TTP Diagrams of Graphitization of Creep Ruptured Carbon Steels and 0.5Mo Steel

Graphitization in carbon steels should be avoided because it results in the degradation of material performance. Safety management standards state that graphitization occurs at 698 K for carbon and carbon-Mo steels, although some standards state it to be above 738 K for carbon-Mo steels. However, recently, graphitization was found at 673 K in creep ruptured 0.3C steel. Herein, we investigated the graphitization behavior of creep ruptured 0.3C, 0.2C, and 0.5Mo steels. It was confirmed that the graphitization occurred below the specified temperatures of 673 K for the 0.3C and 0.2C steels and 723 K for the 0.5Mo steel. In addition, time-temperature-precipitation diagrams for graphite were obtained for all the steels. Elongated graphite and spherical graphite were confirmed in the 0.3C and 0.5Mo steels, while only spherical graphite was confirmed in the 0.2C steel. It was suggested that the elongated and spherical graphite were formed due to different mechanisms. The formation of elongated graphite was promoted by a higher carbon content, Mo addition, and higher applied stress, whereas that of spherical graphite was suppressed by Mo addition. Further, to accurately assess the risk of graphitization, time and temperature, as well as the stress level and different formation mechanisms, of the two types of graphite must be considered.