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Thread: Case hardening success

  1. #1
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    Default Case hardening success

    All the photos can be clicked to view bigger. CC BY-SA version 3.0 unported license applies to them all.

    It has taken me some time, but finally I was able to experiment case hardening with my home made heat treatment oven. Now that I have established some basics it is less of a mystery or black magic and I hope that others can benefit from this also.

    All this was done at home in my little 'shop room' and produced no odors, smoke or anything so wife is happy and so am I.

    For the record, I'm after actual case hardening and NOT color case hardening, as this is going to be used on tools and not decorative items, so depth of case, hardness and time are of interest. Tools will be turned, milled, ground or polished after hardening, so the oxidation colors will be gone if there is any.

    When I tried to find information on this 'lost art', all I got was for color case hardening, which frankly is black magic to 'get correct'. All their recipies for the mixture included anything from plain charcoal to one-winged bats eyelashes thrown in to the mix. And color case hardening isn't so much after the deep case as it is for the colors (which by the way are awesome).

    What I finally found was actual books on the subject, dating back to the early 1900's when this pack carburization or pack case hardening was done. These detailed many aspects and the process itself, so I read a whole week this new-old-information and armed with this knowledge I fired up my oven and got to work.

    What is case hardening?

    In case hardening the idea is to have a low carbon steel (like structural steel) workpiece that won't harden on its own. The workpiece is cooked in a carbonaceous material (like charcoal) at high temperature for X long period of time to diffuse more carbon to the surface of the steel, thus making a layer of high carbon steel that can be hardened by heating and quenching.

    Why would you do it?

    To get a hard, wear resistant surface on steel that won't harden on its own. Or to get a workpiece that has a hard skin but tough core, thus combining the benefits of both the hardness and toughness. Ordinary low carbon steel is soft to work with, doesn't cost a fortune like highly alloyed tool steels and hardening only parts of the workpiece is very easy.

    What happens?

    The carbonaceous material burns to produce carbon monoxide as there is more carbon than oxygen inside the container. When the temperature is high enough, this carbon monoxide breaks to produce carbon that diffuses in to the steel and the oxygen recycles again to carbon monoxide. The longer the steel is at the high temperature, the deeper the carbon can diffuse. The higher the temperature, the faster this happens.

    If one includes carbonates in to the mix, they will speed up or enchance the formation of carbon monoxide, thus providing a better atmoshpere inside the container. Usual carbonates ae those of sodium, calcium, potassium and barium, though I would not recommend the barium carbonate due to its toxicality. Sodium carbonate is the same as washing soda and calcium carbonate is the same as chalk, both easily obtained and very cheap.

    Container

    First you need a box or container that is large enough. Large enough meaning that there should be about 25-50 mm of space around the workpiece when inserted. Bigger works too, but requires more heating, so a suitable sized container is the best. The container can be made from your basic steel by welding together, like from round or rectangular tubes and plates.

    In the following photos you can see that my containers were made from 102 x 3.5 mm steel pipe with caps welded on as the bottoms. The bottoms and lids were sawn off from 100 mm diameter bar and cleaned up on the lathe (not necessary, but bugged my eye). To the lids I turned a small should, 5 mm long and 0.5 mm smaller than the tube inside diameter so it fits on easily but isn't excessively loose either. The open end of the tube I turned on the lathe to get the lid to sit flat on it.

    Source of carbon

    Next one needs a source of carbon. Charcoal is quite cheap, easily obtainable source for carbon, but the usual form that it comes in is briquettes. These are formed from whatever organic material was at hand and pressed together with some binder. Do not use them, but instead find a charcoal bag that says "100 % something wood", something being preferably a hard wood.

    This charcoal has to be crushed somehow and it produces fine powder that will mess up everything. I crushed a 2 kg bag of it with a chunk of steel, but I would advice something else, especially if a machine can be used. After this I used an ordinary kitchen sieve to get the fine dust out of it (did it near a vacuum cleaner) and collected the rest in a plastic container. The 2 kg bag got me about 6 liters of crushed charcoal that doesn't contain dust.

