[twoartistic]

We are having issues with a shaft for a new heavy service industrial application.

The shaft has an arm attached on the end. there is overhang to accommodate a grease seal. The arm has a device on the end with some significant weight, and through normal use, in an environment of intense vibration, it imparts some torsional force, but most of the force is parallel to the axis of the shaft. Basically it is like putting a vibrator motor out on the end of the arm, and then smacking it every so often with a carnival mallet. Another issue is we have a very limited area for the shaft bearings, so the shaft has been sized as large as we can go. This is also a harsh environment with lots of minerals and salt to contend with. So we are chrome plating.

Here is the problem. We selected 4140 hardened and tempered to 48 Rockwell, but the shafts are breaking, so we dialed back the hardness to 32-33 Rockwell and we are getting bending. I am not a metallurgist so I am wondering about some different steels and hardness that might work better for this project.

[txcncman]

Why did you go all the way from 48 down to 32? Have you tried chrome moly? What about a deep case hardening? What about a PH stainless? Contact a metallurgist.

[twoartistic]

Why did you go all the way from 48 down to 32? Have you tried chrome moly? What about a deep case hardening? What about a PH stainless? Contact a metallurgist.

The Company owner decided the hardness both times. Since he is my customer, and I am just a lowly design engineer, I question him not.

No, we have not tried chrome moly, I wanted the advice of a metallurgist before trying things outside my expertise.

We are through hardening, then tempering back. We have another shaft that we induction case harden, but it does not have the same function. This is new territory for the company. I read on another forum that sharp corners can act as a heat sink and effect the hardness when quenched. The problem is at the threaded end of the shaft, should we machine the threads after hardening?

No, we have not tried PH stainless, for the same reason stated for chrome moly.

This is an attempt at metallurgist contact. Sorry, I thought that was obvious.

[mactec54]

txcncman
Have you tried chrome moly

4140 is chrome moly all the 41 series is referred to as chrome moly

twoartistic
The problem is at the threaded end of the shaft

Machine a relief radius at the end of the thread down to the root of the thread, it should blend with the thread, so there is no sharp edges/corners

Do not case-harden the 4140, Just use normal heat treatment

[twoartistic]

Thanks Mactec54

I would have never guessed that sharp corners on threads would make a difference. Any finger in the wind guess on a hardness to shoot for?

Do you know if the shafts we have can be reworked (radius the threads) then re-hardened and tempered back? Sorry for such a basic question, I really don't know anything about metal hardness.

[txcncman]

Good luck finding a real metallurgist among a group that consists of more than 50% hobbyists.

Actually, I was thinking alloys other than the one mentioned. But what the hey. You are right on track.

Threads and other sharp features can be masked during heat treat if absolutely needed.

Having actual drawings with actual forces applied would help tremendously in making recommendations. Otherwise, it is just a WAG.

[mactec54]

twoartistic

You may be able to remachine just as they are, your hardness is not to high to remachine them, always any stressed parts need a nice smooth blended radius were ever you can put them, never have any sharp corners Etc, do this before heat treatment is best

For hardness 40RC would be a good starting point, Temper at 600 to 650 to get to around 39/40RC, make sure the parts are oil quenched, or you will get cracking

[twoartistic]

Thanks again Mactec54,

I went to my Solidworks group quarterly meeting tonight. And talked to some of the engineers there. They confirmed what you are saying here. I love projects like this that take me outside my regular work flow, and challenge me to learn new things.

[twoartistic]

We are going to order pre-hardened shaft stock and machine it down to our needs. Our machinist does not think it will be that much more to do it that way, and we don't have to worry about the heat sink issues for heat treating.

Thanks for the advice, it was a big help in getting us further down the road.

[RICHARD ZASTROW]

You should still put radii instead of sharp corners. Sharp corners are mechanical stress points as well as heat treating stress points. Cracks and breaks start in sharp inside corners.

Dick Z

[twoartistic]

Yes, I agree. We had a slight radius on the original parts, but it was for mechanical reasons like you mentioned, not intended to lessen heat sink issues.

We just had the shafts lab tested, outside hardness was 32 - 35 RC, internal hardness, went as low as 22. That seems like a huge swing. Like I said, my skill set is design and engineering specialty mechanisms, and manufacturing machinery. I don't know much about metal hardness, and properties. Is that kind of swing typical in 4140?

Now ask me about wood, I can help you out.

[txcncman]

Depends on the heat treat process (method). Sounds like it was heated and quenched.

[Geof]

......Cracks and breaks start in sharp inside corners.Dick Z

This is most likely your problem. You need to do a bunch of reading about stress raisers and fatigue failure.