Carbides in Fe-30.8Ni-26.6Cr alloy

Table 1: Chemical composition of the alloy (by wt.%; analyzed by Emission Spectroscopy).

Figure 1: Microstructure of as-received material. Scale bar: 100 µm.

Figure 2: Microstructure of as-received material. Scale bar: 10 µm.

Table 2: EDS analysis of as-received alloy (by wt.%).

Figure 3: SEM micrograph of as-received material. Scale bar: 10 µm.

Figure 4: Microstructure of specimen after aging at 800C for 1 hour. Scale bar: 10 µm.

Figure 5: Microstructure of specimen after aging at 800C for 24 hours. Scale bar: 10 µm.

Figure 6: Microstructure of specimen after aging at 00C for 24 hours. Scale bar: 10 µm.

Figure 7: SEM micrograph of specimen after aging at 1000C for 10 hours. Scale bar: 10 µm.

Figure 8: SEM micrograph of specimen after aging at 1000C for 24 hours. Scale bar: 10 µm.

Table 3: EDS analysis of individual phases after aging at 1000C for 24 hours (by wt.%).

Table 4: EDS analysis of primary carbides after aging for various aging time (by wt.%).

Carbide name: No data
Record No.: 852
Carbide formula: No data
Carbide type: No data
Carbide composition in weight %: See the tables and text.
Image type: LM, SEM
Steel name: Fe-30.8Ni-26.6Cr alloy
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Ni-Cr alloys
Steel composition in weight %: See the table 1.
Heat treatment/condition: The as-received alloy was produced by the casting process. The initial alloy was still not proper microstructure and does not have good mechanical properties as desired after this manufacturing. Thus, the following heat treatment is necessary to fulfill the material requirements. Therefore, various conditions according to the tested program were carried out to the alloy as following: 1) Aging at 800C for 1, 3, 10 and 24 hours; 2) Aging at 900C for 1, 3, 10 and 24 hours; 3) Aging at 1000C for 1, 3, 10 and 24 hours; 4) Aging at 1100C for 1, 3, 10 and 24 hours.
Note: This work has an aim to study and investigate the relationship between heat treatment conditions on microstructural evolution and mechanical properties in the iron-based alloy, Fe-30.8 Ni -26.6 Cr alloy, strengthened by carbide precipitation. Various aging temperatures (800, 900, 1000 and 1100C) with various aging times are systematically introduced to the as-received alloy. After aging, it was found that the secondary carbides precipitated early near the primary carbides, which are chromium and niobium/titanium carbide networks. The secondary carbide precipitations were also found in the dendrite cores. The amounts of needle-like carbides and secondary carbide films increased with time and temperature of aging. However, by EDS analysis, the composition of secondary carbides was almost the same as that of primary carbides. It can be summarized that the heat treatment conditions have greatly effect on shape, size, dispersion and the location of secondary carbides in microstructure and result in the different mechanical properties such as hardness, yield strength and tensile strength. Aging at 800 and 900C, the very fine precipitates of secondary carbide particles locate and concentrate in the area close to primary carbide. Aging at 1000 and 1100C, the coarser secondary carbides disperse to the cores of dendrites. The needle-like and film carbides were found in heat-treated specimens at 900, 1000 and 1100C. The precipitated secondary carbides precipitated after various heat treatment conditions are chromium carbide, which its chemical composition is similar to primary chromium carbide. It could be concluded that the uniform precipitation and dispersion of fine secondary carbides result in the higher ultimate tensile and yield strengths as well as hardness. However, the obtained result of some mechanical tests did not show any significant effect of aging conditions, especially in ductility and modulus of toughness. The most proper heat treatment condition to maximize tensile strength is aging at 1100C for 10 hours.

Microstructure of as-received alloy: The received microstructure consists of primary carbide networks in austenitic matrix, as shown in Fig. 1. The dendrite structure indicates the characteristic of casting microstructure. However, no secondary carbide was detected in the microstructure, see Fig. 2. From SEM analysis, it was found that the primary carbide networks could be classified in two types as black and white phases, Fig. 3. Using EDS to analyze the chemical composition of each phase is concluded that the black phase consists of 70.59 % of chromium and white phase consists of 19.93 % titanium, 32.60 % niobium and 0.61 % chromium.

