Graphitization Behaviors of Creep-ruptured 0.3% Carbon Steel at 673 to 773 K

Abstract

Graphitization in carbon steels must be prevented because it reduces the amount of carbon in the matrix, which degrades the material performance due to loss in strength. In addition, when graphite particles are aligned, they can cause fracture by their linkage. The safety management standards for carbon steels in high-temperature applications state that graphitization should be considered at 698 K and above. The number of reported cases on graphitization in steels below 698 K is limited, and the mechanism has not yet been well investigated. This paper reports the finding of unprecedented graphitization at 673 K in creep-ruptured carbon steel and an elongated form of graphite that appears after a much shorter time at 673-773 K than other previously reported times. Furthermore, the formation mechanism of this elongated graphite is discussed. Dislocations and inclusions in the vicinity of grain boundaries may facilitate graphitization kinetics at these temperatures.

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

“…This indicates that the elongated graphite becomes a preferential formation site of crack. 7) In contrast, the graphite in the longer-term side at 773 K are spherical, as shown in Fig. 4(k).…”

“…However, recently, graphitization cases at 673 K in the gauge portion of a creep ruptured 0.3C steel plate have been noted. 7) The graphite found at 673 and 723 K has an unprecedented elongated form, whereas that at 773 K found after crept for 27 872.2 h and more is spherical. 7) The relationship between the graphite form and creep conditions has been discussed, 7) and it was suggested that stress higher than the proof stress promotes the formation of elongated graphite along the grain boundary perpendicular to the tensile axis.…”

“…7) The graphite found at 673 and 723 K has an unprecedented elongated form, whereas that at 773 K found after crept for 27 872.2 h and more is spherical. 7) The relationship between the graphite form and creep conditions has been discussed, 7) and it was suggested that stress higher than the proof stress promotes the formation of elongated graphite along the grain boundary perpendicular to the tensile axis. 7) The graphitization behavior of other carbon and carbon-0.5Mo steels, which have different applied stress levels, must be investigated for confirming the effect of the stress on graphitization kinetics.…”

“…The subfigures in the left side indicate the shortest time to rupture that was The graphite precipitated at 673-723 K and that at 773 K in the shorter-term side have an elongated morphology perpendicular to the stress direction and often coexist with cracks. 7) The existence of cracks is recognized in the EDS maps due to the weaker signal of carbon around the cracks, as indicated by arrows in Fig. 4.…”

“…7) The relationship between the graphite form and creep conditions has been discussed, 7) and it was suggested that stress higher than the proof stress promotes the formation of elongated graphite along the grain boundary perpendicular to the tensile axis. 7) The graphitization behavior of other carbon and carbon-0.5Mo steels, which have different applied stress levels, must be investigated for confirming the effect of the stress on graphitization kinetics. In addi-tion, the investigations have another motivation from an engineering perspective, i.e., whether graphitization occurs below the specified temperature for carbon steels (698 K) and carbon-0.5Mo steels (738 K) and whether Mo addition can reduce the graphitization susceptibility.…”