Surface Cracks in 4340 Ground Washers

[damc]

We are reverse engineering a steel adjustment washer and lab analysis determined the original to conform to AISI 4340 per ASTM A29-99. Hardness 53HRC. A rough turned bar was heat treated prior to finish machining. The washer was stress relieved at 374 degrees F. after parting off. After finish surface grinding we detected 3 surface cracks .005" to .010" deep in one part out of ten that were mag particle inspected.
Any ideas what could cause these cracks? As always, any help is greatly appreciated

 

[swall]

My first thought would be grinding cracks.

 

[unclesyd]

What was the specific Heat Treatment? Times, Temperature, quenching media, time to tempers, tempering times and temperature, etc.

What size bar?

How were the washers parted off?

There several possibilities for having cracks in 4340 even though fairly forgiving.

 

[damc]

The size is 2.5 X 2.0 X .100. Parted on engine lathe with cobalt parting tool