2021 Volume 61 Issue 11 Pages 2822-2831
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.
The duplex structure of ferrite and ferrite/cementite eutectoid lamellar (pearlite) provides appropriate mechanical properties to carbon steels used for structural components. However, since cementite is a metastable phase in the Fe–C binary system,1) it decomposes into iron and carbon during long-term exposure at elevated temperatures. This results in the precipitation of the equilibrated graphite (graphitization)2) and degrades the mechanical properties of steels2) by reducing the volume fraction of cementite and lowering carbon solubility in ferrite.3) In addition, sometimes, precipitated graphite facilitates the formation and propagation of cracks.4) Therefore, when carbon steels are used at elevated temperatures, such as in power or petrochemical plants, graphitization must be avoided.
The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code5) suggests that graphitization risk arises over long periods of time above 698 K for both carbon and carbon–0.5Mo steels. In contrast, the description of Japan Industrial Standard (JIS)6) is slightly different, i.e., graphitization occurs at above 698 K for carbon steels and 738 K for carbon–0.5Mo steels during their long-term exposure. 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 27872.2 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. In addition, 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.
Since graphitization around the specified temperature requires extremely long incubation time, most of the reported cases are those reported in actual plants, and to the best of our knowledge, systematic investigation of graphitization behavior at around the specified temperatures (698–738 K) has never been performed. Therefore, we systematically investigated the graphitization behavior using specimens crept for up to 250005.9 h under temperatures of 673–823 K, as taken from the NIMS Creep Data Sheet Project.8) Three heats of 0.3C steel plates,9) 0.2C steel tubes,10) and 0.5Mo steel tubes11) are selected, and the time-temperature-precipitation (TTP) diagrams are established for the conservative risk assessment of graphitization of the steels.
Here, 0.3C silicon-killed steel plates,9) 0.2C silicon-killed steel tubes,10) and 0.5Mo silicon-killed steel tubes11) were used as samples. Their heats are CaH, CAB, and LAG in NRIM (National Research Institute for Metals, currently NIMS) creep data sheets,9,10,11) respectively. Table 1 shows the chemical composition and processing and thermal histories, while Table 2 lists the 0.2% proof stress of the steels.9,10,11) Creep tests were performed at 673, 723, and 773 K for the 0.3C and 0.2C steels and 723, 773, and 823 K for the 0.5Mo steel under a constant load of 69–412 MPa in air using specimens having 50-mm-gauge length and 10-mm-gauge diameter for the plates (0.3C steel) and 30-mm-gauge length and 6-mm-gauge diameters for the tubes (0.2C and 0.5Mo steels). The gauge and grip portion of the crept 0.3C steel specimens and only the gauge portion of crept 0.2C and 0.5Mo specimens were subjected to analysis. Coupons cut from the crept specimen were ground, polished, and etched using 3% nital for optical microscopy, and those for scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (SEM-EDS) were finished using colloidal silica. Here, SEM was conducted with an accelerating voltage of 1–15 kV using the secondary electron (SE) mode or in-lens SE mode. Then, elemental mapping was obtained using SEM-EDS with accelerating voltages of 10 or 15 kV.
| C | Si | Mn | P | S | Ni | Cr | Mo | Cu | Al | N | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.3C | 0.29 | 0.22 | 1.00 | 0.016 | 0.011 | 0.05 | 0.07 | 0.34 | 0.05 | 0.011 | 0.0035 |
| 0.2C | 0.20 | 0.28 | 0.60 | 0.018 | 0.008 | – | 0.045 | 0.010 | 0.05 | 0.005 | 0.0066 |
| 0.5Mo | 0.20 | 0.28 | 0.59 | 0.012 | 0.010 | 0.02 | 0.02 | 0.54 | 0.07 | 0.003 | 0.0054 |
| Processing and thermal histories |
|---|
| Hot rolled → 1183 K/5 h AC → 898 K/5 h FC |
| Hot extruded and cold drawn → Normalized |
| Hot extruded and cold drawn → 1193 K/1 h → 953 K/1.5 h AC |
*AC: Air cooling, FC: Furnace cooling
Figure 1 shows the relationship between stress and time to rupture curves. The slope of the plots for the (a) 0.3C steel gradually increases with both time to rupture and temperature, indicating the premature failure by microstructure deteriorations. The time to rupture for the (b) 0.2C steel is shorter, and the slopes of plots become steeper from the shorter-term side than those of the 0.3C steel under same temperatures. The plots of 0.2C steel at 673 and 723 K seem to be exhibiting the inverse S shape, suggesting that they reached the inherent creep strength in the longer-term side.12) The creep strength of the (c) 0.5Mo steel is higher than that of the 0.3C steel, although premature failure also occurs in the longer-term side. The differences in the creep strength according to steel type is due to the effect of Mo content,12,13) as shown in Table 1.
