I respectfully disagree. That's a brittle fracture due to quench-hardening. I am not sure if it is actually due to accidental case-hardening in the coke, you would normally have to actually try to get a significant case depth, or whether the steel used actually has a reasonably high carbon-equivalent content. Do you know the actual steel grade, Steve?
You would need a lot more strain (shape change) than that for significant work hardening, but I am actually basing this opinion on the colour of the fracture surface.
(In a previous life my job revolved around heat-treatment, cold-work and fracture mechanics research)