that's is an auto hamon, the blade failed to through harden. You could up the hardening temp a bit, might be your oven is a bit cold or add time to your soak that might fix the issue as well. It could also be caused by over thermocycling, to small a grain reduces hardenability, this is especially the case if the normalizing cycle isn't high enough temp or a long enough soak to reset/grow the grain after forging. 

MP

Thanks Matthew, appreciate the explanation. I think the issue must have been the thermocycle. I only did one, but I don't think it got high enough temperature as I was concerned about overheating the blade edge. New heat treating oven so I don't think it was there. Fortunately, it is all near the spine so I am hoping that the edge hardened fine and will hold an edge. Will definitely pay closer attention to this on the next ones!

doubtful it would be from too low a temp in thermocycleing I would look at the normalizing. after forging with so many cycles especially if you do the last few at  cooler temps you can easily over refine grain lowering hardenability (and making a nasty mix of structures that can react poorly to hardening) a good normalizing cycle 1650 with a 20 min soak resets that. If i don't have access to an oven ill do one cycle at forging temp (2000deg ish) with out a soak to get a similar effect.  Then two cycles at or around critical (1475 deg) to re refine the grain. if hardening from 1475 you need to hold at hemp for at least 15 min 20 is better. or if you bump the temp to 1500 (you wont get grain growth with out a soak) you just need to equalize the temp before quenching. 

Matthew,

Thanks for more information. I have heat treating oven so will be able to do the 1650 and 1475 normalizing, and longer soak for hardening you suggested on the next knife.

Very interesting new video by Dr Larrin Thomas discussing soak times and other factors and his thoughts on salt pots and HT ovens.

https://youtu.be/iwYS4cD9Y_o?si=RZ7bS1MIIAEWXxF4