Choosing Heat Treat and Case Depths
Choosing Heat Treat and Case Depths
(OP)
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?





RE: Choosing Heat Treat and Case Depths
RE: Choosing Heat Treat and Case Depths
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)
RE: Choosing Heat Treat and Case Depths
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.
RE: Choosing Heat Treat and Case Depths
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.
RE: Choosing Heat Treat and Case Depths
RE: Choosing Heat Treat and Case Depths
AGMA 2001 would recommend 0.013-inch - from Figure 15 of the 2004 edition.
RE: Choosing Heat Treat and Case Depths
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?
RE: Choosing Heat Treat and Case Depths
RE: Choosing Heat Treat and Case Depths
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.
RE: Choosing Heat Treat and Case Depths
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.
RE: Choosing Heat Treat and Case Depths
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."
RE: Choosing Heat Treat and Case Depths
RE: Choosing Heat Treat and Case Depths
RE: Choosing Heat Treat and Case Depths