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  1. #1
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    Default Case Hardening Steel

    Awhile ago Ray asked if I would let everyone know how I got on with my kiln and pack case hardening.

    Well I finally got a temp controller wired onto the kiln and today I did my first experiment with pack case hardening..

    There is not a lot of info on the net about home brew case hardening and a lot of it conflicts with one another.... I eventually went with what was written in a NSW forestry commission publication about hardwood charcoal..

    In it, it said.

    Case-hardening Compounds

    Case-hardening charcoal must be reactive, have high structural strength and be a
    suitable size. Charcoal from hardwoods is preferred though charcoal made from
    coconut shells is also used. These charcoals are desired because of their high
    density and comparatively low burning rate.

    The main function of charcoal in carburisation is to ensure a continuous supply
    of carbon monoxide to the work being carburised and to remove from the work
    the resultant carbon dioxide. It is necessary that the charcoal be in quite close
    contact with the work but also that the particles be ofsuch a size as to permit free
    circulation of the gases. Too fine a charcoal would result in a high concentration
    of carbon dioxide, retarding carburisation. The charcoal size normally specified
    is about 8 mm and seldom larger than 12 mm.

    In addition to the charcoal a catalyst, usually a carbonate which decomposes on
    heating, is required. This serves as the initial source of carbon dioxide which
    reacts with the heated charcoal to form the carbon monoxide necessary for the
    carburising action. The selection of specific carbonates depends on temperature
    and time of carburising. Less stable carbonates such as calcium and sodium
    carbonate evolve their carbon dioxide at lower temperatures than the more
    stable barium carbonate.

    For high temperature carburising and deep case-hardening a stable carbonate is
    required. Generally a mixture of these three carbonates, calcium, sodium and
    barium, is used to take advantage ofthe carburising which occurs through a large
    temperature range. Sodium carbonate however is kept to ~ minimum as it attacks
    the alloy containers used in the process. Rate of carburisation depends on the
    percentage catalyst in the compound, up to 8%, after which little increase in rate
    occurs. As the process involves a considerable loss of catalyst it is necessary,
    however, to commence with a higher percentage:

    A typical specification for a case-hardening compound is as follows:

    Hardwood charcoal 85%
    Sodium carbonate (soda ash) 2%
    Barium carbonate (precipitated) 10%
    Calcium carbonate (whiting) 3%


    The exact percentages may vary to some extent with the carburising method
    used. The carbonates are usually added to the charcoal in a powdered condition.
    They may be introduced into the pores of the charcoal suspended in water, after
    which the resultant material is dried.

    It is, however, considered by some workers that this carbonate/ charcoal bond is
    too weak, resulting in high loss of catalyst. As this aspect of carburising
    operations is very important to the economy of the process the use of binding
    materials is usually warranted.

    One such method is to coat the charcoal granules with a mixture ofcarbonates in
    molasses. Alternatively, powdered charcoal can be intimately mixed with
    catalyst and binder and pressed into tablets by machinery. Often coke, anthracite
    and other diluents may. be used in the formulation of these compounds. Their
    function appears to be cost reduction rather than technical.


    continued next post
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  2. #2
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    Default

    I collected some charcoal from a long since dead half burnt hardwood tree, most likely an iron bark... The barium carbonate came from ebay... Sodium carbonate from hardware store in the pool chemical section and calcium carbonate was very finely crushed marble..

    What size to crush the charcoal to was a guess.... Some other instructions say it must be very finely crushed. These instructions say larger chunks around 10mm... After following how the process works I went with the larger chunks... Reason being the materials all react to create a carbon monoxide gas which the carbon in the gas penetrates the steel when the steel is above a certain temperature (a bit over 800 degrees C) If the charcoal is fine it could hinder the CO from reaching the steel or that was my reasoning which could be untrue.

    Anyway I crushed to charcoal to a size and sieved out the fines and the large bits...

    Measured up the % amounts by weight...

    425 grammes charcoal
    50 grammes Barium Carbonate
    15 grammes Calcium Carbonate
    10 grammes Sodium Carbonate

    mixed with the charcoal in a water slurry mixture, then heated the mixture until the water evaporated...

    Made up a steel container to hold the test pieces out of 3" water pipe. the container was vented each end with a 3/16 hole but the lid was only held on with wire..... The test piece was a toolmakers clamp I made awhile ago out of mild steel.. dead soft.. Surface ground most of the surfaces. I cleaned the parts with acetone before putting them in the container to remove all traces of contaminants... Then did not handle them with bare fingers. Overkill probably..

