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M7C3 carbides in WeldoverlayA white cast iron


Table 1: Bulk chemical composition, chemical composition of the primary carbides and austenitic matrix for WeldoverlayA.


Figure 1: Metallographic examination of the surface of WeldoverlayA. (a) General overview of the metallographic sample showing the extent of check cracking and the location where micrographs were taken. (a) Micrograph of Area 1 showing primary M7C3 carbide, eutectic M7C3 carbides and austenite. A halo can be seen surrounding the primary M7C3 carbides, 500x magnification. (b) Micrograph of Area 2 showing the complex regular carbide microstructure, 500x magnification. Scale bars: 50 µm.


Figure 3: Secondary electron micrographs of WeldoverlayA. The examination surface is parallel with the wear surface. (a) Micrograph showing the primary and eutectic M7C3 carbides in an austenitic matrix. (b) Higher magnification micrograph of the boxed region in (a) showing martensite laths extending from regions surrounding the eutectic carbides into the matrix. Scale bars: 10, 2 µm.

Carbide name: M7C3
Record No.: 822
Carbide formula: M7C3
Carbide type: M7C3
Carbide composition in weight %: See the table 1.
Image type: LM, SEM
Steel name: WeldoverlayA
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: As-received condition.
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.

Figures 1(b) and (c) show the respective microstructures of Area 1 and Area 2. The microstructure of Area 1 consists of primary M7C3 carbides surrounded by an austenite halo and a eutectic of M7C3 carbides and austenite. In the micrograph the primary carbide rods are nearly perpendicular to the plane of polish. The microstructure in Area 2, Figure 1(c) consists of the complex regular carbide morphology.
WeldoverlayA is a commercially deposited high chromium white iron weld overlay deposited on a steel substrate. The sample was taken from an ex-service alumina plant spool. The hypereutectic microstructure consists of primary M7C3 carbides of different orientations and a eutectic of M7C3 carbides and austenite.

Further microstructural examination was done on lightly etched sample in an electron mi- croscope, 2. The high magnification secondary electron images more clearly illustrate the eutectic M7C3 carbides and the precipitate free austenite halo regions surrounding the primary carbides. Typically the primary carbides are greater than 10Ám in diameter and the eutectic carbides are much smaller being less than 2 Ám in diameter. The spacing between the eutectic carbides is also small, being less than 2 Ám. What is also evident from the high mag- nification micrograph shown in Figure 2(b) is the presence of martensite that surrounds the eutectic carbides and martensite laths extending into the eutectic austenite.

The bulk chemical composition and the chemical microanalysis results of the primary carbides and matrix for WeldoverlayA are given in Table 1. The carbon composition being 5.5% and chromium composition of approximately 25%, giving a chromium to carbon ratio of 4.5. With no significant addition of hardenability alloying elements this alloy is classified as a AS2576 23XX austenitic chromium carbide iron hardfacing. The primary M7C3 carbides were found to contain 42% chromium, 1.2% manganese and negligible traces of silicon, nickel and molybdenum. The eutectic M7C3 carbides were not analyzed due to the small eutectic carbide size being less than the machine resolution. The matrix chemical com- position represents the composition of the halo regions, as the close spacing of the eutectic carbides did not allow the analysis of the eutectic austenite. The halo regions contained 1.3% carbon, 8% chromium 3% silicon and 1.2% manganese. The manganese of the matrix was found to be similar to the primary carbides. The carbide volume fraction of the primary and eutectic carbides was measured to be 59%.
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