The matrix consists of 35.74 % iron, 31.59 % nickel, and 24.54 % chromium, see Table 2. The alloy consists of 30.8 % nickel, which is high enough to stabilize the austenitic matrix microstructure. The primary carbide networks could form during slow cooling of solidified alloy by the combination of carbon and chromium, niobium and titanium. Titanium and niobium would form as niobium-titanium carbide, which precipitated at higher temperature comparing to chromium carbide resulting in high ratio between Cr and C. Therefore, the presence of primary carbide precipitation type is M23C6. They locate near austenitic grain boundary networks.

Microstructure of the alloy after heat treatment: From SEM micrographs, generally, all microstructures after various heat treatment conditions were found in similar manner. Most of microstructures consist of primary carbides as the as-received microstructure. However, very fine precipitations of secondary carbides were found locating in the matrix, usually, in areas close to primary carbides, Fig. 4. After aging at 800C, the secondary carbide particles concentrate in the zones adjacent to the primary carbides. The amount of secondary carbide particles increase with time of heat treatment (10 and 24 hours), as shown in Fig. 5. Furthermore, the film and needle-like carbides were also observed. It should be noted that these secondary carbide particles precipitate in higher concentration near the primary carbide particles and more precipitation disperse toward the dendrite core when aging time increase.

After aging at 900C for various aging time, the heat-treated microstructures are similar to those aged at 800C but are different in amounts of secondary carbide precipitation. After short-term heat treatment (1 and 3 hours), secondary carbide are in high concentration near primary carbides. However, when aging time was increased up to 10 and 24 hours, the previous precipitation of secondary carbide particle would agglomerate to become in coarser sizes and there are more very fine secondary carbides in the center of dendrite core, Fig. 6. Coarsen needle-like and film carbides are found as well.

The microstructures, after aging at 1000 and 1100C for various aging time, are quite similar to those aged at 800 and 900C, Figs. 7 and 8. The secondary carbides are in round shape and precipitate toward the dendrite core. The secondary carbides are in high concentration to the primary carbides in case of exposed time of 1, 3 and 10 hours. For aged microstructure for 24 hours, very fine particles of secondary carbide would agglomerate as coarsening size. However, using SEM investigation in all cases, precipitates free zones (PFZ) were found close to primary carbides because of low chromium content in these areas, where chromium precipitated during previous secondary carbide precipitation.

From EDS analysis of primary and secondary carbides as well as in matrix, it is summarized that the amounts of chromium in the matrix decreases after all heat treatments as chromium forms the secondary carbides. However, amounts of other elements in matrix are quite constant, see Table 3. Furthermore, it was also found that the amount of chromium (70.72 %) in secondary carbides is very close to that of primary carbide (the black phase). This could imply that no significant phase change in primary carbide after all heat treatment but only loses some amount by the decomposition of partial primary carbides to secondary carbides. For the needlelike secondary carbide, its chemical composition is as following: 37.86 % Fe, 32.33 % Ni, and 27.92 % Cr, as can be seen in Table 3.

After all heat treatments, the chemical composition of primary carbide is almost constant due to no phase transformation, see Tables 2 and 3. However, the another type of primary carbide (the white phase), which consists of 32.6 % Nb, 0.61 % Ti, 1.7 % Si, and 21.88 % Ni, the amount of niobium has a trend to decrease while the amounts of nickel and silicon increase after aging at 800, 900, 1000, and 1100C for 24 hours. This might be due to the occurrence of phase instability during long-term aging at high temperatures. Finally, it is summarized that temperature and time of aging have significant effect on size, shape, and dispersion of secondary carbides. The most precipitated secondary carbides are the same type as primary carbide, M23C6. Furthermore, niobium and chromium also influence the shape of secondary carbides as well. High amount of niobium and chromium in iron base alloy induces to needle-like secondary carbide formation at long-term exposed conditions. Therefore, the control of niobium and chromium is very important. The proper and careful control of amount of both elements reduces the tendency of needle-like carbide formation, which causes the brittle fracture later.

Aging time and aging temperature, especially in the range of 800 - 1000C, can decrease amount of niobium in niobium/titanium carbide while increase amount of nickel and silicon, Table 4. In this temperature range of 800 - 1000C, niobium/titanium carbide is not stable and susceptible to the transformation of G-phase or nickel-niobium-silicide (Ni16Nb7Si16) according to previous study. However, the G-phase transformation would rarely occur partially because titanium could inhibit the phase transformation. The G-phase probably is considered as the weak point for creep-rupture strength.
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