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Designing a Novel Graphitic White Iron for Metal-to-Metal Wear Systems

  • Jie Wan
  • Jingjing Qing
  • Mingzhi XuEmail author
Article
  • 75 Downloads

Abstract

Metal-to-metal wear systems are widely used in various industries, but heat-induced adhesive wear has been limiting the lifetime of the components for many years. An idea of introducing interconnected flake graphite networks into white iron was developed by the authors, which can potentially solve this problem by increasing the overall thermal conductivity. To optimize the thermal conductivity and wear resistance, five alloys with different chromium and carbon contents were designed, produced, and investigated to develop the first generation of graphitic white iron. Mathematical models were developed to correlate the graphite phase concentration and cooling rate with carbon equivalent. It was shown that graphite volume percent needs to be higher than 7 pct to have a consistent thermal conductivity increase. Hardness model developed in this article suggested that M7C3 has a higher hardness than the plate cementite, and hardness increases with increasing chromium content in the carbides. The as-solidified microstructure was characterized using a SEM, and solidification sequence was established for this novel alloy system. Unexpectedly, for the first time, study of alloy with 11 wt pct Cr shows that M7C3 was formed during eutectic reaction and then transformed into cementite at a lower temperature.

1 Introduction

White cast iron exhibits a white, crystalline fracture surface because the fracture occurs along the iron carbide plate,[1] which is a result of no graphitization and fast cooling rate. Due to the relatively high hardness of cementite carbide (800-1100 HV),[2] white cast iron is extensively used in wear resistance applications. However, it is difficult to obtain sufficient cooling rate to produce white cast iron in heavy sections or relatively large castings, thus high alloy additions are added to promote carbide formation at a slower solidification cooling rate. Typically, high-alloyed white cast irons fall into two major groups[3]: Ni-Cr white iron and Cr-Mo white iron. Among all grades of white cast irons, high Cr white iron is the most popular type due to its higher hardness resulted from M7C3 carbides (1100-1600 HV).[2] Numerous studies have been conducted on the effects of alloy additions on high Cr white cast iron. Additional vanadium is added for grain refinement purpose.[4,5] Silicon is also added for grain refinement and it also increases the volume fraction of the eutectic carbides.[6] Titanium is used to refine primary M7C3 carbides.[7, 8, 9, 10, 11] In addition, niobium was found to be effective to improve the fracture strength by changing the morphology of eutectic carbides from plate/rod-like shape to isotropic shape.[12]

Gray cast iron exhibits a gray fracture surface because the fracture occurs along graphite flakes,[1] which can be formed as primary graphite or eutectic graphite. Generally, flake graphite in gray iron is divided into five type based on their distributions, morphologies, and patterns.[13,14] In specific, type A graphite flakes have a random orientation and are intended in gray iron, type B graphite has a rosette pattern, type C graphite is kish graphite which is formed in hypereutectic cast irons, type D graphite is randomly orientated very fine interdendritic flakes, and type E graphite is very fine interdendritic flakes with a preferred orientation.[14] Due to interconnected structure of graphite, gray iron always presents higher thermal conductivity as compared to the with other graphitic irons,[15,16] which also can be concluded from the cooling curves for different cast irons.[17, 18, 19]

High Cr white cast iron is commonly used in metal-to-metal wear applications, where significant heat induced by friction causes severe adhesive wear.[20,21] To take the advantage of both high wear resistance from carbide and high thermal conductivity from interconnected graphite, the authors proposed to design a mottled graphitic white iron. Typically, mottled cast iron refers to mottled nodular cast iron,[22, 23, 24, 25, 26, 27] generally used for mill rolls.[22, 23, 24] But limited work has been done on designing mottled cast iron containing flake graphite.

In the present work, the authors introduced interconnected graphite networks into the as-solidified matrix to increase the thermal conductivity of white iron. Specifically, five mottled cast iron alloys were designed and produced in an induction furnace. The five alloys were studied metallographically to determine the graphite and carbide phase fraction. Solidification sequences were studied by rationalizing the thermodynamic equilibrium calculation results with optical microstructures. Meanwhile, energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) were used to further verify the solidification sequences. Hardness of different carbides in all five alloys was measured. Numerical models were constructed for guiding future alloy design.

