Cracking 4140 shafts when induction hardening

[Limbowhale]

We make a clutch shaft from a 3-3/4 dia x 20" long piece of 4140 HT'd to 270-320 as the base matl. The part has a splined section requiring induction hardening to 50-55 HRC. We are experiencing a very high percentage of scrappage after induction hardening due to cracking in the splines.

We have worked with two different heat treaters now to resolve the problem but are getting nowhere. After this last scrappage the heat treater suggested that maybe the shafts should be tempered after quenching.

Does anyone have any suggestions? Using 8620 and carburizing didn't work becuase of the part geometry. The shafts kept turning into bananas. Starting with a thru hardened material was a step in the right direction but the induction hardening process is killing us.

We are wondering whether we should abandon the induction hardening approach and switch over to a nitriding treatment in lieu of that. We are confident we can get the 50-55HRC by nitriding but the case depth will only be about .020 deep. We have very little experience with nitriding and are a little hesitant to put parts out in the field that might not stand up. The application is in a hydraulic pump drive where this splined shaft coupled with a splined gear takes the input torque from a diesel engine and transmits the torque to the output gears in the gearmesh. Most of our applications are in the 400-600HP range but we do see applications up to 1000HP

 

[swall]

If you through harden the shaft, with an oil quench,and 400F temper, you will get surface hardness around Rc50. You will not fully harden a section of this size, obviously, which may work in your favor in retaining some toughness in the core. Have you considered this?

 

[metengr]

Limbowhale;
Can you describe your induction hardening process in more detail? Also, are the cracks in the root of the spline?
What temperature are you reaching during induction heating before quenching? Are you using an oil quench or water quench after induction heating?

 

[redpicker]

Yes, some more information on the process would help. How deep of a case are you trying to achieve with your induction hardening?

If they are using water for the quench, you can probably solve the problem by switching to a polymer quenchant. I wouldn't advise oil as it would be a severe fire hazard.

If they are using a polymer quenchant, as has been mentioned, too high of an austenitizing temperature can cause cracking. So can too high inclusion content, mainly sulfide stringers.

You might consider a lower carbon steel (4135, for example). It could lower your maximum hardness, but would be less likely to crack.

Tempering after treating can help, but I'd consider this mainly if delayed cracking was a problem (they were crack-free after heat treatment, but failed in service, or even on the shelf).

rp

 

[mfgenggear]

In addition to what has been said

stress relieve prior to induction hardening but after finish turning & milling the parts.

4140 if not tempered within one hour can crack
yes snap temper then temper it will be a big help.
also if the heat treat procedure is not done correctly
parts will crack

When the geometry requires it I specify to our vendors to uses mar temper in oil or salt, this helps
with the cracking and distortion. But adds cost.
The geometry is the factor.

It will crack between teeth or in the general area.
extra precautions are required.

Nitriding is more desirable with nitraloy 135 material
core harden, machine then nitride.
Nitrided surfaces are brittle. it with chip easily.
However, it is very hard and has good wear resistance

 

[dbooker630]

I would also check the splines before heat treat - are they rolled or hobbed? I have seen cracks initiating from sharp spline roots or hob corners after IH.

At our facility we routinely IH this stock size material to a .300" case depth without any issues, beginning with a pretreat (austenitize, oil quench and temper), followed by IH and temper within two hours of polymer quenching. The polymer concentration should be on the high side of the recommended range for alloy steel.

 

[MWQCM]

Have you tried stepping up to 4340? It takes less oven time to achieve the hardness you want, and hits RWC 50-53 easily.

 

[dhan12]

This problem is only related with process control in Induction Coil design and Current, Quenching process. Please send the details if possible.

 

[peuray]

Limbowhale ?can we see the photo of the workpiece which has cracking?
I think this would help us to analyse how the cracking happend?