3.2. Graphitization Behavior of the 0.3C SteelFigures 2(a) and 2(b) show the optical micrographs of the 0.3C steel in the as-received condition. Both rolled and stress directions are along the horizontal direction of the paper. The as-received steel has a duplex structure of ferrite (bright contrast) and pearlite (dark contrast), and the banded structure of ferrite and pearlite are clearly seen. Figures 2(c) and 2(d) show the optical micrographs of the gauge portion of the 0.3C steel ruptured after crept for 232983.5 h at 673 K under 294 MPa. Even though the duplex structure is maintained, elongated graphite is observed at the ferrite-pearlite interface. In contrast, the spheroidization and decomposition of cementite is significantly progressed, and spherical graphite is observed in the gauge portion of the steel ruptured after crept for 85699.2 h at 773 K under 88 MPa, as shown in Figs. 2(e) and 2(f).
Optical micrographs of the 0.3C steel in the (a), (b) as-received condition, and gauge portion of specimen ruptured after crept for (c), (d) 232983.5 h at 673 K under 294 MPa and (e), (f) 85699.2 h at 773 K under 88 MPa.
Figure 3(a) are the representative SE image and SEM-EDS mapping for (b) C and (c) Fe around the graphite precipitated in the gauge portion of the 0.3C steel ruptured after crept for 85699.2 h at 773 K and for (d), (e) enlarged view of the graphite. The regions of Fig. 3(a) enlarged in Figs. 3(d) and 3(e) are indicated by arrows. The darker contrast phase in the SE images is the graphite, and the brighter one is the matrix. One of the features of identification of graphite is the carbon enrichment, as shown in Fig. 3(b). In addition, the interface between graphite and the matrix, indicated by an arrow in Fig. 3(d), are serrated. Furthermore, as can be seen in the dashed circle in Fig. 3(e), the graphite surface shows a laminated structure of plates, although it was carefully polished by colloidal silica. It results from the preferential growth orientation of graphite lattice and their mechanical exfoliation among laminated layers.14,15) By considering these morphological features and results of SEM-EDS analyses, graphitization behaviors in the crept specimens were investigated.
(a) SE image of graphite precipitated in the gauge portion of the 0.3C steel ruptured after crept for 85699.2 h at 773 K under 88 MPa; its EDS mapping for (b) C and (c) Fe; and (d)(e) enlarged view of (a). (Online version in color.)
Figure 4 shows the SE images and SEM-EDS mappings for carbon around the graphite, obtained from the gauge portion of the creep ruptured 0.3C steel. The subfigures in the left side indicate the shortest time to rupture that was observed for graphite, and those in the right side indicate the longest time to rupture at 673 K, 723 K, and 773 K. 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. 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). This difference is due to the applied stress.7) When the stress above the proof stress is applied, dislocations are inevitably introduced at the beginning of the creep testing. Therefore, decomposition of cementite16,17) and/or the diffusion of carbon atoms along the tensile axis are facilitated, which result in earlier graphitization.7) In addition, the elongated graphite are formed along the grain boundary due to much faster diffusion kinetics of carbon atoms than that of iron and thermodynamic instability of grain boundary.7)
SE image and SEM-EDS mapping for carbon of the gauge portion of the 0.3C steel: creep ruptured at 673 K after (a)(b) 45856.4 h under 353 MPa and (c)(d) 232983.5 h under 294 MPa, at 723 K after (e)(f) 4377.8 h under 294 MPa and (g)(h) 122103.2 h under 196 MPa, and at 773 K after (i)(j) 324.0 h under 235 MPa and (k)(l) 85699.2 h under 88 MPa. (Online version in color.)
