I am using EN8D unalloyed medium carbon steel material bright bars (dia 22 mm) for manufacturing a component which is used just like a gear.
Component Manufacturing process flow is turning, milling & drilling, gear teeth hobbing, induction hardening and OD grinding.
The problem which I am facing is, few component gets cracked during different stages of machining.
Earlier, i had doubted my hardening process but, for few components, cracks could be observed in soft form too. So that's not the root cause.
Crack is such minor that it's quite difficult to see by naked eye. In few components, cracks are not visible before hardening but cracks gets expand after hardening and grinding of components.
As material inspection method, I am doing following metallurgical testings before processing material: BHN Hardness test, Spectro chemical analysis for verification EN8D compliance, micro structure test to verify whether micro structure is normalised or not.
But problem exists even if material is OK as as per above mentioned testing methods.
Later on, I tried to test material in raw form with ultrasonic testing for detection of internal cracks. And found that material is satisfactory. But even after taking such care, components gets cracked during different stages of machining.
Can anyone please guide me regarding what might be the root cause for this issue. Also, please suggest me any further methods to identify defect at incoming stage of inspection.