Choosing Heat Treat and Case Depths 3

[Tigerdawg]

On 4140 or 4340 metric gears, what is the most effective heat treat method? If nitriding, what is a reasonable case depth? I have seen .15 x module in a recent post for carburizing, but is that valid for nitriding?

 

[Tigerdawg]

I've done a bit more looking and nitriding is probably the way to go, but what about case depth?

 

[Carburize]

Case depths for nitrided gears are typically much thinner than carburized parts - in large part due to process limitations, it is not economically feasible to get case depths greater than about 0.030-inch.
AGMA 2001 and ISO 6336-5 have recommended core hardness values and case depths for the various rating numbers.
Typically nitrided gear teeth have between 0.010 and 0.020-inch case depth depending on pitch (module)

 

[diamondjim]

We induction harden both materials successfully.
Most of the time we do it not so much for strength
but for durability as the stresses do not require
exceptional depth. Where it is need for strength
or to overcome subsurface stresses, I believe the
4140 seems to respond better to case depth as well
as greater surface hardness.

 

[Tmoose]

The big INA catalog has some formulas for load vs contact stress depth for needle bearings, etc. Might be applicable for gear contact.

My last boss had an experience with hard working Nitrided gears shedding the shallow case. Scarred him for life.

 

[Tigerdawg]

We currently call out for a .020 nitrided case depth on a set of module 3 gears. We had a failure and have the set out for testing. Verbally, the metallurgist has said that our nitriding was no where near .020 deep. I'm preparing for the big fight between our purchasing agent, my boss, and our vendors after we get the final report.

 

[Carburize]

ISO 6336-5 for gears would recommend a nitride case depth of (0.008 to 0.014-inch) for a 3 module tooth - from figure 18 of the 2003 edition

AGMA 2001 would recommend 0.013-inch - from Figure 15 of the 2004 edition.

 

[diamondjim]

I would be a little more concerned with
the basic hardness or core hardness of the
material as well as the surface hardness.
Do the gears have full fillet radii?
What was the surface finish before
nitriding? Where did the tooth break?
In root, near pitch diameter, side opposite
the load direction?

 

[Tigerdawg]

The teeth were ground. I do not have the documentation in front of me, but the surface finish visually compared with a 16 to 32 finish based on a sample I have handy. The fillets look typical to what I have normally seen. The motion is reciprocal, and the teeth did look like they broke at the root. I do not have the report yet, but the case was very shallow, something like Rc 40 at .020 deep.

 

[diamondjim]

I kind of thought case depth was defined
at 50Rc or 48Rc hardness transition points.
The case would help create compressive
stresses which would help offset the tensile
stresses. You did not answer what the core
hardness is as it may be the determiner of
the practical strength of the tooth since
the case depth is shallow. Do you have any
idea of the load and its intensity that
caused the fracture? I assume it was some
type of shock condition. Keep us posted.

 

[Tigerdawg]

That was what I was trying to get at. At .020, we are already near the core hardness of ~32. The drawing called out for .020 case depth, deep for a 3 module gear but not impossible.

Also, the load is a shock load. To make matters worse, the machine is in a plant with a disgruntled union. We cannot yet prove intentional misuse, but all the circumstantial evidence is there.

 

[Carburize]

Different types of case hardening processes have different ways in which the depth of the case is defined.
In nitrided cases the effective case depth is defined in AGMA 923 and ISO 6336-5 for gear teeth as:
"The effective case depth is measured from the finished surface to a location where the hardness number is equivalent to 40.8 HRC( 421 HK500 or 400 HV500min)by conversion from a microhardness test result. If the core hardness is more than 38.9 HRC (391 HK500 or 380 HV500min, core hardness plus 54 HK500 or 50 HV 500min (4.6 HRC) may be used as the definition of nitrided effective case depth."

 

[Tigerdawg]

Thanks C, now I just have to wait for the metallurgy report for the actual numbers.

 

[Tigerdawg]

Any ideas where I can find a set of standards inexpensive enough for an account to approve?

 

[Carburize]

AGMA 923 which collects together all the metallurgical requirements for steel gearing is available from

http://global.ihs.com/

for $65