2 Materials and Methods

2.1 Rational for Chemistry

In the present investigation, a series of 2 wt pct Si-0.5 wt pct Mn-C-Cr-Fe alloys were designed and studied. Based on thermodynamic calculation using software FactSage Equilibrium module (version 7.1, database FSstel), the primary phase formed during solidification over 0-30 wt pct Cr and 0-8 wt pct C is plotted in Figure 1. It is shown that carbon promotes the formation of graphite, while chromium promotes the formation of M7C3 and ferrite. It is well known that an interconnected graphite network in metal can provide high thermal conductivity, for example, the type A flake graphite in gray cast iron. Carbides are known for a relatively high hardness, which increases a casting’s wear resistance. To take the advantage of both graphite and carbides, three hypereutectic and two hypoeutectic chemistries were proposed. The alloys were named with their individual Cr content and marked as different colored dots in Figure 1. Here, Cr is working as a regulator to control the competition of carbon between graphite and carbides.
Fig. 1

The primary phase formed during solidification at different contents of C and Cr; colored dots represent the alloys studied in this research; numbers in each callout represent the chemistry of C and Cr individually (Color figure online)

To accurately control the chemistry, high-purity charge materials including induction iron, Desulco graphite, low carbon ferrochrome, ferrosilicon, and ferromanganese were melt in an induction furnace to produce five alloys with various Cr and C contents. Metal was cast into no-bake sand molds with 200 °C superheat for each alloy. The chemical composition of the five produced alloys is given in Table I. Additionally, 0.15 wt pct graphite inoculant was added into the sprue well prior to each pour to promote graphite formation. The pertinent temperature information is shown in Table II.
Table I

Composition Analyzed Using Spark Emission Spectrometer and Leco C/S Analyzer

Alloys

Chemical Composition (Weight Percent)

Inoculant (Weight Percent)

Leco C

Leco S (ppm)

Si

Mn

Cr

3Cr

4.45

154

2.06

0.44

2.79

0.15

5Cr

4.72

157

2.00

0.47

5.11

0.15

7Cr

4.88

184

2.07

0.49

7.08

0.15

9Cr

4.93

198

2.05

0.50

9.08

0.15

11Cr

5.00

170

2.03

0.50

11.03

0.15

Table II

Pertinent Temperature Information for All Studied Alloys

Alloys

Temperature (°C)

A1

Solidus

Liquidus

Pouring Temperature

3Cr

798

1142

1305

1505

5Cr

818

1139

1304

1504

7Cr

834

1134

1292

1492

9Cr

834

1138

1256

1456

11Cr

834

1138

1282

1482

2.2 Sample Preparation and Characterization

All specimens were sectioned from the bottom part of the casting to minimize the amount of shrinkage porosity. Metallography samples were machined from the same location to minimize the variation due to different cooling rates. Two sets of samples were ground on silicon carbide papers to 1200 grits and then polished using 3 µm followed by 0.1 µm diamond paste. The first set of as-polished samples were used to study the graphite fraction, and the second set of samples were etched with 2 pct Nital to measure carbide fraction as well as microhardness. Microstructure was analyzed using a Nikon FX-35DX camera. Microhardness was conducted with Duramin-5 microhardness tester. ASPEX-EDS was used to determine the composition of each carbide phase. EBSD was performed on alloy 11Cr to discriminate different types of carbides in it. FactSage 7.1 (FSstel database) was used to calculate the phase diagrams and phase equilibrium step diagrams to study the solidification sequences.

