Interface Formation Mechanism of Cemented Carbide Dipped in Molten Cast Iron

Abstract

When cemented carbide contacts molten cast iron during the insert casting process, the binder phase of the cemented carbide is thought to melt even if the molten temperature of the cast iron is lower than the solidus temperature of the cemented carbide (1593 K). It is important to understand the melting mechanism to clarify the interface formation mechanism, and subsequently control the interface structure. The purpose of this study is to clarify the interface formation mechanism from the microstructural change of cemented carbide dipped in molten cast iron. A round bar specimen made of cemented carbide was dipped in molten cast iron at 1473 to 1596 K, and pulled up after a predetermined time. Microstructure observation, elemental analysis, and hardness test were performed on the cross-section of the specimen. The specimen changed from a homogeneous sintered structure to a two-layer structure, the center side was a non-reacted layer that did not change, and the outer side was the transition layer where melting had occurred. The diffusion of Fe and C is thought to have decreased the solidus temperature of the binder phase significantly that the binder phase melted. The non-reacted layer radius could be expressed by the rate equation derived from the Nernst-Brunner equation. Structural changes were seen at the interface such as increased outer diameter of the cemented carbide round bar specimen, occurrence of shrinkage cavities in the transition layer, and characteristic concentration of Co at the boundary. These are thought to be due to liquid phase migration occurring in the molten binder phase and decreased WC solubility due to increase in Fe concentration.

 

This Paper was Originally Published in Japanese in J. JFS 93 (2021) 67–73. The background, experimental procedures, results, and discussion have been revised.

1. Introduction

Insert casting is one of the processes used to improve the wear resistance of cast iron. This is a simple manufacturing process in which a core material, which has better wear resistance than cast iron, is placed in a mold and molten cast iron is poured into the mold. A desired region can be strengthened selectively. Hard materials such as cemented carbide and cermet are used as the core material. Various studies focused on insert casting with these hard materials have been reported.^1–5)

In previous studies, we conducted a dipping experiment.^6,7) A cemented carbide round bar was dipped in a molten high-chromium cast iron at 1596 K. The homogeneous sintered structure of the cemented carbide changed to a three-layer structure. The thick layer (referred to as the diffusion layer) was formed in the cemented carbide even with short-time dipping. A thick diffusion layer was also formed when the cemented carbide round bar was dipped in molten gray cast iron. If all of the binder phase comprising the cemented carbide is melted by the heat of the molten metal, elements of the binder phase and the molten metal should diffuse in the liquid phase and the corresponding distribution shows a monotonous increase or decrease. However, the elemental distribution of the specimen suggested that the binder phase melted only in the diffusion layer. Although the molten metal temperature was almost the same as the solidus temperature of the cemented carbide, a thick diffusion layer was formed during short-time dipping. This indicated that melting of the binder phase can occur even if the molten metal temperature is lower than the solidus temperature of the cemented carbide.

According to the Fe–C binary phase diagram and the WC–Co pseudo-binary phase diagram, the solidus temperature of cast iron is lower than that of cemented carbide.^8–11) In the actual insert casting process, the temperature of molten cast iron poured into a mold decreases with time. The liquid phase of the cast iron exists even at temperatures below the solidus temperature of the cemented carbide (1593 K). Therefore, considering the above study, the melting of the binder phase that occurs below the solidus temperature of the cemented carbide must be taken into consideration. Horikawa et al. reported that Si of the molten cast iron reduces the melting point of the binder phase to approximately 1473 K, thereby promoting melting of this phase.¹²⁾ However, the mechanism controlling melting of the binder phase below the solidus temperature of the cemented carbide must be clarified. This melting mechanism is important for clarifying the interface formation mechanism between cemented carbide and molten cast iron. The expectation is that this clarification will lead to interface microstructure control.

The purpose of this study is to clarify the interface formation mechanism governing contact between cemented carbide and molten cast iron at temperatures below the solidus temperature of the cemented carbide.

