MC, M2C, M6C and M7C3 carbides in 2%C5%Cr5%V5%Mo5%W5%Co white cast iron

Table 1: Chemical composition of test specimen.

Figure 1: SEM microphotographs of basic multi-component white cast iron with Fe2%C5%Cr5%V5%Mo5%W 5%Co (mass%) in different states. Scale bars: 10, 1 µm.

Figure 2: X-ray diffraction patterns of basic multi-component white cast iron. (a) As-cast, (b) Annealed, (c) Hardened-Tempered.

Table 2: Types of carbide identified by X-ray diffraction.

Figure 3: TEM image, diffraction pattern and EDS profile of crystallized carbides in basic multi-component white cast iron. As: As-cast, HT: Hardened-Tempered. Scale bars: 0.5, 1 µm.

Figure 4: TEM image of transformation from M2C carbide in Fe2%C5%Cr5%VMoW5%Co (mass%) multi-component white cast irons. Scale bars: 1, 0.5 µm.

Table 3: Alloy concentration of carbides with different morphology, shapes and crystal lattices in as-cast, annealed and hardened-tempered states of basic multi-component white cast iron.

Figure 4: SEM and EPMA microphotographs of eutectic carbides before and after heat treatment in basic multi-component white cast iron. 1273 K x 7.2 ks, 1.3 x exp10(-3) Pa heat and quenching. Scale bars: 5 µm.

Figure 5: SEM microphotographs of transforming eutectic carbides after heat treatment at 1273 K in basic multi-component white cast iron. Scale bars: 2 µm.

Carbide name: MC, M2C, M6C, M7C3
Record No.: 702
Carbide formula: MC, M2C, M6C, M7C3
Carbide type: MC, M2C, M6C, M7C3
Carbide composition in weight %: See the table 3.
Image type: SEM, TEM
Steel name: 2%C5%Cr5%V5%Mo5%W5%Co
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: White cast irons
Steel composition in weight %: See the table 1.
Heat treatment/condition: The test pieces were made of multi-component white cast iron containing the target contents of 2% carbon and 5% each chromium (Cr), vanadium (V), molybdenum (Mo), tungsten (W) and cobalt (Co). The specimen was prepared by subjecting the raw materials such as the mother alloy, electrolytic iron and ferroalloys to atmospheric melting in a high-frequency induction furnace at 1873 K and pouring the melt into a preheated CO2 mold at 1843 K to make specimens 100 mm in diameter and 300 mm in length.
The columnar specimen was bisected and one of the bisected pieces was annealed by holding at 1153 K for 54 ks and by cooling slowly to room temperature in the furnace, which is the same annealing cycle as that of the actual roll manufacturing process. This annealed specimen was bisected in turn, and one section was austenitized at 1273 K for 54 ks, hardened by fan air cooling to room temperature, and subjected to double tempering at a repeated 798 K for 54 ks (herein after referred to as hardened-tempered treatment).
At the positions 50 mm away from the bottom end and 10 mm from the periphery of each specimen, rectangular specimens 10 mm square and 20 mm long were cut out in radial direction.
Note: A multi-component white cast iron was developed for rolling mill rolls. The morphology and alloy concentration of carbides precipitated during solidification were investigated using X-ray diffraction, SEM, TEM and EDS analysis in the cast iron with typical chemical composition of Fe2%C5%Cr5%V5%Mo5%W 5%Co (mass%). When the iron solidifies, petal-like MC carbides with face-centered cubic lattices and platelike M2C carbides with hexagonal lattices crystallize. During heat treatment, M2C carbide (hexagonal) reacts to gamma -Fe and transforms to M6C (fcc), M7C3 (orthorhombic) and MC (fcc) carbides, but the reaction is not followed by a change of carbide morphology. MC carbides mainly consist of V, and M7C3 carbides are mainly formed by Cr and Fe. M2C carbides contain 2025 atomic% each of Cr, V, Mo, and Fe, and 12 atomic% of W. M6C carbides are composed of approximately 33 atomic% of (Mo+W), and 50 atomic% of Fe and (Cr+V) in the balance.

