Which is better material for One way clutch system casing SAE 8615 (8620) or 20CrMo

[ndsram]

I have 2 materials for one way clutch system, SAE 8615 (modified C% 0.1-0.14 or 8620 - Nickel Chromium Molybdenum Steel) and 20CrMo. This clutch is being used in starter motor. I could see failure of 20CrMo (Chromium Molybdenum Steel) like the casing getting crack in one particular variant. However on using 8615, there is no failure. I would like to understand why 8615 survives and 20CrMo do get crack. Irrespective of achieving case hardness , core hardness and case depth as per our requirement, I have crack onthe casing of 20CrMo material.

From metallurgical perspective, i would like to understand the superiority between 8615 and 20CrMo.
So I request someone to let me understand the difference

 

[Tmoose]

Got drawings?
Got detailed closeup pictures of the failure(S) ?
Was more than one part made of each material ?

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