The H22 tapered drill rod is a commonly used rock drilling tool in mining, highway, and railway tunnel engineering construction. Its performance and durability play a vital role in the progress and quality of the project. However, in rock drilling, the drill rod is subject to axial compression, tensile, torsional, and bending stress. The rod and the rock crash with each other, the inner wall of the core hole of the rod is corrosion by mineral water, the use of harsh environmental conditions, the stress condition is complicated, and it is easy to produce early failure phenomenon. It not only affects construction efficiency but may also cause safety hazards. So, what are the reasons for the early failure of the H22 tapered drill rod?
The early failure of the H22 tapered drill rod has the failures of the shank end, the shoulder area, and the breakage of the drill rod.
Causes of shank end failure
The H22 tapered drill rod shank consists of a shank and a drill shoulder. The major function of the shank is to connect with the drill sleeve of the rock drill to withstand the impact energy, torque, and propulsive force output by the rock drill. The drill shoulder can position the impact point of the rock drill piston and cooperate with the drill clamp of the rock drill to prevent the drill rod from falling off during use, and at the same time, it can cooperate with the rock drill to withdraw the drill rod from the rock hole when the rock drilling operation is finishing.
The failures of the shank of the H22 tapered drill rod during use are phenomena such as blowing top and stacking top.
The reason for stacking top
Under the high-frequency impact of the rock drill piston, if the hardness of the end of the shank is low, it may cause the end-to-edge size to become large, resulting in the drill rod not being taken out properly in the hexagonal sleeve of the rock drill, which is called the phenomenon of shank stacking top. The main reason for the shank stacking top is the low hardness of the shank of the drill rod.
The reason for blowing top
Under the high-frequency impact of the piston of the rock drill, the local part of the shank end of the drill rod is subjected to multiple impact stresses, causing the end surface to continuously experience contact fatigue spalling and block falling, which is the phenomenon of shank blowing top.
The reasons for shank blowing top are as follows:
1. Shank expanding water pinhole heating temperature is too high or heating time is too long, which leads to overheating, overburning, or quenching heating, overheating, and overburning, resulting in the quenching process producing small cracks.
2. During the quenching process of the shank, the heating rate is too fast, the heating temperature is uneven, the cooling rate of the selected quenching medium is too large, the temperature of the quenching medium is too low, and the cooling rate is too fast, etc., which leads to higher quenching internal stress or generate microcracking.
3. Tempering after quenching is not timely or the tempering time is not enough, the tissue stress is not fully relieved, and the tempering temperature is low, resulting in the hardness of the shank exceeding 57 HRC and the change of the tissue stress area caused by the impact of the rock drill piston during the use of the drill rod to blowing top.
4. Non-vertical processing of the end surface of the shank of the drill rod, the poor flatness of the end surface, and heavy metallurgical defects in the hollow steel may lead to excessive local stress on the end of the shank, resulting in the phenomenon of blowing top during use.
Reasons for the failure of the drill shoulder part
The failure of the drill shoulder is mainly a fracture, and the failure parts mainly include the front of the drill shoulder, the middle of the drill shoulder, and the root of the drill shoulder.
The drill shoulder is the part where the cross-sectional area of the drill rod connected by the H22 tapered drill rod changes sharply. Therefore, the shape design of the drill shoulder must be reasonable, and the forged shape requires full filling. The irregular shape of the drill shoulder will cause stress concentration at the root of the drill shoulder and lead to early fracture.
Cause of fracture failure
The failure mode of the H22 tapered drill rod during use is a fracture, which is a brittle fracture and fatigue fracture.
Brittle fracture
Most of the main reasons for brittle fracture are defects in the hollow steel used to make the drill rod, coupled with flare, overburning, improper heat treatment, and other factors generated during the forging drill shoulder and subsequent processes, resulting in a reduction in the strength of the drill rod, poor plasticity or serious regional stress concentration, so the crack develops very quickly, resulting in the sudden fracture of the drill rod in use.
Fatigue fracture
The fatigue fracture point of the drill rod is usually at the fatigue source site, and the fatigue source is located more in the weak area of the material. Most of the vulnerable parts of the drill rod come from the non-metallic inclusions, bubbles, and white spots inside the steel hollow of the drill rod as well as the steel surface defects, cracks, scars, decarburization, corrosion cracks, and the drill shoulder structure of the drill rod which is unreasonable design, flaws generated during forging and heat treatment.
The fracture of the H22 tapered drill rod, especially the breakage of the rod part, has a great relationship with the metallurgical quality and type of hollow steel used, in addition to the defects in the processing.
Other causes of early failure of drill rods
Surface condition of drill rod
During transportation and handling, the surface of the drill rod may sustain damage by scratches or impacts. Although these damages may seem minor, they may affect the overall strength and stability of the drill rod. In addition, the straightness of the drill rod is also an important consideration. If the drill rod is bent or twisted during manufacturing or transportation, it will be subjected to uneven stress during the rock drilling process, thus accelerating its failure.
Cooperation between drill rod and rock drill
If the accessories of the rock drill experience severe wear, the drill rod will endure eccentric impact loads during the rock drilling process, leading to premature breakage. In addition, the wear of internal parts of the rock drill will also accelerate, forming a vicious cycle.
Operator skill level
Experienced rock drill operators adeptly adjust the operational parameters of rock drilling equipment according to varying rock drilling scenarios. It not only enhances drilling efficiency but also prevents the drill rods from overheating during use, thereby averting premature failure due to changes in the material properties of the rods. Therefore, strengthening the skill training of operators and improving their operating level is also an effective way to extend the service life of the drill rod.
How to use the drill rod
As a rock drilling tool, the drill rod should be used strictly according to its designed purpose. Avoid using it as a crowbar or other tool to avoid reducing its use value. At the same time, during use, you should avoid hammering the drill rod with heavy objects or making inappropriate marks on the drill rod. These actions will leave scars and sharp notches on the surface of the drill rod, leading to the early failure of the drill rod.
Conclusion
There are various reasons for the early failure of the H22 tapered drill rod, involving materials, processes, usage conditions, and other aspects. To extend the service life of the drill rod, we need to start from multiple angles, use high-quality hollow steel, equip specialized forging and brazing equipment, strictly control the process, strengthen the quality control of the drill rod, improve the skill level of the operators, and adhere strictly to the instructions for using the drill rod. At the same time, we must also intensify research and analysis into the causes of drill rod failures to effectively prevent and address early failure issues. By doing so, we can offer more dependable and efficient rock drilling tools for mining, tunnel construction, and other projects. Only through this approach can we significantly diminish the incidence of premature drill rod failures and guarantee the safety and efficiency of construction endeavors.
