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Question about case hardening


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I have tried searching to forum and have had no luck answering the question I am asking. I have tried the site:www.bladesm..... Search tool and have had no luck. When referring to the depth of hardening when an object is case hardened is it referring to total depth or the depth of any given side?

 

Example: item A is case hardened at 1/16 thick. Does that mean each side hardened to a depth of 1/32 or that total depth of hardness is 1/8?

 

I'm assuming that uniform depth of hardness is likely not going to happen but for ease of explanation let's assume it does.

 

Also, I see a lot of post about how horrible case hardened files are, but have yet to too see an explanation why. I guess what I'm having a hard time understanding is that you are looking at a piece of metal with a cross-section of 1/8 or less. If, as noted in my example above, the latter is correct wouldn't that mean a thorough hardening? If the former is correct how is that different from a shallow hardening steel like 1095 or W2?

 

I know a lot of blades on this forum are made with simple steels with a cross-section somewhat similar to this. I have heard that steels such as 1095 and W2 harden very shallow, in comparison to case hardening from what I can tell. Please correct me if I am mistaken, I would rather know the correct theory than remain ignorant to the subject.

 

If there is a thread explaining this and I missed it please point me in the right direction!

 

Thank you!

Edited by Josh Brannen
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Case hardening is far shallower than that, typically measured in thousandths of an inch. That is one reason case hardened files are not good, once you wear a little off the tips of the teeth that's it. The other reason is that you can't forge a good blade out of them afterwards because the overall carbon content is too low.

 

Your 1/16" example is pack carburizing, not case hardening. Same idea, but over a much longer time period at higher heat to allow carbon migration to penetrate the steel or iron if you're trying to make shear steel. A thick hardened zone for modern case hardening is around .010" .

 

That help?

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Oh, and I suspect you got shallow hardening mixed up with case hardening as well. In that regard, the 1/16" depth of hardening is inwards from every surface. That means on a blade you will have through-hardening anywhere the total thickness is less than 1/8".

 

Case hardening is only for steels that do not have enough carbon to harden in any quench and is a very thin skin. There are exceptions of course, 4140 is often given a hard surface via case hardening in industry to improve wear resistance and maintain toughness, but in general it is not something that makes a good knife.

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Another thing with shallow hardening steels like the two that you mentioned is that when you get to thin objects like knives things start to get weird. Let's say that with a given austinization temperature and grain size that the hardening will only be to 1/16". Now let's say that your blade is 1/4" at it's maximum. One would think that upon quenching that you would end up with a blade that is hardened 1/16" deep all the way around a core of unhardened steel with a maximum thickness of 1/8". This is not what happens, however. Because there is little difference in the cooling rates between the surface and core of something as thin as a knife blade the blade will form martensite (aka harden) the maximum thickness of

~ 1/8". Thicker than that the steel will remain pearletic (not harden) and you can get that mysterious hammon even though you did not clay coat the blade or one that didn't come close to depth that you quenched at in edge quenching.

 

Larger grain in a shallow hardening steel will prevent this but you don't want large grain in a knife blade as it makes a weaker blade and sometimes causes that dreaded "tink" on quenching. You can get around this with a higher alloy too.

 

As Alan pointed out, this has nothing to do with case hardening even though it seems like it would.

 

Doug

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That is very cool! A bit over my head but was able to grasp a small portion of it. Thank you very much for pointing me towards the thread!

 

I'll need to sit down with some technical material to understand it better. I might try to get my hands on the book mentioned in the thread called "Metallurgy for the Non-Metallurgist".

Edited by Josh Brannen
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  • 5 months later...

On old thread but seemed the best place.

 

I saw a how its made for straight razors, the segment is viewable on youtube:

 

 

At about 1:13 they do this graphite coating dipped into melted lead thing that i believe is for case hardening. I'd never seen this approach and thought perhaps of interest.

 

Regards

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There are several things in there that just aren't right. Such as the die that "condenses the metal to strengthen it". The lead (if it is lead) is really super-heated and is being used like people use salt pots, to quickly and evenly heat the metal before quenching. The graphite is added just to protect the steel from the lead. She makes it sound like there is a hardening treatment after the lead pot heat and oil quench, but that was in fact the heat treatment for hardening.

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  • 2 weeks later...

Heating materials in lead to get them to austenitizing temperature is very old school in an industrial setting, but yes it was done, and yes the lead is superheated so you have significant lead vapor in the area even with good ventilation, lead pot covers, and filters. A liquid bath was used because it had minimal affect on surface carbon levels and provided rapid heat transfer. Around 1988, I visited Heller Files in Ohio to support our industrial gas salesman on replacing their lead pots with controlled atmosphere furnaces supported by nitrogen/methanol atmospheres. At that point in time, they were using lead pots to harden their files. Workers in the heat treating area had to get lead blood tests on a regular basis, and if their blood lead level exceeded a certain maximum, they were restricted from working in the area until on retesting it had dropped below the maximum. The push for nitrogen/methanol was the possibility to eliminate the lead pots and get away from the environmental oversight that running lead pots required.

 

A nasty process, that has pretty much been replaced in the USA. and Europe, not certain about the 3rd world.

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