Case Hardening

Hi, I read on a thread here a wee while ago about case hardening and someone mentioned using charcoal, as in wooden embers from a fire, as a hardening medium. Using my Bodges Logic, I have now thought of using charcoal, as in lump wood charcoal bought for the barbie, pounded down to dust, packed in a tin with a lid with the item to be hardened and then heated, would be the same.

Would this work? Would the charcoal dust catch fire, or would the fact that the lid was keeping the oxygen out stop the charcoal igniting?

How hot would the tin and contents have to be heated to - I had something in my mind that said red hot for 20 minutes, but I could have dreamt it!

What thoughts?

Chris

I have successfully case hardened to a depth that allowed grinding on the outer race of a vintage motorbike bearing made in my workshop. A tobacco tin was used containing a mixture of bonfire charcoal and bits of chopped up leather and was sealed with clay dug from my garden. The tin needs to be airtight for good results. I can't remember how long it was kept at a very bright red going on yellow heat but the case depth is dependant on the time. Parts treated like this should be allowed to cool slowly and then hardened as this refines the grain structure. Colin

Hi Colin, thanks for that. The clay used for sealing - where exactly was that put? Was the clay put around the outside of the tin or inside around the tin lid? This is all a new chart for me, never done case hardening before!

Chris

Edited By ChrisH on 20/05/2015 21:02:56

Hi Chris, The clay is just to seal the tin's lid and is plastered around the edges to fill any gaps. It is usual fo only use this process when a deep (more than 20 thou) case is required ; usually to allow for grinding. I assume you know it is quicker to case harden with a propriety material under a blowtorch for a shallow case. Colin

Hi Colin, thanks, all now clear! Won't have much to case harden and haven't got any propriety material, but have got charcoal for the barbie, always have that!

Chris

Chris, charcoal is not required as you can just use any carbonising materiel, preferably old leather cut into small slivers and packed around the item. It also has the advantage of not scaling the steel., it comes out clean when you reheat and dip in water at the end.

What is it about old leather that makes it a good case hardening medium? Is it something to do with the chemicals that are used for tanning?

Hi Chris, have a look at this link and the links within the post there is a lot of detail and you can search the forum for case hardening, you will find a lot more posts.

**LINK**

John

I think leather is just a good source of carbonifereous materiel, bones and other animal products also. Its just that leather is easy to get and works. There must be a lot of chemicals in leather being animal sourced?

Clive

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Amazingly wide range of properties available from Carbon, depending on how it is 'sourced' ... It would appear that burnt animal material [bone/hoof/leather] has a very fine structure [see here for an interesting modern application]; which probably makes it ideal for use in case-hardening.

If anyone can provide substantiating [or refuting] evidence, then I would be very interested.

MichaelG.

Never a day wasted when you learn something.

I now know that close contact with the steel and the carbon rich pack is not as essential as I had always imagined.

I was interested to learn that carbon went from carbon monoxide into the steel freeing up the oxygen to make more carbon monoxide and so on.

Clever innit?

One of Guy Lautard's Machinist's Bedside Readers has an article about case hardening. Fill the hole in the bottom of a small clay flowerpot with fireclay; put bonemeal in tha pot converging the item you want to harden; stick another clay pot over the top with clay and seal that one too. Then heat to red in a fire for a few hours, it will smell ghastly. Then drop the whole red hot assembly in a bucket of water and fish out your hardened item. Recommended.

Have you seen the price of clay pots lately ? maybe worth more that the job.

Clive