    Edit: The charcoal is best if it is 100 % hardwood (usually the packaging label says this) to ensure that it isn't made of whatever organic material was laying around. The pieces in my sack were 100 % birch and one could find bigger pieces that had the wood grain patterns still showing.

    The other good source would be just buying activated charcoal, as it usually comes in a pill form and is thus ready to use. I haven't tested it yet, but it probably will be my source in the future.

    If one wants to add carbonates, a 10-20 % should be enough. My own containers pulled 175 grams of charcoal, so I added 25 grams of carbonate to it and mixed well to get a 15 % mix.

    Packing the container

    Packing the parts in to the container needs some prep work, like cleaning them from oils and dirt. Then the container is first filled with the carbonaceous material, about 20-50 mm bed is good. The container is tapped on the table or with a hammer on the side to settle everything. Then the part is inserted in the middle of the container and more material poured around it to support it. It is advisable to tap the container once in a while while adding to make sure the part sits tight with inside the pack. Then on top of the part is again poured a thick layer of the carbonaceous material, 30-50 mm is enough. This extra is allowed for shrinkage, as the carbon slowly sinks in the container a little.

    Once the container is full, put the lid on and make sure it closes down and doesn't bear on the carbon mix inside or that the seal area doesn't have the mix on it.

    Carburising

    Put the container(s) in to the oven, preferably to the middle to provide even heating. It would be best that the container have some legs under it or that it is located on two pieces of scrap iron so that the hot air inside the oven can heat the container from the below also. Close the oven, heat up your oven to 900 degrees Celsius and let it soak for 4 hours. After the 4 hours is up, cut off power to the heating elements and let the oven cool down on its own.

    The parts have been now carburised. They are still soft, but the surface layer is of high carbon content steel. The parts can be machined further now to make threaded holes or other features to so that they will not harden later on.

    In this picture the containers have been removed from the oven after they have cooled down. The surface on the lid looks like it has been just come off from the mill, scaled badly and the scaling comes off just by touching it. Quick cleanup on one of the lids and check with a vernier caliper shows no appreciable change in thickness.



    Once the containers are opened, the drop in carbon level can be seen. Next to the lids is my control piece that is the same size as the parts in the containers but it didn't receive any heating. The carbonates can be seen in a few containers as non-black substance.



    Some fishing and cleaning later I had my test specimen in a row. In this picture from left to right is control piece (K), plain charcoal (C), calcium carbonate (CA), sodium carbonate (NA) and barium carbonate (BA). Nothing special off them, a little darkness on the surface, the sodium one being black for a reason unknown. The coloring on the control piece is due to me playing with copper coating and sulfuric acid. All the pieces were cut off from a 25 mm diameter cold drawn S355 steel bar.

    Hardening

    I put the carburised pieces to the oven and set my oven to heat up to 800 degrees Celsius and while waiting, went to the local hardware store to buy the cheapest pipe tongs I could find. Back home removed the plastic handles from them, straightened the handles in a vise and cleaned them up a little. Lastly I welded pieces of 16x1 mm stainless tube over the handles with a MIG. Cheap and easy way to get long tongs. Also seen is my welding gloves to protect my hands. What is missing in the picture is a face shield.



    Once the oven reached 800 degrees Celsius, I waited for 30 minutes for the heat to soak through the parts. Rule of thumb is that heat goes in 25 mm per hour, so a 25 mm piece heats up in 30 minutes.
    Last edited by Jaakko Fagerlund; 06-20-2013 at 04:37 PM. Reason: Added details of the charcoal and changed wording from briquette.
    Amount of experience is in direct proportion to the value of broken equipment.

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    After 30 minutes I put on my face shield, gloves and took the tongs ready in my hand and opened the door. It is best to remember where you put those workpieces, as everything in the oven is the same color, more or less. So I grabbed one of the pieces quickly with the tongs and dunked it in to the water and swirled a little until most of the sizzling went away and dropped it in to the bottom of the metal mesh basket I had hanging in the water bucket. Did this same with all the pieces, probably took about 15 seconds in total and closed the oven door and shut it down.

    With the parts in the basket I swirled them around more until they had cooled down. Picked them out, dried and took my best file to test them out. Yup, glides like a lubed granny on ice, they are extremely hard.

