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

Abstract

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.

Prediction of Graphitization Behavior during Long-Term Creep in Carbon Steels

“…[19][20][21] Their graphitization behavior has been systematically investigated in our previous studies. 13,14) The graphitization condition in the gauge and grip portions for the steels are summarized in Fig. 1 and Fig.…”

“…12) Recently, Hatakeyama et al systematically investigated the graphitization behavior of carbon steels and 0.5Mo steel using creep ruptured specimens obtained through the NIMS Creep Data Sheet Project. 13,14) They established the TTP diagram of these steels and pointed out that graphitization behavior changes with, not only time and temperature, but also the chemical composition of the steels. 14) In addition, they discovered two types of graphite, i.e., elongated and spherical graphite, and suggested that elongated graphite formation is especially accelerated by applied stress.…”

“…13,14) They established the TTP diagram of these steels and pointed out that graphitization behavior changes with, not only time and temperature, but also the chemical composition of the steels. 14) In addition, they discovered two types of graphite, i.e., elongated and spherical graphite, and suggested that elongated graphite formation is especially accelerated by applied stress. 13,14) However, their research investigated only 3 heats of steel, and was therefore, insufficient to predict discuss the graphitization risk of any steel composition.…”

“…14) In addition, they discovered two types of graphite, i.e., elongated and spherical graphite, and suggested that elongated graphite formation is especially accelerated by applied stress. 13,14) However, their research investigated only 3 heats of steel, and was therefore, insufficient to predict discuss the graphitization risk of any steel composition. Nevertheless, investigating all available heat is quite time consuming -although still insufficient for the individual prediction of graphitization kinetics.…”

“…Nevertheless, the prediction of the graphitization condition using the limited available data is beneficial to efficiently determine the threshold condition through an experiment with minimal effort. Therefore, in this study, quantitative evaluation of the contribution of temperature, time, stress, and chemical composition on graphitization was performed using the available data points 13,14) and a prediction formula for graphitization behavior was recommended. Furthermore, the accuracy of the prediction formula was evaluated by investigating the graphitization behavior of other heats of creep-ruptured carbon steels that had not been used to establish the prediction formula.…”