3 Metallography Results

As-polished microstructure is shown in Figure 2 to reveal different graphite morphology and fraction among five alloys. Type A (green arrows in Figure 2) and type C (red arrows in Figure 2) graphite are observed in alloy 3Cr, 5Cr, and 7Cr. Type D graphite (purple arrows in Figure 2) is found within the dendritic structures for all five alloys. Another set of specimens that were etched with 2 pct Nital to reveal various phases. Figure 3 shows the microstructures under a lower magnification. Few skinny plate carbides are observed in alloys 3Cr. Longer and thicker plate carbides are found in alloy 5Cr. In alloy 7Cr, besides plate carbide, cluster carbides are noticed. As the Cr content increases, in alloys 9Cr, more hexagonal-shaped carbides and fewer plate carbides are observed. However, in alloy 11Cr, a lot of hexagonal-shaped carbides but minimum plate carbide was observed. In Figure 3, the plate carbides are indicated by red arrows, cluster carbides are indicated by purple arrows, and hexagonal-shaped carbides are indicated by green arrows, respectively. Furthermore, pearlite was differentiated in all five alloys under a higher magnification shown in Figure 4 and there was no obvious difference in pearlite morphology among all five alloys.
Fig. 2

Graphite morphology in as-polished microstructure: (a) 3Cr; (b) 5Cr; (c) 7Cr; (d) 9Cr; (e) 11Cr (Color figure online)

Fig. 3

Carbide morphology in 2 pct Nital etched microstructure at a lower magnification: (a) 3Cr; (b) 5Cr; (c) 7Cr; (d) 9Cr; (e) 11Cr (Color figure online)

Fig. 4

Pearlite structure revealed by 2 pct Nital etched microstructure at a higher magnification: (a) 3Cr; (b) 5Cr; (c) 7Cr; (d) 9Cr; (e) 11Cr

4 Discussion

4.1 Graphite Phase Fraction vs Carbon Equivalent

FactSage 7.1 (database FSstel) was used to calculate the amount of graphite and carbide during solidification in all studied alloys. The weight percent of both graphite and carbides were calculated at a temperature right above the critical temperature A1 during the equilibrium solidification. No eutectoid reaction takes place at such temperature, consequently no eutectoid product was considered in the calculation. In another word, only primary carbide and those carbides formed during metastable eutectic reaction in ledeburite were included. Then weight percent for each phases were transformed into volume percent using following densities: ρ(graphite) = 2266 kg/m3, ρ(Fe3C) = 7730 kg/m3, ρ(M7C3) = 7230 kg/m3, and ρ(austenite) = 7915 kg/m3.

ImageJ software was used to measure the amount of graphite and carbide on optical micrographs for all five alloys. Graphite was measured from the as-polished microstructures shown in Figure 2. The contrast threshold was adjusted to include both primary graphite and eutectic graphite. Carbides were measured using the 2 pct Nital etched microstructures shown in Figure 3. Similarly, the contrast threshold was carefully adjusted such that only primary carbides and eutectic carbides were highlighted. Carbides in pearlite formed during the eutectoid reaction were not included in the ImageJ measurements. As a consequence, neither FactSage calculation nor ImageJ measurements considered the eutectoid reaction products. Figure 5 shows an example of an adjusted threshold for both graphite and carbides.
Fig. 5

Contrast threshold was adjusted in ImageJ to include (a) graphite and (b) primary and eutectic carbides; scale bars were removed to avoid interference with the threshold

As shown in Figure 6, the volume percent of graphite and carbides calculated using FactSage was compared with that measured with ImageJ. Red bars represent FactSage equilibrium calculation results at a temperature above critical temperature A1. Blue bars represent ImageJ measurements averaged from ten micrographs. The error bars represent 95 pct CL uncertainty range.[28]
Fig. 6

Phase percent comparison among five alloys for: (a) graphite; (b) carbide (Color figure online)

Overall, volume fractions measured by ImageJ is similar to those calculated by FactSage. Following conclusion can be drawn: graphite percent decreases and carbide percent increases with increasing Cr content. Nevertheless, as the Cr content increases, graphite concentration predicted by FactSage at A1 temperature decreases at a slower speed compared to ImageJ measurements. One possible explanation is that at higher Cr levels, FactSage equilibrium module does not have a graphite growth model via diffusion built in. It is worth noting that the measured carbide volume percent for all five studied alloys is smaller than the calculated values, which is consistent with Hecht’s work.[29]

