This paper analyzes the material defects, working environment and stress characteristics of drill string in railway drilling in turn, and then discusses five specific manifestations of drill string wear combined with the above analysis. Finally, some views on reducing drill string wear are put forward. Key words: date of receipt of drill string wear in railway drilling: 2007- 10- 10 In railway drilling, drill string wear is a very serious problem. When the drill string is worn to a certain extent, it should be replaced with a new drill string. At present, the diameter of drill string has not reached the level of automatic monitoring with instruments, and most of them are visually observed by technicians on the spot and measured with vernier calipers. However, the use of specialized technicians to monitor the diameter of drill pipes is often aimed at drilling holes with deep holes. When the drill string is seriously worn and neglected in maintenance, it is easy to cause the drill string to break when it encounters abnormal stress concentration. In a good rock and soil layer, it is relatively easy to salvage the drill string if the verticality of the borehole is good, otherwise it will waste a lot of time and cause economic losses, especially for deep-hole drilling. The study of drill string wear has been highly valued by many scientific and technological workers. The external and internal causes of drill string wear in railway drilling are analyzed, as well as the specific manifestations of wear. First, the material defects of the drill pipe From the material point of view, the inside of the drill pipe is defective. There is a heat treatment process in the production workshop when producing drill pipes. During heating and cooling, the internal structure of drill pipe will change. Heat treatment usually eliminates the coarse-grained structure in the drill pipe, making its structure refined and able to bear greater stress. But there are always some defects. Observing the grain structure of drill pipe under high power electron microscope, it is found that it is composed of many spatial lattices with ions and atoms arranged according to certain rules, and the lattices are generally in a stable equilibrium state. There are usually some defects, such as inclusions and voids between grains. In these grains, even in the elastic range, plastic deformation may occur when the force is not too large. Second, the working environment of drill pipe rotary drilling is mostly used for railway drilling, and impact drilling is also used for rock strata with difficult coring, such as sand layer and fully weathered layer. In the work of drill string, the main media interacting with drill string include drilling fluid and rock and soil layer. Railway drilling drilling fluid is generally water-based drilling fluid, which is a multiphase unstable system. Water is the dispersion medium (continuous phase), clay is the dispersed phase (solid phase), and certain chemical treatment agents or weighting materials are added. Its components include water, bentonite, chemical treatment agent (such as filtrate carboxymethyl cellulose), gas (dissolved oxygen, carbon dioxide gas and hydrogen sulfide gas) and other corrosive media such as Cl-, SO42-, Ca2+, CO32- and HCO3-. Common rock cores in railway drilling include soil layer, sandy pebble layer, fully weathered rock layer, strongly weathered fractured rock layer and weakly weathered rock layer. The wear of drill string in complex strata such as sandy pebble layer and hard and strongly weathered layer is more serious than that in other strata. Three. Working stress characteristics of drill string During the working process, the motion modes of drill string include rotation and revolution around the drilling center, which usually exists in deep hole drilling. The stress on the drill string is a compound stress, which mainly includes the following sub-items: tensile stress caused by the weight of the drill string, bending stress caused by lateral stress, shear stress caused by torque, axial and lateral stress caused by the vibration of the drill string, friction with rock stratum and interaction with drilling fluid. Four. Drill pipe wear form 1. Abrasive Wear Abrasive wear is the phenomenon of surface wear and surface material falling off due to the micro-peaks of external hard particles or hard surfaces during the relative motion of friction pairs. The surface characteristics of drill string are wear and tear, longitudinal stretch marks, local peeling and cracks. The wear mechanism of abrasive particles is micro-cutting, multiple plastic deformation leading to fracture and micro-fracture. Under the condition of compound stress, the drill string is prone to bending, and serious friction occurs between the protruding part and the hard rock after bending. The quenching thickness of drill string surface is generally 1 mm, and the later wear rate will be accelerated after the quenching layer is worn off. In deep hole drilling, sometimes there are obvious scratches on the surface of drill pipe, which is the performance of abrasive wear. 2. Adhesive wear Adhesive wear is a wear phenomenon caused by the material transferring from one surface to the other or falling off the surface when two pairs of surfaces of a friction pair slide relatively. Because the actual contact area between the two pairs of surfaces of the friction pair is very small, the stress at the contact point is very large. When the two sides of the friction pair are in this state of high temperature and high stress, the surface of the rod breaks, which leads to the adhesion of the contact micro-peak, and then the adhesion point is cut off during sliding. The bending of drill pipe causes local friction with rock stratum, and the high stress of local contact point constitutes the condition of adhesive wear. 3. Fatigue wear Fatigue wear refers to the rolling or rolling of two pairs of surfaces of a friction pair (page 185).

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