M7C3 and M6C5 carbides in white cast Fe-Cr-C-V alloy


Figure 1: Optical micrographs of Fe-Cr-C-V alloy containing: (a) 1.19% V (b), 3.28% V. Scale bars: 20 µm.


Figure 2: SEM micrographs of deep etched sample showing morphology of M7C3 carbides in Fe-Cr-C-V type alloy containing 3.28% V: (a) eutectic colonies consisting of a larger number of long M7C3 carbide rods which grow along their longitudinal axes; (b) eutectic colonies when viewed perpendicular to their fastest growth direction (mainly composed of very fine rode-like carbides in the center, becoming coarser rod-like or bladelike with increased distance from the centre). Scale bars: 20 µm.


Figure 3: Optical micrographs of Fe-Cr-C-V alloy containing 1.19% V (etched with Murakami). Scale bar: 5 µm.


Figure 4: SEM micrograph of as-cast microstructure in Fe-Cr-C-V alloy containing 1.19% V (a) and corresponding vanadium distribution map (b). Scale bars: 10 µm.


Figure 5: SEM micrographs of deep etched sample showing morphology of vanadium carbide (a) and EDS spectrum of this carbide (b) in Fe-Cr-C-V alloy containing 1.19% V. Scale bar: 2 µm.


Figure 6: Transmission electron micrograph of the Fe-Cr-C-V alloy containing 1.19% V showing vanadium carbide and selected-area diffraction pattern (in the corner) from the region in this micrograph, (markings on the micrographs: sf-stacking fault, M-martensite). Scale bar: 200 nm.


Table 1: Chemical composition of tested Fe-C-Cr-V alloys.


Table 2: The volume fraction and size of phases in the microstructure of the examined Fe-Cr-C-V alloys.

Carbide name: M7C3, M6C5
Record No.: 829
Carbide formula: M7C3, M6C5
Carbide type: M7C3, M6C5
Carbide composition in weight %: No data
Image type: No data
Steel name: White cast Fe-Cr-C-V alloy
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 chemical composition of tested alloys is listed in Table 1. The melting of various alloys has been conducted in induction furnace. Test samples for structural analysis have been cut from the bars (200 mm long and 30 mm in diameter) cast in the sand molds.
Note: Experimental results indicate that vanadium affects the solidification process in high chromium iron. Vanadium is distributed between eutectic M7C3 carbide and the matrix, but its content in carbide is considerably higher. Also, this element forms vanadium carbide. TEM observation reveals that vanadium carbide present in examined Fe-Cr-C-V alloys is being of M6C5 type. DTA analysis found that with increasing vanadium content in tested alloys, liquidus temperature is decreasing, while eutectic temperature is increasing, i.e. the solidification temperature interval reduces. The narrowing of the solidification temperature interval and the formation of larger amount of vanadium carbides, as a result of the increase in the vanadium content of the alloy, will favour the appearance of a finer structure. In addition, the phases volume fraction will change, i.e. the primary gamma-phase fraction will decrease and the amount of M7C3 carbide will increase.

The as-cast microstructure of examined alloys contain primary austenite dendrites and eutectic colonies composed of M7C3 carbides and austenite (Fig. 1).
The SEM micrographs of deep etched sample revealed that single M7C3 carbides in all tested Fe-Cr-C-V alloys were rod or blade shaped (Fig. 2), where the blades are basically consist of multiple rods (Fig. 2(b)). A larger number of long carbide rods within the eutectic colonies usually grow along their longitudinal axes (Fig. 2(a)). When viewed perpendicular to their fastest growth direction, the M7C3 carbides within the eutectic colonies are very fine rod-like at the center, but become coarser rod-like or blade-like (Fig. 2(b)) with increasing distance from the center.
When the reagent for selective etching of carbide was used (whereby the M7C3 carbide become dark brown and vanadium carbide white), fine, white particles were noticed in the structure of alloy containing 1.19% V (Fig. 3). Figure 4(b) shows not only that the vanadium is distributed between eutectic M7C3 carbide and the matrix and that its content in carbide is considerably higher, but also the area of high vanadium concentration, corresponding to particle marked C from Fig. 4(a).
Vanadium carbide has nearly spherical shape (Fig. 5(a)). EDS analysis (Fig. 5(b)) indicates that this carbide (Fig. 5(a)) contains 50.86 mass% V, 15.94 mass% Cr i 15.73 mass% Fe.
Vanadium carbide present in examined Fe-Cr-C-V alloys was identified as M6C5 type carbide (Fig. 6). Stacking fault is clearly visible within the carbide (Fig. 6).

The volume fraction and size of phases in the microstructure of the examined alloys are shown in Table 3. With increasing vanadium content in the alloy, the volume fraction of primary austenite is decreased, whereas the amount of M7C3 and M6C5 carbides are increased. In adition, dendrite arms spacing (DAS) and size of eutectic M7C3 carbides are decreased, while the size of M6C5 carbides is increased with increasing vanadium content.
Links: No data
Reference: Not shown in this demo version.

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