The issue of high-temperature graphitization in metal pipes

The graphitization of metal pipes at high temperatures is a phenomenon of material degradation, especially in carbon-containing steel materials such as low-alloy steels like 20# steel. Under high-temperature conditions, carbon atoms gradually aggregate to form graphite; this process alters the material’s microstructure, thereby affecting its macroscopic mechanical properties. Indeed, graphitization leads to a decrease in the fracture elongation of pipeline materials. Strain at break refers to the maximum amount of plastic deformation that a material can withstand before breaking. A decrease in this value indicates that the material is more likely to fracture rather than undergo plastic deformation when subjected to tensile forces. As for tensile strength and yield strength, these two parameters reflect a material’s ability to resist deformation and fracture. The microstructural changes caused by graphitization reduce these two performance indicators of the material, as graphite has a lower strength compared to the other components of steel, which in turn reduces the overall load-bearing capacity of the material. The allowable stress of a material is a safety value determined based on the material’s properties, such as yield strength and tensile strength. If graphitization leads to a decline in these performance parameters, the corresponding allowable stress should also be reduced to ensure the safety of the structure during use. Regarding the issue that the GB20801 standard does not impose restrictions on use above 427°C, it is likely because the standard took into account the changes in performance and safety factors of different materials under various conditions during its formulation. The standards may consider that although there is a risk of graphitization above 427°C, under certain conditions, proper design and material selection can still ensure the safety of the structure. However, for specific application scenarios and conditions, it is indeed necessary to conduct an in-depth analysis of the graphitization trend and its impacts of the materials to ensure safety during long-term use. If there is concern that graphitization may reduce the allowable stress of the material, potentially exceeding the range of secondary stresses intended in the original design, it is recommended to conduct detailed tests and evaluations of the material’s properties, or to consider using materials with better high-temperature resistance as a substitute. .

GB20801 states that 20-grade steel and 0.5 chromium-molybdenum steel undergo graphitization, while 1.0 chromium-molybdenum steel undergoes spheroidization. However, the DL series standards state the exact opposite. Who is right after all?