.graphitization

 Graphitization is a microstructural change that occurs in carbon or low-alloy steels exposed to temperatures of about 425–550°C for several thousand hours.

It causes the metal to weaken and be susceptible to cracking failures. The steel tends to break down to form iron and carbon (graphite); carbon will migrate to the material’s grain boundaries forming graphite nodules, which causes the metal to become brittle, losing strength, toughness, creep resistance, and ductility.

Creep resistance is solid material’s ability to resist “creep”

Creep: tendency of a material to slowly deform over a long period of exposure to high levels of stress.