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Seasoning cast iron: accidental proof of principle

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  • Seasoning cast iron: accidental proof of principle

    We had a thread a while ago in which the principle of seasoning cast iron to relieve casting stress was discussed. Some maintained that it was a myth and some agreed that it must have some merit to be practised for as long as it has.

    I am on the side that it does actually relieve the strains produced by cooling after casting. My reasoning is based on the fact that common cast iron undergoes a ductile to brittle transition at temperatures near freezing. By reducing the the amount of energy required to propagate a crack by as much as 90 percent the ordinary changes in size with temperature are enough to cause micro cracks that will reduce the strains built into the part during casting.

    This "seasoning" usually involves leaving the cast piece outside in the weather for a full season so that it is exposed to a full range of temperatures from hot weather to below freezing. Of course this only works in a moderate climate where it does actually freeze in the winter. The part is not under any external load so that the formation of micro cracks does not result in a significant reduction in strength since the cracks do not propagate far. They also will "heal" somewhat as the molecules of iron and carbon rearrange positions to fill the voids left by the cracks.

    At least, that is the conjecture.

    I have inadvertently tested this theory. I left a pair of cast iron C clamps holding a 2x4 piece of lumber that was to become a hand rail on one of my sections of ornamental iron railing. That was a couple of years ago and I (ahem) have been too busy attending meetings of the procrastination society to complete the handrail installation.

    The C clamps were used to hold the wooden rail in place on the top metal rail of the iron railing. They weren't clamped particularly tight because it would mark the wooden rail, but they were under moderate compression for two years. During that time the temperature has covered a range from -40F to +100F with many freeze/thaw cycles.

    These particular C clamps are a 3" clamp sold by the local Canadian Tire store. They sell a wide variety of tools of decent quality, certainly not bottom of the line imports quality. I have at least 50 of this brand of clamps and have never had one fail, until now. The two clamps are from different batches and of considerably different age but they were both outside and under medium pressure for the same length of time.

    When I removed them them the other day everything seemed fine. However, when I went to use them to clamp some parts for welding they both failed easily and in precisely the same way. They both cracked at the part of the clamp where the greatest stress is brought to bear under load. I repeat, I have never seen this sort of failure before regardless of how much I tighten these type of clamps.

    The only reasonable explanation is that the seasoning process really does produce micro cracks that relieve casting stresses. In this case the cracks propagated because of the additional external stress cause by the clamping force.

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  • #2
    I'd have sworn this was a pretty well established fact. It's discussed here:
    http://www.ehow.com/list_6045435_eff...radiators.html

    It's not a new problem.

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    • #3
      could another possible culprit be the wood swelling
      --
      Tom C
      ... nice weather eh?

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      • #4
        The old Romans knew a very simple technique to crack rocks:
        Make a rectangular hole, drive wood in and then keep the wood whet.
        That's how the C-clamps cracked.


        Nick

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        • #5
          Originally posted by Astronowanabe
          could another possible culprit be the wood swelling
          -Doubtful the wood could produce sufficient force to stress the clamps that much. I've cranked down on those things before (not that exact brand, but similar midrange imports) and even after using cheater pipes to tighten and beating on the clamped part with heavy cudgels, they never failed. (Or at least, haven't failed yet.)

          I'm not yet convinced of the "microcracks" theory, but I doubt it was swelling wood. I suppose I could see damp wood swelling from ice formation during the cold, combined with the cold-related contraction of the iron frame, combined with the cold-induced weakening of the metal, but even that's just a theory.

          Doc.
          Doc's Machine. (Probably not what you expect.)

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          • #6
            The wood was actually already fully finished and sealed. It is also softwood, pine to be exact and it can't exert enough force to break the clamps even if it did swell. This is a very dry climate, especially in the winter. Wood shrinks here.

            Also, the clamps were not visibly cracked. They didn't fail until I applied significant force.
            Last edited by Evan; 05-10-2010, 04:23 AM.
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            • #7
              but they were under moderate compression for two years. During that time the temperature has covered a range from -40F to +100F with many freeze/thaw cycles.
              The wood was actually already fully finished and sealed. It is also softwood, pine to be exact and it can't exert enough force to break the clamps even if it did swell. This is a very dry climate, especially in the winter. Wood shrinks here.
              And in summer, it swells. It doesn't matter wether the wood was sealed, it still shrinks and swells. I once made a table with an iron flat underneath (my fault) and the wood cracked in winter. I needed a 10 ton press to get the crack back together. That wood was sealed.