Figure 1 shows SEM microphotographs of the specimens in as-cast, annealed and hardened-tempered states. In all specimens, nodular or petal-like (hereafter generically described as nodular) MC carbides 5 to 20 µm in diameter crystallized in and at the grain boundaries of pro-eutectic austenite; and rod-like or plate-like (hereafter generically called plate-like) M2C carbides 10 to 50 µm in length crystallized at grain boundaries. The outward appearance of the plate-like carbides is analogous in all the specimens. While the plate-like carbides are white and single phase in the as cast state, those in the annealed specimen are rugged at the periphery and they show a mixture of white-and-gray phases under a high magnification, though they seem to be an aggregate of small particles under a low magnification.

Identification of carbides by X-ray diffraction: Figure 2 and Table 2 show the results of carbide identification by X-ray diffraction. The as-cast specimen shows strong peaks of M2C and MC and weak peaks of M7C3. In the annealed specimen, very strong peaks of M6C appear, and the M2C peaks are also strong but weaker than those in the as-cast specimen. The hardened-tempered specimen shows strong peaks of M6C and MC with some weak peaks of M7C3, and M2C peaks disappear. The peak positions of the MC carbide agreed with the JCPDS Card for V8C7 (No. 35-0786), and the other carbides were identified by using JCPDS Cards for W2C (No. 35-0076), Cr7C3 (No. 26-1482) and Co3W3C (No. 27-1125). It is clear that MC carbide is present as the main carbide in all specimens. Annealing decreases M2C, which is also one of the main carbides in the as-cast specimenindeed, it almost eliminates M2C and, instead, forms M6C. Annealing increases the quantity of M7C3 in addition to that existing in a very small amount in the as-cast specimen. Hardened-tempered treatment decreases the amount of M7C3 but M7C3 is still present in small quantities.

Morphology and crystal structure of carbides by TEM observation: The morphology and crystal structure of individual carbides were identified by TEM. As crystallized and precipitated carbides differ greatly in size, relatively larger carbides that are 2 µm or more in length or diameter are regarded as carbides crystallized from the melt as primary and eutectic. Though the carbides in the hardened-tempered specimen are approximately 1m m in size, carbide aggregates observed as a part of the plate-like eutectic carbide (hereafter referred to as string-like carbides) were regarded as carbides transformed from eutectic carbide. Figure 3 shows typical examples of TEM bright-field images, diffraction patterns and EDS profiles of crystallized carbides in the as-cast and hardened-tempered specimens. Figure 4(a) shows a TEM microphotograph of the Figure 3(a) shows the results of investigations on nodular carbide in the as-cast specimen shown in Fig. 1(a). As can be identified from the morphology of the carbide in Fig. 1(a), the MC carbide and its crystal structure was a facecentered cubic lattice (fcc). As this type structure of carbide continues to exist even after heat treatment involving hardening and tempering (Fig. 3(b)), all MC carbides were found to have absolutely the same morphology and crystal structure.
Next, the white and plate-like carbide in the as-cast specimen (Fig. 1(a)) was investigated. Based on the morphology and alloy concentration, carbide with this type of microstructure could be the M2C type. The results of TEM analysis also indicate it has M2C carbide with a hexagonal lattice as shown in Fig. 3(c). The same type of carbide was observed in the annealed specimen as shown in Fig. 4(a), but not in the hardened-tempered specimen.
Figures 3(d), 3(e) and 3(f ) show the results of analyzing the string-like structure in the hardened-tempered specimen that is shown in Fig. 1(c). The string-like structure has a profile similar to that of the plate-like carbide observed in the as-cast specimen, and it can be seen as an aggregate of small carbides under high magnification. The TEM analysis revealed that it consisted of three types of carbides, MC with a face-centered cubic lattice (Fig. 3(d)), M6C with a face-centered cubic lattice (Fig. 3(e)), and M7C3 with an orthorhombic lattice (Fig. 3(f )).
According to SEM observation of the annealed specimen (Fig. 1(b)), bright and dark phases around the plate-like M2C carbide similar to that observed in the as-cast specimen (Fig. 1(a)) are revealed. They are considered to have formed due to the transformation. The results of investigation for the bright and dark phases are shown in Fig. 4(b). It is clear that the transformed phases involved mixes of M6C carbide with a face-centered cubic lattice, MC carbide with a face-centered cubic lattice and M7C3 carbide with an orthorhombic lattice.