2. Experimental

2.1 Materials

The chemical composition of the cast iron used in this study was Fe–4.0 mass%C–2.2 mass%Si–0.03 mass%Mn–0.022 mass%P–0.009 mass%S. Hereinafter, mass% is abbreviated as %. A commercial WC–13.7%Co cemented carbide round bar (outer diameter: 4 mm and length: 50 mm) was used. The roughness of the cemented carbide round bar in the length direction was R_Z 1.9 µm. Moreover, the WC particles were rectangular-shaped with a length of approximately 1 µm. The η phase (W₃Co₃C) and the free C were not observed in the cemented carbide round bar. They are observed when the C content is lower and higher than the stoichiometric content, respectively. The C content of the cemented carbide round bar is considered the stoichiometric content.

2.2 Dipping experiment

The equipment used for the dipping experiment was the same as those employed in previous studies.^6,7) Molten cast iron temperatures of 1473, 1523, 1573, and 1596 K were considered. The WC–Co pseudo-binary phase diagram indicates that the solidus temperature of the cemented carbide round bar is 1593 K.^9–11) The cemented carbide round bar at room temperature was rapidly dipped in the molten cast iron at each of the aforementioned temperatures. However, the depth of the molten cast iron was approximately 25 mm, and hence only part of the cemented carbide round bar was dipped in the molten cast iron. After holding for 30∼300 s, the cemented carbide round bar was pulled up using a motor (speed: 200 mm/min). It took approximately 5 s to pull up the part used for microstructure observation, elemental analysis, and hardness test from the molten cast iron. The dipping time is corrected to the value obtained by adding 5 s to the holding time.

2.3 Microstructure, element analysis, and hardness

The dipped part of the specimens became thicker than the undipped part, and the lower tip was especially thick. Some specimens cracked during cooling, owing possibly to the difference in the coefficient of thermal expansion between the center and the outer circumference of the specimens. The area 10 mm from the lower tip was moderately thick. This suggested that the molten cast iron through the cross-section of the lower tip had no effect on the microstructure of this part. Therefore, microstructure observation and elemental analysis were performed on this part. When dipped in the molten cast iron at 1596 K for 185 s, the specimen was deformed to an S-shape and a part of the outer circumference appeared to be peeled off. This specimen was excluded from the analysis. For microstructure observation using an optical microscope, etching was performed with Marble solution (4 g of copper sulfate, 20 mL of hydrochloric acid, and 20 mL of water). Microstructure observation was also performed via scanning electron microscope (SEM) at an acceleration voltage of 15 kV. Elemental analysis at an acceleration voltage of 25 kV was performed using a field emission electron probe microanalyzer (FE-EPMA).

Specimens used for microstructure observation and elemental analysis were also used for a Vickers hardness test. The hardness was measured in 100∼200 µm intervals in the radial direction from the center of the specimens. The measurements were based on JIS Z2244: 2009, and a test load of 0.9807 N and a holding time of 15 s were employed.

3. Results

3.1 Microstructure observation and elemental analysis

Figure 1 shows the cross-section of the specimens observed with the optical microscope. The homogeneous sintered structure changed to a two-layer structure. The outermost reaction layer observed in a previous study was absent.^6,7) Arrows indicate the boundary between these two layers. With increasing molten metal temperature or increasing dipping time, the boundary moved increasingly toward the center of the specimens. After dipping, the center side of the specimens remained unchanged compared with before dipping. Black spots and white mesh-like patterns were observed on the circumferential side of the specimens. Hereafter referred to as the non-reacted layer and transition layer, respectively. The boundary refers to the region between these two layers.

Fig. 1

Optical micrographs of cross-section of cemented carbide round bar dipped in molten cast iron.

Figures 2(a) and (b) show an optical micrograph and an SEM image, respectively, of the specimen dipped in the molten cast iron at 1473 K for 305 s. In the non-reacted layer, the WC particle spacing was narrow and the space between particles was filled with the binder phase, which also filled the spaces between the WC particles in the transition layer. However, the WC particle spacing in the transition layer was wider than that in the non-reacted layer. The black spots are voids that remained unfilled by the WC particle and the binder phase. These voids are considered shrinkage cavities that appear when the liquid phase solidifies.

Fig. 2

Microstructure of cemented carbide round bar dipped in molten cast iron at 1473 K for 305 s (a) optical microscope and (b) SEM.

