Premature cracking of 8620 component
Premature cracking of 8620 component
(OP)
We are experiencing cracking from the top corner of a keyway in a load carrying component that mounts to an rotating output shaft. The problem is that the cracking is occuring at 1/10th of the actual operating load during a controlled assembly prove out process.
The part is fabricated from AISI 8620. It is carburized to 61 Rc to a depth of .015" and a 46 Rc core is resulting. The component does not have an especially thick cross-section, .25", and this is reduced by 1/3 in the region of the keyway.
This is a legacy material choice that has been in use for many years,so it has a decent track record. It appears that something has change in the processing that could create or promote cracking at the keyway corner but I am uncertain where to start. Any help would be greatly appreciated.
The part is fabricated from AISI 8620. It is carburized to 61 Rc to a depth of .015" and a 46 Rc core is resulting. The component does not have an especially thick cross-section, .25", and this is reduced by 1/3 in the region of the keyway.
This is a legacy material choice that has been in use for many years,so it has a decent track record. It appears that something has change in the processing that could create or promote cracking at the keyway corner but I am uncertain where to start. Any help would be greatly appreciated.





RE: Premature cracking of 8620 component
RE: Premature cracking of 8620 component
RE: Premature cracking of 8620 component
RE: Premature cracking of 8620 component
Metallurgical indicates a 91 15N case, .011 deep, with a 23 Rc core. Is a lower core hardness more typical of 1117 versus 8620. We hear heat treaters telling us that the core follows the case but that is certainly not true of 1117. Is that more true of the 8620 as we try to achieve a 40 Rc core?
RE: Premature cracking of 8620 component
RE: Premature cracking of 8620 component
RE: Premature cracking of 8620 component
Yes, 8620 has higher hardenability than 1117, and therefore will have a higher core hardness when processed to a given case depth. 1117 will not be able to achieve a core hardness of 40 HRC.