According to previous published works,[30,31] following multiplying factors were reported for carbon equivalent calculation: \( G_{\text{C}}^{\text{S}} \) = 0.4, \( G_{\text{C}}^{\text{Si}} \) = 0.33, \( G_{\text{C}}^{\text{Mn}} \) = − 0.027, \( G_{\text{C}}^{\text{Cr}} \) = − 0.25, so Eq. [1] is established to calculate carbon equivalent in these studied alloys.
$$ C_{\text{Eq}} \, = \,C_{\text{C}} \, + \,0.4C_{\text{S}} \, + \,0.33C_{\text{Si}} \, - \,0.027C_{\text{Mn}} \, - \,0.25C_{\text{Cr}} $$
(1)
To fully understand how the graphite volume percent is affected by the carbon equivalent, Eq. [1] was used to calculate the carbon equivalents for all five alloys. Then the relation between carbon equivalent and graphite volume percent for those alloys is plotted in Figure 7. It turned out that graphite volume percent increases with increasing carbon equivalent by a linear relation following Eq. [2]. And the coefficient of determination is 0.9696, which indicates that Eq. [2] is fitting very well and can be used for future works.
$$ V_{\text{p}} \left( {\text{Gr}} \right)\, = \,4.1046C_{\text{Eq}} \, - \,9.0513. $$
(2)
Fig. 7

Relation between carbon equivalent and graphite percent

Cooling curves can be used to predict the graphite morphology and cast structure by studying the cooling characteristics and critical points.[19] One way to qualitatively study the effect of graphite on thermal conductivity is to compare the cooling rate among different alloys during solidification. Figure 8 correlated the cooling rate at 900 °C with graphite volume percent for all five alloys. Figure 8(a) shows the cooling curves obtained from thermal analysis cups and Figure 8(b) shows the relation between graphite volume percent and cooling rate at 900 °C measured from cooling curves in Figure 8(a). A linear relation between cooling rate at 900 °C and graphite volume percent is established and shown in Eq. [3] for all five studied alloys. As is shown in Figure 8(b), as the graphite volume percent increases, cooling rate at 900 °C also increases, which proved that introducing graphite into white iron increases its thermal conductivity. However, cooling rates at 900 °C for alloy 3Cr and alloy 5Cr are much higher than the rest three studied alloys and once the graphite volume percent is lower than 7 pct, which is the minimum measured graphite volume percent for alloy 5Cr, the corresponding cooling rate at 900 °C is scattered around 1.1 °C/s. One can conclude that graphite volume percent needs to be higher than 7 pct in order to increase the thermal conductivity consistently in a graphitic white iron alloy.
$$ V_{\text{p}} \left( {\text{Gr}} \right)\, = \,2.8358R\, + \, 1. 6 4 1 9. $$
(3)
Fig. 8

Cooling rate study for: (a) cooling curves obtained from ATAS quick cups; (b) relation between graphite volume percent and cooling rate measured from temperature curves at 900 °C (Color figure online)

4.2 Characterizations of Carbides

Plate carbides found in alloy 3Cr and 5Cr were cementite, which is proved by FactSage thermodynamic calculation in Sections IV–C–1 and IV–C–2. However, different morphologies of carbides were observed in alloys with higher Cr contents. Initial FactSage calculation indicated those carbides are cementite and M7C3 carbides. Solidification equilibrium was calculated for alloy 7Cr, 9Cr, and 11 Cr, and the different ratios of Fe/Cr(wt pct) between cementite and M7C3 can be used to differentiate the types of carbide under an electron microscope with an EDS detector. Figure 9 shows some of the examples of EDS measured carbide morphologies. Table III summarizes the calculated and measured Fe/Cr(wt pct) ratios within cementite and M7C3 carbides across all samples. It is noticed that the Cr concentration in both cementite and M7C3 carbides increases with increasing Cr content in the alloy. By comparing the EDS measured results with the equilibrium calculation, it can be concluded that the plate carbides are primary cementite and hexagonal-shaped carbides are M7C3 carbides. It is worth noting that the difference on the exact chemistry between EDS and thermodynamic calculation is a result of electron reaction volume in the SEM.
Fig. 9

Fe/Cr(wt pct) ratio of: (a) plate carbide in 7Cr; (b) plate carbide in 9Cr; (c) cluster carbide in 7Cr; (d) hexagonal-shaped carbide in 9Cr; (e) hexagonal-shaped carbide in 11Cr