Figures 3 and 4 show the FE-EPMA line analysis results of the specimens dipped in the molten cast iron at 1473 and 1596 K, respectively. The analysis (spot diameter: 5 µm and step interval: 5 µm) was performed from the center of the specimen to the circumference. The characteristic X-ray intensity of C was similar to that of W, and the characteristic X-ray intensity of Si was similar to that of Fe. Therefore, C and Si are excluded from the figures. The characteristic X-ray of Mn, P, and S were undetected. Downward triangle marks in the figures indicate the position of the boundary. In the non-reacted layer, the characteristic X-ray intensities of W and Co remained constant, and the corresponding intensity of Fe was undetected. From the boundary toward the circumference, the characteristic X-ray intensities of W and Co decreased, whereas the intensity of Fe increased. At 1596 K, the characteristic X-ray intensity of Co showed a characteristic peak near the boundary (the formation mechanism of this peak is described in a subsequent section).

Fig. 3

FE-EPMA line analysis in radial direction of cemented carbide round bar dipped in molten cast iron at 1473 K.

Fig. 4

FE-EPMA line analysis in radial direction of cemented carbide round bar dipped in molten cast iron at 1596 K.

3.2 Image analysis of cross-section

Figures 5 and 6 show the dependence of the outer diameter d_O and the non-reacted layer diameter d_N on the molten metal temperature and the dipping time, respectively. These dependencies were determined through image analysis of the cross-section observed with the optical microscope. The higher the molten metal temperature, the larger the outer diameter after short-time dipping. The outer diameter increased approximately 11% after 35 s of dipping at 1596 K, whereas the non-reacted layer diameter decreased by approximately 30%. The transition layer thickness δ_T is defined as follows:   

\begin{equation} \delta_{\text{T}} = \frac{d_{\text{O}} - d_{\text{N}}}{2} \end{equation} (1)

The dependence of the transition layer thickness on the molten metal temperature and the dipping time is shown in Fig. 7. The transition layer thickness increased in the same way as the outer diameter, and was approximately 0.8 mm after 35 s of dipping at 1596 K.

Fig. 5

Relationship between dipping time t and outer diameter d_O.

Fig. 6

Relationship between dipping time t and the non-reacted layer diameter d_N.

Fig. 7

Relationship between dipping time t and transition layer thickness δ_T.

3.3 Vickers hardness

Figure 8 shows the Vickers hardness of the specimens dipped in molten cast iron at 1473 K. The hardness before dipping (indicated by the broken line) is also shown in the figure. The hardness, which was measured from the center of the specimen to the circumference, decreased sharply at the boundary. Moreover, compared with the hardness before dipping (1913 HV), the hardness of the non-reacted layer (1899∼1930 HV) was the same, whereas the hardness of the transition layer (1071∼1157 HV) was approximately 40% lower.

Fig. 8

Vickers hardness of non-reacted layer and transition layer of cemented carbide round bar dipped in molten cast iron at 1473 K.

4. Discussion

4.1 Melting mechanism of cemented carbide

As shown in Figs. 3 and 4, elements of the cemented carbide and the molten cast iron diffused in the transition layer. In addition, the WC particle spacing increased and black spots that were considered shrinkage cavities were observed in the transition layer. Therefore, the binder phase was melted in the transition layer even when the molten metal temperature was below the solidus temperature of the cemented carbide. The change in the solidus temperature induced by the mixture of WC–13.7% Co cemented carbide and Fe–C alloys was calculated with the integrated thermodynamic calculation software Thermo-Calc. The concentrations of P and S in the cast iron were extremely small and were therefore excluded from the calculation. Figure 9 shows the solidus temperature T_S corresponding to the mixture of WC–13.7%Co with Fe, Fe–4%C, and Fe–4%C–2.2%Si alloys in various proportions. The solidus temperatures of Fe–4%C and Fe–4%C–2.2%Si were almost the same. In addition to the above alloys, solidus temperatures were also calculated for Fe–1∼3%C and Fe–4%C–2.2%Si–0.5%Mn. The solidus temperature corresponding to the mixture of WC–13.7%Co and Fe–C alloy was <1473 K for C concentrations of 2% or more. In the lowest case, the solidus temperature decreased to 1413 K. The solidus temperature of Fe–4%C–2.2%Si–0.5%Mn was almost the same as that of Fe–4%C–2.2%Si. From the above calculations, the cause of the melting of the binder phase at temperatures lower than the solidus temperature of the cemented carbide is considered to be the lowering of the solidus temperature of the cemented carbide due to the diffusion of Fe and C.

