Failure Modes of Cemented Carbide Rock Drill Teeth and Countermeasures

The cemented carbide rock drilling teeth violently impact the rock during rock drilling to achieve the purpose of drilling holes in the rock. During this process, the carbide teeth collide and rub against the rock, which inevitably causes wear. In the absence of fracture, the wear is a normal failure mode of the alloy teeth.

The wear is usually due to the friction between carbide teeth and the rock during the drilling process. The hard particles in the rock enter the softer binder phase of the carbide teeth to preferentially remove them, further losing the viscosity in the subsequent cutting motion. The phase-protected WC grains are peeled off, thereby grinding away a small portion of the carbide teeth. Under the loading of the rock drilling machine, the wear and tear, the relative movement of the alloy and the rock and the contact area are also increasing, and the wear of the carbide teeth is further accelerated.

In the process of using carbide drill teeth, thermal fatigue occurs, such as oil fields, mining, drilling wells, etc. Thermal fatigue is caused by the impact and friction between the carbide teeth and the rock during rock drilling, which causes the surface temperature of the teeth to rise. Under the condition of high contact pressure, speed and frequency, the temperature can reach 700 degrees or so. Water or other cooling medium acts as a "quenching" to the alloy, creating cracks.

During use, spherical tooth spalling also occurs. Fragments or debris of various sizes continually fall off the surface of the ball. Large pieces of spalling can greatly shorten the life of the bit, which is catastrophic. Generally speaking, for alloys with high hardness and poor toughness, obvious large pieces of flaking occur.

Of course, internal cracks may also occur during use, which may cause failure in the early use of the rock teeth. If the crack is generated in a deep hole, the crack may smash upward and cause longitudinal cracking of the upper part of the ball.

1. Dealing with abrasive wear

When no other failure mechanism plays an important role, the life of the bit depends on the wear of the alloy teeth. The wear resistance of the ball teeth can be improved appropriately, and the wear resistance of the teeth is higher than that of the middle teeth. Improve the rock drilling life of the bit.

2. Dealing with thermal fatigue

When thermal fatigue plays a major role, several or even hundreds of cracks can be seen on the surface of the ball. A bit design that reduces contact time between the ball and the rock minimizes thermal fatigue damage. Therefore, the drilling method has a great influence on the generation of thermal fatigue cracks. In the case of unreasonable drilling methods, it can be seen that there are severe thermal cracking cracks in some parts of the alloy, but not elsewhere.

3. Dealing with spalling

If excessive spalling occurs, this can generally be improved by increasing the toughness of the alloy (with a higher critical stress intensity factor KIC) and reducing porosity. Increase the content of cobalt in the alloy while refining the WC grains. Of course this will sacrifice a little wear resistance.

4. For internal cracks

To some extent, all of the spheroidal alloys have internal cracks. Under normal conditions, these cracks are themselves small and in small quantities. However, if the operating conditions are poor and the impact and shear stress on the alloy teeth are large, these small cracks will cause cracks due to stress concentration, which will cause early failure of the bit. Therefore, the better the toughness of the material, the less sensitive the crack is.

5. For the case of non-naked parts fracture

During the processing of the bit, the smoothness of the alloy ball after grinding should be ensured. It should not be rounded and no grinding crack should be produced. At the same time, the perforations of the bit head housing must also be rounded, without taper, and the bottom of the perforations must have an appropriate supporting shape that conforms to the bottom surface of the ball teeth. It is also an extremely important factor to select the appropriate amount of the tooth diameter and the hole diameter when cold pressing or hot cogging.