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Tapered drill rods and integral drill rods
10 Jun 2025
Tapered drill rods and integral drill rods generally refer to drill rods made of H22 and H25 hexagonal hollow drill steel, which are suitable for shallow hole drilling operations of small pneumatic rock drills or hydraulic rock drills. Integral drill rods are drill rods with carbide inlaid on the head, which are widely used in developed countries and some developing countries. The head of the tapered drill rod is conical, and it can only be used for rock drilling after being connected and matched with the drill head with a tapered hole. In my country, almost all shallow hole rock drilling is done with tapered drill rods, which are also used in Canada and some developing countries.
The most widely used steel for this type of drill rod abroad is 95CrMo, which belongs to the alloy tool steel category, and the same category includes 95Cr, etc. Alloy structural steels include 40SiMnCrNiMo, 38CrNi3Mo, 38SiCrV, 40MnMo, etc. Some countries also use spring steel 55Si2Mn and carburized steel 18CrNi3Mo. After a long period of screening in China, it is believed that spring steel 55SiMnMo is a relatively ideal steel grade. The T8 and T8Cr steels that were widely used in the early days have now been basically eliminated.
The main requirements for the steel grades of tapered drill rods and integral drill rods are:
(1) High fatigue strength and good toughness combination.
(2) Low notch sensitivity and low fatigue crack growth rate.
(3) A hardness of about HRC40 can be obtained under controlled rolling and controlled cooling or normalizing conditions to ensure the elasticity of the drill rod. After quenching and tempering, the hardness reaches HRC50~54 to ensure that the drill tail does not produce top pile or top explosion under the impact of the rock drill piston.
(4) There is a small tendency to decarburize when heated.
1.55SiMnMo steel
55SiMnMo steel is a special steel type for tapered drill rods for shallow-hole rock drilling developed successfully in my country. It is a bainitic steel improved from 55Si2MoV. It is alloyed with silicon-manganese, and has high elastic limit and fatigue limit. The addition of molybdenum has a strong inhibitory effect on the transformation of steel from austenite to pearlite, which is conducive to the formation of bainite. At the same time, molybdenum can reduce the overheating sensitivity and graphitization tendency of silicon-containing steel. Under the condition of hot rolling and air cooling, a residual austenite structure dominated by granular or lath bainite and rich in carbon is obtained. Research by Chinese scholars has shown that the carbon-rich residual austenite of 55SiMnMo steel, whether in strip or block form, has a good inhibitory effect on the extension of fatigue cracks. A large number of tests, production and use have proved that the H22mm drill rod made of 55SiMnMo steel has created the highest record of the service life of tapered drill rods in my country.
