4140 hardness change mystery

Old Elsmar Cove guy here. Glad I found this forum!
What am I missing? I am unable to do a controlled study at this moment because of lack of parts but I have a mystery that I can't explain. Three different times we sent 4140 material forgings to be heat treated to 18-24 Rc and they were certed to that spec. receiving inspection verified it all three times. The parts were then shot blasted to clean them and then Magnetic Particle Inspected to test for cracks. The customer then finds 100+ pcs. testing from 10-15 Rc. They sent a sample, I test it to be 10-15 Rc depending where I test it. First thought...customer had a process that changed it before they tested it.
Fourth lot of parts sent to HT, certs say they are in spec., I verify correct hardness at receiving, parts are shot blasted & Mag Inspected, I retest 5-6 pieces and every one is 10-16 Rc.
Is either of our process capable of softening the parts like that?
Any ideas would be appreciated!

 

Thanks Miner! Interesting read.
For both receiving inspection and pre-shipping inspection, I ground about .030" off to get both a flat surface and to get below the surface to test hardness. I also sacrificed a couple parts and they were soft into the middle so I am quite confused. I know all about the nose of an auditor but are we all picking mis-processed parts out to test and leaving the goods ones untouched?

 

Yes, depth is critical in this, I would expect the surface to be harder (case hardening). Removing 0.030" probably gets down into softer material.

 

I know what case hardening is and normally that is something done with low carbon steels that get carbonized to provide a hard shell. This is 4140 material and is supposed to be hardened through. I wouldn't think the hardness would be lowered because of tumbling in a shot blaster.

 

Not always - see this discussion - http://www.eng-tips.com/viewthread.cfm?qid=324850