Table III

Comparison of Fe/Cr (Weight Percent) Ratio Between FactSage Calculation and ASPEX-EDS Measurement for Fe3C and M7C3

Fe/Cr (Weight Percent)

3Cr

5Cr

7Cr

9Cr

11Cr

Fe3C

     

 FactSage

7.4/1

6.2/1

5.5/1

5.5/1

5.6/1

 EDS

12.7/1

6.7/1

6.5/1

6.5/1

M7C3

     

 FactSage

1.5/1

1.5/1

1.5/1

 EDS

3.0/1

2.4/1

2.0/1

Previous work by Xing et al. suggested that M7C3 has a hollow hexagonal structure at high chrome ratio.[32] As shown in Figure 10, higher magnification images show that the cores of the hexagonal M7C3 carbide were pearlite, which was austenite before eutectoid reaction. Similar to M7C3 carbides in 7Cr and 9Cr. Sha Liu’s work claimed that primary M7C3 is irregular polygonal shape with several hollows in the center and gaps on the edge.[33] However, Figures 9(e) and 10(c) show that with increasing Cr content in the alloy, the primary M7C3 carbide becomes more regular and close to hexagonal in the shape and there is no gaps on the edge of M7C3 in alloy 11Cr.
Fig. 10

Pearlite in the center of M7C3: (a) 7Cr; (b) 9Cr; (c) 11Cr

4.3 Solidification Sequence

4.3.1 Alloys 3Cr

By relating the FactSage Equilibrium step diagram, as shown in Figure 11(a), to the microstructure, one can construct the solidification sequence. Specifically, during the solidification, graphite was the primary phase thus type A and type C graphite were observed. As the temperature decreases, the solidification reached the stable eutectic reaction, where austenite and type D graphite were formed. As the solidification continues, cementite (Fe3C) was precipitated as the primary phase for the metastable reaction, followed by the metastable eutectic reaction. In the metastable eutectic reaction, ledeburite was formed which contains cementite and austenite. When the temperature reached A1, the austenite transformed to pearlite, as shown in Figure 4(a).
Fig. 11

Schematics of equilibrium step diagram calculated using FactSage for: (a) 3Cr; (b) 5Cr; (c) 7Cr; (d) 9Cr; (e) 11Cr (Color figure online)

4.3.2 Alloy 5Cr

Similar to alloys 3Cr, during the solidification of alloy 5Cr, graphite precipitates firstly, as shown in Figure 11(b). As a result, type A and type C graphite were observed. As the solidification continues, primary cementite was formed as the primary phase for the metastable reaction. With decreasing temperatures, the solidification reached stable eutectic reaction where liquid transformed to austenite, type D graphite. Then ledeburite was formed which contains cementite and austenite during the metastable eutectic reaction. Finally when the temperature reached A1, pearlite structure was formed from the austenite, as shown in Figure 4(b).

4.3.3 Alloy 7Cr

As shown in Figure 11(c), during the solidification of alloy 7Cr, graphite was still the primary phase. Type A and type C graphite were expected. Then M7C3 precipitated into cluster carbide instead of cementite as a result of high Cr content compared with alloy 5Cr. After that, primary cementite solidified as the primary phase for the metastable reaction. Austenite and type D graphite were formed as metastable eutectic products after the solidification of primary cementite. Subsequently, ledeburite was formed during the metastable eutectic reaction. At last, as shown in Figure 4(c), eutectoid reaction occurred and austenite transformed into pearlite at temperature below A1.

4.3.4 Alloy 9Cr

With regard to alloy 9Cr, M7C3 precipitated first from the liquid metal. As the temperature decreases, primary austenite solidified as the primary phase for the hypoeutectic reaction. At a lower temperature, primary cementite formed as the primary phase for the metastable hypereutectic reaction. Then, the stable eutectic reaction occurred, where austenite and type D graphite were formed, followed by the metastable eutectic reaction, where ledeburite was formed which contains cementite and austenite. And pearlite shown in Figure 4(d) was formed as a eutectoid product at last.

