M7C3 carbides in Fe-25Cr-3.1C-2.0Mn-0.7Si and Fe-25Cr-5.2C-2.1Mn-0.4Si high chromium cast irons


Table 1: Experimental heat compositions spectrographic results.


Figure 1: Thermal analysis cooling curve for the 25-3 alloy.


Figure 2: Solidification region of the thermal analysis curve for the 25-3 alloy.


Figure 3: Thermal analysis cooling curve for the 25-5 alloy.


Figure 4: Solidification region of the thermal analysis curve for the 25-5 alloy.


Table 2: Liquidus and solidus temperature measurements


Figure 5: Measured liquidus & solidus temperatures for the two alloys.


Table 3: Physical Property results for specimens.


Figure 6: BSE images for the 25-3 alloy in the as-cast condition. Scale bars: 10, 1 µm.


Figure 7: BSE images for the 25-3 alloy after solution treatment at 1200C. Scale bars: 10 µm.


Figure 8: BSE images of the 25-3 alloy in the age-hardened condition. Scale bars: 10, 1 µm.


Figure 9: BSE images of the 25-5 alloy in the as-cast condition. Scale bars: 10, 1 µm.


Figure 10: BSE images for the 25-5 alloy after solution treatment at 1200C. Scale bars: 10, 1 µm.


Figure 11: BSE images for the 25-5 alloy in the aged condition. Scale bars: 10, 1 µm.


Table 4: EBSD results for the 25-3 alloy in the solution treated condition.


Table 5: EBSD results for the 25-3 alloy in the age-hardened condition.


Table 6: EBSD results for the 25-5 alloy in the solution treated condition.


Table 7: EBSD results for the 25-5 alloy in the age-hardened condition.


Figure 12: 25-5 Alloy solution treated sample; (a) microstructure, (b) EBSD patterns and (c) indexed EBSD result for primary carbide, (d) EBSD pattern and (e) indexed EBSD result for the austenite phase. Scale bar: 10 µm.


Figure 13: 25-5 Alloy age-hardened sample; (a) microstructure, (b) EBSD patterns and (c) indexed EBSD result for eutectic carbide, (d) EBSD pattern and (e) indexed EBSD result for the primary carbide, (f) microstructure showing martensite, (g) EBSD patterns and (h) indexed EBSD result for the martensitic matrix. Scale bar: 20 µm.


Table 8: Results from dilatometry to 1200C for 2 hours.


Table 9: Results from dilatometry to 950C for 4 hours. Note: The martensite start temperature is denoted as Ms.


Figure 14: Dilatometer cooling curve for the 25-3 alloy after solution treatment at 1200C for 2 hours.


Figure 15: Dilatometer cooling curve for the 25-3 alloy after heat treatment at 950C for 4 hours.


Figure 16: Dilatometer cooling curve for the 25-5 alloy after solution treatment at 1200C for 2 hours.


Figure 17: Dilatometer cooling curve for the 25-5 alloy after heat treatment at 950C for 4 hours.

Carbide name: M7C3
Record No.: 996
Carbide formula: M7C3
Carbide type: M7C3
Carbide composition in weight %: No data
Image type: SEM, EBSD
Steel name: Fe-25Cr-3.1C-2.0Mn-0.7Si, Fe-25Cr-5.2C-2.1Mn-0.4Si
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Cast irons
Steel composition in weight %: See the table 1.
Heat treatment/condition: The material used for this work was cast from a 25kg induction melting furnace into sand moulds to produce cast plates approximately 13mm thick. Two alloy compositions were studied.
The alloys are referred to throughout this report by their nominal compositions; ie. the 25-3 alloy and the 25-5 alloy, which represent 25%Cr with 3% carbon, and 25% chromium with 5% carbon, respectively.
Thermal analysis was conducted during the casting process. This was achieved by pouring approximately 2kg of molten metal into an insulative alumina fibre cup supported by silica sand. An R-type thermocouple (protected by a thin silica-glass sleeve) was inserted into this molten sample and connected to a datalogger to monitor the temperature of the sample throughout solidification and cooling to approximately 400C.
Test coupons were sectioned from the cast plates using a water-cooled metallurgical cut-off saw. The as-cast surfaces were removed using a water-cooled swing grinder with a carborundum grinding cup. Small specimens were initially produced via cylindrical grinding followed by centreless grinding, and later by wire cutting to overcome difficulties with machining of these small brittle specimens. Disc specimens were sliced using a water-cooled, low-speed precision saw fitted with a diamond-coated metal blade.
Note: Two high-chromium white cast irons, with compositions of Fe-25Cr-3.1C-2.0Mn-0.7Si and Fe-25Cr-5.2C-2.1Mn-0.4Si, were cast and subjected to various heat treatments. The microstructures were investigated using scanning electron microscopy and electron back-scattered diffraction techniques. The microstructure and properties were found to vary profoundly upon the increase in carbon content from 3 to 5%, changing from near-eutectic to hypereutectic and from a ferrous matrix of austenite partially transformed to martensite to one entirely transformed to pearlite. Age hardening followed by rapid cooling produced secondary carbides and a predominantly martensitic matrix in both alloys.

