Pitting corrosion steel plates

Discussion in 'Materials' started by naserrishehri, Dec 22, 2022.

  1. naserrishehri
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    naserrishehri Senior Member

    May the surface grinder change steel grade or decrease it's strength?
     
  2. Rumars
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    Rumars Senior Member

    If you respect the feed rates (speed) and use proper lubrication, no, nothing will happen. It's like using a giant angle grinder or a milling machine, as long as you don't overheat the steel all properties will remain the same.
    But, I repeat, the size you want is custom made, it's unlikely to find a used one, and new they are very expensive.
     
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  3. seasquirt
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    seasquirt Senior Member

    If the steel is ground dry, with no coolant flushing, then hot spots can be created, changing the hardness and temper of the steel around that local surface, and inducing stresses; and too much heat can warp the plates while grinding, so that when cooled, they're not flat but hollowed. A horizontal axis surface grinder over a reciprocating table, flushed with coolant could do it, but they are usually made for accuracy, not speed. A vertical axis rotary 'slash grinder', flushed with coolant will remove material faster; instead of a single stone wheel, it has a spinning steel plate holding many individual coarse stones, and is made for grinding rough bulky pieces like oxy cut plate, where a 'surface grinder' is more often used for fine grinding, like tool making. Your plate sizes may be too large for either machine anyway, but a large milling machine will accommodate large plates, and remove material much more efficiently. A medium sized toolroom surface grinder will remove about 1/2 kilo of steel an hour, a slash grinder will remove maybe 2-3 Kg steel an hour, a large milling machine with carbide cutters on a thick plate, well secured, maybe 50 Kg an hour, and much less for thin plate.
    The rusting, pitting, ageing, and machining all decrease its strength, so new plate of the same eventual thickness would be stronger than your salvaged same thickness plate, and also have a known grade and paperwork acceptable to whatever building regulations apply. Instead of the plate surface being hot or cold rolled in a consistent manner, your surfaces would have disturbed crystalline structures from being worked, and possibly surface micro cracks, waiting to initiate bigger problems.
    Your old steel's only applicable 'grade' is scrap, unless you spend heaps of time and money on it. You need to nail down costs and benefits of keeping and reworking old plate, or selling it and buying new. If it's a commercial enterprise, buy new to save on bureaucracy issues.
     
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  4. gonzo
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    gonzo Senior Member

    I think that in a shipyard corroded plates can be useful. There is always need of temporary bracing, building stations, etc. where the corrosion would not matter.
     
  5. fallguy
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    fallguy Senior Member

    This is a great post. The big problem when you want to do rework is to find cheap ways to do it and this is what will happen..someone would run the material fast.
     
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  6. Barry
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    Barry Senior Member

    More than likely not
    While you need to identify the alloy of the steel, I would suggest that most ships are built using low carbon content hot rolled steel
    (But could be cold rolled ) Ad Hoc?
    Carbon is the most common element that when added to steel can modify the characteristics, strength, ductility, toughness and the list goes on, but a heat treatment with a quench or quench tempering process is required. (certainly other elements can change characteristics)
    Low carbon content being less than about .25% carbon but normally less than 0.15% C

    So the low carbon steels characteristics are not Significantly impacted with normal machining processes. When the carbon content begins to get into a range of . 4% and above, cooling machining surfaces gets more important and these levels , mid level carbon content steels react well with high temperature level heat quench and tempering processes
    At .8%, high carbon content, you are approaching the tool steel range and heat treatment processes, can have profound impacts on its characteristics.

    Of course at high level’s of carbon, specific additions of other elements, are normally added.

    You have asked for advice wrt to mechanically removing the 2mm pitting depth and the consensus is that it would more than likely
    be more expensive than either selling it to a company that can use the material where the pitting can be removed by say sandblasting
    and where the thickness variation is irrelevant.
    This is great advice and should not be discounted or overlooked.
     
    Last edited: Dec 30, 2022
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  7. naserrishehri
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    naserrishehri Senior Member

    I have attached the picture of the grinding machine. It's hot grinding by belt.
    Does it change the strength of steel plates?
     
  8. gonzo
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    gonzo Senior Member

    The picture did not attach. However, hot grinding will change the material properties of steel. The temperature and time will determine how much.
     
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  9. Barry
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    Barry Senior Member

    exactly how without knowing the alloy of the steel?
     
    Last edited: Dec 30, 2022
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  10. Barry
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    Barry Senior Member

    Without knowing mainly the percentage of the carbon in the steel, the question "Does it change the strength of steel plates" cannot be realistically answered.

    It is a common misconception that if you heat ANY steel, that the properties will change significantly and or heating and quenching ANY steel will increase its strength significantly. Remember that a low carbon steel comes out of the foundry molten and red hot and then is rolled while hot. As it cools, it possesses certain specifications. Grinding will not "significantly" change the specs of steel unless there is a high level of carbon

    So find what the alloy is, and then ask the question.

    Stating that the carbon content is the most significant component of steel whose percentage is simplified. Many other alloys can be added to enhance specific properties. But this is another intensive course that cannot be dealt with
    in a short post. Or a long post for that matter.

    Ad Hoc, what alloy do you use when building steel boats. Quite curious
     
    Last edited: Dec 30, 2022
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  11. Rumars
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    Rumars Senior Member

    Most ships are made from mild steel with a carbon content of max. 0.21%. There are different grades, but we are basically talking about a version of structural steel as used in general construction. If you want to know more there are several standards, for example ASTM A 131M.
    Example of chemical composition for grade A: carbon max. 0.21, manganese min. 2.5x%C, silicon max. 0.5, phosphorus & sulfur max. 0.035. Yield strength 235 N/mm2.
     
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  12. Ad Hoc
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    Ad Hoc Naval Architect

    Correct.
    Too many unknowns.
    Same as I do when designing with aluminium.
    The base most common grade, Grade - A, or J235... etc.

    I always aim to design any vessel using the most common grade and the grade that is most forgiving....generally with its elongation that's the key.
    It is the elongation, just like with O/H111 temper in aluminium, that gives you the reserve capacity for further deformation (i.e factor of safety) before signs of failure.
     
  13. gonzo
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    gonzo Senior Member

    In this case, would you say toughness and elongation are directly related?
     
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  14. Ad Hoc
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    Ad Hoc Naval Architect

    No, not at all.

    Elongation is a measure of the ability of a material to 'stretch' (strain) from it original form until it yields or fails. It is a measure of a material's ductility.
    Toughness is the ability of a material to absorb energy; some call it the ability to resist cracking.
     

  15. gonzo
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    gonzo Senior Member

    I understand. However, toughness can be measured as the area under the stress/strain curve. Doesn't ductility increase the curve to the right?
     
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