    Afterwards I did a not-so-scientific test to see how deep the case hardness went by turning them on the lathe until file soft appeared. I used a Kennametal ceramic insert, cut like a butter and gave off that nice spark show, yet again confirming that it indeed is hardened. After all the test specimen were turned, I measured the diameters. There were no apparent difference between the ones that were in carbonate mixtures (around 0.9 to 1.0 mm thick layer), but the plain carbon one showed only a layer of 0.7 mm thick.



    So it seems that the carbonates do speed up the process compared to just plain carbon, but there is no distinct difference between the different carbonates. So with this in mind I would not recommend using barium carbonate in any mixtures, as it has no benefit that would overrule its toxicity. Even plain charcoal works, though a little slower, so it is up to you if you want to wait a little longer or speed up things by using the safer carbonates of calcium or sodium.

    Methods to stop carburizing

    Sometimes there is a need for a part that is partly hardened, like only a bearing position or surface. Or a part has to be machined by conventional methods further after hardening. With case hardening this is easily done in several ways.

    One way is to have only the portion of the work to be hardened inside the carbon container, thus only that portion receives carbon. This of course requires a special container to be built and for a one off piece might be too much work.

    Other way is packing the part partly in sand. For example, if the part has a large area on one side that needs to stay soft, the bottom part of the carburising container is filled with fine sand and the part put on that and then filled with the carbon mix. Later on separating the carbon ix and sand can be a little problematic, but works for one off parts easily.

    Covering areas to be left soft with heat resistant clay is one option, for example covering portions of a blade or holes or threads.

    For protecting threads one can also just screw in a bolt or nut over it to stop the carbon from penetrating. Copper bolts and nuts are recommended, but ordinary non-plated ones will work also.

    Other option is first making the part without any holes or threads, carburise it and after that make the holes and threads. This effectively removes the carburisation in those areas, so they will not harden.

    The last technique that I like the most is copper deposition. This puts a small film of copper on the part that prevents carbon from diffusing. Later on this copper can be removed by machining or with acid bath or with fine emery, as it is very thin layer only. What you need is 1.5 % sulfuric acid solution to which copper sulfate is added as long as it dissolves. Sulfuric acid can be had from auto parts store as battery acid that can be mixed 1:20 with water. Copper sulphate can be had from pottery or ceramics supply.

    The workpiece is first cleaned from grease and oils and then either dipped in the copper sulphate solution or it can be painted on also. The copper layer emerges instantly on the part. Rinse the part in clean water to remove any traces of the copper suplhate solution and acid and dry it to prevent rust formation.

    The copper deposition can be stopped with for example nail polish from areas that need hardening. For example a Dickson style quick change tool holder that is to be hardened only from the V-grooves. Nail polish is applied to the V-grooves, dried and then dipped in the coppering solution, rinsed with water and the nail polish removed with acetone. Now the part is ready for the carburisation an only gets carbon on the V-grooves.

    The copper deposition method is the best when the part has features that are hard to protect otherwise or there are more than one part to protect.

    Here is an example of a piece of steel dipped in the solution and immediately raised. It was not cleaned in any way (staged photo) and this shows as black deposits on the otherwise good copper surface.



    All in all case hardening isn't rocket science and can be easily done at home, if one just has an oven and can manufacture the container for carburating the parts. The methods outlined above are not The Only and Ultimate Truth(tm), but merely the way I did it and had success.

    Pros
    -low cost
    -easily done at home
    -only cheap steel is required
    -selective hardening easily accomplished
    -easily controlled penetration depth
    -no odors or fumes
    -no dust if one sieves the charcoal before use

    Cons
    -takes a few hours of time at heat
    -requires actual workpieces and not just playing with cut offs
    -the author is a d*ck

    You may be interested in reading my other thread on building the heat treatment oven, in case you don't have one already or you want a third one. Here is the link: http://bbs.homeshopmachinist.net/thr...photo-heavy%29

    TL;DR: Well shame on you

    List of sources:
    -ASM Handbook vol. 4 on heat treating (that is a 46.4 MB PDF)
    -an old heat treating book called Casehardening
    -a couple of books in Finnish (sorry, no translation exists for them and the stupid copyrights forbid me from doing that) from the library, dating back to 1920's.
    -Google/Youtube searches with words like pack case hardening, pack carburising/carburisation/carbonizing/etc. and all kinds of relative searches that I could come up with. It is also a good idea to search through the image search for anything resembling pack carburising as it may give links to pages the regular search didn't come up with.
    -Wikipedia has a nice article on case hardening worth reading.

