Carburizing iron knives

Metalhead0483

Metalhead0483

Joined
Jan 17, 2008
Messages
539
Hey all. So as many of you know I have been playing with early and medieval knives. Specifically with welded wrought iron and steel construction. If you're curious I've got a couple of completed WIP threads on this forum.

Recently I've been watching a lot of Clickspring videos on youtube (mostly because I find machining videos to be relaxing ;) ) He's done several videos where he has case hardened (carburized) mild steel to make files and drill bits in much the same way as Theophilus describes in "On Divers Arts." It got me thinking... Is there any reason I couldn't do the same thing to a wrought iron blade? Forge the blade to shape, grind it to maybe 85% or 90%, carburize it (pack with a mix of charcoal, flour, and salt and encase completely in a clay vessel, heat and hold at critical temp for a time) then heat treat and finish as I normally would.

I realize it likely wouldn't give the same performance as a modern blade. And that there would likely be fairly uneven carbon absorption due to the lack of homogeneity in the iron. My reasoning here is if they used this technique to make files (and we know they did), why not blades? Or am I missing something?
 
This is commonly known as "case hardening."
The carburized layer is very thin.
Once case hardened, you do not further heat treat.
 
That was my first thought too Bill. But I would encourage you to watch the videos in question. Chris is getting martensitic steel several millimeters in from the surface after carburizing, after which he performs a backyard heat treat like we are all familiar with. His results seem very solid.

If for the sake of argument say he's getting carbon penetration 1.5 millimeters in from the surface, that's essentially full penetration on an 0.125" blade, and say about 75% penetration on a 1/4" blade, leaving a soft inner core. Why wouldn't that work?

Again, I know it's not going to perform with one of my modern blades, or even with one of my welded edge wrought iron pieces. But I'm curious enough to give it a try.
 
Have you by chance seen the Aristotle furnace and the resulting steel? I’ve thought several times about trying it and using some wrought iron. It’s not quite the same process as what you’ve described, however it is a means (and a very old one at that) for adding carbon to iron. The resulting steel “puck” can be forged down and used for blades and can leave (in my opinion) very aesthetically pleasing effects when etched and sanded out.

Good luck with it all, I’m curious to see what you come up with.


Jeremy
 
12345678910 said:
try a san mai with a real hardenable steel core.
Click to expand...

Sigh... I have done san mai blades in the past. And any number of monosteel and welded blades. This isn't about creating a "real" knife with a "real hardenable steel core." I'm trying to use a medieval period technique to create a useable (hardenable) knife out of solid wrought iron. If nobody has done it before or has real world examples of why it wouldn't work I'll just try it and see what happens.
 
Why would that be the case? If I get even just 0.04" of carbon penetration (and my research suggests that I should be able to get significantly better than that) that's a good percentage of the blade...
 
I would try it. It sounds like a fun project and you're not trying to win a BladeSports competition with the product. Given that technique, I would expect that there would be a carbon gradient from surface to core unless you kept it at temperature for a long time to equilibrate. If you do, you have limited control of how much carbon you introduce (possibly associated with availability as well as inhomogeneity of your wrought iron). So you may range from mild steel all the way to cast iron depending on the procedure used. I would expand on his techniques, try to harden your product, and hardness check the results.

Mike
 
I think it sounds fun. The guy on forged in fire that had a mild steel blade I kept thinking it might have been worth trying to case harden it.
 
knife to a gunfight said:

Here's the video being referenced, for those interested. Skip to around 4 minutes if you want to see the pertinent info.
Click to expand...

Yep! That's the one. He has three videos that reference the process. One is about making files, and the other drill bits. All three are totally worth watching.

Thanks for the link by the way. I was at work and couldn't access YouTube :-\
 
As you mentioned Click Spring's videos on the subject are excellent.

But also check out Green Beetle on Youtube. He has a number of videos where he has successfully experimented with case hardening knives made of iron and various other low carbon steel objects.

Here is one of them:


~Paul
My Youtube Channel
... (It's been a few years since my last upload)
 
Case hardening ( carburizing) is a surface effect. But, when the surfaces are as close together as they are on a knife bevel, the effect can be through the entire metal. There may be a small softer core at the spine and upper bevels on thicker blades, which would actually be a good thing. The technique has been used for a long time to make hardened knife blades.

Now, as to the question about re-hardening after carburizing:
When steel is carburized, carbon is added to hot steel. The parent steel usually has low to nearly no carbon content. If placed in an environment filled with free carbon ( like packing in bone meal), the carbon migrates into the steel. This will continue to a degree of around .50% carbon fairly easily, and if held at a high enough temperature and long enough, can attain eutectic equilibrium. Holding longer can gain a bit more carbon, but the eutectic is a good goal. This is carbon rich austenite. Upon cooling, depending on the rate it becomes pearlite, martensite, and various other structures. Any martensite formed would be brittle if the carbon content was above .50%. Thus a tempering is needed. If one was to merely take the blade out of the carburizing bed and cool it, it would not be likely to last well as a blade. The grain would very likely be coarse, and the metal brittle. Tempering could deal with the brittleness, but the grain would still be very coarse.

However, if the carburized blade was given a subsequent full HT of grain reduction, austenitization, quench, followed by temper, it would create a blade equivalent to any modern 1084 blade.
 
I've watched some Green Beatle videos, but I hadn't seen those, Thanks!

And thanks Stacy, That's exactly what I was thinking. I'll be curious to see how using wrought iron with it's lack of homogeneity will effect things.
 
I think you may want to research “shear steel” rather than case hardening if you are looking for a historical approach to making steel from wrought iron.
 
Metalhead0483 said:
I've watched some Green Beatle videos, but I hadn't seen those, Thanks!
Click to expand...


Interestingly enough this was just uploaded yesterday.

It shows "Shurap" case hardening a RR spike knife which may also give you some ideas.


--------------------------


I also agree that shear steel is a good idea for historical knives. Although that is basically how Green Beetle made that "'Murica" knife in the video above, except he used pieces of pure iron as opposed to wrought iron.

~Paul
My Youtube Channel
... (It's been a few years since my last upload)
 
lanternnate said:
I think you may want to research “shear steel” rather than case hardening if you are looking for a historical approach to making steel from wrought iron.
Click to expand...

Shear steel is one I'm familiar with. Cut and welded blister steel, which was created by a similar, but far lengthier process than what I'm talking about here. Also it's quite a bit later in history than I'm working with (17th century as opposed to 10th-12th century)

ColdSteel I love Shurap's channel. I follow him semi-religiously. I hadn't seen the new one yet though. The timing is kind of funny... ;)
 
I had a similar question about making springs out of mild steel this way...

It worked like a charm for me even though I rushed the process and overlooked some major things Stacy mentioned above.

Having read that, I feel confident on making knives this way next.

Thanks Stacy!
 
You must log in or register to reply here.
Back
Top