4140 shaft distortion during heat treat
4140 shaft distortion during heat treat
(OP)
I have a fluid power transmission shaft made from 4140. The shaft has a 3.75" diameter center section with helical spline and steps down on each end to roughly 2" diameter with bearing supports and straight spline outputs on either end. We currently induction harden only the sealing and bearing diameters which has proven insufficient to achieve the required service life so we've chosen to carburize the entire shaft.
I need to minimize distortion during heat treat so I specified 4140 annealed material with a rough turn op, then thermal stress relieve, finish turn and gear cutting, heat treat (vertically oriented), and grind to final diameters on sealing and bearing surfaces.
My heat treater says that won't solve the problem but doesn't offer any assistance. Any process help would be appreciated. I'm not limited by the choice of material at this point either, but I must cut the splines prior to heat treat. We have not seen any problems related to heat treating the splines.
I need to minimize distortion during heat treat so I specified 4140 annealed material with a rough turn op, then thermal stress relieve, finish turn and gear cutting, heat treat (vertically oriented), and grind to final diameters on sealing and bearing surfaces.
My heat treater says that won't solve the problem but doesn't offer any assistance. Any process help would be appreciated. I'm not limited by the choice of material at this point either, but I must cut the splines prior to heat treat. We have not seen any problems related to heat treating the splines.





RE: 4140 shaft distortion during heat treat
You mention that you don't get the expected service life by induction hardening the 4140 fluid power drive shaft. Are you failing the shafts or have in service wear problems with the journals? Do you specify a heat treatment for the shaft material prior to local induction hardening?
I would think for this application that you would want to have a quench and temper heat treatment specified for this fluid power transmission shaft. In fact based on the stated shaft diameters, I would look at using an AISI Type 4340 alloy steel with an oil quench and temper heat treatment to provide a hardness range of 28 to 32 HRC (Rockwell C scale). The quench and temper heat treatment will result in optimum drive shaft strength, good toughness and fatigue resistance.
I don't believe just carburizing the entire drive shaft is going to help.
RE: 4140 shaft distortion during heat treat
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RE: 4140 shaft distortion during heat treat
Can you provide some additional details on the hardening that has been performed so far? What is the hardened depth? Is wear a problem, or just strength? Is failure through the splines?
RE: 4140 shaft distortion during heat treat
The second problem I have is fatigue cracking at the output ends which typically starts at the spline relief diameter. This shaft currently has a reciprocating cycle (180 degree rotation forward loaded, backward unloaded) life of 1 million cycles transmitting approximately 30,000-40,000 in-lb of torque evenly divided between both shaft ends in an environment that produces frequent shock loading. The design target is 3 million cycles under the same conditions.
So my concerns are improving the wear characteristics on the helical spline and improving fatigue resistance at each end. Due to the gear cutting limitations, I must treat the shaft after machining and before grinding and still maintain reasonable dimensional stability.
I have considered cryogenic treatment, but I am under a tight budget and cannot add "significant" expense to this component.
RE: 4140 shaft distortion during heat treat
For the fatigue improvement a shot peening of the area where the cracks start should improve fatique life.
RE: 4140 shaft distortion during heat treat
That said, there are two ways to increse fatigue life.
1) Increase the Tensile strength of the material. I dont think that you want to increase just the case hardness. I would be concerned about the overall strength of the material. You mention shock loading, the severity of the shocks will place an upper limit on the (core HRc) hardness that your application can withstand. Your HRc 32 corresponds to a Tensile Strength of ~140ksi, I dont have S-N curves handy for your steel, however I'll offer a guess that you'll want to keep peak stresses below 50-70ksi to get the 3M cycles.
2) Surface preparation. By removing crack initiation points and creating a compressive residual stress concentration in the surface you can dramatically increase the fatigue life. (In my work I see a 20x increase in life cycles to failure through surface preparation alone.) As israelkk says shot peening will help increase the fatigue life.
HTH (hope this helps)
nick
RE: 4140 shaft distortion during heat treat
While the additional details on the process were helpful, you still didn't provide information regarding the surface hardness or depth of hardening. 4140 is capable of surface hardening considerably in excess of 40 HRC when using the induction process. My initial guess is that induction hardening and low temperature tempering should be used, with a surface hardness requirement of 45-48 HRC minimum. The depth of hardening should be on the order of a couple of mm, say 2-3 mm, before returning to the core hardness.
NickE's suggestion of higher core strength/hardness is worth noting-- you may want to increase to something like 32-34 HRC minimum. Feel free to post additional details, as this is an interesting topic. See the following thread for more discussion on a similar application:
Thread330-83411
RE: 4140 shaft distortion during heat treat
I'm very appreciative of all you who have and are contributing to this discussion. Will supply more info as needed. Thanks!
RE: 4140 shaft distortion during heat treat
We also bump up our fatigue life by shot peening critical areas.
RE: 4140 shaft distortion during heat treat
Thanks for the additional information. I think that your hardened depth of 0.6-1.3 mm is too small-- recommend increasing to something more like 2-3 mm. And I would definitely investigate increasing the hardness (both surface and sub-surface) of the splines. Austempering is one technique for achieving higher hardness while minimizing distortion, and may be attractive if you want to start with normalized or as-rolled/cold-drawn bar for easier machining.
RE: 4140 shaft distortion during heat treat
If you can machine at 35-38Rc get the core that high first, nitriding isn't going to distort that much (and leaving the white layer gives greatly incresed wear resistance.)
Alex