The Steel
For an underground Tophammer button bit, the steel used for the bit body is one of the crucial ingredients. During drilling the bit body is subjected to different types of wear.
Flushing of rock cuttings are forced to flow by the bit body at a reasonable velocity which can be seen as machining it with a grinding paste. A certain amount of rock cuttings are always trapped in between the bit head and the rock and during impact, parts of these are crushed by the bit head which can be compared to using the bit body as a rock crusher. A certain amount of rock cuttings are continuously squeezed in between the bit gauge and the rock wall which is a sort of crude and rough grinding operation. Naturally the steel needs to be able to withstand all of these types of wear but it should not completely withstand it. It should allow just the right amount of wear to be in balance with the wear of the carbide buttons. If the steel wears too fast, the buttons will drop out or the body will crack, and if the steel wears too slowly the required drill string torque will gradually increase until the body will get stuck in the hole.
Except for the wear, the bit body is subjected to a large amount of mechanical stresses trying to tear it apart. Majority of which is of course originating from the hammer impacts from the rock drill which forces the bit into the rock with the equivalent of 20-30 tons of weight being put on and removed from the bit some 100 times each second. From these numbers it is easy to understand that the bit body performs an immense amount of work for every meter drilled. This takes its toll on the steel body that need to be able to take this beating without being fatigued.
In order to meet the above requirements, Epiroc R&D used what was developed for the Powerbit surface drill bit range and adapted it for the smaller underground bits and its specific needs and achieved a steel body which has more than 10 percent greater fatigue strength and gives more than 20 percent greater wear resistance. It is clear that this increased wear- and fatigue resistance can be used together with the improved PUG design features to keep the wear balance and the increased fatigue strength allow greater freedom in the design to achieve that.
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