How is Ultra-Coarse Grain Cemented Carbide for Geological and Mining Applications Developed in Meetyou Carbide®?

Ultra-coarse grain cemented carbide for geological and mining use has emerged as a key research direction in recent years. Compared with traditional geological and mining cemented carbides, it boasts comparable performance indicators, a slightly lower cobalt content, coarser grains, and superior wear resistance and impact toughness. Currently, ultra-coarse grain cemented carbide has demonstrated significant advantages in its unique application fields such as coal mining and road construction. The classification of coarse grain sizes references standards from domestic and foreign manufacturers: medium-coarse grain: 2.0–2.4μm, coarse grain: 2.4–3.2μm, ultra-coarse grain: 3.2–5.0μm, and extra-coarse grain: >5.0μm.

 

Failure Analysis of Geological and Mining Cemented Carbides

Geological and mining cemented carbides operate in harsh environments with mixed hard and soft rock formations that are complex and variable, leading to diverse forms of scrapping.

1. Poor Toughness

Impact fragmentation occurs, mainly caused by excessive tensile and shear stresses acting on the carbide (see Figure 1).

This necessitates improving the flexural strength and toughness of the carbide by increasing its cobalt content and grain size. The flexural strength and toughness of cemented carbides are not entirely equivalent—high flexural strength does not necessarily mean good impact toughness. Although the flexural strength of ultra-coarse grain cemented carbides is not high and is basically comparable to that of carbides with the same grade of cobalt content, their toughness is significantly improved. For example, an ultra-coarse grain cemented carbide with 8% cobalt content can achieve toughness levels similar to those of carbides containing 10% or even 11% cobalt.

2. Abrasive Wear

During operation, the binder phase Co is first worn away, followed by the fragmentation and shedding of WC—this is the common issue of poor wear resistance.The relationship between the hardness and wear resistance of cemented carbides is also not entirely equivalent. The wear resistance of carbides is closely related to the wear mode during use.

Carbides used in rotary drilling, such as geological exploration bits and roller bits, mainly undergo grinding during operation, with abrasive wear as the primary form—higher carbide hardness corresponds to better wear resistance. In percussion drilling, thermal fatigue effects and plastic deformation are the main forms. Due to their lower hardness and coarser grain size, coarse grain cemented carbides are more conducive to improving resistance to thermal fatigue and thermoplastic deformation.

3. Impact Fatigue Cracks

During operation, carbides are subjected to high-frequency impacts. Even if the impact stress does not reach the strength limit of the cemented carbide, repeated impacts can cause fatigue cracks. The propagation of these cracks leads to the fragmentation and shedding of the carbide (see Figure 2).

Under the action of repeated impact stresses, inherent defects in the carbide itself—such as pores, cavities, Co pools, graphite inclusions, and excessively large coarse grains—will accelerate the growth of fatigue cracks, which spread from local areas to the entire structure.

4. Thermal Fatigue

During operation, the impact and grinding between the carbide and rock generate heat. High temperatures can reduce wear resistance accordingly. Due to the significant difference in thermal expansion coefficients between the binder phase Co and WC, Co has lower thermal conductivity than WC.

The inconsistency in thermal expansion and contraction rates and timing—especially the repeated alternation of local instantaneous high and low temperatures—easily causes shrinkage changes at the contact interface between Co and WC, leading to microcracks. These microcracks extend and propagate inside the carbide along the WC-Co interface and Co phase, forming thermal fatigue cracks, commonly known as “crazing” or “snake skin” (see Figure 2).

These cracks spread from the surface to the depth, resulting in carbide failure. Reducing Co content, increasing grain size, and improving thermal conductivity can effectively reduce the thermal fatigue effect of carbides.

5. Plastic Deformation

At lower drilling speeds, fine grain carbides experience less wear than coarse grain ones. At high drilling speeds, however, coarse grain carbides have higher wear resistance than fine grain ones. Under high temperature and pressure conditions, fine WC grains undergo grain sliding, leading to plastic deformation.

 

When the grain sizes are comparable, carbides with lower cobalt content have better wear resistance than those with higher cobalt content. Even when the hardness is comparable, carbides with lower cobalt content still outperform those with higher cobalt content in wear resistance. Sometimes, even if low-cobalt carbides have slightly lower hardness than high-cobalt ones, they may still exhibit better wear resistance. By reducing the cobalt content while increasing the grain size, the carbide can not only maintain sufficient toughness but also achieve good wear resistance.