    I hope that's a good start, but the best source for information probably is from the library, especially those very old machining books as they contain more than pretty pictures that they nowadays do.
    Last edited by Jaakko Fagerlund; 06-20-2013 at 03:03 PM.
    Amount of experience is in direct proportion to the value of broken equipment.

  3. #3
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    IMHO, charcoal briquettes designed for the BBQ is a terrible source of charcoal for case hardening.

    Depending on the brand those briquettes can contain charcoal (derived, from basically anything organic that can be rendered to charcoal: hardwoods, soft woods, saw dust, fruit pits, coconut husk, peat, shells, rice or peanut chaff etc, …think cheapest source), coal/coal dust, corn starch or other binders to hold the briquettes together, lighter fluid, Borax, Sodium Nitrate (accelerant), wax (binder& accelerant) limestone ( ash whitening agent). So as you can see there can be a lot of undesirable stuff in charcoal briquettes.

    If you’re going to continue to do case hardening, I recommend that at a minimum you locate some “Lump Hardwood “ charcoal. The wood charcoal that I purchase is made specifically for case hardening, made from hardwoods, with no softwood, binders, waxes or fillers, and sieved to size.

    I would also recommend locating some bone char; Ebonex is the largest supplier here in the US, I understand that they have their product made in the UK.. so it’s available “across the pond” .

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    Forgot to say, but the charcoal I bought is not the cheapast sack I could find, but specifically made from hard wood (and made only of that, according to the manufacturer).

    Quote Originally Posted by Mike Hunter View Post
    I would also recommend locating some bone char; Ebonex is the largest supplier here in the US, I understand that they have their product made in the UK.. so it’s available “across the pond” .
    Could care to elaborate as to the why? Bone is mainly calcium phosphate and some carbonates, so it doesn't do much good (actually phosphates should be kept out of the mix) that a pure carbonate would not do.
    Amount of experience is in direct proportion to the value of broken equipment.

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    Nice writeup on the case hardening procedure and heat treat oven build.

    Color case hardening is case hardening. The end result is no different in surface hardness or depth of case than non-color case hardening. The difference is that CCH is done under more closely controlled temperatures and quenching procedures than plain case hardening resulting in the attractive colors. Granted, there are a lot of distractions on the web regarding CCH, but, it is a straight forward process.

    The depth of the case is dependent on time and temperature. The longer the time and higher the temperature (within limits), the deeper the case.

    Mike Hunter does excellent restoration and color case hardening of collector quality firearms, and has spent much time and effort in refining the process. His advice is not to be taken lightly.
    Jim H.

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    Nice work, Jaakko. You probably had more time putting this together than you did with the whole hardening process. Well done.
    Wayne

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    Believe it or not, I’m not trying to argue with you, but actually trying to provide advice. I’ve been doing case hardening and color case hardening for about 10 years now, who knows maybe I might have learned a little something.

    On your briquettes, unless the manufacturer cuts the charcoal into cute little briquette shapes, there are binders/fillers etc, what else holds the charcoal dust in that shape? Unless it’s food or medicine manufacturers are not required to list the ingredients.

    The process of case hardening can and has filled volumes, and I’m not going to quote them all. Mr Gaddy, did an interesting study about 20+ years ago, here are some of his observations/conclusions:


    “Pure carbon or wood charcoal alone is not an effective carburizing agent as it produces very little carbon monoxide in a closed container at elevated temperatures. Another substance, usually called the energizer, is added to the wood charcoal to facilitate the production of carbon monoxide in the closed carburizing container. This is the main function of bone charcoal in the carburizing mixture.”