Thermal analysis simply involves monitoring the temperature of the metal during cooling. As a material cools, if there are no phase transformations, the cooling rate follows a continuous exponential cooling curve. However, any changes to the structure upon cooling will normally produce a deviation from the ideal cooling curve since the change is either exothermic or endothermic. Deviations from the ideal continuous cooling curve are observed and can be interpreted as phase changes. For example, the solidification reactions are exothermic so the liquidus and solidus temperatures can be readily identified as thermal arrest points. Another possible example in these alloys is the transformation of austenite to pearlite. The pearlitic transformation is also exothermic and can be seen as a thermal arrest on the cooling curve at temperatures of approximately 650-700C.

25-3 Alloy (ie. Fe-25Cr-3.1C-2.0Mn-0.7Si): At a first glance, the thermal analysis curve for the 25- 3 alloy (Figures 1 and 2) appears to be typical of an eutectic alloy. Eutectic alloys show only a single thermal arrest during solidification. However, the curve for this alloy shows a very small initial peak at 1267C, followed by a much larger arrest at 1265C. The small initial peak corresponds to the first solidification reaction. It represents the liquidus temperature, which is the measurable exothermic reaction evolved due to the solidification of a small amount of primary phase according to the following equation: Liquid -> liquid + primary M7C3 carbide (s)
Therefore, Alloy 1 (25-3) is a near-eutectic alloy rather than a purely eutectic alloy. The small amount of heat generated at the liquidus temperature indicates that the volume fraction of primary phase is very small. An observation that is typical of near-eutectic alloys is that the liquidus temperature is very close to the solidus temperature; in this case the liquidus is only 2C above the solidus. The larger arrest at 1265C is due to the heat generated by the exothermic eutectic reaction according to the following equation: Remaining liquid -> eutectic (s).
Where: eutectic(s) = eutectic austenite(s) + eutectic carbide(s)
The relatively large size of the thermal arrest at the solidus temperature indicates a relatively large proportion of the eutectic constituent. Solidification is complete at the end of the arrest at the solidus temperature. Upon further cooling down to around 400C, no solid state transformations are evident from the thermal analysis curve for this alloy at the observed cooling rate.

25-5 Alloy (ie. Fe-25Cr-5.2C-2.1Mn-0.4Si): The 25-5 alloy (Figure 5 and 6) is hypereutectic so it contains a primary carbide phase. Solidification of the primary phase begins at 1346C according to the following reaction: Liquid -> liquid + primary carbide (s)
The liquidus temperature for Alloy 2 (25-5) is higher than the liquidus temperature for the near-eutectic Alloy 1 (25-3). Solidification of the remaining liquid occurs at the solidus temperature of 1239C according to the equation: Remaining liquid -> eutectic (s) Where: eutectic (s) = austenite (s) + eutectic carbide (s).
It can be observed that the solidus temperature for the 25-5 alloy is lower than that of the 25-3 alloy. This indicates that, for Fe-25Cr-C alloys within the composition studied, the eutectic temperature decreases with increasing carbon content. In other words, the solidus line on the Fe-Cr-C phase diagram is not perfectly horizontal but slopes down slightly as the carbon increases. Upon further cooling, the solid-state pearlite transformation begins at 680C according to the following equation: Austenite (s) -> pearlite (s).
Where: pearlite (s) = lamellar ferrite (s) + lamellar carbide (s).
Subsequent microstructural examination revealed that all of the austenite transformed to pearlite.

Liquidus and solidus temperatures for 2 kg metal samples were measured and shown in Table 2. Figure 5 shows graphically the measured liquidus and solidus temperatures for the two alloys and these curves constitute a small part of the phase diagram for the Fe-25Cr alloy system.

