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pack steel in an airtight container, fill with carbon bearing compound soak at a red heat for a week or save yourself the time, trouble, fuel and effort and start with high carbon steel. I think if you do a google search you can come up with the exact process, or search this forum I believe it was covered here a couple months ago.

Edited by Woody
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Correct me if I am wrong on a few points here, relevant to carburising. I am sure someone will put me right if I am misinformed, but I understnd the following;

Carburising does not make mild steel into tool steel, it is merely a process for allowing case hardening to be undertaken after the process,

Some of the carburised surface can be removed by machining to leave surfaces that can be treated/hardened mainly for wear resistance (ie cams on machinery)

The depth of carbon penetration into the material is relatively small and penetration depends on the length of the soaking time, (proportionally I suppose thinner material will have a greater depth in relation to thicker material, although the actual dimension is the same) Up to 1/8" being a maximum expected penetration

There is little point in trying to temper a case hardened item

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It is possible to carburise a piece of relatively low-carbon steel. As already mentioned, it's placed in a carbon-rich low-oxygen environment (e.g. a stone or iron chest filled with charcoal) and kept at a cherry red for a length of time. The depth of the carburised layer is proportional to the time it spends in there. A matter of hours and the item is said to be case hardened or case carburised, with applications as already mentioned.

If you extend this heating time to, say, a week the depth of carburisation is dramatically increased. This is the method mostly used in the past to make steel. When wrought iron is used, the impurities in iron (slag mainly) cause gas to form and bubble out of the steel while it's soft, and so it's called blister steel. This is one of the steps that was used in making tool steel until relatively recently. After carburising the carbon content is heterogeneous (varies through the piece of steel), and so it must be refined. One way is by piling, where the steel bars are broken into lengths, wired on top of one another, and hammer-welded together. This produces shear steel; repeating the process produces double shear steel; repeating it again produces triple shear steel. Huntsman crucible steel is the result of taking blister steel, breaking it into small pieces, and melting it in a coke furnace to remove impurities.

To directly answer your question, yes you can carburise mild steel into a higher-carbon one. However if you're looking for something a bit better than case-hardening it will cost you a lot more in time, fuel and burned steel to make it yourself rather than just go and buy some. However if you want to do it for the challenge, go right ahead! There are some resources available on the internet I believe form bladesmiths who have made their own shear steel. If you do, make sure you document the process and let us know about it!

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Case hardening or, carburizing as some of you call it , is generally reserved for a specific application.It is used when you need something with a very high wear surface but a tough interior that is shock resistant. A good example is for a pin in a draw bar or coupler pin on the old railroad coupling knuckles. This is a very old method used many,many years ago before the higher alloy steels were created.
They used mild steel pins that are very shock resistant and placed them into clay or steel containers along with a carbonizing compound, and yes old animal hoofs were sometimes used as was leather and lamp black, or charcoal and heated the containers up to 1500 to 1600 degrees for several hours. The outer skin of the mild steel would absorb carbon from the compound raising the overall carbon content quite significantly. However, this skin was perhaps only about thirty Thousandths of an inch deep after about eight hours of soaking. The piece was removed from the container and quenched in water. This gave the outer surface of the steel a very hard surface yet left the inner core of the steel relatively un-affected.
This made the outer skin Very Resistant to wear and greatly extended its useful life and at the same time still retained its ability to withstand sudden shock loads and shear strength in its normal use. The pieces were not tempered after hardening as there was no need, and no machining was done to the piece due to the fact the hardened outer skin was so shallow in depth.
With todays modern alloys you can now heat treat the steel to get the hard wear surface you require and still retain the toughness in the steel to prevent breakage so the use of case hardening is somewhat redundant .

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You can also make orishnagane by cycling small bits of wrought iron or low carbon steel through a charcoal forge several times and then welding them up into a billet.

Ric Furrer demonstrated "3 ways of making steel" one quad-state: Blister Steel, orishnagane and crucible steel; he was able to get a material that sparked like 1095 starting with old WI scraps.

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