Carbides in 2.25Cr-1Mo steel

Figure 2: Microstructure of ferritic-pearlitic sample: (a) as received, (b) after 2000 h at 600°C. Scale bars: 50 µm.

Figure 3: Microstructure of ferritic-bainitic sample: (a) as received, (b) after 2000 h at 600°C. Scale bars: 50 µm.

Table 1: Chemical composition (%wt) of the studied steel and the ASTM A 335 T22 specification.

Figure 4: Ferritic-pearlitic steel scanning electron micrographs at 600°C and 100, 500, 1000 and 2000 h of aging. The arrows indicate the carbides coarsening effect. Scale bars: 2 µm.

Figure 5: Ferritic-bainitic steel scanning electron micrographs at 600°C and 100, 500, 1000 and 2000 h of aging. The arrows indicate the carbides coarsening effect. Scale bars: 2 µm.

Figure 6: Ferritic-pearlitic steel transmission electron micrographs at 600°C and 100, 500, 1000 and 2000 h of aging. Scale bars: 2 µm.

Figure 7: Ferritic-bainitic steel transmission electron micrographs at 600°C and 100, 500, 1000 and 2000 h of aging. Scale bars: 2 µm.

Figure 8: EDS spectra of the carbides identified in the ferrite-pearlite aged at 600°C for 1000 h sample.

Figure 9: TEM carbon replicas images of phases identified in the samples aged at 600°C and 1000 h: (a) and (b) ferritic-pearlitic, (c) and (d) ferritic-bainitic. Carbides are identified by colors: M2C (green), M23C6 (black), M7C3 (yellow), M3C (blue), M6C (red). Scale bars: 500 nm.

Carbide name: M2C, M23C6, M7C3, M3C, M6C
Record No.: 1535
Carbide formula: M2C, M23C6, M7C3, M3C, M6C
Carbide type: M2C, M23C6, M7C3, M3C, M6C
Carbide composition in weight %: No data
Image type: LM, SEM, TEM
Steel name: 2.25Cr-1Mo
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Creep resistant steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: Table 1 shows the chemical composition of the steels according to the ASTM A355 standard. Both steels were supplied as tubes (324 mm diameter and 25 mm wall thickness), and samples were cut in the longitudinal sections. The aging was conducted at 500, 575, and 600ēC for 100, 500, 1000 and 2000 h. After each aging test samples were polished and etched with Nital 2% followed by observation with optical microscopy and SEM. Precipitates were characterized by TEM using extraction carbon replicas and energy dispersive spectroscopy (EDS). To obtain the extraction carbon replicas, polished samples were etched with Nital 20% and then carbon coated; after they were crossed out and submitted to Vilella’s etchant (5 ml of clorhidric acid, 1 g of picric acid and 100 ml ethanol 95%) and then immersed in ethanol. The precipitates were identified by EDS analysis by comparing the results with the characteristic spectra of each type of precipitate presented elsewhere6-9. The identification was done with the ferrite, pearlite and bainite grains, as well as in the grain boundaries. To compare the mechanical properties between each microstructure, creep tests were conducted on the samples as received. The tests, in accordance to ASTM E-139, were performed at 575ēC under 100 MPa.
Note: 2.25Cr-1Mo steels are widely used in thermoelectric power plants. Under operational temperatures, their properties degrade due to microstructural changes related to carbide coalescence and stoichiometric transformations. The extent of such microstructural changes is controlled by stress, temperature and time. Therefore, these factors can be used to evaluate damage and as life assessment tools for the individual component. In the past, ferrite-pearlite was the predominate microstructure in commercial Cr-Mo steel products, owing to the well-known methodologies for remaining life assessment based degradation. Currently, the ferrite-bainite microstructure obtained through a more economical route is most commonly used for this steel grade. However, there is no consensus in the literature about microstructural changes that can be used as a degradation pattern for ferrite-bainite steels. This paper compares the aged microstructures and creep properties of ferrite-pearlite and ferrite-bainite 2.25Cr- 1Mo steels. Aging was conducted at 500, 575 and 600ēC until 2,000 h, and creep tests were performed at 575ēC under a stress of 100 MPa. Microstructural changes were characterized by optical microscopy scanning electron microscopy. Metallographic observations of the ferrite-bainite steel show a more stable behavior at the ageing temperatures and time considered. However, creep tests revealed that the ferrite-pearlite microstructure possesses a better rupture time performance. Carbide size distribution and stoichiometric evolution of the carbides provided by transmission electron microscopy support the creep behavior. These results show that the current techniques for evaluating microstructural degradation of 2.25Cr-1Mo steels must be reconsidered.

