What is case hardening?

[WolfieParticipant @wolfie]

Errr ok what is case hardening and is it the same as normal hardening by heating and quenching?

[WolfieParticipant @wolfie]

[jason udallParticipant @jasonudall57142]

in short no.
Carbon steels can be hardened by heating then quenching.
Tempering reduces that final “glass” hard state to something less hard more resiliant..the trade off determines the temper .
Ok now try this with mild steel..no joy.
To make the surface layer of the mild steel hard and tough case hardening heats the steel in a carbon rich envionment and forms a layer of higher carbon steel on the outside.
You might ask if this layer can be hardened and tempered in itself…never heard of it . Anyone?

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Case hardened jobs are sometimes given further heat treatment which approximates to tempering so as to alter the hardened layer . Basically it is usually a small loss of hardness in return for an increase in toughness .

 

The hardened layer is fully hardened to start with unless its gone wrong and I’ve never heard of any application for rehardening .

 

MW

[jason udallParticipant @jasonudall57142]

thanks .
Michael..I hadn’t heard of heat treating post hardening myself..but when writing considered the possibility. Wolfie the is another thread going on homebrew caseharding compound..I can’t post link here to it maybe MW can.

[JohnFParticipant @johnf59703]

Hi Wolfie, Broadly this is correct but it is more involved, to keep it simple there are two methods of case hardening, both involve “putting” a skin of carbon steel on the outside of a low carbon [mild] steel.

 

The simple one for small parts requiering a thin skin say 0.002″ to0.005″ is to use a prepared compound from I understand, Blackgates or Chronos [Used to be Kasenit] and a good sized gas torch, heat the work to red heat and dip in the compound–it should stick to the hot metal, might need to dip again to get enough on the work. Then heat in the torch for several minutes and quench in water. You will now have a hard case. You will rarely need to temper with this process. What you have done is add carbon to the surface layer thus enableing it to harden, the core will remain soft.

 

For the more complex pack hardening where you can have a case of 0.020″ to 0.030″ or more if needed see this post http://www.model-engineer.co.uk/forums/postings.asp?th=32838

With this method there are considerations due to holding the steel at elevated temperatures for long periods which give rise to grain growth in the core which weakens the material–it can be overcome by secondary treatments–see the link.

[Anonymous]

It’s probably not useful to try and harden and temper the surface if starting with mild steel. But if using steels intended for case hardening, then a form of hardening and tempering may be used. See for instance EN32:

 

http://www.westyorkssteel.com/EN32.html

 

or EN36:

 

http://www.westyorkssteel.com/EN36.html

I’ve not used EN36, but I have used, and case hardened, EN32. We hardened and tempered it, after a fashion, after the initial heat treatment to get carbon into the outer layer.

 

Regards,

 

Andrew

[Author]

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