M3C, M7C3 and M23C6 carbides in Cr white cast iron

Figure 1: Scanning electron micrographs of heat treated high chromium white iron castings after deep etching (a) A 9Cr-3C alloy destabilized at 8000 C for 4 hours. Secondary M3C carbides (marked 1) have precipitated within the prior austenite. The larger carbides are the eutectic M7C3 carbides (marked 2). (b) A 18Cr-3.1C-1.1Mo alloy destabilized at 10000C for 4 hours. Secondary M7C3 carbides (marked 4 and 5) have precipitated within the prior austenite. The secondary M7C3 carbides marked 4 are discrete rods and carbides marked 5 are plate-like in shape. (c) A 29Cr-2.5C alloy destabilized at 10000C for 4 hours. Secondary M23C6 carbides have precipitated within the prior austenite. The secondary M23C6 carbides are very fine fibers (fibrous) which appear to be connected (Powell and Laird II, 1992). Scale bars: 5 µm.

Figure 2: Scanning electron micrographs of heat treated high chromium white iron castings. Sample is a 18Cr-3.1C-1.1Mo alloy destabilized at 10000C for 0.25 hours (same alloy as Figure 1(b)). The secondary M23C6 carbides have a cubic faceted morphology (Powell and Bee, 1996). Scale bars: 2.5 µm.

Carbide name: M3C, M7C3, M23C6
Record No.: 814
Carbide formula: M3C, M7C3, M23C6
Carbide type: M3C, M7C3, M23C6
Carbide composition in weight %: No data
Image type: SEM
Steel name: Cr white cast iron
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 %: No data
Heat treatment/condition: No data
Note: This thesis investigates the development of microstructure in high chromium white irons typically used in the Australian Alumina Industry and how variables such as the bulk chemical composition, cooling rate and heat treatment can be used to vary the microstructure. Microstructural characteristics that influence wear and corrosion were investigated by undertaking corrosion and erosion-corrosion wear tests in a sodium aluminate solution representative of what is found in the alumina processing industry. The corrosion of high chromium white irons in sodium hydroxide solution was compared with their corrosion in sodium aluminate solution to investigate the influence aluminate ions have on corrosion.

In low chromium white irons of less than approximately 10% Cr, the secondary carbides are M3C (Powell and Laird II, 1992). The M3C carbides heterogeneously nucleate within the austenite as growth does not occur on the eutectic carbides. As the time at temperature increases, the number of secondaryM3C carbides increases without a change in morphology. The morphology of the secondary M3C carbides is plate-like, Figure 1(a).
The secondary carbides in 15 to 20% Cr hypoeutectic high chromium white irons have been confirmed to be M7C3 (Pearce, 1983, 1984, Powell and Laird II, 1992, Tabrett et al., 1996, Wiengmoon et al., 2005a). However, Powell and Bee (1996) found that in 18Cr-3.1C-1.1Mo alloy having an austenitic matrix composition of 10.1Cr-1.2C-0.4Mo, that after a short dura- tion at a destabilization temperature of 10000C, extensive M23C6 secondary carbides had het- erogeneously nucleated within the austenite and not the expected M7C3 carbide. Sub-grain boundaries, formed in the as-cast state due to the difference in the coefficient of thermal contraction of carbides and austenite matrix, have been found to be preferential nucleation sites for the initial precipitation of secondary carbides (Bee et al., 1994). The secondary M23C6 carbides have the appearance of discrete cubes in two-dimensions and their cuboidal shape was confirmed when deep etched and examined in an electron microscope, Figure 2 (Powell and Laird II, 1992). The preferential formation of M23C6 rather than the equi- librium M7C3 carbide was explained in terms of the reduced activation energy required for nucleation due to the good lattice matching between austenite and M23C6 carbides. After a longer destabilization time of 4 hours, further precipitation and growth of the secondary car- bides occurs, and the equilibrium M7C3 carbide, having the morphology of discrete rods of hexagonal cross section or plate-like shapes, had also precipitated, Figure 1(b). It would appear that due to the kinetics of nucleation and growth, it is possible to have a destabilized microstructure consisting of the non-equilibrium carbide (M23C6) and equilibrium carbide (M7C3).
In high chromium white iron alloys of 25-30%Cr, the secondary carbides that precipitate within the austenite have been confirmed to be M23C6 (Pearce, 1983, Powell and Laird II, 1992, Tabrett et al., 1996, Wiengmoon et al., 2005a). The M23C6 secondary carbides have the appearance of fine fibers and are rod-like in shape and tend to join together to form an interconnected network, Figure 1(c) (Powell and Laird II, 1992, Wiengmoon et al., 2005a). Higher destabilization temperatures result in coarser secondary carbide particles and a greater tendency for a network of carbides to form (Wiengmoon et al., 2005a, 2004).
In a 30Cr-2.4C alloy, Pearce and Elwell (1986) found that the eutectic M7C3 carbides would undergo an in-situ transformation to M23C6 during normal destabilization heat treatments. In the heat treated condition, the eutectic M7C3 rods were surrounded by a complete or partial shell of M23C6, forming a duplex carbide. It is believed that the M23C6 carbides nucleate at the original interface between the eutectic M7C3 and the matrix before growing inwards, consuming the M7C3 carbide. M23C6 carbides were also found to precipitate within the prior dendritic austenite matrix as well as forming the duplex carbides. Wiengmoon et al. (2005a) also confirmed the transformation of eutectic M7C3 to M23C6 in a 30Cr-2.3C white iron with and without additions of vanadium.
There exists an optimum destabilization temperature to provide maximum hardness for white iron alloys (Maratray and Poulalion, 1982, Sare and Arnold, 1995, Tabrett et al., 1996). The maximum hardness has been found to depend on the ratio of martensite to retained austenite,(Maratray and Poulalion, 1982). At higher than optimum destabilization temperatures, the hardness decreases due to an increase in the amount of retained austenite and the secondary carbides are fewer and coarser (Sare and Arnold, 1995, Tabrett and Sare, 1998). The increase in retained austenite is due to the increase in the solid solubility of carbon in austenite with increasing temperature (Figure 1.11) which decreases the driving force for secondary carbide precipitation and therefore the lowering of carbon and chromium within the austenite. The higher carbon and chromium content of the destabilized austenitic matrix results in a lowering of the MS temperature, which decreases the amount of transformation to martensite. However, the martensite that has transformed is of a higher carbon content and therefore of a higher hardness. At lower than optimum destabilization temperatures, secondary carbide precipitation is usually extensive, causing a significant re- duction of carbon within the austenite. On cooling, near complete transformation to marten- site occurs, but the carbon content of the martensite is low and therefore has a lower hardness (Tabrett et al., 1996).
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