The microstructure evolution during aging was characterized by optical microscopy and Figure 2 shows them for ferrite-pearlite steel in the as received condition and in the more severe 600ēC for 2000 h, aged steel. Figure 3 shows the microstructure evolution for ferrite-bainite steel in the same conditions as above. The microstructural evolution of 2.25Cr-1Mo steel reveals expected ferrite-pearlite progressive cementite spheroidization until complete pearlite dissolution and, also, precipitation coarsening in the grain boundaries, in agreement with the literature4. The ferrite-bainite microstructure, in turn, does not show a remarkable difference between the as received and aged conditions.
Figure 4 shows the microstructural evolution at 600ēC as a function of aging time, observed by SEM for the ferritepearlite microstructure. The white arrows indicate the carbides along the grain boundaries which coalesce along the time. In the same way, Figure 5 shows the microstructural evolution under the same conditions for the ferrite-bainite microstructure, precipitation coarsening is also signaled by the white arrows. Comparing the images of the 100h aging of both microstructures, the coarsening effect in ferrite-bainite is more pronounced, and it is clear that the precipitate distribution in ferrite-pearlite was more effective. That behavior was observed for all ageing samples and it explains the better performance of ferrite-pearlite steels in the creep tests presented at Figure 1.

Figures 6 and 7 show TEM carbon replicas images of the precipitation present in, respectively, ferrite-pearlite and ferrite-bainite steels aged at 600ēC for 100, 500, 1000 and 2000 hours. Those images were used to select specific areas for the phases characterization. According to the literature8-11, carbides present in 2.25Cr-1Mo steels have characteristic EDS spectra, which allow the correct identification of each type. Figure 8 presents the EDS spectra, obtained from the ferrite-pearlite microstructure aged at 600ēC for 1000 hours, which correspond to the five types of carbides identified in this steel: M2C, M3C, M7C3, M23C6 and M6C. Figure 9 shows, for both microstructures aged at 600ēC for 1000 h, the carbides’ identification by colors which enables the observation of their distribution and morphology in the selected regions. The same procedure was adopted for the all aging conditions and for the as received samples and, as result, the precipitation evolution could be studied.

As observed, the ferrite-bainite microstructure does not present M3C phase in any moment of its aging, but it initially presents M2C, M7C3 and M23C6 phases. The M23C6 was identified after 1000h at 550 and 575ēC and after 500h at 600ēC for ferritepearlite microstructure. The M6C phase was the last identified in both microstructures, but the ferrite-bainite presented it before than the ferrite-pearlite, for all conditions of aging. It is important to say that as deleterious phases, like M23C6 and M6C, coalesce, the others decrease their contents, but they are still present even after the most severe aging conditions. That behavior has already been registered in the literature12: the authors observed that as the M6C content increased, the M7C3 content decreased, but, in spite of this correlation, the M7C3 carbide remained present in all aged samples. The presence of large M23C6 carbides for the ferrite-bainite steel in the as cast condition should be noted, especially at the grain boundaries. Although the M23C6 carbide is a common phase in this class of steel and can be present in a relatively large volume fraction, its presence has generally been associated with a loss of creep resistance. The M23C6 precipitation removes alloying elements from the matrix, decreasing the solid solution hardening contribution to the creep strength, which corroborates the observed behavior in the creep curves. Moreover, the difference in size and spacing of precipitates present on each microstructure also confirms the behavior of the creep curves. In the ferrite-bainite microstructure, the carbides are larger and more spaced apart, especially in the grain Creep tests indicate that ferrite-bainite steel is less resistant than ferrite-pearlite steel. The reasons for this difference are because a ferrite-bainite microstructure presents deleterious phases, M23C6 and M6C, earlier and the difference in size and spacing of precipitates.
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