What is the critical point?

Reference the FAQ here: https://legacy-fm.com/answer/what-is-the-hardening-of-steel/

The upper critical temperature is the point below which cementite or ferrite begins to form. This occurs when steel is cooling from the normalizing temperature. Depending on carbon content, this point rests between 1333–1670°F.

https://www.reliance-foundry.com/blog/steel-grades#:~:text=The%20upper%20critical%20temperature%20is,between%201333%E2%80%931670%C2%B0F.