Graphitization | Materials And Corrosion Control
| Damage Mechanism | Graphitization |
| Damage Description | · Graphitization is a change in the microstructure of certain carbon steels and 0.5Mo steels after long-term operation in the 800°F to 1100°F (427°C to 593°C) range that may cause a loss in strength, ductility, and/or creep resistance.
· At elevated temperatures, the carbide phases in these steels are unstable and may decompose into graphite nodules. · The most important factors that affect graphitization are the metal chemistry, stress, temperature, and time of exposure. · Temperature has an important effect on the rate of graphitization. Below 800°F (427°C), the rate is extremely slow. The rate increases with increasing temperature. |
| Affected Materials | Some grades of carbon steel and 0.5Mo steels. |
| Control Methodology | · Graphitization can be prevented by using chromium containing low alloy steels for long-term operation above 800°F (427°C).
· Scattered/random graphitization may have little or no effect on mechanical properties of material · “Eyebrow” or aligned graphitization is most severe form and will have negative impact on mechanical properties |
| Monitoring Techniques | · Evidence of graphitization is most effectively evaluated through removal of full thickness samples for examination using metallographic techniques.
· Metallographic replication is suitable to check for graphitization at surface. But note that damage may occur midwall. · Advanced stages of damage related to loss in strength include surface breaking cracks or creep deformation that may be difficult to detect. |
| Inspection Frequency | · As advised by local corrosion engineer or CSD Metallurgical Specialist. Note: Operators do not usually carry out examinations to intentionally check for this damage mechanism; however, old vintage assets made of low-quality steels operating at high temperature represent the highest potential for this damage mechanism. |
| KPIs | · Number of graphitization inspections/examinations |
| Reference Resources (Standards/GIs/BPs) | · API RP571 (DM #15)
· ASM Metals Handbook, “Properties and Selection: Iron, Steels, and High-Performance Alloys,” Volume 1, ASM International, Materials Park, OH. |