'Seasoning' of cast iron
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  1. #1
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    I was told years ago that it was good practice to leave new castings outside for as long as possible before machining to 'season' (to allow residual stresses to relax).

    I was a bit sceptical, and thought that stress relief was the way to go. No doubt 'seasoning' helped, providing it was longer than the 10 minutes favored by the works' accountants.

    Anyway, for machine beds that weren't stress-relieved, what happens to their shape after decades of seasoning in the shop? Any facts and figures available?

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    Rusty Iron would go through a decent thermal cycle out in the sun. And for free or very little cost.

    Bill

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    Who can afford to hold castings in stock these days to season with inventory taxes and j.i.t. schedules?

    WE did rough out, release, reshim, and finish machine cycles on machine beds and frames in the box machinery business years ago.

    We seldom had problems with distortion.

    It is hard to tell where "distortion" comes from.
    Was it distorted in clamping, heat distortion from the machione cycle, or from released stresses?

    Stress relief may be the answer on critical castings like lathe beds etc.

    You still have to rough out, release, and finish cut even with a stress relief.

    It is very hard to clamp a rough casting in a "free state" for the first cut.

    Kap

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    Hi John,
    Just got the Myford ML7 bed back after an inexpensive top grind- £35 in our money!
    There is no doubt that castings should be rough machined and allowed to weather- for years.
    The quicker method seems to be to heat them red hot once again- and then machine.
    I think that you will be aware of the Quorn Tool and Cutter grinder. When I did my boring- to close limits- everything was fine. The 1" Precision bars went in fine- until I tried to split them. The so and so's nipped and without a reamer, it took ages to sort out. Again, I did the head casting of a Westbury- not a Dore Westbury and the bored casting again nipped when cut.

    I haven't a clue of where you are heading casting wise but there is some merit in boring on the plus side rather than to the lowest side.
    For another view, read George Thomas on his Universal Pillar Tool- and he altered the original design and castings!

    Kindest regards- keep well

    Norman

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    An iron casting poured in a sand mold is going to have some "locked in stresses". The sand mold is madeusing sand mixed with clay and has some water content. The sand is then faced with a carbon dust (typically, powdered soft coal) to try to keep the sand from vitrifiying (fusing to glass) at the surfaces of the mold cotnacted by the molten iron. Nevertheless, when the molten iron gets near the sand mold surfaces, there is alocalized rapid cooling. This produces a stressed and somewhat harder "skin" on iron castings poured in sand molds. When the castings are machined, if portions of the skin ar eleft intact while other portions are machined off, there is bound to be some "settling out" due to the stressed portions of the skin becoming partially untrestrained due to the machining.

    The term "Seasoned Iron Castings" was used in advertising for machine tools and auto engines. Years ago, on a job out in Ohio, I rented a house from an oldtimer who had worked for Willys-Overland Motor Company. The oldtimer had worked for Willys-Overland in the 1920's and 1930's, mostly delivering new cars. What he did tell me was that Willys-Overland always had huge stacks of iron castings outdoors "seasoning". Willys, it seemed, had taken over several other smaller automakers. I think Hupmobile may have been one of them. In taking over the other automakers, Willys got piles of rough castings which they left stacked outdoors until they could figure out what to do with them. Some of the piles of castings went into engine series that Willys based on some other makers' block designs. By that point, the castings had been outdoors for several years. They found the castings machined a lot easier than "green castings" fresh from the foundry. As it was, Willys and the other automakers in that era did not make significant changes to the engines from one model year to the next, using the same basic block and head castings. The castings would be poured, cleaned and snagged ( sprues, gates and risers ground off). The castings would then be stacked outdoors and left there for at least a year. The weather did the rest. The oldtimer told me the belief was that repeated freeze/warming cycles plus the changes of the seasons would make for high mileage engines. In the 1920's or 1930's anyone who got near 100,000 miles on an automobile engine without a rebuild including a rebore of the cylinders thought that was remarkable. It could well be that the automakers' advertising departments figured to get some mileage out of the fact that the engine castings were left outdoors for a year or two by hyping the business of "seasoned iron castings". This was the 1920's and the public was a lot more familiar with seasoning of wood, so could relate to the business of "seasoning" engine block castings. People passing the auto plants saw the castings stacked like so much cordwood and began to wonder about buying a car with an engine made from rusted old castings. Advertising the outdoor storage of engine castings as "outdoor seasoning" made for good advertising. Remember, this was an era when the US was a manufacturing nation. The public in general had a bit more knowledge of how things were made and took some interest in it. Look at ads for that time period for cars- things like "Tocco Hardened Cranksahft Journals" or "Superfinished surfaces" or "Seasoned Iron Castings" were the kind of things that were advertised to consumers.

