Fe3C, M23C6 and VC carbides in X20CrMoV121 and P91 steels


Figure 1: Microstructure of the X20CrMoV121 steel at the initial (as-received) state. Scale bar: 1 µm.


Figure 2: Microstructure of the X20CrMoV121 steel after 8760 h of tempering at 650 C. Scale bar: 1 µm.


Figure 3: Microstructure of the X20CrMoV121 steel after 8760 h of tempering at 750 C. Scale bar: 1 µm.


Figure 4: Microstructure of the P91 steel at the initial (as-received) state. Scale bar: 1 µm.


Figure 5: Microstructure of the P91 steel after 8760 h of tempering at 650 C. Scale bar: 1 µm.


Figure 6: Microstructure of the P91 steel after 8760 h of tempering at 750 C. Scale bar: 1 µm.


Table 1: Chemical compositions of the X20CrMoV121 and P91 steels in mass %.

Carbide name: Fe3C, M23C6, VC
Record No.: 700
Carbide formula: Fe3C, M23C6, VC
Carbide type: M3C, M23C6, MC
Carbide composition in weight %: No data
Image type: SEM
Steel name: X20CrMoV121, P91
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Creep resisting steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: The samples were cut from the pipelines with diameter = 38 mm 8 mm and diameter = 82 mm 14.5 mm. Before extracting the specimens for the room-temperature tests and examinations, the samples of both steels were tempered for discrete times up to 17520 h at 650 C and for a shorter time up to 8760 h at 750 C to simulate the changes in the microstructure that take place under real operating conditions in the power plants, and their effect on the room-temperature tensile properties and hardness.
Static-tensile tests at room (ambient) temperature were performed on the specimens extracted from the previously tempered samples. The tests were initially performed on the specimens prepared from the as-delivered tubes and then on the specimens tempered for 2 h, 4320 h and 8760 h at 650 C and 750 C, and up to 17520 h at 650 C.
Note: The effect of tempering time and temperature on the room-temperature tensile properties and hardness of two martensitic creep-resistant steels, X20CrMoV121 and P91, was investigated. Samples cut from industrial tubes were tempered for 17520 h at 650 C and for 8760 h at 750 C. On the tempered samples the yield stress, tensile strength, and hardness at room temperature were determined and an SEM examination was carried out.
It was found that the effect of tempering at 750 C on the microstructural changes, room-temperature tensile properties and hardness was greater for both steels than the effect of tempering at 650 C. The changes in the yield stress, tensile strength and hardness of both steels at a given tempering temperature were found to be very similar. Therefore, a general mathematical expression with specific coefficients for each property was deduced. These results are part of a larger investigation aimed at establishing a correlation between the particle spacing, yield stress, creep rate and hardness, which could be useful in an evaluation of the lifetime issues relating to the thermal-power-plant components.

The influence of tempering on the microstructures of both steels is greater after tempering at 750 C than at 650 C (Figures 1 to 6), because the diffusivity of the carbide-forming elements (Cr, Mo, Fe, V, and Nb), found in the solid solution in the ferrite matrix is temperature dependent, and is higher at higher temperatures.
Due to tempering, both the size and the inter-particle spacing of carbide particles increase. In addition, the carbide-particles distribution changes from stringers along the grain and sub-grain boundaries to a uniform structure. Images in Figures 1 and 4 show the as-delivered- state microstructures of the X20CrMoV121 and P91 steels, respectively. In both cases, the majority of particles are cementite Fe3C, containing also chromium, or they are chromium carbides Cr23C6, containing also iron and molybdenum. The carbide particles are found in the stringers distributed along the grain and sub-grain boundaries of martensite, and there is no difference between the size of the Fe3C, M23C6 and VC particles. After 8760 h of tempering at 650 C the precipitates are almost evenly distributed and there is a difference between the size of VC (small white particles) and M23C6, which grow larger in both steels (Figures 2 and 5). In addition, the grain and subgrain boundaries of martensite are much less pronounced and some of them have already disappeared.
From Figures 3 and 6 it is obvious that the tempering at 750 C for 8760 h causes much greater changes in the microstructures of both steels, where the size of the M23C6 particles and the spacing between them is drastically increased. On the other hand, the number density of these particles has clearly dropped, so the Ostwald ripening effect, where larger particles coarsen at the expense of smaller ones is quite obvious in this case.
Links: No data
Reference: Not shown in this demo version.

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