Cemented carbide is crucial for button bits, playing a key role in enhancing the drill bit’s longevity and cutting efficiency due to its impressive wear resistance and hardness. However, in complex working environments, cemented carbide may experience multiple failure modes. By analyzing its failure modes, the performance and service life of the button bit can be effectively improved.
Types and characteristics of cemented carbide
Cemented carbide is a composite material composed of hard particles and binders. Commonly used cemented carbides for button bits include WC, TiC, and B4C. These cemented carbides exhibit elevated hardness, exceptional wear resistance, and robust high-temperature stability, enabling them to maintain optimal performance in demanding working conditions characterized by high temperatures, high pressures, and high speeds.
Failure modes of cemented carbide
Abrasive wear
Wear without fracture is a normal failure for carbide buttons. The wear is usually due to the collision and friction between the carbide button and the rock during drilling. The hard particles in the rock first plow into the softer bonded phase part of the carbide button to make them preferentially worn out, and in the subsequent cutting, movement to further make the WC grains lose the protection of the bonded phase flake off, thus wearing off a small part of carbide button. Under the loading of the rock drill, the carbide button wears continuously, and the relative motion and contact area between the carbide and the rock also increase and the wear of the carbide button will be further accelerated.
Thermal fatigue
Thermal fatigue is a phenomenon that can be seen in all kinds of applications where cemented carbide button bits are used. Thermal fatigue is due to the impact and friction between the carbide button and the rock during rock drilling, increasing the surface temperature of the carbide. This temperature can reach about 700°C when the contact pressure, speed, and frequency are high. Water or other cooling media can “quench” the carbide. Due to the low thermal conductivity of the carbide, the heated surface will generate extremely high thermal stresses, and a two-dimensional stress field will be formed during the rapid cooling process when a crack is formed in one direction to release the stress in that direction and the perpendicular direction of the crack still retains high stress. Therefore, further stress release is possible only if a crack forms in a direction perpendicular to that crack. This thermal stress field is small in extent and decreases rapidly with crack growth.
Flaking
Flaking is the constant falling off of the surface of the carbide button of various sizes of debris or chips. Large pieces of flaking can greatly shorten the service life of drill bits. Even if the damage to the button is not catastrophic, it will greatly reduce the service life of the drill bit. In general, for carbides with higher hardness and poor toughness, obvious large flaking occurs. The more ductile carbide flaking presents itself in a form similar to the contact fatigue that occurs in bearings looks like microscopic chipping. The size of carbide flaking is related to the composition of carbide, the WC grain size, and the average free range of the drilled phase, but not to the rock being drilled.
Internal cracks
Fractures caused by internal cracks usually appear in the early stages of drill use, also known as early failure. This damage originates from the cracks generated inside the carbide. When the drill bit is working, the internal cracks act as stress concentration points, so the fracture marks caused by this damage are easy to distinguish.
Button non-exposed part fracture
The fracture of the non-exposed part of the button usually has a smooth and relatively unmarked section. This section is perpendicular to the axis of the button. The fracture can occur from the top of the non-exposed portion of the button to the deepest part of the button bore. The fracture of the non-exposed part of the button is usually caused by an improper fixing process or out-of-round fixing hole and button, resulting in a large tensile stress concentrated on a point on the button. If there is a crack in the cylinder of button bit itself, it will also cause this fracture.
Shear fracture
It is generally not easy to be found when a shear fracture occurs. Because the drill bit can continue to work after a shear fracture occurs, it will greatly shorten the service life of drill bit. The shear fracture section is messy and has no obvious characteristics. Usually, the shear fracture is easy to occur at a certain angle with the axis of the button, and the sharp corner of the in-line carbide sheet that is sensitive to shear fracture is particularly prone to occur. It is necessary to carefully observe the section to determine whether it is a shear fracture. Under normal circumstances, there will be a characteristic of tensile damage at a place far from the smaller shear area.
Surface cracks
Surface cracking is similar to shear fracture in that it occurs on the exposed part of carbide button. It is only in this form of fracture that its initiation usually occurs at the place where the compressive stress is the greatest and the rock is most effectively broken, but it is difficult to find the crack initiation point on the fracture. The characteristics of this fracturing can usually be roughly understood by analyzing several spherical buttons in different stages. This type of surface cracking is a common form of damage to the button, which may be mistaken for a normal form of damage, but is caused by the structure of the drill bit head, the drilling method, the position of the button, the structure of the rock being drilled, etc.
There are various failure modes of cemented carbide in button bits. To improve button bits’ performance and service life, we should take corresponding preventive measures and technical improvements for different forms of failure. By optimizing the design, selecting high-quality carbide materials, and adopting advanced manufacturing technology, the incidence of various failure modes can be effectively reduced, thereby improving the overall performance and service life of the button bit.
