The exploitation of various mineral resources, the construction of energy transportation such as railways, highways, water conservancy, and hydropower, as well as the departments of geology and construction, are inseparable from rock drilling tools. As a highly expendable tool in rock drilling tools, the drill rod is subjected to high peak value and high-frequency impact load in the process of rock drilling, and coupled with its harsh working conditions, its service life is generally short. Therefore, improving the service life of drill rods has become one of the urgent problems for modern industrial high-efficiency and low-consumption rock drilling operations.
Although many factors affect the service life of drill rods, the most important one is the quality of raw materials. Hollow steel is a steel specially used to manufacture drill rods and its intrinsic quality, rock drilling conditions, and use of technology all affect the service life of drill rods. Among them, the intrinsic quality of hollow steel is of decisive significance.
The intrinsic quality of hollow steel is determined by the geometric structure parameters, steel type, and production process, and the production process is the main reason for the surface and inner hole defects of drill rods.
Causes of surface defects of drill rod
First, in the production process of hot rolling of solid bar stock, due to improper heating temperature, uneven heating time, improper cooling, and other reasons caused by the surface decarburization of the bar stock, and the possibility of crack generation due to excessive rolling stress, this defect and retained to the subsequent processing process.
Secondly, with the solid bar hot piercing, the hot rolling process is related to the process. The defects existing in the production of bar stock in the hot piercing – reduction process of the intense deformation process, due to excessive reduction ratio after hot piercing, the outer diameter of the hollow steel is compressed, resulting in convergence, stacking deformation, and easy to produce folding. When working in service, such folding of the drill rod will form fatigue cracks and reduce the fatigue strength of the rock drilling tool, thus affecting the service life of the rock drilling tool.
Causes of the inner hole defect of drill rod
The cause of cracks in the bore and the formation of severe decarburization on both sides of the cracks are mainly related to the solid bar in the piercing and reduction process. When the bar is hot pierced, the heating temperature is too high, which will cause decarburization of the inner hole. In addition, due to the poor steel quality of the bar, there are defects such as bubbles, inclusions, and cracks. In the hot piercing, the plug and the hole wall produce a large frictional heat. Too high frictional heat causes the inner hole to overheat, intensifying the tendency of decarburization. At the same time, due to the severe deformation of the perforation and diameter reduction process, the defects are easy to expand and form inner hole cracks.
To sum up, the reason for the surface and inner hole quality defects in drill rods is inseparable from the production process of hollow steel. Therefore, to improve the production quality level of hollow steel, it is necessary to control the whole process from smelting to finished product in order to get products with good surface quality and internal quality, uniform composition, and organization. Only by using high-quality hollow steel can we make long-service life drill rods.