Fig. 9

Change of solidus temperature T_S by mixing WC–13.7%Co with Fe, Fe–4%C, and Fe–4%C–2.2%Si.

The hardness of the transition layer was higher than that of general cast iron and wear resistant cast iron, but was approximately 40% lower than the hardness of the cemented carbide before dipping.¹³⁾ It is considered that the wear resistance is lower than expected for insert castings. However, the presence of the transition layer is considered beneficial as this layer is bonded strongly and is therefore similar to welding.¹⁴⁾

4.2 Melting rate of cemented carbide

The melting that progresses toward the center of the cemented carbide round bar results from dissolution of the binder phase in the molten cast iron. The rate at which a solid metal dissolves in a liquid metal follows the Nernst-Brunner equation (see eq. (2)). The rate equation was derived by modifying eq. (2) under various assumptions.^15–22)   

\begin{equation} n = n_{\text{s}}\left\{1 - \exp \left(- K\frac{A}{V}t \right) \right\} \end{equation} (2)

Where, n is the concentration of the solid metal in the liquid metal at time t, n_s is the saturated concentration, K is the rate constant of dissolution, A is the area where the solid metal and the liquid metal are in contact, and V is the volume of the liquid metal. The applicability of this rate equation to the experiment yielding a thick transition layer was verified.

The following assumptions were made: the solid metal is cylindrical in shape, the radius of the non-reacted layer decreases, a part of the cylinder is dipped, and the amount of binder phase is insufficient for saturating the molten metal. The following expressions were derived, based on these assumptions:²¹⁾   

\begin{equation} Kt = \frac{1}{\sqrt{ab}}\left\{\arctan \left(\sqrt{\frac{b}{a}} r_{0} \right) - \arctan \left(\sqrt{\frac{b}{a}} r \right) \right\} \end{equation} (3)

  

\begin{equation*} a = \frac{\rho_{\text{L}}VC_{\text{s}} - 100\rho_{\text{S}}\pi hr_{0}{}^{2}}{100\rho_{\text{S}}V} \end{equation*}

  

\begin{equation*} b = \frac{\pi h}{V} \end{equation*}

Where, r₀, r, ρ_L, C_s, ρ_S, and h are the initial radius of the cemented carbide round bar, non-reacted layer radius at the dipping time t, density of the molten cast iron, saturated concentration of Co in the molten cast iron, density of the cemented carbide round bar, and length of the dipped portion of the cemented carbide round bar, respectively. For the present experiment where a thick transition layer was formed, ρ_L and V were unclear and hence correction of these terms with the progression of dissolution was necessary. To understand the dissolution process occurring in this experiment, these terms were considered constant and the non-reacted layer radius and the dipping time were varied during the dipping experiment. The corresponding relationship is given as follows:   

\begin{equation} t \propto \arctan r \end{equation} (4)

Figure 10 shows the relationship between the dipping time and arctangent r. A linear relationship occurred at all molten metal temperatures. Furthermore, the time dependence of the non-reacted layer radius is described by eq. (3). Therefore, the Nernst-Brunner equation is applicable in the experimental temperature range even in the melting of cemented carbide where a thick transition layer was formed.

Fig. 10

Relationship between dipping time t and arctan r.

4.3 Increase of outer diameter and occurrence of shrinkage cavity

The increase in the outer diameter is attributed to the wide WC particle spacing and the formation of voids in the transition layer (see Fig. 2). To obtain a dense microstructure in a cemented carbide that is produced by liquid phase sintering, high solubility of the solid phase (WC) in the liquid phase and a small contact angle between the liquid phase and the solid phase (wetting is good) are required.^23,24) These properties lead to strong attraction between WC particles. In the liquid phase sintering of Cr₃C₂–Ni–Cu, the shrinkage rate varied depending on the ratio of Ni and Cu.²³⁾ The level of shrinkage increased with increasing fraction of Ni that can dissolve a large amount of Cr₃C₂. To compare the ease of WC dissolution in liquid Co and liquid Fe–4%C, the saturated concentration of WC was calculated using Thermo-Calc. The temperature was set 30 K higher than the respective solidus temperatures. The saturated concentration differed significantly between the two solvents, 23.7% for liquid Co at 1623 K and 6.7% for liquid Fe–4%C at 1443 K. However, the contact angle of WC with each solvent is 0°.^25,26) In this experiment, the decrease in the solubility of WC is attributed to the increase in Fe concentration of the binder phase. Therefore, the WC particle spacing increased in the transition layer and the outer diameter increased. The occurrence of shrinkage cavities indicated that the capillary force weakened, owing to the increased particle spacing and non-filling of the liquid phase.

