4140 straightening problem

[gowrishankarz]

Hello everybody,
We are having a lot of problems with heat treating round 4140 material. This part is .5" in dia & 25" long. It has a V groove machined into it. We just need the edges of the V groove to be hardened to about 55 Rc. Right now we are flam hardening the edges & this makes the part to bow severely. We then straighten the part in a manual press. I was wondering if it will be better to through harden the hole part instead of flame hardening? Is there any better way to straighten the part after heat treating? Can sombody help me with this?

Thank you

 

[TVP]

Can you please provide the exact process used to manufacture the parts? The following would be an example:

1. Obtain 4140 steel in the form of cold drawn bars xx.xx mm in diameter.

2. Machine features using single spindle lathe, etc. Final dimensions are...

3. Cut v-groove using vertical milling machine. Final dimensions are...

4. Flame harden using y velocity, z heating parameters

5. Quench into water with temperature = i, no agitation, remove after 10 minutes, etc...

 

[swall]

You will have trouble hitting Rc 55 if you through harden this part. But you might try through hardening and tempering at about 800F to give you a core hardness around Rc 40. Then go back and flame harden your groove. Or you could induction harden the whole surface and skip the pre-hardening step.

 

[gowrishankarz]

TVP,
Here is the process

1. We use pre-hardened 4140 turned, ground, and polished material. The diameter is .5" & 25" long.

2. Then we cut the V-groove in a vertical milling machine. We remove about 25% of material.

3. Then we flame harden the edges. We just use a torch & to flame harden the edges. We run cold water through the part as we move the torch across the part. This is purly a manual process. I know there is no real control over the process. But, this is all I have got to work with. We don't have a proper flame hardening equipment. Any recommendations for controlling the process better?

4. Then we straighten the bar in a mechanical press. Some times it would take about 3 hours to get the bar straight.

Thanks for your help TVP

 

[gowrishankarz]

swall,
The core doesn't have to be hardened. I was just wondering if the through hardening process would have better control. I cannot induction het treat this part because I cannot put centeres on them. Do you think induction hardening would reduce the bowing?

 

[EdStainless]

I like the idea of induction hardening. Why would you need centers? I have done flat ended pieces. Use a high frequency so that you only heat treat the skin. You can set up to hold the part straight (vertical) and scan with coil and quench ring.
The part may still bow some, but it shouldn't be nearly as much.

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[swall]

Ed--in my experience with induction hardening of shafts, scanning is more likely to result in bowing that single shot hardening on long skinny shafts. Gowrishankarz--can you chuck this part in a lathe and rotate it while you are flame hardening? I think this would help quite a bit.

 

[gowrishankarz]

swall,
I don't have any room in the ends to hold these parts in a chuck. The outter surface of the bar doesn't have to be hardened. We are only concerned about the V-groove surface. Is there anyother material which is as strong as 4140 & has better resistance to heat treat distrosion?

 

[TVP]

The distortion is caused by several different factors, not because of the specific grade 4140. Uncontrolled flame hardening is the perfect method for producing large distortions. If the 4140 has already been hardened prior to the flame hardening process then you have the optimum microstructure for minimizing distortion. It appears that you will have to retain some type of secondary process for correcting the distortion. Good luck.

 

[swall]

I agree with TVP that you are probably stuck with a straightening operation. At my previous employer,we induction hardened axle shafts. Most were around 1" diameter. Anything over 30" long was given a straightening operation. I would also add Mag Particle inspection, as straightening is a good way to produce cracked parts.

 

[gowrishankarz]

swall,
I understand there I cannot eliminate the straightening process. All I am trying to do is minimize the distortion so that the straightening process doesn't take as much time as now. What would be your recommendations to minimize the distortion?

Thank you

 

[TVP]

Use the highest water temperature that will still produce acceptable hardness. Add a suitable polymer to the water (5% Aqua-Quench 365 to 95% water).

 

[gowrishankarz]

Thanks for all of your replies.

TVP,
If we were to use a proper flame hardening equipment, what would be your recommendations as far as y velocity & z heating parameters?

Thanks again