4140 Pinion Pros & Cons?

[aujwc]

Hi Folks

I am dealing with some fellows who want to case carburise 4140 for a pinion for a slew gear. I thought this is too high in carbon for case carburising.

Secondly 4140 being low on the hardenability compared with something containing Ni - could the core hardness be a problem?

JWC
Mechanical/Structural
Brisbane, Australia

 

[dbooker630]

You are correct in that 4140 is typically not carburized. It can be nitrided, but I'm not sure that will fit your application.

You may want to consider induction hardening the teeth which should allow you to maintain a reduced core hardness.

 

[TVP]

I agree with dbooker630: 4140 is typically induction hardened or nitrided when used in gear applications. If the pinion needs to be case carburized then a more appropriate grade (4118, 8620, 4320, etc.) should be chosen.

 

[gearcutter]

I see no reason why it can't be done but I don't understand why you'd bother as there are several things you'll have to watch out for:

Typical case hardening steels will generally develop a core hardness of around 28-42 Rc with 35-38 Rc being the usual result. Any harder than this and you'll start reducing the tooth's ability to handling shock loading. Putting 4140 through a case hardening process will effectively through harden it to around 50-55 Rc.

There is also a point where too high a core hardness can actually reduce the tooth's resistance to bending fatigue. In case hardening steels the hard case & "softer" core creates tensile stresses which improves the tooth's resistance to bending fatigue. A harder core relative to the case will reduce this.

Typical case hardening steels are produced specifically to be more dimensionally stable during the quench cycle helping to reduce distortion. 4140 is generally not as stable although there are procedures that can be implemented to help keep the distortion to a minimum.

You'll also have to ensure that your heat treater attempts to take the necessary precautions to ensure that the risk of quench cracking is kept to a minimum, although I feel this will be extremely difficult to do with 4140.

You'll still end up with the same case depth and hardness using 4140 but I don't see why it would be necessary to use as you'll end up with a far better result using a case hardening steel.

Here in Australia the 3 most common types are 8620, En36a & 4317.

 

[dimjim]

If the 4140 is normalized before carburizing,
I do not think you will have any problems.
What size are you talking about and I assume
you have full fillet radii. They hardened
tapered roller cups and cones by carburizing and
I thought it was rare to see depths greater than
3/32 deep. If your dp is large ie small teeth, then
you may harden across the tips which may be bad.

 

[aujwc]

Apparently they've bought the steel already. I understand we're changing to flame hardening now, which seems appropriate. This is a pinion with an OD = 12" and face width 5". Gear specification probably Q8-A-FA14. How does that sound?

JWC
Mechanical/Structural
Brisbane, Australia