Fatigue fracture is the most common kind of failure for Rock Drill Extension Rod, which primarily occurs in the threaded part and transition groove region. The drill rod among them breaks approximately 2/3 in the thread and the transition groove and about 1/3 of the rod body. The bulk of fatigue fractures is seen outside the Rock Drill Extension Rod, while internal fatigue fractures are rare (except for the metallurgical defects on the inner surface). The Rock Drill Extension Rod's primary failure mechanism is fretting wear and impact wear, which combine to generate a fatigue source, which then grows into a fatigue crack, and the fatigue crack's progression results in the drill rod's eventual fracture.
Consequently, the Rock Drill Extension Rod needs to fulfill the requirements listed below:
(1) High toughness and wear resistance;
(2) Strong fatigue resistance and enough stiffness and flexibility;
(3) Low fatigue crack propagation rate and low notch sensitivity;
(4) Needs to be resistant to high-temperature softening and have a particular high-temperature hardness;
(5) Must possess excellent corrosion resistance as well as fatigue and corrosion resistance.
There is currently no steel grade among all steel grades that can completely match these specifications. Some performance requirements are still not reached despite the use of many heat treatment techniques. Only low-carbon high-strength alloy steel is currently integrally carburized after thread forming, and quenching and low-temperature tempering are carried out on both ends of the thread and the transition groove after air cooling, according to a thorough investigation and analysis of Rock Drill Extension Rod in various countries (or controlled cooling). To achieve a longer and more stable life, the inner hole and the outside surface are sandblasted or shot for anti-corrosion treatment.