Figure 5(a) shows the in-lens SE image and SEM-EDS mappings of (b) boron (B), (c) carbon (C), and (d) nitrogen (N) obtained from the grip portion of the 0.3C steel ruptured after crept for 232983.5 h at 673 K. As the images obtained by in-lens SE detector sensitively reflect the surface and element information,18,19) two types of precipitates are confirmed to exist in Fig. 5(a). Figure 5(c) reveals that the dark precipitate is graphite, which occurred at 673 K without the aid of applied stress. As the graphite size is in the order of hundreds of nanometers, it was considered that the graphite observed in the grip portion of 0.3C steel after crept for 232983.5 h at 673 K is that immediately after the nucleation. In addition, Figs. 5(b) and 5(d) indicate that the bright precipitate adjacent to the graphite consists of B and N. It is considered that the precipitate is boron nitride (BN), which is known to accelerate the graphitization by providing the opportunity of epitaxial nucleation of graphite on (0001)BN because of their crystal structure similarity.20) Nevertheless, B is not intentionally added to the steel, and its concentration was below the detection limit. Figures 5(e)–5(h) are the SE images and EDS mappings for carbon in the grip portion of the 0.3C steel observed at 723 and 773 K. Even we previously concluded that the elongated graphite forms only in the gauge portion,7) careful SEM observations clarified that it can form without the aid of the stress at ferrite-pearlite interface, as clearly seen in Fig. 5(g).
(a) In-lens SE image and SEM-EDS mapping for (b) B, (c) C, and (d) N of the grip portion of the 0.3C steel ruptured after crept for 232983.5 h at 673 K, SE image and SEM-EDS mapping for carbon of that after crept for (e)(f) 35127.6 h at 723 K, and (g)(h) 27872.2 h at 773 K. (Online version in color.)
Graphitization condition in both (a) gauge and (b) grip portions of the creep ruptured 0.3C steel are summarized as TTP diagrams in Fig. 6. The plots are classified into the formation of no graphite (open symbol), elongated graphite (solid gray symbol), and spherical graphite (solid black symbol). The mixture of elongated and spherical graphite was regarded as the formation of spherical graphite. The graphite shown in Fig. 5(a), grip portion after crept for 232983.5 h at 673 K, is regarded as the elongated one because it nucleated and grown at the BN-matrix interface. The first lines in the diagrams indicate the formation of any graphite, and the second indicate that of spherical graphite. The time and temperature regions after the risk of the start of graphitization in carbon steel weldments, predicted by Foulds and Shingledecker,21) are shaded in each figure for comparison. It should be noted that the graphitization susceptibility in weldments is higher than that in the base metal because weld thermal cycles result in carbon supersaturation, which provides the driving force for graphitization.21)
TTP diagram of the graphite in (a) gauge and (b) grip portion of the creep ruptured 0.3C steel with the graphitization risk region of carbon steel weldments reported by Foulds and Shingledecker.21)
The time required for the graphitization shifts toward the shorter-term side with temperature under the investigated conditions, suggesting that the nose temperature is 773 K or above; the nose temperature for graphitization in high-carbon steels is reported to be approximately 923 K.22,23) It was revealed that the precipitation of elongated graphite in the gauge portion is much earlier than that in the grip portion as well as the predicted risk21) due to applied stress.7) In contrast, the graphitization kinetics in the grip portion shows a good agreement with the predicted risk,21) although the specimens investigated were not welded. Therefore, it was considered that the systematic investigations by several crept specimens permit to find the very early stage of graphitization as shown in Fig. 5(a), and graphitization risk in the base metal is in the same level with that in the weldments. The formation of the spherical graphite in the gauge portion is slightly earlier than that in the grip portion, while both plots are in the predicted risk region.21) This suggests that the formation of spherical graphite is less affected by the applied stress in the 0.3C steel.