    At Brooklyn technical High School in the 1960's, we had machine shop teachers who had come through their time as toolmakers or machinists in industry. These guys had been around some before they settled down to teaching us kids. They told us that time along with the seasoning of iron castings or any other precsion parts was a necessity to get dimensional stability. They kidded about, telling us that to make things like "Jo" blocks and similar, it was necessary to "get the molecules to nearly stop moving around". The way this happened was to let things "season" using time and thermal cycling. Some of the teachers told us when they were apprentices or young toolmakers, they had made their own 1-2-3 blocks, sine bars, and similar. In so doing, they left the parts out on window sills in winter weather for days or weeks, letting the freeze/warming cycling do its work.

    Within the last couple of years, I was talking with Mr. Robert Yancey out at Yancey Machine Tool. Mr. Yancey has been around heavy machine tools and heavy machine tool building for many years. We were discussing the building of heavy precision machine tools. We shared the view that the best way to insure accuracy and rigidity in a machine tool is to use properly designed massive iron castings. I mentioned that the older machine tool builders had specified that they used "seasoned iron castings". Mr. Yancey said the effectiveness of "seasoning" iron castings to get dimensional stability was debatable. He held that if iron castings are proplery designed and then properly cooled, the outdoor seasoning may not really be what makes the casting so stable. He felt that automobile makers had started the idea of "seasoning" castings simply by having to store enough castings outdoors for a production run. At that time, as Mr. Yancey pointed out, the methods of cleaning castings were not what they are today. In addition, castings came from the foundry with a kind of harder "skin" where the iron had solidified against the damp molding sand. This skin also held some additional stresses. Letting the castings sit outdoors in the weather for a year or two also let some rusting of this skin occur. This rusting tended to get partially through the skin, letting the stresses in the skin relieve over much more of the surfaces of the castign than would have occurred through machining only slected areas. The other idea was that the rusting got through the harder skin,making for easier machining.

    My own sense of the things is that the best dimensionally stable castings are the ones which have been outdoor seasoned for a couple of years. During WWII, machine tool builders were cranking out machine tools from what amounted to "green castings"- castings that had been poured only days or weeks earlier. Those machine tools went into use and held close tolerances. That would seem to blow the theory that "outdoor seasoning" of iron castings was a necessity for dimensional stability. Perhaps, with machine tool castings needed as fast as possible, these castings were put into a "soaking" furnace and brought back up to heat and allowed to normalize by slow/furnace cooling. This would achieve a similar result to the "outdoor seasoning". However, these same machine tools built during the WWII era simply had more massive castings to start with, so dimensional stability was easier to maintain.

  6. #6
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    I remember Singer Industrial Sewing machines as being promoted as being made from castings seasoned for a year. I have also seen large vibrating tables used for stress relieving large weldments by shaking them.

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    C. W. Ammen wrote that when iron works moved to the South they had difficulty in producing castings suited for high precison apparatus for lack of cold winters.

    Modern metallurgists maintian that anything that time and weather can do in years can be done in a furnace and cold box in a few days. My experience agrees with that assertion. I've machined large (40 ft) stiff plant equipment weldments that were thermal cycled. I re-inspected them 20 years later. When supported by the same points the dial indicator readings from the reference pads were within a thou ot two of the original inspection.

    Remember that this fabrication has been banged around, stored out of doors, suffered small accidents, and been blasted and painted at least five times. Now that's stability.

    These days, few castings go right from the sand to the machine shop without stopping for a thermal cycle of some kind. It only costs a couple of hundred dollars to raise a car bottom furnace full of castings to 1200 degrees and slowly cool over a weekend. Stability in the workpiece is sufficient incentive for a manufacturing process to include thermal cycling. That's plain economics.

    I checked around once when I was thinking of having some castings made. Sub-zero treatment is now a low cost treatment in most foundries these days.

  8. #8
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    Forrest Addy, you mentioned sub zero treatment, with LN2 what kind of treatments could be done, i suspect stress relieve, but other treatments normally require hunfreds++ more degrees above zero, and LN2 is only -320 degrees Fahrenheit, so is it a rule that the minus temp gives the same result as the plus temps, or do the minus temps cause totally different behaviour?