    “Only samples prepared with percentages of bone charcoal greater than 50 percent produced measured hardness greater than the cold-rolled steel substrate. A sample produced by carburizing in 100 percent bone charcoal for two hours at 715 degrees C. and quenched at this temperature was cross-sectioned and examined under a metallurgical microscope. The surface-hardened layer was found to be .002 inch thick and had undergone a true transformation to the Martensite phase of steel indicating that true hardening had occurred. Since the percentage of bone charcoal also affects the production of carbon monoxide in the pack and thus the amount of carbon introduced into steel, even thinner layers would be expected with steel casehardened with smaller percentages of bone charcoal under similar heating and quenching conditions.”

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    Quote Originally Posted by Jaakko Fagerlund View Post
    Afterwards I did a not-so-scientific test to see how deep the case hardness went by turning them on the lathe until file soft appeared. I used a Kennametal ceramic insert, cut like a butter and gave off that nice spark show, yet again confirming that it indeed is hardened. After all the test specimen were turned, I measured the diameters. There were no apparent difference between the ones that were in carbonate mixtures (around 0.9 to 1.0 mm thick layer)
    Did you read that article I posted from the ASM Metals Handbook about case hardening? Barium Carbonate is used extensively in industry for a case hardening accelerator. They quote the acceleration in case depth per minute.

    Like the others have said, BBQ charcoal is a poor source of carbon. You'll probably get a much deeper case depth with a pure carbon source like activated charcoal. The guys on iForgeiron often use aquarium charcoal and it's said to work great.

    I still have a ton of Kasenit, and Cherry Red is still widely available, so I'm less motivated to develop home brew casehardening recipes

    Edit: Jakko, Mike Hunter is a well-known gunsmith who specializes in restorations. You should listen to him
    Last edited by lazlo; 06-20-2013 at 01:07 PM.
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    Sure I take any advice given, thank you for those Mike, but there is a reason for using everything. Bone charcoal is used as an energiser because it contains some carbonates, but it is mainly calcium phosphate which doesn't do anything to promote the carbon monoxide formation and that is why it is used in such large quantities in the mix. Easier and cheaper is to just add the carbonates as such, thus making a more pure mixture. Calcium carbonate and sodium carbonate can be bought from just about any hardware store and relatively safe to use.

    The barium carbonate I don't suggest using due to its toxicity and even industry has changed from using it to something other. And from my test the barium carbonate didn't differ in any way to the other carbonates, so there appears to be no sane reason to use it.

    I edited the first post about the details of the charcoal I'm using, but yes, it is called BBQ briquettes in here, even though it isn't compressed from dust. It is actually chunks of wood turned to charcoal, one can even see the wood grain patterns and the biggest pieces in the sack look like fire wood. And the law in here states that the ingredients have to be listed on the package or it can't be sold. On my charcoal package it states 100 % birch charcoal and it looks just like that.

    JCHannum, I may have conveyd my distinction between CH and CCH a little bit wrong. What I meant is that in color case hardening you basically dump the whole carburising pack straight to the water, whereas in 'normal' CH you don't have to do it, thus making the process nice and clean and so that it can be done indoors as there is no smoke, sparks and splashes.

    And what I've read is the color case hardening is usually done in lower temperatures and with shallower depth, as most of the parts are thin to begin with and don't require 1+ mm deep case. Tools that are to be ground require a deep case so that there actually remains something hard after grinding the warped pieces back to flat, so it is quite different from the gunsmithing objectives.

    I have to check if I can find the 1 kg bag of activated charcoal I remember once having, a leftover from making alcohol. If I can't find it, I'll just go and buy a new bag, costs probably 6-7 EUR. Quaranteed to be pure, as it is food grade.

    Edited to add: The bone and leather and hoof and horn charcoals used in CCH are mostly used because they contain nitrates, phosphates and god knows what else that is one ingredient to giving nice colors. But in plain CH they are not needed or wanted due to the impurities they contain.
    Last edited by Jaakko Fagerlund; 06-20-2013 at 02:11 PM.
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    Thank you, Jaakko. That was most interesting and informative, as was your post on making your heat-treating oven.

    You mentioned that you did a lot of study of the case-hardening process. Would you mind sharing with us what sources you consulted? The little I know about case-hardening (I have not tried it myself yet), I learned from the book "Gunsmithing", by Robert Dunlap.

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