4.3.5 Alloy 11Cr

During the solidification of alloy 11Cr, as shown in Figure 11(e), the hexagonal-shaped M7C3 formed first as the primary phase. As the temperature decreases, eutectic reaction occurred and formed M7C3 and austenite into a ledeburite-shaped structure. As the temperature reduced, very few primary graphite was formed as the primary phase for stable eutectic reaction followed by the stable eutectic reaction, where austenite and type D graphite were formed. Meanwhile, part of the M7C3 carbide was transformed into cementite (Fe3C), as shown in Figure 11(e) at temperatures below 1126 °C. Further ASPEX-EDS analysis in Figures 9 and 12 confirmed that the carbide in ledeburite-shaped structure for alloy 11Cr has a similar Fe/Cr ratio with the cementite in alloys 7Cr and 9Cr. Electron Backscatter Diffraction (EBSD) was used to confirm the phase constituent in alloy 11Cr. As shown in Figure 13, EBSD analysis was conducted on an area including both carbide in the ledeburite-shaped structure and primary hexagonal-shaped carbide. And Figure 13(b) shows that most of the carbides in ledeburite-shaped structure are cementite, while the primary hexagonal-shaped carbide is still M7C3 except few cementite around it. More details are discussed in later section. Finally, starting at the critical temperature A1, austenite transformed into pearlitic structure shown in Figure 4(e).
Fig. 12

Fe/Cr(wt pct) ratio for carbide in ledeburite-shaped structure for alloy 11Cr

Fig. 13

Schematics of: (a) SEM image of alloy 11Cr taken by an EBSD forescatter detector; (b) EBSD phase mapping including spots where Kikuchi pattern was taken for each phase; (c) indexed Kikuchi pattern for ferrite obtained at spot 1; (d) indexed Kikuchi pattern for Fe3C obtained at spot 2; (e) Indexed Kikuchi pattern for M7C3 at spot 3 (Color figure online)

Overall, the schematics of solidification sequence for each alloy are shown in Figure 14.
Fig. 14

Schematics of solidification sequence: (a) 3Cr; (b) 5Cr; (c) 7Cr; (d) 9Cr; (e) 11Cr

4.4 M7C3 to Fe3C Transformation in Ledeburite-Shaped Structure in Alloy 11Cr

Since both Skobir[34] and Inoue[35] claimed that M3C can transform into M7C3 during a tempering process in high Cr white iron, to further verify that the Fe3C in ledeburite-shaped structure was originally transformed from M7C3, a heat treatment was done on alloy 11Cr. A small piece of metal sectioned from alloy 11Cr was sealed with inert argon gas in a quartz tube to avoid decarburization during the heat treatment. The sample was heat treated for 4 hours at a temperature 1122 °C, which is right below solidus temperature, in a SiC box furnace and then quenched in agitating water. EBSD technique again was used to confirm the phase constituent in alloy 11Cr after heat treatment. As shown in Figure 15(a), EBSD analysis was performed on the ledeburite-shaped structure to verify the phase constituent in the ledeburite-shaped structure in this heat-treated alloy 11Cr. Figure 15(b) is an EBSD phase mapping and shows both M7C3 and Fe3C are existing in the ledeburite-shaped structure after the heat treatment, which means this specimen was quenched in a state where M7C3 and Fe3C are co-existing. The Cr EDS mapping in Figure 15(c) shows that the M7C3 in the ledeburite-shaped structure has a higher Cr content compared with Fe3C. The Cr content in both M7C3 and Fe3C in the ledeburite-shaped structure was measured using SEM-EDS, and compared with the Cr content calculated from FactSage equilibrium modulus in Figure 16. It is shown that the Cr content in M7C3 is about twice of that in Fe3C, although the measured Cr contents are slightly lower than the calculated Cr contents, which is due to the reaction volume effect in the SEM-EDS. Consequently, M7C3 is proved to be the initial carbide in the ledeburite-shaped structure during the solidification and was transformed into Fe3C at a lower temperature. It is worth noting that the temperature range between solidus temperature and M7C3 in ledeburite-shaped structure transformation temperature is too narrow to capture a stage where only M7C3 but no Fe3C exists in the ledeburite-shaped structure. The red-dashed line in Figure 16 represents the temperature used for this heat treatment. The difference to Inoue’s[35] work could be resulted from different Cr contents. In this work, at lower Cr content (11 vs 18 wt pct), Fe3C is more stable at temperature below 1100 °C.
Fig. 15