4.4 Characteristic X-ray intensity change of Co at the boundary

The diffusion flux is proportional to the concentration gradient, and hence the concentration distribution of the target element generally increases or decreases monotonically. Therefore, explaining the change in the characteristic X-ray intensity of Co at the boundary shown in Fig. 4 based on the diffusion phenomenon is difficult. This change is macroscopic and is considered a kind of segregation. The assumption is that macroscopic flow of the liquid phase, driven by the interfacial energy between the liquid and the solid phases, has occurred. The liquid phase ratio in WC–13.7%Co calculated using Thermo-Calc is approximately 4.3% at 1596 K. The solid phase, which occupies most of the volume of the material, limits the movement of the liquid phase. This state is equivalent to liquid phase sintering in powder metallurgy.

In liquid phase sintering, liquid phase migration (LPM) occurs when the flow of liquid phase Co minimizes the interfacial energy even after the voids in the sintered body are filled.^27–29) The flow of liquid phase Co via LPM in cemented carbide occurs along the following directions:²⁹⁾

1. (1)    From coarse WC particles to fine WC particle
2. (2)    From the higher concentration to lower concentration of C dissolved in liquid phase Co
3. (3)    From higher liquid phase ratio to lower liquid phase ratio

The WC particle size of the specimens remained approximately constant, indicating that LPM induced by the particle size played no role in this experiment. However, the C concentration in the liquid phase Co at the boundary is lower than that of the transition layer in which Fe and C have already diffused from the molten cast iron (and melted). In addition, the boundary is where melting begins, so the liquid phase ratio is the lowest in the transition layer. Due to these two factors, the liquid phase Co moved from the outer circumference to the center by LPM and concentrated on the boundary. Consequently, a characteristic peak was observed for the characteristic X-ray intensity of Co. Figure 11 shows the boundary and the transition layer observed via SEM. The space between WC particles is filled with the binder phase. Moreover, the WC particle spacing at the boundary was wider than that of the transition layer and the proportion of the binder phase indicated that Co is concentrated on the boundary, as previously described.

Fig. 11

SEM micrograph of boundary of cemented carbide round bar dipped in molten cast iron at 1596 K for 125 s.

5. Conclusion

In this experiment, the interface formation mechanism of a cemented carbide round bar dipped in molten cast iron at 1473∼1596 K, which is lower than the solidus temperature of the cemented carbide, was clarified. This clarification was achieved by elucidating the melting mechanism occurring at the interface. In addition, we discussed the melting rate of cemented carbide, the increase in the outer diameter, the occurrence of shrinkage cavities, and the characteristic enrichment of Co at the boundary. The conclusions of this work are summarized as follows:

1. (1)    The microstructure of the cemented carbide round bar dipped in the molten cast iron changed to a two-layer structure consisting of a non-reacted layer at the center side and a transition layer at the outer side. The outer diameter and the transition layer thickness increased with increasing dipping time.
2. (2)    In the molten cast iron with C concentration of 2% or greater, the solidus temperature of the binder phase decreased to <1473 K due to the diffusion of Fe and C. The binder phase melted even below the solidus temperature of cemented carbide.
3. (3)    The progression of melting toward the center of the cemented carbide round bar is considered equivalent to the dissolution phenomenon of a solid metal in a liquid metal. The change in the non-reacted layer radius is expressed by the rate equation derived from the Nernst-Brunner equation.
4. (4)    Owing to LPM and the decrease in WC solubility of the liquid binder phase resulting from the increase in Fe concentration of the liquid binder phase, the WC particle spacing increased in the transition layer. The hardness decreased, the outer diameter increased, and shrinkage cavities were formed.
5. (5)    The liquid phase Co flows toward the center of the specimens by the LPM, driven by the difference in the liquid phase ratio and C concentration in the binder phase. Therefore, Co was concentrated on the boundary and the characteristic X-ray intensity change was observed.

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