluminium and also in the steel, produces 3 times as much deflection in the aluminium than in the steel.

    Bridges etc are made out of steel because steel can sustain a higher load than other materials with minimal deformation. You don’t want a bridge to flex when you simply walk over it, as would be the case if made from Nylon. It would be safe, but anyone walking over it may feel very unsafe as it sways about. So you use a material that can sustain high laods with minimal extension...steel!

    Does this mean steel does not bend..no, of course it does. But that depends upon the load applied and the 'structural stiffness', or EI of the structure.
     
  11. Quatsino Boater
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    Quatsino Boater Junior Member

    I'll add my two cents as a welder and mechanical Enginnering tech. One of the reasons to grind all welds as in Welder Fitters reply

    " ...As well, I was being somewhat facetious when suggesting that I'd grind my hull welds. During a lay-off some years ago, I went to another of my union's client-companies, for a short spell. I arrived as a welder, but they asked me to do a "fitter test" &, subsequently asked me to fit/fabricate for them, instead, as their shop foreman & senior fabricator was on vacation in portugal for 6 months. the company
    http://www3.telus.net/unr3al/EARLS/index.html
    built container spreaders and required that their welders grind 100% of the welds! When a buddy of mine who was welding there told me that they had to do this, I was astonished! Yes, there were specific areas where associated components required that there be flush surfaces, but there are many external welds that didn't need grinding "

    Is probably due to achieving a very stong and long lasting. Due to the operation, you have a cyclic load, a chance of over load, many operations within it's servicable life. Probably the biggest reason to grind all the welds is due to stress risers caused by either undercut or poor profile, not nessisarily out of the reason of over reinforcement or cosmetics. By blending into the parent metal you will form a smooth radius to reduce stress concentrations.

    Ad Hoc , you have covered elastic and plastic deformation very well :) However about that aluminum, you forgot to mention creep and creep rupture determined over time. ( Logarithmic) Kinda the same with polymers. That Nylon for instace if used as a berring. It will creep and extrude . I don't know if it is a factor in boat or ship building but it is noteworthy. Man I like this discussion :)

    Hey quick question, does anyone still does manual engineering calcs anymore? An example is Buckling/slenderness ratios such as masts. Load,shear force and bending moments on structurals or is it all FEA now ? Just how much engineering goes into a vessel , every member? Or again FEA ? Just conversation :)
     
  12. MikeJohns
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    MikeJohns Senior Member

    Poida
    See the post by Ad Hoc. You have your concepts a bit muddled I'd afraid.

    Flex is the elastic material response. The properties you were describing with the term flex and flexure is synonymous with the materials modulus of elasticity.
     
  13. MikeJohns
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    MikeJohns Senior Member

    We usually design to scantling rules published and backed by class societies.

    If you don't adhere to their rules and still want class compliance you need to submit specific design to first principles based on their published minimum loads.
    That can be something as simple as section modulus calcs based on a panel load, or a detailed buckling study conducted using FEA.
     
  14. Poida
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    Poida Senior Member

    Ad Hoc and Mike Johns as I respect you both as I assume excellent ship builders and I have never built a steel boat, my posts are purely based on physics and metalurgy.

    Ad Hoc you said I was confused, and then you said I was correct (and you reckon I'm confused)

    I posted: steel is very elastic because it does, "up to a point" return to its original length.

    AdHoc said I am wrong because: All materials have a "point of elasticity."
    Exactly what I said.

    I used the term flexing rather than the term deflection to try and keep it simple. But when a load is placed on an "I" beam sometimes called a universal beam the beam will deflect. When it does the bottom flange will stretch. The beam will be designed so it doesn't exceed its maximum allowable deflection or the allowable elasticity of the bottom flange.

    If it did it would not return to its original length and would remain bent.

    Mike: elasticity = molecular bonding. The ability of a material to return to original length after stretching is due to its molecular bond.

    Now Mike: Mate. I am assuming maybe incorrectly that the original post was about a person building a standard boat. Not a battle ship, or an ice breaker or a tug. My response was based on that assumption. You are talking about purpose built vessels and it is like comparing a common car with an armoured vehicle.

    Ad Hoc I had a bit of a laugh and some nolstagia (if that's how you spell it) I remember in the 60s Professor Julius Sumner Miller (I am that old) had a series on TV the Summer School of Science where he spent quite a bit of time pointing out that elastic bands should be called non elastic bands because stretching is not a characteristic of elasticity. It is the ability of the material to return to its original length and elastic bands don't do that.

    And no Mike I wont build a boat out of nylon, I'm still trying to work out why you brought that into discussion.

    Anyway your a couple of great guys and I'll catch you later
     

  15. MikeJohns
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    MikeJohns Senior Member


    Do you understand that the amount a beam deflects to a given load varies with it's sectional stiffness (second moment of area or I) and its modulus of elasticity? Both these values are constants and don't change.
    The "allowable elasticity" is the material definition not the loading criteria.


    Absolutely there's no question of this at all. All the material properties are down to molecular and atomic bonds but the reductionist definition may not be all that helpful to basic concepts.

    I was illustrating that for strength not only can you fully weld, but you need to. It doesn't make the structure weaker, it makes it stronger. you only stich or chain weld to reduce distortion and welding cost, it's the minimum allowable not the maximum.
    You were wrong there, thinking the more it was allowed to flex the stronger it became.


    I was proffering a simple comparison as to why steel doesn't get "it's strength from its flexibility" its a very good structural material because of it's stiffness not its flexibility. The stiffer a material is the more useful it is in a structure since buckling is the principle mode of structural failure.
     
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