In a hardening process, the hypo-eutectoid steel is

• heated from 30° C to 50° C above the upper critical temperature and then cooled in still air
• heated from 30° C to 50° C above the upper critical temperature and then cooled suddenly in a suitable cooling medium
• heated from 30° C to 50° C above the upper critical temperature and then cooled slowly in the furnace
• heated below or close to the lower critical temperature and then cooled slowly