Schematics of heat-treated alloy 11Cr: (a) SEM image taken by an EBSD forescatter detector; (b) EBSD phase mapping on ledeburite-shaped structure; (c) Cr EDS mapping (Color figure online)

Fig. 16

Change of Cr content in wt pct vs temperature in different phases calculated using FactSage equilibrium module. The blue regions represent the Cr wt pct measured by SEM-EDS for M7C3 and Fe3C both in ledeburite-shaped structure. The red-dashed line represents the temperature used for heat treatment (Color figure online)

4.5 Vickers Indentation Hardness Analysis

Hardness is related to wear resistance of the metal. To understand the hardness change at various chemistry levels, Vickers indentation hardness was taken for primary plate cementite, hexagonal-shaped M7C3 cross face and side face in accordance with ASTM standard E92.[36] A Vickers scale of HV 0.05 (50gf test force) was used along with 10 seconds press time and 40 times objective magnification to ease the measurements and ensure the testing condition was coherent. Figure 17 shows an average of ten measurements with an error bar showing 95 pct CL uncertainty range.[28] By comparing the Vickers hardness of each phase and their Fe/Cr(wt pct) ratio listed in Table III, one can conclude that (1) hexagonal-shaped M7C3 is harder than plate cementite; (2) hardness of hexagonal-shaped M7C3 and plate cementite increases with increasing Cr content; (3) cross face is harder than side face in the hexagonal-shaped M7C3 which indicates the close-packed plane is the hardest plane in HCP structure for M7C3, and this is consistent with both Wang’s and Coronado’s investigations, respectively.[37,38]
Fig. 17

Schematics of Vickers indentation hardness with a parameter of 490.6 mN press load, 10 s press time, and 40 times objective magnification (Color figure online)

5 Conclusion

Five graphitic white iron alloys have been designed, cast, and investigated. Graphite and carbides (excluding carbides in pearlite) percent measured using ImageJ matches thermodynamic calculation results using FactSage equilibrium module. Two numerical models were constructed to guide future design efforts: (1) graphite volume percent increases linearly with increasing carbon equivalent; (2) graphite volume percent needs to be at least 7 pct to be effective on increasing the thermal conductivity of the alloys.

Solidification sequences for each alloy have been determined based on microstructures and FactSage equilibrium step diagrams. Over the investigated five alloy compositions, as Cr content in bulk materials increases, (1) the Cr content in M7C3 carbide increases, (2) M7C3 carbides start to be the primary phase, and the morphology of M7C3 carbide becomes more regular and closer to hexagonal, (3) carbon competition is more favorable towards carbides formation. At higher Cr content, alloy 11Cr, M7C3, and austenite were formed during the metastable eutectic reaction at early stage, then as the temperature decreases, M7C3 in the ledeburite-shaped structure was transformed into cementite. Moreover, the presence of M7C3 in ledeburite-shaped structure at high temperature during the early-stage metastable eutectic reaction was verified by EBSD analysis on the heat-treated specimen.

It was also observed that M7C3 carbide has a transformed austenite core structure along its [0001] direction. Vickers hardness measurements show that, in these austenite-cored M7C3 carbides, cross face is harder than side face. When comparing the hardness between different phases and chemistries, M7C3 carbide is harder than plate cementite, and hardness of M7C3 and plate cementite increases with increasing Cr content.

Notes

Acknowledgments

This work was financially supported by Caterpillar Inc. The authors are grateful for the technical discussion with Dr. David C. Van Aken. Perrin W. Habecker is acknowledged for his assistance with the experiments and sample preparations. The FEI Helios NanoLab EBSD was obtained with a Major Research Instrumentation grant from the National Science Foundation under contract DMR-0723128.

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Copyright information

© The Minerals, Metals & Materials Society and ASM International 2018

Authors and Affiliations

  1. 1.Missouri University of Science and TechnologyRollaUSA
  2. 2.Georgia Southern UniversityStatesboroUSA

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