In rock drilling, because of the unevenness of the rock, the irregularity of the bottom of the hole, and a series of factors will directly shorten the service life of the extension rod. If the drill rod fails early, it will not only affect the progress of the construction and endanger the safety of operators but also increase the auxiliary time of rock drilling operation, reduce the efficiency of rock drilling, and consume a large amount of high-quality steel. Therefore, it is of great significance to improve the service life of the extension rod.
Service conditions of extension rod
The extension rod is a threaded drill rod whose nominal diameter is equal to the nominal diameter of the hollow steel body and is usually used for multi-branch connections. Because it is the major workpiece that transfers energy in the drilling process, its working environment and service conditions are very harsh.
During work, the extension rod mainly bears three kinds of stress:
1. Axial compressive and tensile stresses generated under multiple impact loads.
2. The torsional stress generated during the rotation process.
3. Since the extension rod is connected with multiple drill rods for rock drilling, there will be a certain degree of bending of the drill rod group, and the lateral vibration generated during the impact will cause the drill rod to produce complex bending stress.
In addition, it is also subject to contact fatigue and wear. Under such a complex stress state, the drill rod is required to have good comprehensive mechanical properties. The surface requires high hardness and maintains residual compressive stress so that it has good wear resistance and multi-stroke fatigue resistance; the core should have sufficient strength and toughness so that the drill rod is not easy to produce bending plastic deformation during use and early fracture.
Factors affecting the service life of extension rods
Many factors affect the service life of the drill rod, such as raw materials, heat treatment process, equipment selection, personnel operation, and other factors. We only introduce the metallographic organization of the material.
Formation of reticulated carbide
The reticulated carbide is formed when the carbon concentration is too high during carburizing, the cooling rate is too slow during cooling, and the carbide precipitation is along the original austenite boundary. These reticulated carbides are an important reason for the pitting and peeling of the extension rod when the surface wear is light during the dry period. The appearance of pitting pits accelerates the generation of crack sources and crack expansion, reducing the service life of the drill rod.
Formation of reticulated quartzite
Carbide and brittleness are networked, and brittleness is forming on both sides of the carbide, which is caused by the precipitation of carbide along the austenite grain boundary during cooling, resulting in the surrounding carbon-poor, reducing the stability of austenite, easy to form a non-martensitic organization. The surface layer appears brittle-nite, reducing the surface macro hardness and surface compressive stress, thereby reducing the fatigue resistance and wear resistance of the extension rod.
How to improve the service life of extension rod?
To improve the metallographic organization of the material and increase the service life of the drill rod, the following measures can be taken:
Control the carbon concentration during carburizing
High carbon concentration during carburizing is easy to form reticulated carbide, which increases the brittleness of the carburized layer and reduces the fatigue resistance. Control the carbon concentration of carburizing within a range. Carburizing can be divided into two stages: strong carburizing and diffusion, and adjust the amount of carburizing agent drops and holding time in these two stages accordingly.
Increase the cooling rate after carburizing
The cooling speed after carburizing influences the precipitation of reticulated carbides and the formation of reticulated brittleness, and can be increased by increasing the cooling speed after carburizing to prevent the precipitation of reticulated carbide and the forming of reticulated brittleness, improve the surface hardness of the material, and increase the surface compressive stress to improve the wear resistance and fatigue life of the drill rod.
Improve the straightness of the extension rod
The straightness of the drill rod has a significant impact on its service life. Poor straightness of the drill rod will cause the drill rod to work in an eccentric state, which not only loses impact energy but also produces bending stress, which increases the probability of drill rod fatigue fracture. To reduce the additional bending stress generated during the use of the extension rod, the straightness of the extension rod should be improved as much as possible.
