STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Bits and Downhole Tools 777
IADC CODE 11 1 IADC CODE 121 IADC CODE 131 IADC CODE 21 1 IADC CODE 31 1
Tooth Profile
IADC CODE 11 1 IADC CODE 131 IADC CODE 31 1
Figure 4-141. Steel tooth bit cutting structure design [44]. (Courtesy SPE.)
relatively short life of steel tooth bits in such formations. In this type of bit,
tungsten carbide and forged alloy steel are combined to produce a cutting
structure having a high resistance to abrasive wear and extremely high resistance
to compressive loads. Compacts of cylindrical tungsten carbide with various
shaped ends are pressed into precisely machined holes in case-hardened alloy
steel cones to form the teeth. The grain size and cobalt content of tungsten
carbide inserts is varied to alter the impact toughness and abrasion resistance
of the cutter. Softer formation inserts, which are usually run in less abrasive
rocks at higher rotary speeds, require increased toughness to resist breakage of
the relatively long cutters. A cobalt content of 16% and average grain size of 6
pm is typical for such inserts. Hard formation inserts are generally run in more
abrasive rocks at higher WOB levels. Hard formation inserts have a more
breakage-resistant geometry so abrasion resistance becomes the most important
factor. Thus the cobalt content is reduced to about 10% and the average grain
size is approximately 4 pm.