3.3. Graphitization Behavior of 0.2C SteelFigures 7(a) and 7(b) show the optical micrographs of the 0.2C steel in the as-received condition. Both extruded and stress directions are along the horizontal direction of the paper. Similar to 0.3C steel, a duplex structure of ferrite and pearlite is also observed, and the banded structure of ferrite and pearlite are clearly seen. However, the volume fraction of the pearlite is smaller due to its lower carbon content, as shown in Table 1. Figures 7(c) and 7(d) show the optical micrographs of the gauge portion of the crept 0.2C steel ruptured after 250005.9 h at 673 K under 157 MPa. The cementite spheroidization and decomposition is recognized even at 673 K. In addition, as will be shown in Fig. 8, spherical graphite is observed with inclusions. The microstructure deteriorations are markedly progressed, and significantly coarsened spherical graphite nobles are observed after crept for 23159.8 h at 773 K under 88 MPa. The size of the spherical graphite in the 0.2C steel observed after crept for 23159.8 h at 773 K is the largest, as shown in Fig. 7(e), even though its time to rupture was one fourth than that of the 0.3C steel shown in Fig. 2(e).
Optical micrographs of the 0.2C steel in the (a), (b) as-received condition, and the gauge portion of the crept specimen ruptured after (c), (d) 250005.9 h at 673 K under 157 MPa and (e), (f) 23159.8 h at 773 K under 88 MPa.
(a) SE image and SEM-EDS mapping for (b) C, (c) O, (d) Al and (d) Si of the gauge portion of the 0.2C steel ruptured after crept for 250005.9 h at 673 K under 157 MPa. (Online version in color.)
Figure 8(a) shows the SE images and SEM-EDS mappings of (b) C, (c) O, (d) Al, and (e) Si, obtained from the gauge portion of the creep ruptured 0.2C steel. As shown in Figs. 8(a) and 8(b), graphitization occurs in the 0.2C steel tube at 673 K during long-term exposure. Different from the graphite observed in the 0.3C steel at 673 K, shown in Figs. 4(a) and 4(c), that of 0.2C steel has a spherical shape. One of the causes of this difference is the applied stress of 157 MPa lower than the 0.2% proof stress of 166 MPa at 673 K (see Table 2). In addition, the graphite was frequently observed with elongated inclusions as indicated in Fig. 8(a). Figures 8(c)–8(e) reveal that these inclusions are rich in O, Al, and Si. This supports the contribution of the inclusions toward graphitization as indicated before.7,24)
From the systematic observations, TTP diagram of the graphite in the gauge portion of the creep ruptured 0.2C steel is established and shown in Fig. 9. Only spherical graphite was observed in the steel whose formation is indicated by dashed lines. The time and temperature region after the risk of the start of graphitization in carbon steel weldments, predicted by Foulds and Shingledecker,21) is shaded in the figure for comparison. Please note that there is an additional plot obtained from the creep interrupted sample discontinued after 117835.5 h at 673 K under 157 MPa. Figure 9 reveals that graphitization occurs earlier at higher temperature, i.e., nose temperature is 773 K or above. In addition, the incubation time for graphitization shows a good agreement with the predicted risk for carbon steel weldments21) at 673 K yet faster by an order of magnitude at 773 K. This suggests that the effect of stress for spherical graphitization markedly appears at higher temperatures.
TTP diagram of graphite in the gauge portion of the creep ruptured 0.2C steel with the graphitization risk region of carbon steel weldments reported by Foulds and Shingledecker.21)
Figures 10(a) and 10(b) show the optical micrographs of the 0.5Mo steel in the as-received condition. Similar to the other two steels, the duplex structure of ferrite and pearlite is observed. However, the banded structure of ferrite and pearlite are less clear. The volume fraction of the pearlite in the 0.5Mo steel is almost identical with that of the 0.2C steel due to their same carbon content (Table 1). Figures 10(c) and 10(d) show the optical micrographs of the gauge portion of the 0.5Mo steel ruptured after crept for 67165.8 h at 723 K under 265 MPa. The duplex structure is maintained, and the elongated precipitates, which are identified as graphite, are observed on the grain boundary. In contrast, as shown in Figs. 10(e) and 10(f), spheroidization and decomposition of cementite significantly progress, and spherical graphite is observed in the gauge portion of the steel ruptured after crept for 16996.9 h at 823 K under 69 MPa. In addition, many cracks are observed on the grain boundaries.
Optical micrographs of the 0.5Mo steel in the (a), (b) as-received condition, and the gauge portion of the steel ruptured after crept for (c), (d) 67165.8 h at 723 K under 265 MPa, and (e), (f) 16996.9 h at 823 K under 69 MPa.
