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.

应避免碳钢中的石墨化，因为它会导致材料性能下降。安全管理标准规定，碳钢和碳钼钢的石墨化发生在 698 K，尽管一些标准规定碳钼钢的石墨化温度高于 738 K。然而，最近，在 673 K 的蠕变断裂 0.3C 钢中发现石墨化。在此，我们研究了蠕变断裂 0.3C、0.2C 和 0.5Mo 钢的石墨化行为。已证实石墨化发生在低于 0.3C 和 0.2C 钢的 673 K 和 0.5Mo 钢的 723 K 的指定温度以下。此外，还获得了所有钢的石墨时间-温度-析出图。在 0.3C 和 0.5Mo 钢中证实了伸长石墨和球形石墨，而在 0.2C 钢中仅确认了球形石墨。有人提出细长和球形石墨的形成是由于不同的机制。较高的碳含量、Mo添加和较高的外加应力促进了细长石墨的形成，而Mo添加抑制了球形石墨的形成。此外，为了准确评估石墨化的风险，必须考虑两种类型石墨的时间和温度，以及应力水平和不同的形成机制。