Chapter 3: Physical and Chemical Properties of Cemented Carbide
Tungsten Cemented Carbide has become the core material for cutting tools, wear-resistant parts, mining equipment, aerospace, chemical equipment, nuclear power facilities and deep-sea devices due to its excellent physical and chemical properties. These properties include mechanical properties (hardness, toughness, compressive strength, flexural strength), thermal properties (thermal conductivity, thermal expansion coefficient, high temperature stability, thermal shock resistance), chemical stability (corrosion resistance, oxidation resistance) and electrical and magnetic properties (conductivity, Co phase magnetism), which are derived from the covalent bond rigidity of tungsten carbide (WC) and the plastic synergy of cobalt ( Co) or nickel (Ni) bonding phase.
This chapter analyzes each physicochemical property, inspection and testing methods, factors affecting various aspects of performance, and extended application cases, and deeply explores the theoretical basis, testing technology, regulation mechanism, environmental adaptability, and working condition performance. Through detailed theoretical models, precise experimental data, rich cross-domain cases, and performance impact analysis, this chapter reveals the inherent mechanism of physicochemical properties, providing a comprehensive reference for performance optimization, extreme working condition adaptation, and interdisciplinary research of cemented carbide.
3.1 Mechanical properties of cemented carbide
Mechanical properties are the cornerstone of cemented carbide’s reliability in high load, impact, wear and complex stress environments, and are widely used in cutting, mining, stamping, aviation and deep-sea drilling. This section analyzes hardness, toughness, compressive strength and bending strength one by one, and adds new inspection and testing methods and influencing factors.
3.1.1 Hardness of cemented carbide
Hardness refers to the ability of a material to resist external objects from pressing into or scratching its surface, and is one of the important indicators for measuring the mechanical properties of a material. Hardness generally reflects the strength, wear resistance and deformation resistance of a material, and is often characterized by its ability to resist compression, shear or plastic deformation. Common measurement methods include Brinell hardness (HB), Rockwell hardness (HR), Vickers hardness (HV) and Shore hardness ( HS), and their values are calculated based on the type of indenter (such as steel ball or diamond), load size and indentation area (ISO 6507, ASTM E10). For example, the Vickers hardness of cemented carbide is usually HV 1200-2400, depending on the grain size and bonding phase content. Hardness is closely related to the material microstructure (such as grain size, phase composition) and heat treatment, and is a key basis for selecting materials and evaluating durability.
The Vickers hardness (HV) of cemented carbide is 1500 – 2500 ± 30, far exceeding high-speed steel (HV 800 – 1000), ceramics (HV 1200 – 1800) and titanium alloys (HV 300 – 400), which is the core of its wear resistance. The hardness comes from the covalent bond network of WC (WC bond energy 6.0 eV ± 0.2 eV, bond length 2.0 Å ± 0.05 Å ), and its hexagonal crystal structure (P6m2, Young’s modulus 700 GPa ± 10 GPa ) provides resistance to plastic deformation. The hardness of cemented carbide containing 10% Co is HV 1800 ± 30, and that of cemented carbide containing 20% Co drops to HV 1400 ± 30, because the softness of the Co phase (HV 300 – 400, face-centered cubic FCC structure) reduces the compressive strength.
cemented carbide hardness is outstanding.
At 600°C, the hardness of cemented carbide containing 6% Co maintains HV 1500±30, drops to HV 1200±20 at 800°C, and drops to HV 1000±30 at 1000°C, which is better than high-speed steel (dropped to HV 500 at 600°C) and ceramic (dropped to HV 800 at 1000°C). Adding Cr ₃ C ₂ (0.5%1%) increases the hardness to HV 1900 – 2200±50 through solid solution strengthening (Cr atomic radius 1.28 Å , lattice strain <2%±0.2%) . For example, a cemented carbide tool (HV 1900) containing 8% Co and 0.5% Cr ₃ C ₂ has a wear amount of <0.1mm±0.02mm and a life of 15 hours±1 hour in high-speed cutting of stainless steel (tensile strength>1000 MPa, cutting speed 200 m/min, friction coefficient<0.3±0.05), which is better than ceramic tools (life<5 hours, wear amount>0.3mm).
The impact of the environment on hardness requires attention.
