This topic comes up time to time, and Im always willing to provide comment when not asked.
The question, as I see it; not whether medium and high carbon steels can be case hardened, but rather should they be case hardened. Medium carbon steels such as 4140 case harden with beautiful colors on the outside, BUT its what happens within the steel that concerns me.
Typical case hardening steels are low in carbon typically less than .20% carbon. Because of this they cannot be hardened on their own i.e.. Heating to red heat and quenching. They will generally remain dead soft.
Thats where case hardening is beneficial, by packing low carbon steels in a crucible with carbon (charcoal) and heating to the correct temp the steel will absorb some carbon, and if quenched at the appropriate temp, will have a thin but hard surface while leaving the inner core soft and malleable.
This has a two part benefit; the hard outer surface provides wear protection with the softer inner core is better able to absorb shocks and resist cracking.
On medium carbon steels such as 4140, there is enough carbon within the steel for the steel to self harden i.e.. Heat to red and quench in the appropriate medium and the steel will be hard all the way thru. What you end up when you case harden med carbon steels is an extremely hard outer layer and a hard inner layer, no soft inner core to absorb shock and prevent cracking.
And as Steven brought up, the steel is no longer flexible.
The other issue I have with folks CCH 4140 steels and quenching in water is that SAE 4140 is an oil quench steel, it was designed that way. Quenching 4140 in water causes the steel to harden too quickly and in a non uniform manner; again with serious potential issues.
So again, just because something can be done, doesnt mean it should be done.
Ive turned down a lot of CCH jobs that involved 4140 or some mystery metal and I explain why, some heed my adviceothers find someone who will do it.
