Increasing points of carbon in the reducing part of the forge

B

Bo T

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Feb 12, 2011
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A couple of questions. Do you need to worry about grain growth while baking a blade to case harden it (i.e. 1000 F in the reducing part of the forge)?

Would it be practical to try to make mild steel into medium steel using this process? My interest is mostly academic.
 
From my understanding, the grain does not grow and become course until you go higher into the hardening temps, ( strictly thinking about high carbon content). It also depends on the chemical make up of the steel. So I would think that 1000F would not create the grain growth that you are thinking about.

I would also like to hear about what others think.... Subscribed.
 
Need to get a lot hotter than 1000f to case harden. At least with the system I used. Need to get it to 1700f and hold it there for an extended period of time. IIRC the carbon will travel aprox .010" per hour. So if you have a 1/4" blade it needs to soak with the proper materials for about 12 hours to get full penetration. So yes you would have grain growth and no it is not practical to try this in a home shop with readily available quality high carbon steel at reasonable prices. Also there is no carbon in propane so if you were to try it it would have to be in a coal or charcoal forge.
 
As an academic question. We did do case treating/carbonizing in machine shop class in college. We did the process for our tooling we made so we could actually harden it. Well we thought we did but the carbon material had actually depleted and we were just spinning our wheels. Once I figured this out I found some good material and wrote a new process to do it properly. So it is possible but not in a forge. We packed this into a sealed (not really perfect seal but enough to keep O2 off) container with the active carbon material. We were shooting for .050-.075 penetration and did a 12 hour process. Took us 1-2 hours to get the furnace up to heat then a long soak. After we hardened the pieces we precision ground them making 1-2-3 blocks, parallels, combination blocks etc.
 
The idea that you can add carbon to a blade by putting it in the reducing part of a forge fire is balderdash that was propagated to try to justify forged blades being superior to stock removal based on misconception of the blistering process and case hardening, both of which involve placing the steel to be carburized in a sealed container with carbon solids at high temperature for extended time.

-Page
 
The carbon rich area of the forge ( reducing atmosphere) will eat up any extra oxygen, and help with scale and decarb, but will add no carbon to the blade.

In the scenario given by Bo T, there would be no effect on the blade at all. It would be softer if anything, not harder.

Case hardening is done by packing the blade in carbon rich material ( bone and charcoal plus some stuff to supply a bit of oxygen) and sealing the container. The box is then heated for hours at almost welding temps. The carbon does not move into the steel, as it is a solid, and solids don't do that sort of stuff. The oxygen in the closed system combines with the carbon as CO and this permeates the skin of the steel....very shallowly. The carbon moves a bit deeper as it joins the steel, and the oxygen goes out looking for some more carbon in the charcoal to do it all again...over and over. It takes a good bit of time just to put a skin of hard steel on the soft metal.
Of course, if you grind or sand the steel , the hard skin of "case" is removed.
 
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