In a hot and humid environment (40°C, 90% humidity, 168 hours), micro-corrosion of the Co phase is induced (weight loss <0.1 mg/cm² ± 0.02 mg/cm² , corrosion depth <1μm±0.2μm), and the hardness decreases by <2%±0.5%; in an extremely cold environment (40°C), the hardness increases slightly by 1%±0.3% due to the embrittlement of the Co phase (plastic strain decreases by <3%±0.3%); in high pressure (>100 MPa, 5000 m deep sea), there is no significant change (decrease <0.5%±0.1%); radiation (nuclear power, 10⁴Gy , γ-rays) induces point defects, and the hardness decreases by <1%±0.2%.
Ni-containing cemented carbide (12% Ni, HV 1700±30) is more stable in marine environment (salinity 3.5%, pH 8, Cl⁻19 g/L), with a hardness drop of <1%. In practical applications, cemented carbide rollers containing 6% Co have a wear depth of <0.05mm±0.01mm and a lifespan of >300 hours±20 hours in mining (granite, rock hardness>1000 MPa, impact frequency>1000 times/min), which is better than high-speed steel (lifespan<50 hours).
Optimizing hardness requires a trade-off with toughness.
Adding TiC (10%15%, hardness 20 GPa±1 GPa ) increases the hardness to HV 2000±50, while reducing the density (to 12 g/cm³ ± 0.1 g/cm³ ) , which is suitable for lightweight aviation parts, such as turbine blade molds (load>2000 MPa, deformation tolerance<0.01mm). In the composite stamping (600°C, frequency>10 ⁴ times/hour), the surface roughness of the cemented carbide mold containing 15% TiC is Ra<0.1μm±0.02μm, and the life is increased by 40%±5%, which is better than steel molds (Ra>0.5μm, life<2000 hours). Cross-domain comparison shows that the hardness of cemented carbide is better than high-strength steel (HV 600800) and titanium alloy (HV 300400), but inferior to diamond (HV >8000).
3.1.1.1 Hardness expression method
There are many ways to express hardness, suitable for different test scenarios and material comparisons:
Vickers hardness ( HV)
Vickers hardness (HV) is a standard method for accurately measuring the hardness of materials. It uses a diamond pyramid indenter (vertex angle 136°) to press into the material surface under a specified load (usually 5-100 kgf , the range can be extended to 1-120 kgf ), and after holding for 10-15 seconds, the diagonal length of the indentation (d, unit mm) is measured to calculate the hardness value. The formula is: HV = 1.8544 × F / d ² , where F is the applied load ( kgf ), d is the average diagonal length of the indentation (mm), and the result is expressed in kgf / mm ² , which is usually directly marked as the HV value (ISO 6507-1:2018).
Vickers hardness is applicable to a variety of materials, including cemented carbide, steel, ceramics and thin layer materials, because of its small indentation (diameter 0.01-1 mm), high accuracy (error <5%) and wide test range (HV 10-3000+). For example, the HV of conventional cemented carbide is 1200-2000, while that of nano-scale cemented carbide (grains 0.05-0.2 μm ) can reach 2000-2400 (Journal of Materials Science 2025). Its advantage is that it can test tiny areas (such as coatings) or thin sheets (thickness>0.1 mm), but the test time is long and the surface flatness requirements are high (Ra<0.8 μm , ITIA 2024).
Rockwell hardness (HRC/HRB)
Rockwell hardness (HRC/HRB) is a common method for measuring the hardness of materials. The indenter (diamond cone or steel ball) is pressed into the material under the initial load (10 kgf ) and the total load (60-150 kgf ), and the hardness value is determined by measuring the difference in indentation depth (ASTM E18-22). It is characterized by fast testing, small indentation, and simple operation, and is widely used in metal materials.
HRC : Use a diamond cone indenter (vertex angle 120°) with a total load of 150 kgf , suitable for high hardness materials (such as hardened steel, cemented carbide). The HRC value is calculated by the formula: HRC = 100 – (h/0.002), where h is the indentation depth (mm). The HRC range is usually 20-70, for example, cemented carbide HRC 60-70, equivalent to Vickers hardness HV 1200-2400.
HRB : Use a 1/16 inch steel ball indenter with a total load of 100 kgf , suitable for softer materials (such as annealed steel, copper alloys). The HRB value calculation formula is: HRB = 130 – (h/0.002), ranging from 20-100. For example, the HRB value of annealed steel is about 70-90 (ITIA 2024).
The advantage of Rockwell hardness is that it can be read directly with high accuracy (error <3%), but it is not suitable for thin materials or small areas (ISO 6508-1:2016).
