Cobalt phase and WC hard phase are included in WC powder, and the toughness is mainly provided by the Co phase. Heat treatment can change the morphology of WC phase, the proximity of grains, and the distribution of internal stress. More importantly, it can change the structure and composition of Co phase (such as inhibiting the martensitic transformation of Co phase and increasing the solid solubility of W atoms in Co phase), thereby enhancing the strength and toughness of the alloy.

There are two crystal forms of Co phase: α-Co (fcc structure with 12 slip planes, good toughness) and ε-Co (hcp structure with 3 slip planes, poor toughness). Determining the proportion and variation law of the two is conducive to developing strengthening and toughening processes, improving the performance of cemented carbide and increasing its service life.

The Co bonding phase in the as-sintered cemented carbide is mainly ε-Co, and quenching can increase the content of α-Co. However, relevant studies have not investigated the influence of the relative content of the two on the alloy performance during the sample preparation process.

In this sharing, the method of electrolytic corrosion combined with X-ray diffraction is used to determine the cobalt phase structure of common grades of cemented carbide under different surface treatment conditions, so as to provide a reference for formulating appropriate heat treatment systems.

 

Experiment

According to the cobalt content and the particle size of WC raw material powder, common grades of alloys in the company (see Table 1) were selected to prepare samples. The cobalt phase composition after surface grinding (with a grinding depth of 2mm), grinding and polishing was analyzed and compared with the as-sintered samples. Here, in order to reduce the masking effect of WC phase on the diffraction peak of Co phase, the electrolytic corrosion method was adopted to remove an appropriate amount of WC on the alloy surface.

The process conditions and methods of electrolytic corrosion are as follows: The electrolyte contains 4 mol/L NaOH solution, 3% (mass fraction) C₄H₆O₆, and 2% (volume fraction) HCIO₄. An industrial DC power supply is used with a voltage of 1.8V. The anode is a cemented carbide block, and the cathode is a copper sheet. When the electrolytic corrosion reaches a certain degree, the cemented carbide block is taken out, cleaned, and dried for X-ray diffraction analysis until the diffraction peak intensity of the Co phase is equivalent to that of the WC phase.

The XRD diffractometer model is Panalytical X’Pert PRO with a Co target. The working voltage is 40kV and the current is 40mA. The divergence slit is 1°, the receiving end is an X’Celerator super detector, and the anti-scattering slit is 6.6 mm. The scanning step size is 0.033°, and the dwell time per step is 10s.

 

Results and Analysis

Effect of Electrolytic Corrosion

Figure 1 shows the XRD results of the as-sintered WCCo20 alloy surface before polishing and after polishing followed by electrolytic corrosion. A comparison of the X-ray diffraction patterns before and after electrolytic corrosion reveals that the diffraction peaks of WC are almost undetectable in the X-ray diffraction pattern after electrolytic corrosion. This indicates that within the depth detectable by X-rays, WC has been almost completely corroded away from the cemented carbide matrix.

Figures 2 and 3 show the electron micrograph and energy dispersive spectroscopy (EDS) results of the WCCo20 alloy after electrolytic corrosion, respectively. It can be clearly seen from Figure 2 that the electrolytic corrosion has removed the WC grains from the alloy surface, leaving a Co layer with triangular and square cavities that correspond to the morphology of the original WC grains.

According to the EDS results in Figure 3, the Co content on the alloy surface after electrolytic corrosion is 81.26%, while the W content is only 2.53%. Considering that a certain amount of W atoms are solid-dissolved in the Co phase, it can be concluded that the WC on the alloy surface has been almost completely removed.

It should be noted that different grades of alloys vary in WC particle size and Co content, so the difficulty of completely removing WC grains from the alloy surface via electrolytic corrosion also differs. For XRD analysis, it is sufficient that the diffraction intensities of the Co phase and WC phase on the alloy surface are comparable after electrolytic corrosion.

Analysis of Cobalt Phase Structure

Figure 4 shows that XRD analysis of the cobalt phase on the surface of alloys with different particle sizes and cobalt contents indicates the presence of ε-Co phase in all alloys, among which the high-cobalt WCCo20 alloy has the highest content of hcp-structured cobalt phase. XRD analysis of the cobalt phase on the fracture surface (Figure 5) reveals that the cobalt phase of all alloy grades is basically of fcc structure.

After removing an appropriate amount of WC grains from the fracture surface by electrolytic corrosion (Figure 6), the cobalt phase of WCCo10 and WCCo15 alloys is mainly fcc structure, while WCCo20 contains only a small amount of hcp-structured cobalt phase with fcc as the dominant structure. The above results indicate that after the alloy undergoes surface grinding, lapping, and polishing (i.e., the metallographic sample preparation process), part of the cobalt phase structure on the alloy surface transforms from face-centered cubic (fcc) to hexagonal close-packed (hcp) due to stress or temperature-induced phase transformation mechanisms.

Moreover, the higher the cobalt content, the easier this transformation occurs. It can be seen from Figure 1 that for the polished surface of WCCo20 alloy, even after all surface WC grains are removed, there are still a considerable amount of hcp cobalt phases in the alloy, indicating that the influence of the cobalt phase transformation caused by the metallographic preparation process persists.

Analysis of the cobalt phase structure in the fracture (i.e., the actual interior of the alloy) shows that the cobalt phase structure of different grades of cemented carbide produced by our company is mainly face-centered cubic (fcc) structure. This conclusion differs from the results in relevant literatures [1, 3-6], which may be due to the significant influence of different treatment methods (polishing, fracture, fracture after electrolytic corrosion, etc.) on XRD analysis results.

 

 

When cemented carbide is slowly cooled from the sintering temperature, the cobalt phase can undergo fcc→hcp crystal transformation in a diffusion manner. The starting temperature of the diffusion-type Co phase transformation is higher than that of pure Co (about 1000K). Since the diffusion coefficient of W atoms in the Co phase is very small, the fcc→hcp transformation rate is very low, so the cobalt phase structure of the alloy at room temperature is mainly fcc structure.

At room temperature, the cobalt phase undergoes a non-diffusional martensitic transformation, with its Mₛ (martensite start) temperature above and close to room temperature. Below the Mₛ temperature, any technological process that causes changes in internal stress (enhancement or relaxation) between the WC phase and cobalt phase in WC-Co cemented carbide can lead to an increase in the hcp-structured cobalt phase.

Therefore, the conventional metallographic preparation process will promote the fcc→hcp crystal transformation of the cobalt phase. When the WC particle size in the alloy increases and the Co content rises, due to the increase in the thickness of the Co layer and the diffusion distance of W atoms, the uniformity of distribution in the Co layer decreases. When the internal stress of the alloy changes, it is easier to form hcp-structured crystal embryos, resulting in an increase in the hcp-structured cobalt phase.

 

Conclusions

1.Electrolytic corrosion can effectively remove WC grains on the alloy surface, thereby reducing the masking effect of the WC phase on the diffraction peaks of the Co phase during X-ray diffraction analysis.

2.Different from the descriptions in relevant literatures, the Co phase of common grades of alloys produced by our company is mainly of face-centered cubic structure.

3.The metallographic preparation process will cause part of the Co phase structure to transform from face-centered cubic to hexagonal close-packed structure.