    Can you harden steel with LN2 ?

    Very interesting subject, looking forward to any information about it.

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    In the 1960s, while employed by Westinghouse Marine Division, I observed an accelerated version of "seasoning" big castings.

    Large marine turbine and gear castings were suspended from above, apprentices spent many hours banging on them with lengths of 4 x 4. Quite tuneful sometimes, when they'd get a good rhythm going !

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    Xjenderfloip (How DO you pronounce that?)

    Heat cycling of castings, fabrications, forgings, edge tools, etc from furnace heat to cryogenic termeratures has been the subject of many discussions. In metallurgy, cold is not a replacement for heat nor vice-versa. Cyrogenic cold cycling is only a way to ensure transformation of uncoverted microstructure in materials for which maximum properties are desired.

    If I was a flim flam man I'd set up a flask of lN2 and a big cold box and use it to scam an extravagant life style from knife makers, race car owners, and motorcycle enthusiasts. Then I'd branch out into fishing tackle and golf clubs maybe even sexy underware and food supplements. Where ever people seek an edge over their competetion scam artists are there so skim off flying money. So it is for cryogenic treatment of articles intended for high perfermance.

    A good metallurgist can prescribe a thermal cycling process to enhance to properties of practically any material be it wrought iron to stuff for the space shuttle's successor. It's hard science and it takes a 5 year course of study for a metallurgist to get his degree plus about 10 years of diversified experience to settle his discipline into a body of knowledge. If you have a metallurgical problem like hardening a knife made of mystery metal that turnes out to be German silver or pure electrolytic titanium then all the cryogenic treatment in the world will not make a good knife of it.

    Metals come in standard alloys for which there are books telling you how to roll, weld, forge, heat treat, machine, and work it in practically any way you can imagine. The books give you temperatures, soaking times, ramp rates, and a large body of discussion. These books are extremely well researched and consulted regularly by aerospace engineers, matallurgists, and the other highly trained professionals of science and industry. These books are available in most nay library or through the interlibrary loan program. If you wish to learn a bit about practical home shop metallurgy to support heat treating project peices then a beginning text writtien for engineers and toolmakers would be a good choice.

    Do not consult the pages of car or motorcycle magazines for hard information on materials and processes. They are too often poorly researched except where their information is flat wrong. If you're addicted to car magazines, fine. Use them for inspiration. Research and verify the information you find there before you commit to purchase or manufacture.

    I suggest any newcomers to metalworking to retain a healthy skepticism when probing the murky corners of our trade. Unscrupulous flim-flam men dwell there ready to sell you a transient dream for hard cash.

  11. #11
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    In line with Forrests excellent discription. I worked up in Canada many years ago, and during the Christmas holidays, we were closed for several weeks. sometime during then the main gas line regulator failed and the whole plant went down to below 0 for many days. besides all the water line ruptures, almost all our 5 foot long precision slitter shafts warped. in warping, they cracked all our carbide slitter knives $$$$$$$ !
    We had the shafts evaluated by a metalergical firm, and they said the shafts had never been exposed to temps colder than 65 degree (we confirmed this with with manufacturer!_) and the drop to subzero allowed the movemnet of atoms and the slight, but disasterous shaft warpage..thats why Crogenic treatment is required for critical parts.
    The Chief Metal guy said metals exposed to low temps can move if exposed to lower temps "Then they have been previously exposed to since heat treatment"
    So if you anneal or reheat a component, you need to do the cold thermocycle all over again to gain the stability of Cryo..
    It also explains why Detroit tossed out blocks into the winter weather.
    So if you want to duplicate it , pack some dry ice on your lathe before you rebuild it.

  12. #12
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    My 25th Edition of Machinery's Handbook has a 2 page discussion of sub zero treament of steel. It seems to be used to stablilize precision gage blocks, among other things.

    It also says that it has been used to improve the durability of high speed steels, but "up to the present time, the results of tests by metallurgists and tool engineers often differ considerably and in some instances are contradictory".

    Some years ago, a friend of mine had a side business cryogenically treating cutting tools. He claims to have had success, but I never had any of my tools treated. I do not know if he is still in that business, as it was a sideline for him. He said that crogenically treating (-250F to -300F) was developed at the Watertown (MA) Arsenal during WWII.

    Thermo1


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