Vickers hardness test results and magnetic response are summarised in Table 3.

25-3 Alloy (ie. Fe-25Cr-3.1C-2.0Mn-0.7Si): In the as-cast condition, the 25-3 alloy consisted predominantly of eutectic carbides in a matrix of austenite with some transformation of the austenite to martensite, particularly adjacent to carbides (Figure 6). A small amount of primary carbides were also observed. Virtually no fine secondary carbides were evident.
In the solution treated and quenched condition, the wholly austenitic matrix exhibited relatively large grains with occasional annealing twins (Figure 7). The eutectic carbides appeared to be slightly rounded by the solution treatment. A small proportion of secondary carbides were observed, which indicates that the solution treatment temperature of 1200C, being 65C below the solidus temperature, or the holding time of one hour, was insufficient for the complete dissolution of all of the secondary carbides.
At 950C, the ferrous matrix is supersaturated with respect to carbon and chromium. Therefore, during the aging heat treatment at 950C for 4 hours, secondary carbides precipitate from the ferrous matrix (Figure 8). Consequently, the ferrous matrix becomes depleted with respect to carbon and chromium. The solute-depleted austenite becomes destabilised so that upon cooling the austenitic matrix transforms almost completely to martensite.

Alloy (ie. Fe-25Cr-5.2C-2.1Mn-0.4Si): In the as-cast condition, the 25-5 alloy consisted of primary carbides and eutectic carbides in a matrix fully transformed to pearlite (Figure 9). In the solution treated and quenched condition (Figure 10), the wholly austenitic matrix exhibited austenite grains with annealing twins evident in some grains. The eutectic carbides appeared to be slightly rounded by the solution treatment. Virtually no secondary carbides were evident, indicating that the solution treatment temperature of 1200C, being only 39C lower than the solidus temperature, and the one hour holding time, was sufficient for the complete dissolution of the fine secondary carbides, restoring the carbon and chromium concentrations in the matrix to their maximum levels.
Upon reheating to 950C, the alloy matrix is now supersaturated with respect to carbon and chromium and during the aging heat treatment at 950C for 4 hours secondary carbides precipitate from the ferrous matrix (Figure 11). Consequently, the ferrous matrix becomes depleted with respect to carbon and chromium. The solute depleted austenite becomes destabilised so that upon cooling the austenitic matrix transforms almost completely to martensite.

Electron Back Scattered Diffraction (EBSD): EBSD investigations were carried out on both alloys in the solution treated and in the age-hardened conditions. These studies confirmed the crystal structures of the major phases in both alloys and the results are summarized in the Tables 4-7. The austenite grains in the solution treated samples were face centred cubic (space group 225). Upon age-hardening, the ferrous matrix transformed to martensite with some austenite being retained. Although martensite is body centred tetragonal, the tetragonality is only of the order of a few percent. EBSD studies are not effective for detecting this small degree of tetragonality. Consequently, martensite was identified in these EBSD studies as being body centred cubic. A further limitation of EBSD was that the small amounts of retained austenite in the age-hardened samples did not yield idenitifiable Kikuchi patterns. This may be explained by the presence of martensite, which undergoes a volume expansion upon transformation. This volume expansion in the martensite phase may exert a stress on the retained austenite. This stress would strain the austenite crystal lattice and interfere with the development of Kikuchi lines during electron diffraction.
The primary and eutectic carbides were consistent with a hexagonal close packed structure with space group 186, although they were also similar to the orthorhombic structure with space group 62. Examples of the EBSD patterns for the 25-5 alloy are shown in Figures 12 and 13.

Dilatometry: Dilatometry is a metallurgical tool that is useful for determining coefficients of expansion and for detecting phase changes upon heating and cooling. Dilatometry involves subjecting a specimen to a thermal cycle while continuously measuring the length of the specimen. As the specimen is heated, it expands and this can be measured as an increase in length and the coefficient of thermal expansion versus temperature can be measured. In the same way, the thermal contraction can be measured as the specimen cools. Any phase changes that involve a change in the coefficient of expansion will show up as a change in slope, or inflection, in a plot of extension versus temperature. Any phase changes that involve a change in volume will appear as a dramatic change in the slope of the curve. All data results are shown in Tables 8 and 9, and Figures 14 to 17 for the two alloy types.
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