Earth-boring drill bits comprising one or more polycrystalline diamond compact (“PDC”) cutters are known in the art, and are referred to in the industry as PDC bits. Typically, PDC bits include an integral bit body that can be made of steel or fabricated of a hard matrix material such as tungsten carbide (WC). Tungsten carbide or other hard metal matrix body bits have the advantage of higher wear and erosion resistance when compared to steel body bits. Such matrix bits are generally formed by packing a graphite mold with tungsten carbide powder, and then infiltrating the powder with a molten copper-based alloy binder.
A plurality of diamond cutter devices, e.g., PDC cutters, are mounted along the exterior face of the bit body. Each diamond cutter has a stud portion which typically is brazed in a recess or pocket in the exterior face of the bit body. The PDC cutters are positioned along the leading edges of the bit body so that, as the bit body is rotated in its intended direction of use, the PDC cutters engage and drill the earth formation.
Such PDC bits are formed having a reinforcing/connecting member beneath the bit body that is bonded thereto. The reinforcing member is referred in the industry as a blank, and is provided during the process of making the bit for the purpose of connecting the bit body to a hardened steel upper section of the bit that connects the bit to the drill string. The blank is also used to provide structural strength and toughness to the bit body when the body is formed from a relatively brittle matrix material such as tungsten carbide, thereby helping to minimize undesirable fracture of the body during service.
Conventionally, such drill bit blanks have been formed from plain-carbon steels such as AISI 1018 or AISI 1020 steels because these steels remain relatively tough after infiltration of the bit body material therein (during sintering of the bit). Also, the use of such plain-carbon steels is desirable because they are easily weldable without the need for special welding provisions such as preheating and postheating, for purposes of connecting the bit upper steel section thereto. Additionally, tungsten carbide matrix bits made from plain-carbon steels are less vulnerable to transformation induced cracking that occurs when the drill bit is cooled from the infiltration temperature to ambient temperature. The reason for this is that the plain-carbon steel has a coefficient of thermal expansion that does not produce a drastic volume change during the phase transformation range as compared to the other alloyed steels.
A problem, however, that is known when using such plain-carbon steels for forming the drill bit blanks is that such materials lack a degree of strength necessary for application with today's high performance drill bits. Such high performance bits generate a high amount of torque during use due to their aggressive cutting structures, which torque requires a higher level of drill bit blank strength to provide a meaningful drill bit service life. The low degree of strength exhibited by such conventional steel blanks is caused both by the absence of alloying elements, and by excessive softening that occurs during thermal processes that must be performed during the bit manufacturing process.
It is, therefore, desirable that a drill bit blank be developed having improved strength when compared to conventional plain-carbon steel drill bit blanks. It is desired that such drill bit blanks also provide a degree of weldability that is the same as conventional plain steel drill bit blanks. It is also desired that such drill bit blank undergoes minimal volume change during thermal changes so as to induce minimal stresses in the tungsten carbide matrix material during manufacturing. It is further desired that such drill bit blanks be capable of being formed by conventional machining methods using materials that are readily available.