Re: [CAD_CAM_EDM_DRO] Re: Metal selection 101

Smoke wrote:

> I think the best method for hardening the 8620 is color case hardening.
> the process is simple, leave a nice looking HARD surface and a soft inner
> core.

Not quite true. Color case hardening quenches into water directly from the carburizing mixture and is guaranteed to lead to
microcracking. Indeed this method has been used successfully for years but in today's enviroument with too many attorneys I think
better methods (as below) are more defensible. i.e.
Carburize, cool. reheat and quench or do the double reheat mentioned in my previous post.
According to : "Met Trans", Vol 4, p 1195-1200.

per previous post:
low number of stress cycles high number of stress cycles
1. 200 ksi 155 ksi quench from 1550
2. 250 180 cool, reheat, quench from 1550
3. 325 260 cool, reheat, quench from 1550, reheat quench
from1450.

> treatment #3 had about half the retained austenite of treatments 1 & 2. This is where the dry ice treatment has a chance to
> really do some good in driving down retained austenite. :-)

BTW - surface hardness was somewhere between Rc 60 and 65 for all three treatments.

I doubt whether carburizing in charcoal/calcium phosphate or gas carburizing makes much difference.

I hope this makes it a bit clearer than my orginal post.

Dave

>
> This is ideal for firearms receivers as the soft core helps prevent
> shattering of the receiver(in your face) if an overloaded cartridge happens
> to be fired in it.
>
> Smoke