Re: [CAD_CAM_EDM_DRO] 4130 4140 how to heat treat ?

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I am makeing a sin vice that is 9" long 4" wide and has a jaw depth of
1.750 out of 4130. I am also makeing some bending dies from 4140 and I
have the oven and I have quenching oil that I used to harden O1 drill
rod but I have ben given different conflicting versions on how to heat
treat these to types of metal.

I would agree with Lawrence that you need air hardening material for this
application. Allow me to pontificate.

The decision regarding which alloy class (water, oil or air hardening) to
use is partly governed by the thickness of the section. Hardening takes
place when an alloy is brought below the recrystalization temperature so
quickly that the crystalline structure does not have time to return to its
soft state. In short, it freezes at an intermediate state.

Water hardening steels must be cooled very quickly to achieve this state.
Thick sections will cool quickly on the surface but not quickly enough in
the middle. The result is a soft core and tremendous residual stress which
causes warping and cracking. Oil hardening steels are formulated to attain
hardness under slower cooling and are better for sections over about half an
inch.

Air hardening alloys give the best results where thick sections, odd shapes
or maximum stability are concerns. Hardening takes place slowly enough that
the core has enough time to cool at the correct rate.

4140 is great stuff but it has a few drawbacks. The worst of these is its
intolerance for stress risers such as inside corners and holes. If you are
using it because that's what you have then make sure you avoid sharp inside
corners and holes near edges if possible. 4140 hardens at 1550-1600F,
should be quenched in oil and should be tempered as soon as possible after
hardening to avoid cracking. Cracks can develop within about 24 hours.

I don't have a tempering scale for 4140 but I assume 400-500 degrees will
yield about R55C. Get valid numbers before you start.

Let me clarify one common misunderstanding here. The hardening temperature
is always the same for a given alloy regardless of what you want the final
hardness to be. You must fully harden your part and then temper it back to
the desired value by reheating it appropriately.

Again, based on the description of your project, you really need A2 or a
similar air hardening steel.

Doug Harrison