MC, M2C, M7C3 carbides in Fe-5%Cr-5%Mo-5%W-5%V-5%Co-2.0%C cast steel

Figure 3: Petal-like MC carbide. Scale bar: 20 µm.

Figure 4: Nodular MC carbide. Scale bar: 20 µm.

Figure 5: Coral-like MC carbide. Scale bar: 20 µm.

Figure 6: Fine lamellar M2C carbide. Scale bar: 20 µm.

Figure 7: Coarse plate-like M2C carbide. Scale bar: 20 µm.

Figure 8: M7C3 and M2C carbides. Scale bar: 20 µm.

Figure 9: Solidification sequence of multi-component white cast iron with basic chemical composition (Fe-5%Cr-5%Mo-5%W-5%V-5%Co-2%C). a: Thermal analysis curve, b: quenched microstructures. Scale bars: 50 µm.

Table 1: Solidification sequences of multi-component white cast iron varying chemical composition from the basic alloy.

Carbide name: MC, M2C, M7C3
Record No.: 1539
Carbide formula: MC, M2C, M7C3
Carbide type: MC, M2C, M7C3
Carbide composition in weight %: No data
Image type: SEM, LM
Steel name: Fe-5%Cr-5%Mo-5%W-5%V-5%Co-2.0%C cast steel
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Cast steels
Steel composition in weight %: Fe-5%Cr-5%Mo-5%W-5%V-5%Co-2.0%C
Heat treatment/condition: As cast
Note: For last half a century past, materials of hot work rolls in the steel industry, which are shown as an example in Fig.1, changed from low-alloyed white iron through adamite or hyper-eutectoid Ni-Cr steel and Ni-hard cast iron to high chromium cast iron.[1] On the other side, the abrasion wear resistant cast alloys for the components of cement and mineral pulverizing mills have been changed by following the transition of the roll materials. The high chromium cast iron, which was developed in the nineteen-seventies, is even now used not only for hot working mill rolls but also for pulverizing mill rolls because of its high wear resistance. A typical example of the pulverizing mill roll is shown in Fig.2. According to the demands in upgrading the productivity and the quality of products, the roll materials are being compelled to shift to a new type of material with much higher performance. The multi-alloyed white cast iron or multi-component white cast iron is a new type of alloy which contains plural kinds of strong carbide forming elements such as Cr, V, Mo, W, and resultantly, plural kind of their special carbides with high hardness and strong matrix with secondarily precipitated carbides. This iron was developed around 15 years ago in Japan and has been tried to apply to the work roll of hot finishing stands for the past 10 years.[2] Nowadays, a large portion of work rolls in hot rolling mills is replaced from high chromium and Ni-hard white cast iron rolls, and such types of rolls are now pleading over the world. It is worthy of note that this kind of trial roll was served recently to the pulverizing mills.

Several types of eutectic carbides precipitate in the solidification structure of the cast iron and they show diversities of morphology according to the chemical composition. MC carbide: The MC carbide contains more than 50% V and it precipitates as a eutectic during solidification. This carbide can be classified into petal-like, nodular and coral-like morphologies as shown in Fig.3 to 5, respectively. Petal-like MC in Fig.3 precipitates in a low carbon cast iron where the crystallization of austenite dendrite proceeds before eutectic of (gamma + MC) forms. Nodular MC carbide shown in Fig.4 precipitates in the cast iron with high carbon content. Coral-like MC carbide in Fig.5 consists of primary MC crystals in the center and eutectic MC carbides growing in radial directions from the primary MC. The coral-like MC carbides precipitate in the iron containing high vanadium content, and can co-exist with M7C3 carbide or small amount of M2C carbide depending on the carbon content of the iron. Hardness of MC carbide is about 2800HV and MC carbide may be very effective to the wear resistance of the cast iron, and the nodular morphology of MC carbide may also improve the toughness of the cast iron because of less notch effect. When chemical composition of the iron changes, however, nodularity of MC carbide may deteriorate to chunky or flaky morphology.
M2C carbide: Molybdenum- and tungsten-rich carbide precipitates as M2C type in as cast state and takes fine lamellar and/or coarse plate-like morphology as shown in Fig.6 and 7, depending on the chemical composition of the iron. However, it is reported that the M2C carbide transforms into M6C carbide by heating. [7] Lamellar eutectic M2C carbide exists in isolation and this indicates that the M2C carbide precipitates in last stage of solidification. The plate-like M2C carbide consists of large and thick plates. This type of carbide can be usually seen in the cast iron with high Weq value. In the iron with higher carbon content, the plate-like M2C carbide exists together with M7C3 carbide as shown in Fig.8.
M7C3 carbide: Chromium carbide M7C3 similar to that in high chromium white cast iron shown rod-like or ledeburitic morphology in the iron with higher Weq value and higher carbon content, and it sometimes co-exists with MC and M2C carbides (Fig.4 and 8).
Matrix: Matrix in as-cast state consists of bainite, martinsite and some retained austenite . The volume fraction of retained austenite ranges widely from 5 to 50 % depending on the carbon content of the iron. However, the retained austenite is supposed to transform into the useful phases after heat treatment.

Solidification Sequence: The content of carbon and alloying elements were varied so that several combinations of eutectic carbides could be obtained in multi-component white cast iron and the solidification sequences were investigated by means of quenching test during thermal analysis.[8] As an example, the solidification sequence of the cast iron with basic chemical composition (Fe-5%Cr-5%Mo-5%W-5%V-5%Co-2.0%C alloy), in which the petal-like MC and plate-like M2C carbides co-exist, is shown in Fig.9a, b: a thermal analysis curve in a and the quenched microstructures in b. First of all, the primary austenite dendrite forms at 1590K, and follows by precipitation of (gamma+MC) eutectic at 1558K, and finally (+M2C) eutectic solidifies at 1422K.
The solidification sequences of the other kinds of multi-component white cast irons with more variation of chemical composition than the basic alloy composition are summarized in Table 1.
Links: No data
Reference: Not shown in this demo version.

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