    Put some case hardening mix into the container and added the test pieces, with about 15mm gap between the sides of the container.... Filled to the top with case hardening mix and wired the lid on.

    Dialed in the temperature to 925 degrees and waited.....and waited, then our electricity decided to fail.... Connected to generator for 3/4 hour until the mains came back on...Takes about an hour and a bit to get to 925 degrees.

    5 hours later, with the outside of the kiln quite quite hot (this kiln loses a lot of heat not that I have even used a kiln before though)

    Opened door and was greeted with a lot of heat, grabbed the canister and removed it... Cut the wires removed the lid and poured the contents into a steel tray.... Grabbed the parts and quickly quenched them...

    They were quite clean... Put them on the surface grinder for a clean up.... They had warped and had to take up to 0.15mm off to clean the sides up fully...

    Put them on the rockwell hardness tester... Came back at 60HRC... success!!!!

    No idea of the depth of hardness..
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    Another photo.. hot hot hot..
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  4. #4
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    Ueee is offline Blacksmith, Cabinetmaker, Machinist, Messmaker
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    Default

    Good work RC.
    Did you just do the 2 clamp jaws?
    You could machine a taper on a test piece and test the hardness at regular intervals to see just how deep it has gone. Now you can make nice hard parallels, clamps, vice jaws...the list is endless.
    1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.

  5. #5
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    Hi .RC,

    Excellent thread, and great detailed information.

    I see a lot of applications where you make parts from readily available mild steel and then case harden.. this is a pretty handy capability to add to the workshop arsenal!

    It would be interesting to see what the depth is, I seem to remember the hardening depth is related to the time as well. A few hours carburizing might give you a mm perhaps?

    Once the case hardening is done, and quenched, then I'd guess you could just treat is the same as high carbon steel, and temper back to whatever hardness. Although the only reason you would normally want to do that would be to avoid brittle corners or thin sections.

    Regards
    Ray

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    Hi RC,
    I have a much nastier recipe than that....... I like the sound of yours better.
    How much of the mixture was left you think?

    One thing that might be handy, I've read that there is no need to quench straight away. Let it cool, do some more machining(which of course removes the case) then reheat and quench. Then you have hard where its needed and not where it isnt.

    Stuart

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    Quote Originally Posted by Ueee View Post
    Good work RC.
    Did you just do the 2 clamp jaws?
    You could machine a taper on a test piece and test the hardness at regular intervals to see just how deep it has gone. Now you can make nice hard parallels, clamps, vice jaws...the list is endless.
    Yes just the two jaws at this point in time... Would like to work out how much electricity it used to work out the costs.... Have no idea as to how much power the kiln uses, other then the very light 10 amp cable gets slightly warm after extended use..

    Quote Originally Posted by RayG View Post
    Hi .RC,

    Excellent thread, and great detailed information.

    I see a lot of applications where you make parts from readily available mild steel and then case harden.. this is a pretty handy capability to add to the workshop arsenal!

    It would be interesting to see what the depth is, I seem to remember the hardening depth is related to the time as well. A few hours carburizing might give you a mm perhaps?

    Once the case hardening is done, and quenched, then I'd guess you could just treat is the same as high carbon steel, and temper back to whatever hardness. Although the only reason you would normally want to do that would be to avoid brittle corners or thin sections.

    Regards
    Ray
    Thanks Ray, Yes you could temper it back if needed..

    Quote Originally Posted by Stustoys View Post
    Hi RC,
    I have a much nastier recipe than that....... I like the sound of yours better.
    How much of the mixture was left you think?

    One thing that might be handy, I've read that there is no need to quench straight away. Let it cool, do some more machining(which of course removes the case) then reheat and quench. Then you have hard where its needed and not where it isnt.

    Stuart
    would that be a Sodium Cyanide based recipe? I believe it was very quick rather then taking 5 or more hours to produce a depth of hardness the sodium cyanide would take half an hour... Of course it has a minor drawback of being a highly toxic and if the work was first pickled in acid and if any of that acid were to get into the cyanide tank it would produce hydrogen cyanide gas... and I am sure everyone knows where vast quantities of that was used..


    Of course you cannot compare the toxicity of Sodium Cyanide with Sodium Ferrocyanide which is commonly used as an anticaking agent in salt and is an approved food additive.... Sodium Ferrocyanide is I believe also a major component in the usual modern home brew hardening compounds like Kasenit and Cherry Red that you can buy in the US...

