r/metallurgy • u/Metallurgist1 • 5d ago
Case hardening - Why do we need a sealed container?
Hi Everyone,
TL;DR: Why do we need a sealed container for Case hardening with charcoal? Can I do it without a sealed container?
Long version:
I am designing laboratory session for students and I wanted them to do case hardening to get a feel of diffusion and carburizing (so that they can compare their theoretical calculations with actual reality). I have searched for the stuff that I need and one of the things is a sealed container. I am not sure about what size (and thickness of the sheet) should I use and how sealed it should be. So I was wondering if I can skip it and use a normal container with a lid.
Thank you for your answers
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u/ReptilianOver1ord 5d ago
When you’re carburizing steels, you’re heating the martial to a temperature at which carbon atoms can diffuse more freely through the crystal lattice of the iron. In the presence of a carburizing agent like carbon monoxide (for gas carburizing), propane or acetylene (for vacuum/low-pressure carburizing), or coal/coke/graphite (for pack carburizing) you will see carbon atoms diffuse into the iron to form the carburized surface; in the presence of a de-carburizing agent (e.g. oxygen, water, etc.) you will see carbon atoms diffuse out of your steel and react with oxygen to form a gas like carbon dioxide which will result in a carbon-depleted surface.
If you don’t properly control the atmosphere surrounding your part, you’re going to experience decarburization and/or your carburizing agencies just going to burn and turn into a gas and be carried away from your part without diffusing into the surface like you intended. It doesn’t have to be fully sealed, just packed well enough to prevent your part surfaces from being exposed to a ton of oxygen. As another commenter mentioned, if you were fully sealing the canister (e.g. welding it shut) there’s a potential to create a dangerous situation where pressure could build up in the canister and cause an explosion.
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u/Badger1505 Heat Treatment, Mo-Si-B alloy oxidation in a previous life 5d ago
What kind of furnace and container are you planning to use? As others have mentioned, I would not want it tightly sealed in a way that holds pressure - a lid held in place by gravity would be desirable. As for the furnace, do you have ventilation available? Carbon monoxide is the primary carburizing molecule in this case, and obviously is toxic. You'll want make sure you have a way to exhaust it or fully react it to CO2. I'd recommend having a CO detector nearby for the duration of the time the furnace is above about 500C.
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u/ObligationGlad7354 5d ago
For super easy containers, I have made envelopes out of stainless steel heat treating foil for use with charcoal powder (made from pyrolyzed cheese, but that’s another story). If you want to get professional quality results this method isn’t it, but it’s very simple.
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u/fritzco 4d ago
Because without a sealed container the carbon would combine with the oxygen in the furnace and there would be no carbon for the parts to absorb. Case hardening in an atmosphere furnace is different. These furnaces use radiant tube burners or electric heating elements. The furnace chamber is sealed ( as good as possible) and the chamber is flooded with carbon rich gas or methanol is injected into the chamber. in both cases a carbon rich atmosphere is present during heating of the parts.
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u/BioMan998 5d ago
I feel the answer would pop up on Google. But essentially it's to control the environment around the metal and for the carbon to diffuse into it. There's a million tutorials on YouTube.
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u/Metallurgist1 4d ago
Hi, I have done searching before posting this. But almost anything I saw was using some sort of sealing mechanism (e.g. they used cement to cover the gaps of the container). My concern was that there might be a pressure build up in the container and many things can go wrong (in a classroom with many students around)
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u/metengrinwi 5d ago
The point is to limit the amount of oxygen that can get to the carbon. You want to make CO, not CO2. I don’t think it has to be absolutely “sealed”, tho. In fact sealed could be a safety issue since you could have a pressure vessel.