Submicron cemented carbide attains “double-high” (simultaneously enhanced hardness and strength) performance thanks to its nanometer-level grain size. Yet, compared with conventional cemented carbide, its finer grains lead to problems like high composite powder activity, agglomeration, easy contamination, and severe grain coarsening during sintering. The fine, active raw material powder is prone to oxidation, making precise carbon control difficult and product performance unstable—so carbon content control is key to the quality of submicron cemented carbide preparation.

To produce high-performance cemented carbide, two issues need addressing: inhibiting WC grain growth during sintering to meet grain size requirements, and ensuring a normal alloy structure. Carbon content greatly affects the alloy’s phase composition and properties: it influences sintering temperature, liquid phase amount, and WC grain growth. Abnormal η phase formation (due to carbon deficiency, causing brittleness) or free carbon (from excess carbon) both reduce physical and mechanical properties.

Microwave sintering, a widely used new process, heats materials uniformly through microwave-matrix coupling and dielectric loss, with low temperature, short time, and effective inhibition of grain growth. However, it has decarburization issues. This paper explores how changes in carbon content affect the microstructure and properties of submicron cemented carbide synthesized via microwave sintering.

 

Experiment

Sample Preparation

Performance Testing and Characterization

The density of the sample was measured by the water displacement method, the hardness of the sample was tested using a Rockwell hardness tester, the bending strength of the sample was tested on a material mechanical testing machine, the total carbon content in the sample was measured by the gas volumetric method, and the content of free carbon in the sample was determined by the HF acid method.

The fracture morphology was observed under a scanning electron microscope, and the microstructure morphology was observed under an optical microscope. Then the sample was polished, and the sample was corroded with a saturated solution of ferric chloride and hydrochloric acid. Finally, the microstructure was analyzed with a field – emission scanning electron microscope.

 

Experimental Results and Analysis

Characteristics of Alloy Decarburization Phenomenon in Microwave Sintering

As we all know, for cemented carbide, C is a sensitive parameter, and the fluctuation range of carbon content in the normal two – phase region of cemented carbide is very narrow. Carbon (existing in the combined state) is the key to determining the performance of cemented carbide. Carbon exists in two forms in cemented carbide: free carbon and combined carbon. Free carbon is generally the raw material carbon that does not form an interstitial phase with metal atoms; in contrast, combined carbon refers to the co – lattice compound of the interstitial phase in physics, which exists in the atomic state.

The internal structure of normal cemented carbide should only contain WC phase and γ phase, and any third phase will deteriorate the performance of the alloy. After calculation, the carbon content in the two – phase region of YG8 is about 5.58% – 6.12%. Chemical analysis of the microwave – sintered alloy found that when the total carbon content is 9.28% – 9.71%, the combined carbon content in the alloy is close to the homogeneous phase region (WC + γ), as shown in Table 1. This also confirms that there is indeed a serious decarburization phenomenon in microwave – sintered cemented carbide.

 

Table 1 Changes in carbon content of sintered samples with a total carbon addition of 9.71% before and after sintering %

Treatment state	Total carbon content	Combined carbon	Free carbon	Added carbon
Before sintering	9.71	5.37	0.04	4.30
After sintering	6.86	5.93	0.93

 

Influence of Different Carbon Contents on Alloy Density

Figure 1 shows the density change of microwave-sintered YG8 cemented carbide with different total carbon contents. Under the condition of constant composition, the alloy density is related to the η phase and free carbon in the alloy. In the WC+γ+η three-phase region, as the carbon content decreases, the η phase increases, the Co content decreases, the W content in the γ phase increases, and the density increases.

In the WC+γ two-phase region, as the carbon content decreases, the W content in the γ phase increases and the density increases, that is, the density decreases as the carbon content increases. In the WC+γ+η three-phase region, as the carbon content increases, the free carbon increases (free carbon is called “Type C” pores), and the density decreases.

As can be seen from Figure 1, the alloy density decreases with the increase of total carbon content. When the carbon content is low, decarburized η phase is formed in the sintered sample, the Co content decreases, the W content in the γ phase increases, and the η phase, as a hard and brittle phase, has a relatively high density, resulting in a high alloy density.