Figure 11 shows the SE images and SEM-EDS mappings for carbon around the graphite obtained from the gauge portion of the creep ruptured 0.5Mo steel. The subfigures in the left side indicate the graphite observed in the shortest conditions at each temperature, and those in the right side are the longest conditions. Although some specifications indicate the graphitization risk for the 0.5Mo steel as at above 738 K (40 K higher than 698 K of carbon steel),6) graphitization of the 0.5Mo steel at 723 K was recognized here. In the investigated temperature range, elongated graphite was observed in the shorter-term side, whereas the graphite thickened and spheroidized in the longer-term side. Please note that the elongated graphite is also observed in the longest time at 723–823 K.
SE image and SEM-EDS mapping for carbon of the gauge portion of the 0.5Mo steel: creep ruptured at 723 K after (a), (b) 67165.8 h under 265 MPa, and (c), (d) 126045.9 h under 235 MPa; at 773 K after (e), (f) 5374.2 h under 216 MPa, and (g), (h) 105737.0 h under 98 MPa; and at 823 K after (i), (j) 563.4 h under 177 MPa and (k), (l) 16996.9 h under 69 MPa. (Online version in color.)
From the observations, TTP diagram of graphite in the gauge portion of the creep ruptured 0.5Mo steel is established and shown in Fig. 12. The plots are classified into the formation of no graphite (open symbol), elongated graphite (solid gray symbol), and spherical graphite (solid black symbol). The mixture of elongated and spherical graphite was regarded as the formation of spherical graphite, and the first and second dashed lines in the diagram indicate the formation of any graphite and spherical graphite, respectively. The time and temperature region after the risk of the start of graphitization in carbon steel weldments, predicted by Foulds and Shingledecker,21) is shaded in the figure for comparison. Similar to the graphitization behavior of the 0.3C and 0.2C steels, graphitization occurs earlier at higher temperatures. The formation of the elongated graphite is much earlier than the predicted risk at 773 K and above,21) whereas that of elongated and spherical graphite agrees with the risk curve at 773 K and below.21)
TTP diagram of graphite in the gauge portion of the creep ruptured 0.5Mo steel with the graphitization risk region of carbon steel weldments reported by Foulds and Shingledecker.21)
The lines representing formation of any graphite and spherical graphite, drawn in the TTP diagrams of Figs. 6, 9, and 12, are summarized in Figs. 13(a) and 13(b) for comparison, respectively. As shown in Fig. 13(a), graphitization in the gauge portion of the 0.3C steel is much faster than that in the 0.2C and 0.5Mo steels, which are almost identical at 723–773 K. Nevertheless, due to different graphitization behaviors in the gauge and grip portions of the 0.3C steel, the effect of the applied stress must be considered to evaluate the graphitization susceptibility in the gauge portions among the steels.
TTP diagram for (a) any graphite and (b) spherical graphite in the steels investigated with the graphitization risk region of carbon steel weldments reported by Foulds and Shingledecker.21) (Online version in color.)
As mentioned above, the deviations between the data collected here and the risk curve from the literature21) become significant at higher temperatures, which must be analyzed for the consistency and the reliability of this study. One hypothesis is the difference in the stress level as it is known that the applied stress affects the graphitization.7) According to graphitization cases in actual plants reported in the literature,21) it is considered that the components graphitized at higher temperatures were under much lower stresses than those at lower temperatures. In contrast, the stress level against the graphitization time in the present study are fixed by the creep strength because the data are collected from creep ruptured specimens, i.e., the investigated data correspond to the most conservative graphitization risk of graphitization by considering the stress promoting effect. In other words, there is a possibility of graphitization earlier than the assessed risk line21) under the stress loaded environment. Therefore, not only the time and temperature, but also the stress level must be considered to accurately assess the risk of graphitization.
In addition, the difference in the graphite form should be considered. As shown in Fig. 13(b), the spherical graphite in the gauge portion of the 0.2C steel forms much earlier than those in 0.3C and 0.5Mo steels, which are almost identical. This trend is different from that of the any graphitization, shown in Fig. 13(a). Since the size of the spherical graphite formed in the 0.2C steel, shown in Fig. 7(e), is the largest, it was suggested that spherical graphitization susceptibility in the 0.2C steel is also the highest, even though elongated graphite appears in the shorter-term side in the 0.3C steel. In other words, different formation mechanisms of the elongated and spherical graphite were suggested.