Brinell hardness (HB)
Brinell hardness (HB) is a method of measuring the hardness of a material by pressing a hardened steel ball or tungsten carbide ball indenter (usually 2.5, 5 or 10 mm in diameter) into the material surface under a specified load (F, usually 500-3000 kgf ), holding it for a certain time (10-30 seconds), and then measuring the indentation diameter (d, in mm) to calculate the hardness. The formula is: HB = (2F) / (πD(D – √(D ² – d ² ))), where D is the indenter diameter (mm), F is the load ( kgf ), d is the indentation diameter (mm), and the result is expressed in kgf / mm ² (ASTM E10-18).
Brinell hardness is suitable for softer or medium hardness materials (such as steel, cast iron, non-ferrous metals), and the range is usually HB 30-650. For example, the HB of unhardened steel is about 120-200, while cemented carbide is usually not suitable for this method due to its high hardness (HV 1200-2400) (ITIA 2024). Its advantage is that the indentation area is large, reflecting the average performance of the material, and it is suitable for non-uniform materials, but not for thin parts or high hardness materials (the error is about 3%-5%, ISO 6506-1:2014).
Mohs hardness
Mohs hardness is a relative scale for qualitatively measuring the hardness of materials. It was proposed by German mineralogist Friedrich Mohs in 1812 to assess hardness by comparing the material’s ability to resist scratching. The method uses 10 standard minerals as the benchmark, with hardness graded from 1 (softest) to 10 (hardest): 1-talc, 2-gypsum, 3-calcite, 4-fluorite, 5-apatite, 6-orthoclase, 7-quartz, 8-topaz, 9-corundum, 10-diamond. During the test, scratch the surface of the material with a standard sample. If a mark can be left, the material’s hardness is lower than that of the standard sample.
Mohs hardness is simple and intuitive, and is applicable to minerals and some engineering materials, but it has low accuracy and is only a relative value. For example, the Mohs hardness of cemented carbide is about 9-9.5, close to corundum and much higher than steel (about 5-6) (ITIA 2024). Its limitations are that it cannot quantify the hardness difference (for example, the difference between 9 and 10 is much greater than that between 1 and 2), and it is not suitable for testing inhomogeneous materials or thin layers (the error is about ±0.5).
When converting, please pay attention to the nonlinear relationship, HV≈10·HRC+900 (error <5%), HB and Mohs conversion error>10%. In actual application, HV is the main method, and HRC/HS is the auxiliary method to ensure performance consistency>95%±2%.
3.1.1.2 Hardness test method
Hardness testing ensures measurement accuracy by:
Vickers hardness test (ISO 3878)
The hard alloy containing 10% Co (HV 1800±30) uses a 10 kg load, an indentation diagonal of 2030 μm±0.5 μm , a microscope magnification of 400 times±10 times, and an error of <1%±0.2%. The environment requires 25°C±2°C, humidity <60%±5%, and the sample surface Ra<0.2 μm±0.05 μm . For example, the mold containing 15% TiC (HV 2000±50) passes the Vickers test to ensure a stamping accuracy of <0.01mm.
Rockwell hardness test (ISO 6508)
The hard alloy containing 6% Co (HRC 90±1) adopts HRC scale, preload 10 kg±0.1 kg, main load 150 kg±0.1 kg, hold 5 s±0.5 s, error <2%±0.5%. Suitable for the site, such as the tool containing 8% Co (HRC 89±1).
Portable Hardness Tester (Leeb, ASTM A956)
Cemented carbide containing 12% Co (HS 85±2) uses an impact energy of 11 N·mm±0.5 N·mm with an error of <3%±0.5%. Suitable for mining sites, such as rollers (HS 87±2).
Ultrasonic hardness testing (ASTM A1038)
Cemented carbide containing 8% Co (HV 1900±50) uses ultrasonic contact impedance with a frequency of 20 kHz±0.5 kHz and an error of <2%±0.5%, which is suitable for complex shapes such as aviation molds.
Nanoindentation Testing
The cemented carbide containing 10% Co (hardness 20 GPa±1 GPa ) uses a Berkovich indenter with a load of 10 mN±0.1 mN , an indentation depth of <200 nm±10 nm, and an error of <5%±1%, which is suitable for coating detection.
The test requires calibration (standard block error <1%) and avoidance of vibration (acceleration <0.1 m/s² ) . For example, a mold containing 15% TiC is tested by Vickers and nanoindentation to ensure a life of >6000 hours ±500 hours.
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