When the total carbon content is in the range of 9.28%~9.71%, chemical analysis shows that the carbon content in the alloy falls within the scope of the WC+γ two-phase region, as presented in Table 1. At this point, the microwave-sintered alloy precipitates a normal structure (WC+γ). As the carbon content decreases, the W content in the γ phase increases and the alloy density rises, which means the density decreases as the carbon content increases. With a further increase in the total carbon content, free carbon appears in the alloy and the porosity increases, leading to a further decrease in the alloy density.

Influence of Different Carbon Contents on Alloy Hardness

Figure 2 shows the change of hardness of microwave-sintered YG8 cemented carbide with total carbon content. It can be seen from the figure that the hardness decreases sharply first and then increases slowly with the increase of carbon content, reaching the minimum when the carbon content is 9.28%. This is because when the total carbon content is 8.84%, there is a hard and brittle decarburized η phase in the sintered sample, which is equivalent to a decrease in Co content and refines the WC grains, thus resulting in higher hardness. With the increase of carbon content, the appearance of free carbon (i.e., the appearance of “Type C” pores) leads to a small change in the hardness reduction.

Figure 3 Relationship Curve between Total Carbon Content and Bending Strength (The abscissa is total carbon content x/%, and the ordinate is bending strength / MPa)

Influence of Different Carbon Contents on Alloy Bending Strength

Figure 3 is the relationship curve between the bending strength of microwave-sintered cemented carbide and total carbon content. It can be seen from the figure that the bending strength of the alloy first increases and then decreases with the increase of total carbon content. The performance analysis of the strength when the total carbon content is 9.28% shows that the bending strength is relatively high when the carbon content is 9.28% and 9.71% (Figures 4b and 4c). The structure is relatively uniform, the grains are fine, and there is little free carbon and porosity, which is basically consistent with the obtained law of bending strength.

Comparative Analysis of Microwave Sintering and Conventional Sintering

Conventional sintering experiments were carried out on samples with industrial standard carbon content, numbered A. The results of the conventional sintered Group A samples were compared with those of the microwave-sintered samples with a total carbon content of 9.71% (Group c), as shown in Table 2.

Table 2 Comparison of Properties between Conventionally Sintered and Microwave-Sintered YG8 Cemented Carbide

Table 2 Comparison of Properties between Conventionally Sintered and Microwave-Sintered YG8 Cemented Carbide

Sample No.	Density / (g・cm⁻³)	Hardness (HRA)	Bending Strength / MPa
A	14.8373	90.6	1222.84
C	12.0170	91.1	964.90

It can be seen from Table 2 that although the microwave-sintered samples have higher hardness, their density and strength are lower. This may be due to the serious decarburization that occurs during the microwave sintering of cemented carbide, making it difficult to strictly control the carbon content in the alloy. Moreover, during the decarburization process, a series of physical and chemical changes will occur, which have a certain impact on the microstructure of the alloy and reduce the bending strength of the alloy. These are all important issues that need further research.

The fractures of conventionally sintered and microwave-sintered samples were compared, as shown in Figure 5. The grains of the conventionally sintered samples are coarse, and the phenomenon of abnormal grain growth is obvious, with some grains reaching 10 μm or even larger. However, the grains are dense and the binder phase is uniformly coated. In contrast, the microwave-sintered samples have fine grains, which is conducive to obtaining higher hardness. However, there are more pores and uneven distribution of the binder phase, resulting in lower density and strength of the samples than those of the conventionally sintered ones, which is consistent with the performance analysis.

Although microwave sintering can inhibit the grain growth process and obtain fine grains due to its rapid sintering characteristics, its performance is relatively low. This may be caused by the different interactions between WC, Co, C and microwaves, which have an adverse effect on the γ phase.

Conclusions

Microwave sintering can effectively inhibit the growth of WC grains during the sintering process.(2) In the microwave sintering of cemented carbide, the growth trend of WC grains increases with the increase of total carbon content, and reducing the carbon content has a certain effect on inhibiting the growth of WC grains.(3) Carbon content also has an impact on the binder phase in cemented carbide. Using an appropriate total carbon content (9.28%~9.71%) can obtain relatively high mechanical properties.(4) At present, the mechanical properties of microwave-sintered cemented carbide are not ideal due to the difficulty in controlling C, partial agglomeration of Co and relatively high porosity. However, it has a good research prospect.