It is known that the formation of elongated graphite is facilitated by the stress and mainly occurs at the ferrite-pearlite interface perpendicular to the tensile axis.7) As can be seen in Figs. 2(a), 7(a), and 10(a), the volume fraction of the pearlite in the 0.3C steel is the highest. In addition, austenite grain size number of the 0.3C, 0.2C, and 0.5Mo steels are reported to be 7.8, 5.2, and 5.2, respectively;9,10,11) 0.3C steel has smaller grain size than the others. Furthermore, as shown in Fig. 1, the applied stress for the 0.3C steel is higher than that for the 0.2C steel against the same time to rupture. Therefore, it was considered that the larger volume fraction of pearlite (higher carbon content), smaller grain size,9,10) and higher applied stress of the 0.3C steel than that of the 0.2C steel facilitate the formation of elongated graphite. The absence of the elongated graphite in the 0.2C steel is considered to result from the low applied stress levels; even if sufficiently high stress for elongated graphite is applied, the 0.2C steel suffers creep rupture before graphitization. The formation of elongated graphite in the 0.5Mo steel is understood by the highest applied stress level among the steels, although the grain sizes and the carbon contents of 0.2C and 0.5Mo steels are identical.
Figure 14 show the stress vs. time to rupture curves for the (a) 0.3C,9) (b) 0.2C,10) and (c) 0.5Mo steels.11) The plots are classified into the formation of no graphite (open symbol), elongated graphite (solid gray symbol), and spherical graphite (solid black symbol). The plots for the 0.2C steel (in Fig. 14(b)) suggest that the formation of the spherical graphite is related to the inherent creep strength.12) In addition, the classification according to graphitization behavior enables division of stress vs. time to rupture curves into three regions (no graphite, elongated graphite, and spherical graphite formation), as shown by the dashed lines in Figs. 14(a) and 14(c). The slopes in each region seems to be identical, which become steeper by the stepwise transition to the next classification. This suggests that different forms of the graphite reflect different levels of the microstructural deterioration.
Optical micrographs shown in Figs. 2(e)–2(f), 7(c)–7(f), and 10(e)–10(f) indicate that the specimens with spherical graphite exhibit a marked spheroidization/decomposition of cementite, which is considered to be responsible for the reduced creep strength. This indicates that the carbon atoms for spherical graphitization is supplied from the decomposed cementite. In contrast, as shown in Figs. 2(c)–2(d) and 10(c)–10(d), elongated graphite formation does not accompany microstructural deterioration,7) suggesting that the carbon atoms for elongated graphite formation are supplied from not the decomposed cementite. The carbon solubility in ferrite decreases approximately 100 ppm from the heat-treated temperature (e.g., 898 K for the 0.3C steel) to the creep testing temperature.25) It also markedly decreases by the transition from the Fe–Fe3C system to Fe–graphite system.26) These indicate that the carbon for elongated graphitization is originated from the excess carbon atoms in ferrite. In other words, the loss of the strengthening effect due to the excess carbon in ferrite is one of the causes of the reduced slope in the region of elongated graphite.
As indicated by arrows in Figs. 14(a) and 14(c), elongated graphite appears after 4377.8 h under 294 MPa at 723 K in 0.3C steel, whereas it does not appear even after 21763.0 h in 0.5Mo steel under the same creep conditions. This comparison clarifies that 0.3C steel is more susceptible to the formation of elongated graphite under stress, which can be understood by the carbon content (see Table 1). Because 0.2C steel does not form elongated graphite, the elongated graphitization susceptibility is deduced to be in the order of 0.3C, 0.5Mo, and 0.2C steels.
