Abstract
Thanks to the complex underground environment that coal mines enjoy, drill rods for mining are vulnerable to corrosion during operation. To investigate the impact of the corrosion defects on the residual intensity of the drill rod, a relational expression of stress versus corrosion rate in the conditions of uniform and local corrosions is deduced based on the theory on mechanochemical effects; building on this, a spherical corrosion defect is developed in the joint of a φ73 mm drill rod while it is exerted with a make-up torque, an axial force and a bending moment. So it is found that, when the corrosion defect is under pressure, the bending moment plays a certain role to inhibit its increase, however when it is under tension, the bending moment plays the role to drive its increase so as to quicken its corrosion rate while the impact of the change in the corrosion detect depth is much greater than that of the radius. The result from the research provides a basis to evaluate the residual intensity of the drill rod and theoretical basis to protect drill rods from corrosion.
Highlights
As one of important tools in underground drilling of coal mines, the drill rod is a main component that transmits and bears forces during drilling
When the corrosion defect is 2 mm in depth, 2 mm in radius and under the compressive force, the stress there is as shown in figure 2 and when it is under the tensile force, the stress there is as shown in figure 3. It is understood from figure 2 that, after exertion of the make-up torque, the working torque and the feeding force on the drill rod, the bending moment continues being exerted, the drill rod is first in the elastic stage during which the stress at the corrosion defect rises linearly, when the stress rises to 480 MPa, the bending moment rises within a certain range and the stress goes flattened, meaning the drill rod is in the yield stage; after the stress rises to 560 MPa, with the increasing bending moment loaded, the stress there slumped to 300 MPa, and gets flat, and with the continually increasing bending moment loaded, the stress there rockets up to 580 MPa
When the drill rod is under the negative bending moment, i.e. the stress there is under the tensile force, it drives the change of the stress there to rise, and it is known in combination with the relational expression of stress corrosion rate that the corrosion rate rises
Summary
As one of important tools in underground drilling of coal mines, the drill rod is a main component that transmits and bears forces during drilling. Recent years have been seeing progress in the underground coal-mine drilling technology and increased capability of the drilling machines and tools, and the research on structure and intensity of drill rods in various galleries has made some advances. Subjected to axial force, bending moment and torsional stress, etc., drill rods are required to be strong enough, reliable in operation and long in service life. Because of the action of corrosive medium, the stress corrosion in drill rods[2,3] includes uniform corrosion and local corrosion. Local corrosion will leave stress concentrated at the corrosion defect of the drill rod, and thanks to the bending moment[4,5], the stress at the corrosive place ramps up, accelerating the corrosion. Studying the impact of the bending moment and the corrosion-defect-related parameters on stress in the corrosion defect of drill rod joints is critical to protect drill rods from corrosion and improve the safety of drilling
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