    These days I assume manufacturers requiring what they produce to have a hard outer shell with tough core probably use steels like 8620 which is a low carbon alloy steel containing chrome and molybdenum. The parts are then nitrided, the chrome and molybdenum in the steel reacting to nitrogen in a controlled atmosphere (usually ammonia gas) to produce a very hard outer skin...
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  8. #8
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    One more important point I forgot...

    The canister used to hold the material and part...

    After extended periods in the kiln the canister will oxidise on the outside by a substantial margin...

    This canister had 3mm plate used for the end caps...

    After removing it and the oxidised material removed, the end plates are now only about 1.5mm thick....

    One thing to keep in mind is the canister if made of steel has to have some thickness to it otherwise it will not last the duration of the heat treatment...
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  9. #9
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    Default Power

    Out of interest

    How many kW does your oven use ?

    Mike

  10. #10
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    Do not know Mike... it is a home made job that I am going to have to modify as it loses way too much heat..... The metal exterior gets well over 100C after a few hours at 900C. I may try to put some ceramic fibre wool type insulation in there, to insulate it better...

    I am guessing it has 2.4kW elements...

    I just bought a power measurer on ebay so will connect it to that... Power here is at 25c per kwh, be interesting to know what it costs to run...
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  11. #11
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    Hi RC,
    Nothing that nasty.

    "Place pieces to be hardened in an iron box, made air tight by having all its seams covered well with fire clay, filling the box in with bone dust closely packed around the articles, or (what is better) with leather and hoofs cut into pieces about an inch in size, adding thin layers of salt in the proportion of about 4lbs. salt to 20lbs. of leather and 15lbs of hoofs. in packing the articles in the box, be careful to so place them that when the hoofs, leather, etc., are burned away, and the pieces of iron in the box receive the weight of those above them, they will not be likely to bend from the pressure. When the articles are packed and the box ready to be closed with the lid, pour into it one gallon or urine to the above quantities of leather, etc,; then fasten down the lid and seal the same outside well with clay. The box in then placed in a furnace and allowed to remain there for about 12 hours,"

    I believe the same book mentions quenching in mercury or quenching by hammering the workpiece into a block of lead.

    Its an old boook lol

    You need to copy Joshs design for your furance door.

    Stuart

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    Well the urine would only be for a source of nitrogen... You could use a nitrogen based material like urea fertilizer to achieve the same results... I would not recommend ammonium nitrate for obvious reasons, although that makes me wonder...

    Would placing a part in molten urea @ 500C be like nitriding it? It would only work with chrome moly steels though.....
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  13. #13
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    Hi RC,

    I'm thinking I should send you some Cherry Red, to try as a comparison, the process is simpler than pack carburizing, but I'd think the depth would be less

    Pretty sure you've seen this before, but for others who haven't Instant Steel Case Hardening: demonstration - YouTube

    They also sell a pack carburinzing compound called Wilcarbo WILCARBO Pack Hardening Compound which I think is closer to the technique you have developed.

    I have a small amount of that as well if you wanted to try it.

    Regards
    Ray

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    Hi Ray, Thanks for the offer of some cherry red but you might be better trying it in your furnace... I have to do some work to my kiln to make it a bit more efficient, I purchased some ceramic wool insulation today as there is nothing between the bricks and the outer steel case, So any heat leaking out of the bricks escapes...

    Although on talking to a person that works on kilns this heat leakage is inevitable if you keep the kiln on for long enough at a high enough temperature...

    Was thinking again about urea as a nitriding source, but it melts at only 130C so would be all boiled away by the time 500C was reached....

    You can buy charcoal case hardening compound locally, Fuch's make it, but it is very pricey compared to what you can make yourself...
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  15. #15
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    Another day, another experiment.

    Today I hardened a threading tool I had made out of mild steel, again it ended up 60HRC after quenching...

    In this heat I included a small piece of mild steel rod and a small piece of 4140 rod to use as test pieces..

    The mild steel rod, like the tool I hardened ended up at 60HRC...

    The 4140 rod ended up a bit softer at only 50 HRC... I wonder if the alloying elements in the steel inhibited the amount of carbon the steel could absorb.... I have not read of 4140 being used for case hardening.. It is always the low carbon steels...

    As for depth of hardening... Not sure how to test for this, so I chucked up the mild steel piece in the lathe and attempted to face off the end...

    I did end up cutting some off, enough to reveal a soft core but the hardened outer area seems to be around 2mm thick... Hard to believe this to be the case as the tables say for such a depth of case to be made the amount of soaking time has to be for much longer then I had these int he kiln.
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