As shown in Table 1, carbon contents of the 0.3C, 0.2C, and 0.5Mo steels are 0.29, 0.20, and 0.20%, respectively. Therefore, inferring the graphitization susceptibilities solely from the carbon content is difficult, although elongated graphite formation could be facilitated by the higher volume fraction of pearlite, which results from the higher carbon content. In contrast, the order of the spherical graphitization susceptibility is inversely correlated with Mo content, suggesting Mo addition reduces the spherical graphitization susceptibility. Another possibility of the chemical composition is the Al content, which is known to promote the graphitization;27) however, no marked effect by Al addition was confirmed in this study, as recent reviews have indicated that Al content affects graphitization behavior little.21)
It is known that Mo reduces the solubility of carbon in ferrite.13) In addition, it dissolves and stabilizes the cementite.3) Therefore, it is considered that Mo addition enhances the supply of the supersaturated carbon atoms from the ferrite, while suppresses cementite decomposition. The ejected carbon atoms from ferrite are considered to diffuse along the stress direction and accumulate on the grain boundary. As a result, the elongated graphite preferentially forms along the grain boundary perpendicular to the stress direction.7) In addition, as discussed above, it was indicated that the free carbon atoms ejected from the ferrite form the elongated graphite, while those from the cementite form the spherical graphite. These explain the experimental results of the preferential formation of elongated graphite in the 0.3C and 0.5Mo steels, which have relatively higher Mo content (see Table 1), and the spherical graphite formation in the 0.2C steel, which contains only 0.010% of Mo. Therefore, it is concluded that Mo addition enhances the formation of elongated graphite, while suppresses the formation of spherical graphite. Besides, higher creep strength due to the strengthening effect by Mo addition promotes the preferential formation of elongated graphite in the creep ruptured 0.3C and 0.5Mo steels. In other words, less Mo content permits earlier decomposition of cementite, results in the earlier spherical graphitization. Similarly, as Mn reduces the solubility of carbon in ferrite13) and stabilizes the cementite,3) the effect of Mn for the earlier elongated graphite formation in 0.3C steel than 0.5Mo steel is implied.
Figure 15 shows the elongation and the reduction of area of the steels.9,10,11) The plots are classified into the formation of no graphite (open symbol), elongated graphite (solid gray symbol), and spherical graphite (solid black symbol). As shown in Figs. 15(a) and 15(d), the elongation and the reduction of area for the 0.3C steel with graphitization are obviously lower than that without graphitization, suggesting that the graphitization is detrimental for the ductility. In contrast, as shown in Figs. 15(b) and 15(e), graphitization has a negligible effect on ductility of 0.2C steel. One of the factors that affects the ductility could be the form of graphite, i.e., elongated graphite is detrimental, while the spherical one affects less. This could cause the difference between the 0.3C steel and 0.2C steel ductility. However, the elongation and the reduction of area of the 0.5Mo steel, which have both elongated and spherical graphite, decreased without graphitization, as shown in Figs. 15(c) and 15(f). Nevertheless, due to the low ductility of un-graphitized samples, discussing the contribution of graphitization is difficult. In other words, it is difficult to conclude that the elongated graphite immediately reduces the ductility from the current results due to several other factors that influence the creep ductility. Further systematic investigations are needed.
The graphitization behaviors of 0.3C, 0.2C, and 0.5Mo steels creep ruptured at 673–823 K were investigated, and TTP diagrams for graphite were established. Graphitization was confirmed at 673 K for the 0.3C and 0.2C steels and at 723 K for the 0.5Mo steel, which are lower temperatures than those described in industrial specifications. Both elongated and spherical graphite were confirmed in the 0.3C and 0.5Mo steels, and elongated graphite was observed in the shorter time (higher stress) side. In contrast, only spherical graphite was observed in the 0.2C steel. It was suggested that carbon atoms for the formation of elongated and spherical graphite result from the supersaturated carbon in ferrite and decomposition of cementite, respectively. It was implied that higher carbon content and higher applied stress promote the formation of elongated graphite at the ferrite-pearlite interface. The addition of Mo suppressed the spherical graphite formation by stabilizing the carbide, while facilitated the elongated graphite formation by reducing the carbon solubility in ferrite. In addition, it was suggested that not only the time and temperature, but also the applied stress and different formation mechanisms of the two types of graphite must be considered to evaluate the risk of graphitization.
The authors thank Ms. Yuka Hara (NIMS) and Ms. Akiko Nakamura (NIMS) for their support for the microstructural observations. A part of this study was financially supported by Japan Boilar Association.
