Tungsten cemented carbide balls are spherical materials made by sintering powder metallurgy with tungsten carbide (WC) as the hard phase and cobalt (Co), nickel (Ni), or molybdenum (Mo) as the binder phase. Their toughness performance is influenced by the composition, processing, and microstructure, requiring a comprehensive analysis from the perspectives of impact toughness and fracture toughness.
I. Toughness Performance Indicators of Tungsten Cemented Carbide Balls
1. Impact Toughness
Impact toughness reflects a material's ability to resist damage under impact loads and is linearly correlated with flexural strength. Research has shown that:
Structural defects (such as pores and cracks) significantly reduce flexural strength, thereby weakening impact toughness. For example, the presence of pores in carbon-deficient tungsten cemented carbide balls can lead to stress concentration, forming transverse fracture crack sources.
WC grain size: As WC grain size increases, fracture toughness may increase, but flexural strength decreases, resulting in a decrease in impact toughness. Binder Phase Content: Increasing the cobalt (Co) content thickens the binder phase, improving plastic deformation and thus enhancing impact toughness. However, excessive Co content reduces hardness, so a trade-off between hardness and toughness is necessary.
2. Fracture Toughness
Fracture toughness is a comprehensive reflection of a material's strength and ductility, primarily influenced by the following factors:
Hardness: Higher hardness results in lower fracture toughness, but this range of influences varies. For example, low-Co coarse-grained alloys with similar hardness have better fracture toughness than high-Co fine-grained alloys due to enhanced intergranular bonding.
Microstructural Homogeneity: Homogeneous alloys have higher fracture toughness than heterogeneous ones, but may have lower flexural strength and impact toughness. For example, the homogeneous YG8 alloy performs well in fracture toughness tests, but its flexural strength is slightly lower than that of heterogeneous alloys.
Sintering Process: The pre-sintering temperature affects porosity, which in turn affects fracture toughness. Increasing the pre-sintering temperature can eliminate porosity and improve transverse rupture strength (TRS). However, excessively high carburizing temperatures can lead to abnormal WC grain growth, reducing fracture toughness.
II. Effects of Composition and Processing on the Toughness of Tungsten Cemented Carbide Balls
1. Ratio of Hard Phase to Binder Phase
WC Grain Size: Fine grains (e.g., submicron) improve flexural strength and impact toughness, but may reduce fracture toughness; coarse grains have the opposite effect. For example, YG6 (WC grain size approximately 1.5μm) has better impact toughness than YG8 (WC grain size approximately 2.0μm), but slightly lower fracture toughness.
Cobalt Content: Cobalt is a key element for improving toughness. For example, YG6 (6% Co content) has higher flexural strength than YG8 (8% Co content), but its impact toughness improves due to the increased cobalt content.
2. Sintering and Post-Processing
Vacuum Sintering: It reduces porosity and increases density, thereby improving flexural strength and impact toughness. For example, the transverse rupture strength (TRS) of vacuum-sintered YG6X alloy is higher than that of hydrogen-sintered alloys. Carburizing: Moderate carburizing can eliminate porosity, but excessively high temperatures can cause WC grain growth and cobalt evaporation, reducing fracture toughness.
III. The Conflict and Balance Between Toughness and Other Properties
Increased toughness in tungsten cemented carbide balls is often accompanied by decreased hardness and wear resistance, creating a typical trade-off:
Hardness-toughness trade-off: High-hardness materials (such as ultrafine-grained alloys) have low fracture toughness and are prone to brittle fracture; low-hardness materials (such as coarse-grained alloys) have good toughness but insufficient wear resistance.
Suitable Application Scenarios:
1. High-impact environments (such as mining machinery): Toughness is prioritized, so choose a low-cobalt, coarse-grained alloy (such as YG8C).
2. High-precision machining (such as precision bearings): A balance between hardness and toughness is required, so choose a medium-cobalt, fine-grained alloy (such as YG6X).
3. Extreme wear applications (such as oilfield drill bits): Toughness can be sacrificed to some extent, so choose an ultrafine-grained alloy (such as YG10X).
