Failed HT on 5160

OK, the surfaces in the photo looked very oxidized.

What concerns me most is the variety of harness readings within 25° steps. To me, that bell curve does not seem right at all. Maybe Larin can figure it out.
 
I had a chance to redo the coupons at the same temperatures but quenched in P50. Interesting results. As quenched, topped out at 61.0 HRC @ 1600F w/10 min soak. Blue line on graph:

IMG_2969.jpeg

Not sure if this raises more questions than it answers, but thought I’d post the follow up.
 
The part of that plot from 1525° to 1600° seems right. The sudden drop of 8.5 hardness points with an additional 25° increase seems wrong.
 
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