doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Shooting Fox with annealed frame?

I found data on 8620 stating it will have the core strength raised during casehardening. The data showed a 135ksi core after casehardening.

Arrrgh, my connection cyber ate some terrific nonsense I typed out.

In a nutshell, your reference for 8620 back on page 7 showed most performance measurements were 'typical for steel'. With the ability to look up carbon migration, can 8620 in charcoal, at maybe 1400* for maybe an hour or two, be carburized any deeper than say ten thousandths.

I think there's a reason and affect for gun CCH, but the process would not seem to be able to increase core strength. It's still the same 8620 on the inside. Industrial applications such as ball bearings or gears may be an entirely different situation with the significantly higher temperatures, much longer times and optimized environment.

I also suspect that the performance of a spring, receiver, would have a lot to do with the design. I think very little on a receiver does not act like a spring when a shell is fired. There seems to be an implication creeping in that thin case annealing will automatically cause permanent deformation of the part. It just doesn't seem to make sense if the forces of firing a shell does not exceed the material or design.

Craig,
Just talking about 8620 for a moment, since that is a popular choice among the custom singleshot rifle makers. 8620 has a normalized strength of about 90ksi and that data I referred to earlier states that casehardening will bring the core up to 135ksi. That says that 8620 is core hardening from the low teens in the RC scale up to the high 20's. If true, that would make for a great combination of core and surface hardness for a gun.

The problem with the beam analogy is strain compatibility. The surface layer and core layer need to strain in proportion to their depth within the beam section. But, the gradient from hard case and soft core is supposed to be quite sharp. The cased surface cannot achieve the strain value necessary to get to its higher yield stress without the core yielding first.

I don�t know if a gun frame is like a beam or what results may come from shooting the annealed Fox but, I suspect Dick is right. Case hardening the surface of a steel element should not make it significantly stronger.

Don thanks for the kind words. Well deserved, you are doing a very good job of keeping this discussion on debate points.

A spring's spring rate goes up with the displacement.

If so, please explain how a simple linear "milk" (spring) scale works.

At 2" displacement the spring rate might 100lb/in. at 3" displacement the spring rate might be 500 lbs/in.

Only if the spring is designed to be non-linear. Such springs can be made, but they require collapse of some element(s) of the spring.

So my contention that the receiver gets stiffer (spring rate goes up) as the receiver is flexed more is correct.

Only if the receiver acts as a non-linear spring. Since there would be no element to collapse, I seriously doubt that the receiver would be non-linear in bending.

In the beam example the displacement (stretching) at the bottom of the beam is twice as much at the surface as it is half-way between the surface and the line that is neither in compression or tension.

And since the surface has had the yield point raised by case hardening the surface can stretch more before it yields and goes into plastic deformation. And since the point half way between the surface and the zero tension line is only stretching half as much as the surface it doesn't stretch enough to reach it's yield point even though it's yield point is lower.

I understand you to be saying that the first point to yield would be the area immediately below the case layer; the first point where there is no heat hardening.

If so, I agree. However, this point/layer will yield before the case layer has sufficient deflection to add significant extra force. The layer/case of heat hardened steel (carbon penetration required) would have to be quite thick to add increased force resistance.

DDA







And since the maximum deformation is at the surface and since the spring rate of the metal in the surface goes up as it deforms (like a spring has a higher spring rate at 3" displacement than it does at 1") the bulk of the load is carried by the exterior of the frame.

Don I repeat, it is not a simple matter of calculating the percentage of the cross section that has been case hardened and then pro-rating that increased yield strength into the strength of the frame. The exterior of the frame carries more because it has %deformed more and so is in higher tension than the interior. The interior point at the zero tension line is not providing any resistance at all because it is, by definition, at zero tension.

So case hardening a frame increases it's ability to resist plastic deformation.

Thanks Chuck. Those hardness numbers (rockwell c ?) are in a similar range of what 1018 might be. I believe 8620 will or can get a bit harder, but it may be down to the alloy. You're right, I bet the old time makers would give it a close look.