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Material of Rock Drilling Tools and the Smelting Process
Sep 07, 2018

Material of Rock Drilling Tools and the Smelting Process


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The influence of the material of the brazing steel brazing material and the smelting process


The rock drilling tool has a small cross-sectional area due to the limitation of the diameter of the hole in the drilling and blasting process. However, this slender rod is subjected to high frequency (2000-3000 times/ml.r·) and high impact work (about 294J) rock drill under the conditions of severe wear and corrosive medium. Cyclic stress such as pressure, bending and twisting. Rock drilling life is often only a few minutes to a dozen hours, depending on the load size and rock solidity. For steels used in brazing tools, this basic feature of the work of the tools must first be met.



The main resistance index of the fracture of the drill is the tensile fatigue strength under stress corrosion conditions. The steel for the brazing tool must have sufficient fatigue strength, proper plasticity and toughness, and especially good toughness to ensure the steel has low fatigue notch sensitivity. The drill rod is operated under cyclic stress. The cyclic toughness of the steel is very high. Only when it has good cycle toughness can the steel have good vibration damping performance and reduce the expansion speed of the fatigue crack of the drill. The drill is operated under extremely harsh conditions. The steel for the drill must also have certain corrosion resistance, mainly resistance to atmospheric and mineral water corrosion.



 Carbon tool steel with a carbon content of 0.70% to 1.0% is a brazing steel grade used for shallow hole rock drilling at home and abroad. It matches the low frequency and low impact power rock drill and can still adapt to the drill at that time. Chisel engineering needs. With the development of rock drills, the fatigue strength of carbon tool steels has become increasingly difficult to meet the drilling requirements, although 0.15% has been added to improve the hardenability, grain refinement and toughness of the steel. The carbon tool steel of 0.30% vanadium or 0.47%-0.70% chromium has been reduced in amount, and it is still replaced by alloy brazing steel.


China's medium and large diameter drill rods, drilling 35Si. MnMc, v brazing steel is only subjected to the normalized post-test rod, and the average service life is up to 300m/d. After chemical heat treatment and surface shot peening, the average drilling life can reach 420m / support. However, in the past 10 years, foreign advanced high-frequency, high-power hydraulic rock drilling equipment has entered China in large quantities. The plasticity, cyclic toughness and fatigue strength of the steel grade affect the quality of the steel. The non-metallic inclusions in the steel are fatigue cracks. The source point also destroys the continuity of the matrix, causing stress concentration. The larger the inclusions, the more the fatigue strength is reduced. The content of gas and non-metallic inclusions in vacuum smelting is much lower than that of ordinary smelting, so the fatigue strength is also much higher. Therefore, the smelting of modern hollow steel is carried out by electric arc furnace smelting, vacuum degassing and refining outside the furnace to maximize the purity of the steel. That is to say, the quality of the welding of the raw materials of the brazing material has a great relationship with the advanced degree of the rolling process and equipment. The characteristics of hollow steel produced by the "alloy cast pipe" process are that the alloy steel pipe can be cast more firmly in the hollow steel ingot. After rolling, the inner surface of the hollow steel can form an alloy-lined pipe wall, to a certain extent. Strengthen the inner surface to increase the drilling life of the drill rod. However, in the smelting process, due to the small ingot type, metallurgical defects such as shrinkage holes and inclusions are concentrated on the surface of the steel ingot and the interface between the alloy steel pipe and the base steel. After rolling, these metallurgical defects are extended into strips and linearly dispersed randomly on the cross section of the steel. It is made of drill rod rock drilling, and it is easy to produce crack source under the condition of mine water scouring and stress corrosion. The hollow steel which is rolled by the mechanical center drilling and alloy liner process, due to the low drilling precision and the influence of the core technology and equipment, the liner and the base steel are poorly bonded, and the phenomenon of partial separation is serious. Some produce deformation and folding, forming sharp corners, causing stress concentration, and initiating fatigue crack sources.

    


The inclusions of the brazing steel such as 55SiMnMo and 35SiMnMoV are mainly Ca-containing oxides. When the inclusions are small and diffuse, they have little effect on fatigue fracture, and only have a large effect when concentrated in groups or in a chain distribution. The main failure of drill bit drilling is the internal and external fatigue shape, and the fatigue crack source is generally concentrated on the inner and outer surfaces of the drill rod. Crack source points are also often produced at pits, scratches and folds of a certain size (0.15 to 0.20 ram). These surface defects are highly susceptible to stress corrosion and the maximum stress is in the bottom region of the microcrack. Under the action of the electrode potential difference, the bottom potential is more negative, which promotes the tendency of the crack to expand in the depth. "Steel billet drilling" toughness. Once this tissue is present near the shaft or the shoulder, the life is reduced. In the manufacture, the salt bath furnace is used for tempering and overheating. Due to the heat conduction; when the temperature at the shoulder is raised to 400"C or more, tempered bainite is often formed, which causes the drill rod to break early.


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