Truth to a rumor

[Taye]

In the months that i've been actively smithing, ive come acros a rumor that i would like to resolve. A few people that i know have told me that when you burn coke hot enough, it will add carbon content to a piece of steel. Is there any truth to this? or is it just a rumor?

[Maillemaker]

Kind of. The effect is called carburization. As I understand it, when iron is heated almost to melting and held in a high-carbon environment, the iron will begin to absorb the carbon to create steel. This process takes hours, so carburizating your steel shouldn't be a problem. At least, that's my understanding.

[Nakedanvil - Grant Sarver]

It's a pretty fine line, carbon monoxide will carburize, while carbon dioxide will de-carburize. Also carburizing take much longer than de-carburizing. Not likely to get much carburizing where the carbon is also the fuel. Quite different than carbon "packing".

[pkrankow]

Yes, you can add carbon by having a tall fire and low blast, and put the iron in the top of the fire. A thin edge can absorb carbon readily enough to make a suitable cutting blade, but it will be just a shell. You can also create wear resistant or pieces suitable for sparking with flint in this manner. If the fire burns hollow the work can be undone quite readily.

Pack hardening is a different story, and a whole lot easier. You take steel, put it in a sealed high temperature vessel such as a steel can or a clay jar and fill the space with carbon bearing material such as coke, or organic material such as leather, paper, or flour. Then you "seal" the container, leaving a small hole for it to breathe and heat it up. At first you will for a high carbon shell, and the steel can be quenched and case hardened. Then you will get carbon though out the bar, but the bar will distort and "blister" (this is how blister steel is made) eventually the carbon content will be high enough to be called cast iron (over 2% carbon) but you should have stopped before there. At welding temperature, carbon improvement progresses at about 1/16 - 1/8 inch per hour, but it will also progress at a lower speed at a lower temperature.

Phil

[ThomasPowers]

You can carburize with pretty much *any* carbon based fuel including charcoal and propane.

HOWEVER I've seen a number of people claim they were making high carbon steel from mild *while* forging it when in truth the thickness scaled off was much thicker than the depth of carbon migration *while* forging.

If you are increasing the O2 to make the fire burn hotter for this effect you can also be changing it from a reducing fire to an oxidizing fire and so do just the opposite.

If you are interested in this may I commend to your attention "The Cementation of Iron and Steel" Giolitti, and "Steelmaking Before Bessemer, Vol 1 Blister Steel" Barraclough. Which you should be able to ILL from your local public library in the USA. (can be quite pricey to try to buy)

[Taye]

When you say shell, is that something that would be desireable. A softer inside would absorb shock and stress, and a harder outside would hold an edge right?

[Nakedanvil - Grant Sarver]

Hey Phil! I agree 99%, but I've never seen it claimed that carbon could reach cast-iron levels by this process. In the process used at Sheffield, they started with 5/8 X 3 Norway iron and after a week to 10 days at nearly 2000F they would get about 1.5% near the surface and less than .5% in the center. I don't think it will go much above 1.5% while it's still migrating toward the center. It's affinity for carbon goes down fast as it approaches 1.5% too.

[pkrankow]

When you say shell, is that something that would be desireable. A softer inside would absorb shock and stress, and a harder outside would hold an edge right?

yes and no...

For making a blade, if you carburized the part to a depth of 1/16 of an inch, and your primary bevel is 30 degrees, if you remove 2/16 of an inch (1/8 inch) from the edge you are into the soft metal already. This is why nobody carburizes to make a cutting edge if they have a choice, but may explain why cheap trade knives were sharpened from one side only, sorta like a chisel, historically speaking.

Now, if you were making a frizzen for a flintlock, then this process is used and is historically correct. According to my limited research on the topic both methods have been used successfully by different groups through history.

Case hardening using a modern version of this process is used to improve drive axles in some vehicles, as well as provide wear resistant surfaces on machine parts such as gears. In that process an oven with a controllable atmosphere is used.

The concept is useful and valuable, even if the implementation can be tricky and often means using a solid piece of the better material is easier and cheaper in the modern world.

Phil

[pkrankow]

Hey Phil! I agree 99%, but I've never seen it claimed that carbon could reach cast-iron levels by this process. In the process used at Sheffield, they started with 5/8 X 3 Norway iron and after a week to 10 days at nearly 2000F they would get about 1.5% near the surface and less than .5% in the center. I don't think it will go much above 1.5% while it's still migrating toward the center. It's affinity for carbon goes down fast as it approaches 1.5% too.

I have read about apparent cast iron levels being reached by accident using these processes, but only as part of small scale experimentation.
Phil

[WmHorus]

From my reading and talking to various smiths who have experience doing it, the japanese do something similar using charcoal , They can either raise or lower the carbon content in the steels that they use for knife and sword making. It just depends on where the steel is placed in the fire in relation to the charcoal and air flow.

[ThomasPowers]

I've reached cast iron levels doing a pack carburizing. Sure did act like it when we tried to weld it.

Ric did a pack carburizing and a "run small bits of wrought iron through the forge multiple times and then weld it up" as part of his "3 methods of making steel" Demo at Quad-State one year. (the third was a melt!)

