26C3 Stability/brittleness at 66-67 HRC ?

[Londinium Armoury]

This was the only survivor of its batch, named him the last santokurai.
I assume the white orbs and ghostly images in the hidden activity are the trapped souls of his fallen siblings that cracked in the quench.

Abura Honyaki Hamon no etch

https://imgur.com/a/qfzn3Ui

This knife has been given to my Grandfather who is almost 90, he used it today for the first time.

 

[Londinium Armoury]

UHB 26C3 – Barmond (Special Steels) Limited

UHB26C3 Uddeholm 26C3 is a very high carbon steel with closely controlled alloy content and low impurities. Originally developed for making scalpels and razor blades, it has very fine grain structure and the highest potential for sharpness. When differentially hardened using a clay like satanite...

www.barmondsheffield.com

Here is a reference data sheet for 26C3 if people need something to go by when heat treating and working with it.
basic HRC tempering chart for reference.

 

[Larrin]

UHB 26C3 – Barmond (Special Steels) Limited

UHB26C3 Uddeholm 26C3 is a very high carbon steel with closely controlled alloy content and low impurities. Originally developed for making scalpels and razor blades, it has very fine grain structure and the highest potential for sharpness. When differentially hardened using a clay like satanite...

www.barmondsheffield.com

Here is a reference data sheet for 26C3 if people need something to go by when heat treating and working with it.
basic HRC tempering chart for reference.

I could be wrong but it looks like a summary of the 26C3 study we did to me: https://knifesteelnerds.com/2019/08/12/how-to-heat-treat-26c3-steel/