Advantages of Low-Co Ultra-Coarse Grain Cemented Carbides

Coarse grain carbides can enhance impact resistance, namely improving the strength and toughness of the carbide, which can compensate for the decrease in toughness caused by reduced cobalt content.

Low-cobalt carbides can increase hardness and improve resistance to thermal fatigue and impact fatigue. This is because WC has better thermal conductivity than Co, making low-cobalt carbides more conducive to heat transfer.

 

Dissection of Foreign Samples

The application of coarse grains in foreign geological and mining carbides has demonstrated significant advantages. Below are the dissection and analysis reports of coarse grain products from some well-known foreign companies:

Table 1 Dissection Analysis of Foreign Ultra-Coarse Grain Alloys

Foreign Alloy Sample	Measured Co Content (%)	Physical and Mechanical Properties	WC Average Grain Size (μm)
		Coercive Force (KA/m)	Hardness (HRA)	Density (g/cm³)	Co-magnetism (Co-m)
Sample 1	6.5	5.1	87.5	14.80	5.8	4.0
Sample 2	9.7	4.4	86.1	14.54	9.3	4.2

 

Development of Ultra-Coarse Grain Cemented Carbides by Our Company

Process Route

The company adopts the advanced process of “PEG spray granulation → precision automatic pressing → pressure sintering” to produce carbide products.

High-purity WC raw materials are selected, with PEG as the binder. PEG is easily volatile during sintering and leaves no residue in the carbide.

Spray drying granulation reduces oxidation and contamination of materials during the drying process. The resulting mixed granules have good fluidity and fillability, stable bulk density, high purity, and excellent quality.

Precision pressing by automatic presses effectively ensures consistent weight and dimensions of products, achieving high product precision.

Pressure sintering forms a denser microstructure of the carbide under high temperature and pressure. This minimizes and reduces pores and Co pools in the carbide, improving its density and flexural strength.

Typical Values of Main Alloy Properties and Metallographic Structure

Table 2 Typical Values of Ultra-Coarse Alloy Properties

Alloy Grade	Co Content (%)	Physical and Mechanical Properties	Porosity	Non-Carbide Carbon	WC Average Grain Size (μm)	Co Layer Thickness (μm)
		Coercive Force (KA/m)	Hardness (HRA)	Density (g/cm³)	Bending Strength (N/mm²)	Co-magnetism (Co-m)	Type A	Type B	Type C
DD05SC	6	5.1	87.5	14.80	2520	5.6	A02	B00	C00	4.0	1~3
DD10SC	8	4.9	86.8	14.60	2600	7.6	A02	B00	C00	4.0	1~3
DD30C	10	4.5	86.2	14.45	2750	9.7	A02	B00	C00	4.0	2~3

Application Effects

1 Flat chisel bits of ø40 were fabricated using brazed inserts of DD05SC, DD10SC, YG9C, and YG11C for comparative experiments. The test results show that the service life of DD05SC and DD10SC is significantly superior to that of the traditional grades YG9C and YG11C.

Table 3 Brazed Insert Test Results

Alloy Grade	Rock Property	Number of Bits (pieces)	Total Drilling Depth (m)	Average Drilling Depth (m)	Scrap Form
DD05SC	f=10~14 Granite	5	262	52.4	Normal Wear
YG9C	f=10~14 Granite	5	223	44.6	Normal Wear
DD10SC	f=12~16 Feldspar Porphyry	5	151	30.2	Normal Wear
YG11C	f=12~16 Feldspar Porphyry	5	118	23.6	Normal Wear

2 Road milling teeth made of DD05SC alloy were exported overseas and tested alongside products from a renowned foreign company on asphalt pavements. Their service performance was basically equivalent, while the service life of road milling teeth made of YG8C alloy was significantly insufficient.

3 A domestic coal mine frequently encountered gangue and rocks during coal cutting. YG13C alloy lacked wear resistance, and YG11C alloy often suffered from fragmentation. After adopting our company’s DD30C coarse-grain alloy, the consumption of coal cutting picks per 10,000 tons of coal decreased from 68 pieces to 11 pieces, meeting the user’s needs. Compared with imported products, it boasts high cost-performance ratio.

 

Conclusion

Referring to advanced processes and technologies from domestic and foreign manufacturers and combining with our company’s actual conditions, we have developed and produced a series of ultra-coarse grain cemented carbide grades. The physical and mechanical properties as well as metallographic structure of these alloys are basically consistent with those of foreign products. Since January 2005, our company’s ultra-coarse grain coal cutting picks, brazed inserts, and road milling teeth have been successively launched on the market, gaining widespread recognition and praise from users.