According to Barraclough the temps used in the English blister steel industry were quite a bit cooler than 2k degF. They had a battle between carburization and grain growth both of which like the higher temps!

[Nakedanvil - Grant Sarver]

OK, you made me re-read my "Cementation of Iron and Steel". They do claim that with sufficient time and/or temperature cast iron can be achieved. It does state that a temperature of 1985F (1085C) was favored because it could be guaged by the melting point of copper. I have not seen any reference to concerns with grain growth.

[MattBower]

You're going to grow grain at ~2000 F, no doubt about it, but then your blister steel will be cut, stacked, welded and rolled out -- perhaps more than once -- then forged down to usable sizes. Plenty of opportunity to fix any grain issues.

The one guy I know who has done some real experimentation with blister steel uses thin-walled tubing for the container, thinner than the stock he's carburizing (~1/4"), and runs at near-welding temps to keep the time down. (Like many metallurgical processes, carburizing is much more sensitive to temperature than time.) The tubing is the canary in the coal mine. If it converts to cast iron and starts to melt, it's time to stop. I believe he learned this tip from Ric.

I may be wrong, but I had the impression that the blistering in blister steel had to do with the carbon reacting with silica slag in wrought iron to form carbon monoxide, which wants to get out of the billet somehow. Thus blisters. The only time I've seen such a blister was when I was heat treating a wrought iron hawk with a 1095 bit. A blister popped up out of nowhere, ~1/4"-3/8" behind the seam, where the WI was thin.

[bajajoaquin]

So it sounds like the rumor is true, strictly speaking, but unlikely to be actually useful in forging.

Which reminds me of the saying, "In theory, there's no difference between theory and reality. I reality, there is."

[Ric Furrer]

The blister steel bars I make can be broken with bare hands..large bars take bear hands...and the grain is like gravel. Forge this down by 50% using good forge practice and the issue is resolved.
Once welded into shear steel the issues are all the same as any other steel laminate.

Ric

[Ric Furrer]

Which reminds me of the saying, "In theory, there's no difference between theory and reality. I reality, there is."

Yogi Bera

[MattBower]

The blister steel bars I make can be broken with bare hands..large bars take bear hands...and the grain is like gravel. Forge this down by 50% using good forge practice and the issue is resolved.
Once welded into shear steel the issues are all the same as any other steel laminate.

Ric

I found a great write-up about blister steel grain and judging quality from the color of the broken steel, in a 19th century book on Google books. Now I can't find it. D'oh.

[Ric Furrer]

Matt,
Unless you are using the same starting stock I would think most other references are useless.
color changes with chemistry as would grain size (fracture appearance).

Ric

[Randy]

If you want to witness this process for yourself: take the raw steel and put it on the grinder and note the high carbon sparks. Then place in the fire and take up to a welding heat. Cool off and put on the grinder again. You'll see a lot more carbon sparks and fairly deep. This works on pure iron, too, and in coal or coke fires.

[Randy]

Try in your gas forge and see the results.

[Taye]

Thanks to everyone who posted. it may not be feasible for any knives i plan to make, but i get a feeling like this is a useful tidbit to have nonetheless

[ThomasPowers]

In the UN manual of blacksmithing written for use in Africa they talk of making the simple triangular blade axes out of mild steel. Then heating them and a piece of cast iron up and "crayoning the cast iron onto the mild steel letting it diffuse and form a high carbon steel edge. Another interesting trick.

[Nathan Hall]

I worked for years in a shop that sometimes made prototype tooling and we would often carborize cold roll and use it for form blocks, it will improve the the life of the steel in a wear type situation, this was good for a limited number of uses depending on the application, we would make a suitable sized steel container, place our block of steel in it with charcoal seel with mud and place in our heat treat oven, which lead to a rather exciting ten minutes one day. We sent one of our delivery guys to walmart for some charcoal, and none of us thaught too tell him what type to get, he baught match light kingsford, and none of us noticed until we opened the door on the oven.............. lotta fire then, so although any brand will work just fine please avoid the match light types, sealed container of lighter fluid impregnated charcoal in a 1750 degree oven makes for a heck of a show when you open it up and introduce oxygen.

[MattBower]

I worked for years in a shop that sometimes made prototype tooling and we would often carborize cold roll and use it for form blocks, it will improve the the life of the steel in a wear type situation, this was good for a limited number of uses depending on the application, we would make a suitable sized steel container, place our block of steel in it with charcoal seel with mud and place in our heat treat oven, which lead to a rather exciting ten minutes one day. We sent one of our delivery guys to walmart for some charcoal, and none of us thaught too tell him what type to get, he baught match light kingsford, and none of us noticed until we opened the door on the oven.............. lotta fire then, so although any brand will work just fine please avoid the match light types, sealed container of lighter fluid impregnated charcoal in a 1750 degree oven makes for a heck of a show when you open it up and introduce oxygen.

That's funny. I once got my charcoal kettle grill really hot, with a nice, thick bed of coals, threw a few steaks on, then put the lid on with the vents almost closed. A couple minutes Iater I lifted the lid to check the steaks and WHOOSH -- no eyebrows! After that I changed the way I lift the lid.

Back on topic, I have carburized mild steel in much the same way by packing it in a container with charcoal and leaving it at the bottom of a big wood fire for a good 12 hours or more.