              Nick

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              • #8
                Originally posted by Evan
                The wood was actually already fully finished and sealed. It is also softwood, pine to be exact and it can't exert enough force to break the clamps even if it did swell.
                It was the wood swelling with moisture. "Sealing" doesn't really seal, especially outdoors. Finishes like varnish slow down the movement of moisture but don't stop it. You'd be astounded how much force swelling wood can produce. Like has already been mentioned, it was used to crack large slabs of rock in ancient times.

                The labels "hard wood" and "soft wood" are misnomers - they're classified botanically rather than by hardness. Pine is labelled a soft wood because it comes from conifers. Hard woods come from deciduous trees. Pine is harder than many so called hard woods. Some varieties of pine are harder than oak. Balsa wood is a hard wood.
                Last edited by Punkinhead; 05-10-2010, 07:01 AM.

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                • #9
                  Cast iron is well known for it inability to bend. If the swelling of the wood were sufficient alone to crack the cast iron it would have done so in a very obvious and visible failure. I challenge anybody to take a piece of cast iron and slowly apply stress so that it doesn't actually break the iron or produce visible damage yet reduces the strength by perhaps 50%.
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                  • #10
                    The labels "hard wood" and "soft wood" are misnomers - they're classified botanically rather than by hardness. Pine is labelled a soft wood because it comes from conifers. Hard woods come from deciduous trees. Pine is harder than many so called hard woods. Some varieties of pine are harder than oak. Balsa wood is a hard wood.
                    The softwoods are called softwoods because they are soft. There are very few exceptions regardless of the botanical distinctions. Jack Pine, which used to be the predominate species here is only about 1/3 as hard as the various species of Oak. There is only one species of pine that approaches the hardness of the average Oak variety and that is Caribbean Pine. We don't have that here. We also are VERY familiar with wood products here as we depend entirely on the forest industry for our livelihood. I say that personally as my wife is the general manager of a small business that supplies a wide range of products to the saw filing and planer rooms of many sawmills. Wood is our business.

                    If you check the Janka hardness for a wide variety of wood species you will find the species called hardwoods are consistently much harder than the softwoods.
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                    • #11
                      Originally posted by Evan
                      Cast iron is well known for it inability to bend. If the swelling of the wood were sufficient alone to crack the cast iron it would have done so in a very obvious and visible failure.
                      Magnaflux is used to check for cracks in cast iron that are undetectable by eye.

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                      • #12
                        This turns out to be pretty easy to settle. I broke the piece off completely so I could inspect it for evidence that the crack was pre existing. If there was any trace of a crack while the part was out in the weather the cracked area will be discoloured or rusted. There is no evidence of that. The cracked area is very fresh and all looks the same. If it had cracked from extreme pressure it would have started at the inside of the clamp so the inside would have been exposed to moisture ingress.

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                        • #13
                          the cracked area will be discoloured or rusted.
                          a) I see 3 redish areas in the picture
                          b) The broken clamp in the second picture is the lower one from the first picture. It seems to have been painted. The paint didn't crack


                          Nick

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                          • #14
                            I have seen a lot of broken parts over the years Nick. If there is a pre existing crack it is easy to tell. That paint isn't the least bit elastic. The other clamp is exactly the same. As for the reddish area, either that is primer or the paint DID crack so the rest of the cracked area should also be discoloured.

                            However, I knew you would have some sort of objection so I am going to try to do a relative energy impact test later today. I will compare the seasoned clamps to a similar one that has led a sheltered life.
                            Free software for calculating bolt circles and similar: Click Here

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                            • #15
                              Don't know how well this relates to "seasoned cast iron"

                              Used to be an old engine builder's trick to take a stripped engine block and bury it in the ground for a year. Then to dig it up and perform a complete rebuild on the engine. Theory being 2 fold. All the stresses would be relieved from the engine block and the molecules would "line up" with the earth's poles making for a stronger bond

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