AGMA 2001 gives a graph and an equation for carburized case hardened and nitrided which do you want?
ISO 6336 -5 has equivalent data.
These are general figures which work reasonably well but the most rigorous method is to calculate the depth of maximum sub-surface shear stress and then determine the depth of case necessary to contain the stresses.
I understand the most rigorous method but it seems that most of the time a modul is chosen without consideration of the case depth. As if the case depth is only a matter of the thermal treatment.
Normally, the depth should be chosen to be twice the depth of the maximum shear stress like for bearings.... is it true for gears?
Typically 3 times is used when the detailed analysis is run.
The general equation for carburized case depths in ISO 6336-5 is optimum case depth = 0.15 x Module for tooth sizes between 2 and 10 module. This is a metricated version of the AGMA 2001 case depths.
According to MIL-STD-1876, NITRIDING, GASEOUS ATMOSPHERE PROCESSING FOR,the definition is "The case depth is the distance from the nitrided surface to the point where the maximum of the core hardness specified on the engineering drawing is obtained"
According to MIL-STD-1878, CARBURIZING, GASEOUS ATMOSPHERE PROCESS FOR "For gears, general practice is to specify effective case depth as measured to Rockwell HRC 50, equivalent at the pitch circle contact point of tooth"
According to MIL-S-12515C, SURFACE HARDENING: FLAME AND INDUCTION (FOR FERROUS ALLOYS) the effective case depth depends on the carbon content of the material,
for 0.28 to 0.32 35RC
for 0.33 to 0.42 40RC
for 0.43 to -.52 45RC
for 0.53 and over 50RC
The specification ISO 6336-5 is very helpful... Thank you. Now, it would be intersting to understand the basis of the calculation for the optimal, minimal and maximal carburized case depth (Ehopt=0.15Mn, Ehmax = 0.4Mn, Ehmin>0.3mm and Ehmax<6mm)
Fred,
You are right in that more is not
necessarily better. I assume the max
is based on Jominy Tests for typical
materials. I would assume optimal
could be calculated by examining the
subsurfaces stresses caused by your
max load conditions. Do not have that
standard. Wondering if they give
suggested values where loads are light
but you are looking for high endurance
values. Are the values based on 48Rc
or 50Rc? I know the 48Rc value was used
in Europe for the case transition depths
in the past.
Assuming we are talking carburized cases the maximum case depth is suggested as a way to control case-core separation. A failure mechanism where the top land of the tooth can detach approximately along the intersection of the case and the core. This mechanism is a result of the residual stress field created in the tooth. During quenching after carburizing one of the effects of the high carbon in the case is to delay the martensite transformation at the surface until well after the low carbon core material has fully transformed. The formation of martensite is accompanied by a small volumetric expansion and when the case transforms the expansion is resisted by the already transformed core, building a compressive residual stress field into the case - this, along with the high hardness of the high carbon martensite are what give carburized components such superior load carrying capability in gears for example. The short range high level compressive residual stress in the case is balanced by a longer range, lower, tensile residual stress field in the core - if this tensile stress field becomes too high due to too thick a case the resulting tensile stresses can result in the triggering of the case-core separation failures.
Sorry! - long explanation for a simple question.
Carburize,
Thanks for your explanation. It was not
too long. We normally induction harden,
so come from a different background and
way of thinking about case depth.
