The fatigue strength of the drill rod depends on the weakness of the weakest part of the drill rod and the degree of stress concentration caused by macroscopic and microscopic defects. Therefore, it is often the source of damage due to microcracks in a small part of the drill rod. Therefore, the material of the drill rod and its uniformity, the geometric structure and parameters of the drill rod, the physical and mechanical properties, and the surface condition are all determinants of the fatigue strength of the drill rod. The hollow steel smelting and rolling process, the forging and heat treatment process of the drill rod, and the rock drilling conditions will all have an important effect on the fatigue strength of the drill rod.
Factors that affect the fatigue strength of drill rods include surface quality, alloy composition and non-metallic inclusions, smelting, rolling, and forging.
Surface quality
Drill rod surface quality seriously affects its fatigue strength, and drill rods made from stronger alloy steels are particularly sensitive to surface quality. A small scratch on the surface often becomes a dangerously sharp nick. Drill rod surface defects, such as dents, cracks, bubbles, decarburization, knife marks, and corrosion will increase the fatigue notch sensitivity and reduce the service life of the drill rod. For surface quality problems, shot peening can effectively improve the surface compressive stress and hardness of the drill rod, significantly reducing fatigue notch sensitivity.
Metallographic organization type and grain size on the fatigue strength of the impact are more complex. On the one hand, the more uniform the structure, the finer the grain, the higher the tensile strength of the steel, the greater the toughness, and the smaller the notch sensitivity. The fatigue strength is also higher. Appropriate rod hardness will ensure that the drill rod has a lower fatigue notch sensitivity, thus improving the fatigue strength of the drill rod. In the production process, increase the quenching and heating speed and improve the quenching degree of subcooling, control the growth of grains during the heating process, and phase deformation of the nucleation rate to ensure the organization of the grain size. Through the use of intermediate-frequency induction heating and oil cooling can be achieved by the control of the organization of the grain size.
Alloy composition and non-metallic inclusions
In general, the fatigue strength of steel increases with the increase of tensile strength, and alloying elements can increase the tensile strength to a certain extent, thereby increasing the fatigue strength.
Non-metallic inclusions in steel are the origin of fatigue cracks but also disrupt the continuity of the matrix, causing stress concentration, which will seriously reduce the fatigue strength.
Smelting
The hollow steel produced by the plain carbon steel pipe casting method has a layer of low-carbon steel on the inner wall, equivalent to a layer of “natural decarburization layer”, the source of fatigue is easy to produce in this. Therefore, the drill rod made of this hollow steel has low fatigue strength and low service life. Although the “natural decarburization layer” can not be avoided, we can start from the conditions of its formation of fatigue source to ensure the quality of the drill rod. We can effectively reduce the influence of the “natural decarburization layer” on the service life of drill rods as long as we guarantee the straightness of drill rods.
Rolling and forging
Rolling of hollow steel requires good geometric accuracy and sufficiently high surface quality. The irregular geometric shape of the hollow steel section, unequal hexagons, deflection of the core hole, ellipse, and other conditions will lead to uneven stress distribution on the cross-section of the drill rod, and cause stress concentration, thereby reducing the fatigue strength of the drill rod.
Inappropriate forging and brazing process will make the drill rod produce some irremediable defects, such as over-burning, core hole occlusion at the collar plate, or the appearance of flare, etc., which will seriously reduce the fatigue strength of the drill rod.
For the quality problems caused by the two processes of rolling and forging, we should carry out strict inspection of the transfer material and find obvious quality problems and timely scrapping to improve the quality and stability of the drill rod.