Alloy concentration in carbides: Table 3 shows the alloy concentration in the carbides shown in Fig. 3, as obtained by EDS qualitative analysis, together with the type, morphology and crystal lattice. Figure 5 compares the alloy concentration of carbides in the as-cast, annealed and hardened-tempered specimens. For the sake of simplicity, alloy concentration is expressed in atomic percent (at%).
M2C and M6C carbides: In coarse and plate-like M2C carbides in the as-cast specimen, both the vanadium and molybdenum are approximately 25 at%, chromium and iron are around 20 at% and tungsten is about 12 at%. They remain substantially the same after annealing. M6C carbides around the M2C carbide in the annealed specimen and in the string-like aggregate in the hardened-tempered specimen contain a markedly large amount of iron, approximately 54 at%, with vanadium and chromium, which are 4 and 8 at% respectively (significantly lower than their respective concentrations in M2C carbide in the as-cast specimens). While the concentration of molybdenum is slightly low at 23 at%, that of tungsten remains unchanged at 12 at%.
MC carbide: In the crystallized MC carbide of all the specimens, vanadium constitutes a major portion at 61 to 68 at%, coexisting with iron, molybdenum, chromium and tungsten at about 13, 10, 7 and 5 at% respectively. Alloy concentrations of MC carbide in the as-cast and annealed specimens basically differ little, except for some differences in vanadium and iron contents. Though the vanadium and chromium contents are slightly low and tungsten and molybdenum contents are slightly high, the alloy concentration of the MC carbide in the string-like carbide in the hardened-tempered specimens can be regarded as being basically the same as that of the crystallized MC carbide that existed from the beginning.
M7C3 carbide: In M7C3 carbide that is formed only by application of heat treatment, chromium and iron respectively account for 41 to 47 at%, vanadium 8 at%, molybdenum 3 at% and tungsten as little as 1 at% or below.

Figure 8 shows SEM microphotographs of the typical specimen heat-treated by 1273 K for 7.2 ks and the X-ray images of alloying elements in the heat-treated and polished specimens measured by EPMA. Comparing the SEM microphotographs in Figs. 4(a) and 4(b), the crystallized carbide showed substantially the same shape before and after heat treatment. As can be seen from the X-ray images shown in Fig. 4(c), heat treatment gives all or part of the periphery of plate-like M2C carbide a mottled structure, and it reveals a string-like shape. There are some M2C carbides that transform completely to a string-like composition and also some carbides where the central region remains unchanged. It is presumable from the remarkable distribution of chromium and vanadium shown by mottled patterns that the string-like carbides were aggregates of M6C, M7C3 and MC carbides with significant quantities of alloying elements. The M2C carbide left in the central region of the plate-like carbide shows uniform distribution of elements, with, qualitatively, very low iron. The carbide in the upper left seems to be MC carbide due to its extremely high vanadium content. This proves that the M2C carbide, on being heated to a high temperature, reacts with surrounding g iron and transforms to an aggregate of each carbide in a string-like morphology As this observation was made with regard to the surface of the specimens, it was apparent that oxidation and/or some other reactions had occurred, and that they may have limited adequate atom diffusion. Since the obtained results did not allow quantitative discussions on the degree of carbide transformation, the inside of the specimen was investigated.
Figure 5 shows the process by which the eutectic M2C carbides transform as a function of holding time at 1273 K. As the holding time increases, the portion of white, single phase M2C carbide in the plate-like carbide decreases and instead the string-like aggregate of carbides increases its area from the periphery and occupies the whole area in the end. Firstly the ratio of transformation area (Xarea) is calculated from the area of the string-like aggregate structure in the plate-like carbide revealed by the SEM microphotograph. Then, the ratio of transformation (X) is obtained by correcting it with an increasing coefficient for the carbide reaction.
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